Hyperammonemia in Inherited Metabolic Diseases

Abstract

Ammonia is a neurotoxic compound which is detoxified through liver enzymes from urea cycle. Several inherited or acquired conditions can elevate ammonia concentrations in blood, causing severe damage to the central nervous system due to the toxic effects exerted by ammonia on the astrocytes. Therefore, hyperammonemic patients present potentially life-threatening neuropsychiatric symptoms, whose severity is related with the hyperammonemia magnitude and duration, as well as the brain maturation stage. Inherited metabolic diseases caused by enzymatic defects that compromise directly or indirectly the urea cycle activity are the main cause of hyperammonemia in the neonatal period. These diseases are mainly represented by the congenital defects of urea cycle, classical organic acidurias, and the defects of mitochondrial fatty acids oxidation, with hyperammonemia being more severe and frequent in the first two groups mentioned. An effective and rapid treatment of hyperammonemia is crucial to prevent irreversible neurological damage and it depends on the understanding of the pathophysiology of the diseases, as well as of the available therapeutic approaches. In this review, the mechanisms underlying the hyperammonemia and neurological dysfunction in urea cycle disorders, organic acidurias, and fatty acids oxidation defects, as well as the therapeutic strategies for the ammonia control will be discussed.

Ammonia Metabolism and Detoxification

Ammonia is an important source of nitrogen for cellular synthesis of proteins and essential compounds, being also important to the pH homeostasis. Ammonia is generated in the intestines from protein digestion and deamination by urease-positive bacteria and microbial deaminase (Stewart and Smith 2007; Bosoi and Rose 2009; Romero-Gómez et al. 2009). Several tissue metabolic reactions produce ammonia in the organism, such as glutamate dehydrogenase, glutaminase, and AMP deaminase activities. Glutaminase has a crucial role in the ammonia production since it converts glutamine to glutamate and ammonia in the intestines, kidneys, and brain (Cooper and Plum 1987; Butterworth 2002; Dasarathy et al. 2017).

Part of the ammonia produced by renal deamidation of glutamine in the proximal tubules is excreted in the urine (50%); however, the kidney can also liberate ammonia to blood (Weiner et al. 2015). Ammonia reabsorption is increased in situations of metabolic alkalosis and/or hypokalemia. Some hormones as mineralocorticoids and glucocorticoids may also impair the ammonia excretion by the kidney (Tchan 2018; Eguchi et al. 2021; Weiner and Verlander 2013). Besides, muscle protein metabolism, particularly during exercise, illness, and sepsis, contributes to the ammonia formation, which is incorporated in the alanine structure to be transported to the liver via the systemic circulation (Cooper and Plum 1987; Graham and MacLean 1992; van Hall et al. 1995).

In most extrahepatic tissues, ammonia reacts with glutamate by the action of the enzyme glutamine synthetase, leading to glutamine formation, which is transported to the liver. Hepatocytes have specific enzymes of the urea cycle (UC) that convert ammonia in the nitrogenous urea product, which is released into the blood to be excreted by the kidneys. This enzymatic cycle begins with the mitochondrial reaction between ammonia and bicarbonate (HCO₃⁻), catalyzed by carbamylphosphate synthase 1 (CPS1), to form carbamylphosphate. CPS1 is activated by N-acetylglutamate (NAG), which is produced from glutamate and acetyl-CoA by N-acetyl glutamate synthase (NAGS). In the sequence, ornithine transcarbamylase (OTC) produces citrulline from carbamylphosphate and ornithine. Citrulline is then converted to the amino acid arginine in the cytosol of the hepatocytes through the reactions catalyzed by argininosuccinate synthase 1 (ASS1) and argininosuccinate lyase (ASL). In the final step, arginase cleaves arginine into urea and regenerates the ornithine, making it possible to restart the cycle (Fig. 1) (Walker 2009, 2014).

Fig. 1.

Urea cycle reactions. The urea cycle initiates in the mitochondria of hepatocytes from the reaction between ammonia and bicarbonate to form carbamylphosphate, which requires N-acetylglutamate (NAG) as an allosteric activator. Carbamylphosphate reacts with ornithine to produce citrulline that leaves the mitochondria and originates arginine in the cytosol. Arginine is cleaved by arginase 1 (ARG), restoring the ornithine and forming urea which is eliminated by kidneys. CPS1 carbamylphosphate synthase; OTC ornithine transcarbamylase; ASS argininosuccinate synthase; ASL argininosuccinate lyase

Therefore, urea cycle function is fundamental to maintain low serum concentrations of ammonia (50–150 μM in preterm neonates, 50–75 μM in term neonates, and lower than 50 μM in adults) (Braissant et al. 2013). Impaired ammonia hepatic detoxification [e.g., liver cirrhosis, acute liver failure, portosystemic shunting, urea cycle disorders (UCD) and UC suppressors drugs] can result in toxic blood levels of ammonia (Felipo and Butterworth 2002; Häberle 2013a, b). In the presence of hepatic dysfunction, hyperammonemic episodes are usually provoked by infections, intestinal bleeding, constipation, surgery, glucocorticoids, excessive use of diuretics or catabolic states associated with high protein degradation (Vilstrup et al. 2014).

Ammonia can circulate as ammonium ion (NH₄⁺) or as a small gaseous fraction (NH₃). The ionized form predominates at physiological pH and can cross cellular membranes, including blood–brain barrier (BBB) (Sørensen 2013; Dasarathy et al. 2017). Brain is not able to convert NH₄⁺ to urea, so NH₄⁺ is maintained at low levels by the astrocytic glutamine synthetase activity (Suárez et al. 2002; Cagnon and Braissant 2007). However, severe liver diseases result in toxic accumulation of ammonia leading to a clinical condition known as hepatic encephalopathy (HE) characterized by neuropsychiatric alterations and coma (Fiati Kenston et al. 2019). The extent of brain damage depends on its maturation stage and on the magnitude and duration of hyperammonemia.

Prolonged hyperammonemia or ammonia blood levels between 200 and 500 μM, especially during the two first years of life, usually induce irreversible brain damage (Bachmann 2003; Enns 2008; Msall et al. 1984; Tuchman et al. 2008; Uchino et al. 1998). Brain abnormalities are frequently observed in neonates and infants with severe hyperammonemia, such as cortical atrophy, ventricular enlargement, demyelination, or gray and white matter hypodensities, which are related with severe cognitive impairment, seizures, and cerebral palsy (Enns 2008; Gropman et al. 2007; Tuchman et al. 2008). In these patients, the origin of hyperammonemia can be associated with genetic diseases, such as UCD, classical organic acidurias (OA), fatty acids oxidation defects (FAOD), pyruvate carboxylase deficiency, tyrosinemia type I, glycogen storage disease and others (Summar and Mew 2018). Early detection and correction of hyperammonemia is crucial for the patient survival and a better prognosis.

The mechanisms underlying the hyperammonemia and neurological dysfunction in UCD, OA, and FAOD, as well as the therapeutic strategies for the ammonia control will be discussed in this article review article. In view of the seriousness of these diseases due to the production of toxic metabolites in individuals affected, in order to elucidate the pathophysiological mechanisms of brain dysfunction found in these patients, the present work aimed to review the subject, emphasizing the consequences of toxic metabolism, mainly to the brain tissue. In addition, we intend to address the possible therapeutic targets and approaches.

Mechanisms of Brain Ammonia Toxicity

Ammonia has a central role in the development of brain disturbances during acute or chronic liver diseases and ammonia concentrations above than 150 μmol/L were related with the occurrence of cerebral edema (Bernal et al. 2007; Clemmesen et al. 1999). Ammonia can enter brain cells through K⁺ ion channels and transporters (Na⁺/K⁺ ATPase, H⁺/K⁺ and Na⁺/K⁺/Cl⁻ co-transporters) (Moser 1987; Kelly et al. 2009) or through aquaporin-8 channel (Liu et al. 2006; Saparov et al. 2007) or other ammonia specific transporters (Bakouh et al. 2006). Since astrocytes have a high affinity for potassium (Marcaggi et al. 2004; Bosoi and Rose 2009; Rangroo Thrane et al. 2013), ammonia can compromise astrocyte potassium buffering, resulting in altered brain electrolytic homeostasis (Alger and Nicoll 1983; Brookes and Turner 1993; Marcaggi et al. 2004; Rangroo Thrane et al. 2013).

Many toxic brain effects caused by ammonia excess are related with the accumulation of glutamine in astrocytes. The reaction between ammonia and glutamate by glutamine synthetase in astrocytes protects brain against hyperammonemia; however, this detoxification system creates an osmotic stress in the cells, leading to astrocyte swelling (Hertz and Zielke 2004). This mechanism is related with brain edema and was confirmed in a large number of studies (Gregorios et al. 1985a,b; Ganz et al. 1989; Willard-Mack et al. 1996; Tanigami et al. 2005; Cagnon and Braissant 2007; Jayakumar et al. 2008; Butterworth et al. 2009; Rangroo Thrane et al. 2012). However, astrocyte swelling does not explain all the neurotoxic effects of ammonia and it has been proposed that cerebral aquaporins can regulate the water fluxes, modifying the osmotic effects induced by glutamine (Rama Rao et al. 2010).

Glutamine is taken up by astrocytic mitochondria where it is converted into glutamate and ammonia, which induces changes in pH and in cell membrane potential, rise of intracellular Ca²⁺ levels, inhibition of several bioenergetic pathways and alterations in protein phosphorylation state of various ion channels, transporters, and enzymes (Busa and Nuccitelli 1984; Norenberg 1998; Cudalbu 2013; Rose et al. 2005; Wang et al. 2012; Rangroo Thrane et al. 2013). Ammonia accumulation can disrupt mitochondria metabolism, leading to an excessive production of free radicals (Kosenko et al. 1997; Murthy et al. 2001; Sinke et al. 2008; Norenberg et al. 2009) and nitric oxide (NO) through citrulline-NO cycle (Braissant et al. 1999; Bachmann et al. 2004; Zielinska et al. 2011). These events associated with an increase in mitochondrial Ca²⁺ levels result in the opening of a specific permeability transition pore in the inner mitochondrial membrane, leading to the induction of mitochondrial permeability transition (MPT). MPT provokes a collapse of the mitochondrial inner membrane potential and impairs oxidative phosphorylation and ATP synthesis, leading to the loss of the mitochondria and cell death (Rama Rao et al. 2003; Rama Rao and Norenberg 2012; Albrecht and Norenberg 2006; Jayakumar et al. 2004; Halestrap et al. 1997; Kowaltowski et al. 2001). Ammonia-induced MPT and oxidative stress can also activate the nuclear factor-κB (NF-κB), stimulating an inflammatory cascade that aggravates the neurological damage (Sinke et al. 2008; Marchetti et al. 1996; Bowie and O’Neill 2000; Kyriakis and Avruch 2001).

Ammonia brain toxicity has been also attributed to alterations in the brain mitochondrial energy metabolism, demonstrated by inhibition of α-ketoglutarate dehydrogenase (Lai and Cooper 1986) and other tricarboxylic acid cycle (TCA) enzymes, such as pyruvate dehydrogenase and isocitrate dehydrogenase (Zwingmann et al. 2003). In addition, the detoxification reactions between ammonia and α-ketoglutarate to form glutamate, and subsequently glutamine, cause α-ketoglutarate depletion that compromises Krebs cycle activity (Braissant et al. 2013). Ammonia stimulates glycolysis in astrocytes, as well as it inhibits pyruvate oxidation, resulting in an increased production of brain lactate (Ott et al. 2005; Dam et al. 2013; Bosoi et al. 2014; Zwingmann et al. 2003). High blood and brain lactate levels were verified in patients with HE and hyperammonemia and were associated with increased intracranial pressure and brain edema, the main neurological complications of acute liver failure (Tofteng and Larsen 2004).

Alterations in the neurotransmission also represent an important mechanism of neurological damage induced by hyperammonemia. Astrocytes provide glutamine to the neurons, where it is deaminated into glutamate which initiates an excitatory signal through the activation of the N-methyl-d-aspartate (NMDA) receptor. This process is mediated by pH and calcium alterations induced by astrocyte swelling that induce glutamate release from astrocytes, as well as inhibit the reuptake of this neurotransmitter. As consequence, glutamate causes an excessive depolarization of NMDA receptors, inducing alterations in NO metabolism and in Na/K-ATPase activity, that result in mitochondrial dysfunction, oxidative stress, and neuronal apoptosis (Ott and Vilstrup 2014). In animal models of acute liver failure, the NMDA receptor stimulation and ATP depletion were associated with convulsions and elevated mortality, which were ameliorated by treatment with MK-801, which is a NMDA receptor blocker (Marcaida et al. 1992, 1995). Glutamine is also used by the neurons for the γ-aminobutyric acid (GABA) synthesis, an inhibitory neurotransmitter. After its receptor binding, GABA can be cleared from the synaptic cleft by the astrocytes, where it is recycled into glutamine. Elevated glutamine concentrations in the astrocytes impair this cycle, compromising the GABAergic neurotransmission (Ott and Vilstrup 2014). Clinical evidence reinforces this mechanism since GABA receptor agonists, like barbiturates and benzodiazepines, aggravate the symptoms of HE (Goulenok et al. 2002).

In addition to astrocytes, other cell types have been implicated in the pathophysiology of HE and hyperammonemia (Butterworth 2011). Microglia are immune cells in the brain that release multiple pro-inflammatory mediators when activated (Ransohoff and Cardona 2010). This microglial activation can induce alterations in brain–blood barrier (BBB) permeability leading to brain edema (Jiang et al. 2009a,b; Rangroo Thrane et al. 2012). Therefore, multiple mechanisms can act synergistically to explain the neurological manifestations induced by hyperammonemia, which makes more complex the understanding of these events.

Inherited Disorders Associated with Hyperammonemia

Urea Cycle Disorders (UCD)

Hyperammonemia crisis in UCD patients is the most important clinical hallmark. These diseases occur due to a defect in any of the six enzymes and two transporters involved in the urea cycle pathway, responsible for the detoxification of ammonia in the human body. The enzymes involved in the cycle are located in the mitochondria (CPS1, OMIM #608307; OTC, OMIM #300461) and in the cytosol (ASS, OMIM #603470; ASL, OMIM #608310; ARG, OMIM #608313) and the conversion of ammonia to urea is performed mainly in the liver and to a much lesser extent in the kidney (Brusilow and Maestri 1996; Scriver et al. 2001).

Neonates presenting UCD usually have a total lack or very reduced enzyme activity or a deficient transporter function, whereas late-onset UCD patients still have some residual activity. Individuals affected by this group of diseases, especially neonates, may present severe encephalopathy, which is linearly proportional to the levels and time of the excessive ammonia circulating exposure (Häberle et al. 2019). Usually, newborns exhibit the primary symptoms after the first 24 h of life, when the hyperammonemic crisis can be stimulated by events like the passage from intra to extrauterine life, catabolic events, protein overload, and administration of certain drugs (Häberle et al. 2018). Children who have hyperammonemia in the neonatal period have poor cognitive, adaptive, and behavioral developments (Kido et al. 2012). Late-onset hyperammonemia can be triggered by the same neonatal form events, as for example, protein catabolism, liver failure, exogenous intoxications, portosystemic shunting, “Reye syndrome”, and circumstances that improve ammonia or nitrogen supply in human organism.

Primary therapy consists in removing the extra nitrogen from the body, dietary restriction with low protein intake and amino acids supplementation. Liver transplantation (LT) provides a practical cure for UCD but does not revert preexistent neurological damage and despite the efficiency of cure, the need is always evaluated due to the high risks of survival (Soria et al. 2019).

The encephalopathy presented by UCD patients is the main responsible for the high death rate in these diseases. Ammonia penetrates the BBB through diffusion mechanisms (Cooper et al. 1985) and astrocytic cells are responsible for the brain detoxification, performing the conversion of ammonia and glutamate into glutamine, by the action of the enzyme glutamine synthetase (GS), which is excreted without causing any damage (Auron and Brophy 2012). Studies show that individuals with acute liver failure show a swelling in astrocytic cells (Kato et al. 1992), as well as cerebral edema, increasing intracranial pressure. The swelling in astrocytes contributes to several neurological diseases, such as encephalopathy, epilepsy, migraine, among others (Bemeur et al. 2016; Sepherinezad et al. 2020). When ammonia is elevated in the brain, glutamate concentrations also increase and it causes alterations in NMDA receptors, which seems to be responsible for neuronal damage in acute and chronic hyperammonemia (Cooper 2001). The mechanisms involved in this process are still poorly understood, but it is believed that the increase in ammonia circulation has an important role.

Carbamoylphosphate Synthetase 1 Deficiency (CPS1D)

CPS1D is an autosomal recessive disease with an estimated prevalence of 1/1,300,000 live births. The enzyme carbamylphosphate synthase is responsible for catalyzing the synthesis of carbamoyl phosphate from bicarbonate and ammonia, characterizing the first enzymatic reaction of the urea cycle and the entry of ammonia in this pathway (Diez-Fernandez and Häberle 2017). To achieve the active conformation, CPS1 requires NAG as an allosteric activator and the lack of NAGS activity is limiting for the urea cycle, characterizing another UCD. CPS1D and NAGS deficiencies cannot be differentiated just by following biochemical profile and molecular testing is necessary to make the distinction (Nassogne 2005). CPS1D patients have a biochemical picture characterized by an increase in ammonia and glutamine and a decrease in plasma citrulline and arginine. Untreated hyperammonemia in CPS1D can trigger severe cases of encephalopathy, seizures, coma, and death, as well as severe and irreversible psychomotor disorders.

N-acetylglutamate Synthase Deficiency (NAGSD)

The enzyme NAGS catalyzes the formation of NAG from N-acetyl-CoA and glutamate, which acts as an allosteric activator of the enzyme CPS1, the first enzyme limiting in urea cycle. NAGSD is an autosomal recessive disease with an estimated incidence of 1/3,500,000–7,000,000 live births (Summar et al. 2013). NAGSD is characterized by an increase in ammonia, glutamine, and glutamate in plasma, and a decrease in citrulline levels. This deficiency is the only UCD that can be effectively treated by a drug. N-carbamylglutamate (NCG) as well as NAG can function as activating co-factor of CPS1 and the oral administration can restore ureagenesis and normalize ammonia levels in affected patients (Bachman et al. 1982; Guffon et al. 1995; Morris 1998; Ah Mew and Caldovic 2011). If untreated, hyperammonemia can provoke neurological deterioration that appears within a few hours to a few months of life (Peoch et al. 2020).

Ornithine Transcarbamylase Deficiency (OTCD)

A partial or total lack of OTC activity originates OTCD, the most frequent UCD, an inherited metabolic disease with an estimated incidence of 1/56,500 live births (Summar et al. 2013). This enzyme is responsible for the formation of citrulline from carbamylphosphate and ornithine. OTCD, unlike the other UCD, is inherited in a X-linked manner and when manifested in neonatal-onset form, patients rapidly develop cerebral edema, seizures, and coma, resulting in high rates of mortality and morbidity. Usually, male patients manifest the most aggressive form of the disease because they have only one X chromosome. The phenotype presented by heterozygous women is quite variable, ranging from classical severe form to asymptomatic (Scaglia et al. 2002). High protein intake and/or infectious damage can trigger acute decompensations on asymptomatic individuals (Nassogne et al. 2005).

The biochemical hallmarks of OTCD are elevated ammonia, glutamine, and alanine, and low levels of arginine and citrulline in plasma accompanied by elevated urinary orotic acid. The encephalopathy caused by the exacerbate levels of ammonia in OTCD compromises motor and mental ability irreversibly and individuals frequently show developmental delays, attention deficit hyperactivity disorder, and cognitive function deficits (Lichter-Konechi et al. 2016).

Liver transplantation may be an alternative for OTCD patients who do not respond to conventional treatment with nitrogen scavenger drugs, diet and amino acid supplementation, and with good metabolic outcome and survival rate (Kim et al. 2013). However, due to the severity, the long-term prognosis for early-onset OTCD patients is poor, and the mortality rate for this disease is still extremely high (Brassier et al. 2015).

Argininosuccinate Synthetase 1 Deficiency (ASS1D)

ASS1 catalyzes the condensation of citrulline and aspartate into argininosuccinate. The poor function of this enzyme causes a defect in the urea cycle, triggering a clinical condition known as ASS1D or citrullinemia type 1 (Häberle et al. 2003; Engel et al. 2009) which is an autosomal recessive disease with an incidence approximately of 1/250,000 live births. Outcomes for citrullinemia type 1 are highly variable and the disease can be presented as neonatal (“classic” form), late onset (“non-classic” form), without any symptoms or hyperammonemia, and postpartum form. ASS1D biochemical profile shows low plasma arginine, elevated plasma and urine citrulline levels, and orotic aciduria. Neonatal hyperammonemic crisis provokes progressively encephalopathy, causing lethargy, poor feeding, vomiting, seizure, and coma, which often leads to death (Häberle et al. 2003).

Citrin (Aspartate–Glutamate Carrier) Deficiency (CTLN2)

Citrin transporter (liver‑type aspartate–glutamate carrier isoform 2) is responsible for the exchange of mitochondrial and cytosolic glutamate to provide aspartate supply for argininosuccinate synthetase in the urea cycle. This protein also has an important role in aerobic glycolysis, gluconeogenesis and synthesis of proteins and nucleotides (Saheki et al. 2005). Citrin deficiency occurs with a SLC25A13 gene defect, the human citrin encoding gene (Saheki and Kobayashi 2002).

CTLN2 is an autosomal recessive inherited disorder, presented as three different clinical phenotypes age-dependent: (i) neonatal intrahepatic cholestasis due to citrin deficiency (NICCD); (ii) adult form with recurrent hyperammonemia and neuropsychiatric manifestations (citrullinemia type 2, CTLN2); and (iii) childhood form with failure to thrive and dyslipidemia caused by citrin deficiency (FTTDCD). Citrin deficiency was first described in Asiatic population but currently it is observed in all ethnic groups (Dimmock et al. 2009; Lu et al. 2005). The prevalence of citrullinemia type 2 is still unknown. Hyperammonemic form usually manifests suddenly, in 20–50-year-old adults, who may or may not have a prior history of NICCD or FTTDCD. High levels of ammonia and plasma citrulline are suggestive of CTLN2, as well as increases in pancreatic secretory trypsin inhibitor (PSTI) (Tsuboi et al. 2001). Clinical manifestations are mainly neuropsychiatric symptoms and include disorientation, delirium, aggression, delusion, loss of memory, and coma that can lead to death by brain edema (Saheki et al. 2005).

Argininosuccinate Lyase Deficiency (ASLD)

The enzyme ASL cleaves argininosuccinic acid to produce arginine and fumarate. Enzyme activity defects lead to ASLD that can manifest on the neonatal or late onset form. Usually, the clinical symptoms appear after the first 24 h of life, with newborns developing lethargy, seizures, coma, and death if this UCD is not recognized and the hyperammonemia is not properly treated. ASLD is the second most common UCD, with a prevalence of 1/70,000 live births and an autosomal recessive inherited pattern. Argininosuccinic acid (ASA) is the laboratorial finding hallmark with urinary concentrations greater than > 10,000 µmol/g of creatinine (normal range 0–1 µmol/g of creatinine). Elevated plasma ammonia and citrulline also are found in patient biochemical profiles. Neurocognitive deficits are a quite common outcome in ASLD with an increased incidence when compared to other UCD (Tuchmann et al. 2008). ASLD patients also demonstrate severe neurological complications regardless of hyperammonemic crisis frequency presented in life, including in patients who did not present the metabolic crisis at all, contrasting with other UCD.

Arginase 1 Deficiency (ARG1D)

Arginase 1 catalyzes the hydrolysis of arginine to ornithine and urea, the final step of ureagenesis. Urea (nontoxic) is then transported and excreted in the urine by kidney function. Ornithine is recycled to continue the cycle for further rounds of urea production (Jackson et al. 1986). The reduced activity of arginase 1 characterizes ARG1D or hyperargininemia, a rare autosomal inborn error of metabolism, with an estimated incidence of 1/950,000 (Summar et al. 2013). Elevated plasma arginine, spasticity, loss of mental and motor skills, failure to thrive and seizures are the most important biochemical and clinical findings. When compared with other UCD, hyperammonemia presented by patients with ARG1D is less frequent and severe, and neonatal presentations are rarely reported (Sin et al. 2015). When present, hyperammonemia is linked to be the cause of ataxia, which is rare and intermittent. The poor neurological and developmental outcomes in ARG1D patients seem to be associated with the accumulated arginine levels (Delwing et al. 2008).

Mitochondrial Ornithine Transporter Deficiency (ORNT1D)

Ornithine transporter deficiency (ORNT1D) is a rare autosomal recessive UCD, also known as hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome with approximately 100 cases reported in literature (Martinelli et al. 2015). When this transporter fails, an inadequate quantity of ornithine is provided to the mitochondrial OTC enzymatic reaction and it provokes the accumulation of cytoplasmic ornithine and lysine. The biochemical hallmarks of this disease are the high levels of ammonia and ornithine in plasma, as well as the high excretion of homocitrulline in the urine, although exceptions have been described since some infants cannot excrete significant amounts and hypoproteic diet helps to reduce this elimination (Valle and Simmel 2001). As in other UCD, ORNT1D patients present hyperammonemia with subsequent neurological abnormalities development. As with ARG1D, this disease shows childhood spastic paraparesis as a differential clinical symptom from other UCD, which manifests a little bit later than in ARG1D.

Organic Acidurias (OA)

OA are inborn errors of intermediary metabolism caused by genetic mutations that compromise the activity of specific enzymes involved in the catabolism of amino acids and additional substrates (odd-chain fatty acids, cholesterol and nucleotides). They represent more than 65 different diseases which are mainly inherited in an autosomal recessive manner and affect in conjunct 1 in 3000 live births (Villani et al. 2017; Wajner 2019). Clinical presentation is variable but neonates or infants typically manifest acute and potentially life-threatening metabolic decompensation, which is triggered by conditions that accelerate catabolism, such as prolonged fasting, vomiting, fever, hemorrhage, or infections (Kölker et al. 2013).

Newborn patients affected with OA usually manifest vomiting, convulsions, lethargy, and hypotonia, which are accompanied by severe laboratorial alterations such as metabolic acidosis, elevated blood lactate, ketonuria, thrombocytopenia, hypoglycemia, and hyperammonemia (Filipowicz et al. 2006; Kölker 2015; Häberle et al. 2018; Wajner 2019). Patients who survive the metabolic decompensation crises can present permanent neurological sequelae that cause mental and motor delays. Furthermore, chronic systemic complications such as pancreatitis, cardiomyopathy, and renal failure can be life-threatening (Deodato et al. 2006; Nizon et al. 2013; Thomas 2015).

The metabolic block caused by enzymatic deficiency leads to the accumulation of characteristic organic acids in tissues and biological fluids, depending on the affected metabolic pathway. This metabolites accumulation is related to the pathophysiology of these diseases and allows the diagnosis through the analysis of the urinary organic acids profile by gas chromatography coupled to mass spectrometry (GC/MS) (Wajner et al. 2019, 2020).

Classical OA comprehend three types of inherited disorders of branched-chain amino acids: propionic aciduria (PA; MIM# 606054), methylmalonic aciduria (MMA; MIM# 251000), and isovaleric aciduria (IVA; MIM# 243500), which are caused by deficiency of propionyl-CoA carboxylase, methylmalonyl-CoA mutase, and isovaleryl-CoA dehydrogenase, respectively (Ogier De Baulny and Saudubray 2002; Dionisi-Vici et al. 2006). In each one of these diseases, characteristic organic acids accumulate: propionic acid in PA, methylmalonic acid in MMA and isovaleric acid in IVA, as well as their acylglicine and acylcarnitine derivatives (Wajner 2019).

Hyperammonemia is a common finding of these classical OA during metabolic decompensation and it may result from inhibitory effects exerted by the accumulated organic acids in the urea and Krebs cycles (Filipowicz et al. 2006; Häberle et al. 2018). Isovaleryl-CoA, methylmalonyl-CoA, and propionyl-CoA, which are the main metabolites of IVA, PA, and MMA, respectively, may inhibit NAGS and, consequently, limit the CPS1 reaction (Kasapkara et al. 2011; Dercksen et al. 2014). In addition, due to the impaired synthesis of succinyl-CoA in PA and MMA (Fig. 2), glutamine catabolism is accelerated to produce α-ketoglutarate, in an attempt to improve the Krebs cycle activity. Therefore, these metabolic effects can result in chronic hyperammonemia usually accompanied by low glutamine levels (Thomas 2015; Filipowicz et al. 2006; Häberle et al. 2018; Summar and Mew 2018).

Fig. 2.

Mechanisms of hyperammonemia in MMA and PA. Methylmalonyl-CoA and propionyl-CoA, the main metabolites of MMA and PA, respectively, exert inhibitory effects on N-acetyl glutamate synthase (NAGS). Besides, the enzymatic deficiencies involved in these disorders harm the synthesis of succinyl-CoA, therefore favoring the degradation of glutamine to produce 2-oxoglutarate. The consequence of these metabolic events is a high formation of ammonia which accumulates in blood of patients

Secondary carnitine deficiency that occurs in most organic acidurias can also contribute to hyperammonemia, since low carnitine levels have been associated with a suppressed expression of urea cycle enzymes (Horiuchi et al. 1992; Tomomura et al. 1997). Carnitine supplementation was able to reduce hyperammonemia in patients with carnitine deficiency due to the antiepileptic treatment with valproic acid, reinforcing the important role exerted by this molecule on the blood ammonia control (Maldonado et al. 2017).

There is increasing evidence suggesting that hyperammonemia plays an important role in the neurological dysfunction observed in OA, exerting synergistic toxic effects by accumulated organic acids on cell metabolism (Viegas et al. 2014). Several in vivo and in vitro animal studies have demonstrated that the organic acids accumulated in OA may induce inhibitory effects on the energy metabolism and on the respiratory chain, oxidative stress, and inflammation, resulting in cellular damages that probably contribute to the clinical manifestations of these diseases (Wajner 2019; Wajner et al. 2020). With regards to MMA, it was demonstrated that rats co-injected with ammonia and methylmalonic acid present increased lipid peroxidation in cerebral cortex (Marisco et al. 2003), as well as significant reduction of glutathione (GSH) and sulfhydryl concentrations in cortex and striatum, suggesting that ammonia and methylmalonic acid act in conjunct to promote redox disturbances in the cells (Viegas et al. 2014). Complementing these findings, Royes et al. (2016) showed that ammonia potentiated the duration of convulsive episodes in mice injected with methylmalonic acid and exacerbated the reduction of the mitochondrial membrane potential induced by this organic acid, demonstrating that ammonia aggravates mitochondrial dysfunction in this disorder.

Fatty Acids Oxidation Defects (FAOD)

Fatty acids oxidation (FAO), which the main part takes place in the mitochondria, is essential for energy supply in all tissues, particularly to some organs such as the heart, skeletal muscle, liver, and kidneys. Energy production from fatty acids becomes crucial in periods of catabolic stress related to increased muscular activity, fasting or febrile illness, when most of glucose reserves are consumed by glycolysis. Therefore, fatty acids are used as an alternative energy source when glucose is not available. Fatty acid oxidation comprehends four components: the carnitine cycle, the ß-oxidation cycle, the electron transfer path, and the synthesis of ketone bodies. The mitochondrial β-oxidation cycle consists of four sequential reactions catalyzed by flavin adenine dinucleotide-dependent acyl-CoA dehydrogenases, 2-enoyl-CoA hydratases, nicotinamide adenine dinucleotide-dependent l-3-hydroxyacyl-CoA dehydrogenases, and 3-ketoacyl-CoA thiolases. Each cycle of mitochondrial fatty acid β-oxidation produces one molecule of acetyl-CoA (Scriver et al. 2001; Vishwanath 2016).

FAOD are a group of inborn errors of metabolism (IEM) in which the cells are unable to transport specific molecules or there are deficiencies in enzymes involved in FAO. They are autosomal recessive genetic defects, with an estimated combined incidence of 1:9000 (Kang et al. 2018). In FAOD, the defects can affect any part of the mitochondrial β-oxidation pathway, such as the plasma membrane functions, mitochondrial fatty acid transport or the short-, medium- or long-chain fatty acid β-oxidation pathways. The more prevalent disorders of this group in the clinical practice are medium-chain acyl-CoA dehydrogenase (MCAD), long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD), and very long-chain acyl-CoA dehydrogenase (VLCAD) deficiencies (Ribas and Vargas 2020).

The clinical findings are highly variable with a heterogeneous clinical phenotype at various ages of onset, from neonate to adulthood. There may be a variety of symptoms ranging from mild liver dysfunction, cardiomyopathy, and/or skeletal myopathy to severe liver disease with a recurrent Reye-like syndrome. They include hypoketotic hypoglycemia, lethargy that can lead to coma, seizures, muscle weakness, manifested generally in the neonatal period and after exercise. Cardiac and hepatic alterations also frequently occur. The central nervous system is often affected due to the severity of the hypoglycemia episodes or hyperammonemia, which can be exacerbate under stressors such as fasting, infections, and physical exercise conditions (Houten et al. 2016).

The clinical presentation manifested by these disorders are mainly due to the failure of the metabolism in maintaining a normal energy supply, although toxicity of the accumulating metabolites is growing in importance (Wajner and Amaral 2015). The acetyl-CoA molecules, produced by each cycle of FAO, are the immediate sources of the ketone bodies, therefore a malfunction of any phase of the β-oxidation cascade or the ketone body pathway will produce an insufficient utilization of fatty acids as an energy source. Since gluconeogenesis alone is not sufficient to fulfill the total energy request, the patient will lead up to fasting hypoglycemia. Deficient long-chain and medium-chain enzymes as well as defects of ketogenesis will culminate in hypoketotic hypoglycemia and increased plasma free fatty acid levels. Disorders of short-chain enzymes and of ketone body utilization cause ketotic hypoglycemia. A lack of acetyl-CoA, as observed in the fatty acid oxidation defects, has further outcome: NAG is not produced in adequate amounts, as a result the patient will present hyperammonemia (Fig. 3). Besides, due to the accumulation of fatty acids, hepatic steatosis may develop. This combination of symptoms is usually designated “Reye-like syndrome” (Scriver et al. 2001; Blau et al. 2014).

Fig. 3.

Suggested mechanisms of hyperammonemia in fatty acid oxidation disorders (FAOD) based on Fig. 1. In FAOD and carnitine cycle defects, the reduced acetyl-CoA availability (reduced availability of N-acetylglutamate (NAG) precursors) and the enzymatic inhibition of carbamylphosphate synthase 1 (CPS1; first enzyme of the urea cycle, which transforms ammonia to carbamylphosphate) impair the urea cycle detoxification capacity. The result of these restrictions processes can contribute to hyperammonemia in FAOD

Hyperammonemia is an important feature of many IEM and is common in patients presenting hypoglycemia, encephalopathy, and liver dysfunction (such as ‘Reye-like’ illness) (Häberle 2013a, b). Severe hyperammonemia is particularly well known in complete deficiencies of carnitine acylcarnitine translocase (CACT deficiency, OMIM #212138), carnitine palmitoyltransferase II (neonatal CPT II deficiency, OMIM #608836), and multiple acyl-CoA dehydrogenase (MADD, OMIM #231680) (Olpin 2013).

Carnitine cycle defects include a group of inherited disorders in which different enzymes that help to transport the long-chain fatty acids across mitochondrial membranes are deficient causing impairments in transport of the fatty acids across mitochondrial membranes. As a result of those defects, long-chain fatty acid oxidation is damaged, leading to increment of unoxidized fatty acyl-CoA molecules in cytosol and urea cycle impairment by reduced acetyl-CoA availability and enzymatic inhibition of CPS1 (Fig. 3) (Blau et al. 2014; Häberle 2013a, b).

In most of defects of carnitine metabolism, hyperammonemic decompensations have been often reported as mild but can be severe in single patients, especially those with a defect of CACT and neonatal carnitine palmitoyltransferase II (CPT II) deficiency. CACT deficiency was first reported in 1992 and many patients died with a chronic progressive liver failure and persistent nonreversible hyperammonemia or because of hypertrophic cardiomyopathy and septal heart defects (Rinaldo et al. 2002). CPT II deficiency (neonatal CPT II deficiency, OMIM #608836) results from mutations that either compromise translocation and processing or yield a mature enzyme with no measurable residual activity. These affected patients present coma within a few days of birth due to hypoglycemia and hyperammonemia, which may overreach 1000 µmol/L (Rinaldo et al. 2002; North et al. 1995). The phenotype for this severe disorder involves multiple organs and congenital anomalies and often results in neonatal death. Besides, it is important to highlight that severe nutritional carnitine deficiency can have the same inhibition effect on urea cycle, resulting in hyperammonemic encephalopathy as detected in single patients (Limketkai and Zucker 2008).

Multiple acyl-CoA dehydrogenase deficiency, also known as Glutaric Aciduria type 2 (GA-II), with a worldwide prevalence of 1:200,000, results from the deficiency of electron transfer flavoprotein (ETF alfa and beta) or electron transfer flavoprotein/ubiquinone oxidoreductase (ETF/QO) that transport the electrons from β-oxidation to the respiratory chain. MADD may be accompanied by various symptoms including vomiting, hypotonia, hyperammonemia, hepatomegaly, renal cysts, myopathy, and an odor of sweaty feet and congenital anomalies. Clinical presentation includes three phenotypes: (a) neonatal with congenital anomalies; (b) neonatal without congenital anomalies (severe forms) whose manifestation generally occurs within the first 48 h of life and might be fatal; (c) mild and late clinical form that usually appear in the first months until adult life with broad clinical spectrum and better prognosis (Scriver et al. 2001; Olpin 2013).

The most frequent fatty acid oxidation disorder, medium-chain acyl-CoA dehydrogenase deficiency (MCADD, OMIM #201450), is caused by pathogenic variations in the ACADM gene that encodes the flavoenzyme MCAD (Scriver et al. 2001). Patients with MCADD can be asymptomatic or can present a severe phenotype that may lead to coma or sudden death in the first years of life, characterized by metabolic acidosis, lactic acidemia, hypoketotic hypoglycemia, vomiting, seizures, and lethargy, being provoked by fasting or catabolic stress. Part of these symptoms can be explained by the deficient conversion of fatty acids into acetyl-CoA, which is a substrate for the Krebs cycle. Hepatomegaly and hyperammonemia are also observed in patients with MCADD, contributing to the disruption of brain functions and development of encephalopathy. Myopathy, hypotonia, chronic muscle weakness, and rhabdomyolysis are also common findings in the acute episodes (Anderson et al. 2020; Rinaldo et al. 2002). Hyperammonemia is generally milder, but there are exceptions, such as an adult with MCADD who presented with plasma ammonia concentration of 390 µmol/L, mild hypoglycemia, and cardiac arrhythmias (Feillet et al. 2003). In MCADD, while many symptoms can be assigned to the accumulation of toxic long-chain or medium-chain acylcarnitines, hyperammonemia is probably caused due to the lack of acetyl-CoA resulting from acylcarnitines degradation blockade (Corkey et al. 1988). Moreover, long- chain fatty acyl-CoAs, specifically palmitoyl-CoA, have been described to induce fatty acylation of an active site cysteine residue of CPS1 and successfully hinder CPS1 activity (Fig. 3) (Corvi et al. 2001). This fatty acylation must be considered as part of a strategy from the body during starvation to save nitrogen; however it might be fatal particularly in (very) long-chain acyl-CoA oxidation defects (Häberle 2013a, b).

Even though some OA and FAOD may also cause hyperammonemia, high ammonia levels are much less commonly seen in FAOD (Blau et al. 2014; Olpin 2013). Ammonia concentrations are normal between metabolic decompensation, except in the severe defects mentioned above (Häberle 2013a, b).

It is probable, therefore, that various mechanisms contribute to the hyperammonemia in FAOD (Fig. 3), as in the OA. In defects of carnitine metabolism, disruption of the carnitine cycle results in the accumulation of unoxidized fatty acyl-CoA esters in the cytosol, which are believed to inhibit the urea cycle. The principal mechanism, however, is probably that FAOD reduce the production of acetyl- CoA. This leads to reduced synthesis of NAG and, therefore, hinders urea cycle function, as described for OA and NAGS deficiency (Häberle 2013a, b).

Hyperammonemia Treatment and New Therapeutic Perspectives

Hyperammonemia is a potentially fatal condition, therefore the acute episodes must be immediately corrected to avoid irreversible neurological sequelae in affected patients (even before a definitive diagnosis). Several strategies can be used to correct or control ammonia levels in the blood, that include ammonia removal by dialysis, use of ammonia scavengers [like sodium benzoate (SB), sodium phenylacetate (SP), sodium phenylbutyrate (NaPB) or glycerol phenylbutyrate (GPB)], supplementation with specific amino acids (arginine and citrulline) to stimulate the residual activity of the urea cycle, use of antibiotics (neomycin) and lactulose to reduce the ammonia production by enteric bacteria and nutritional interventions, such as limited protein intake associated with a hypercaloric glucose solution ingestion to enhance anabolism (Auron and Brophy 2012).

Dialysis is the ideal method for rapid ammonia removal and can be performed by hemodialysis and/or continuous renal replacement therapy. Due to the need of specialized centers to perform these procedures and possible complications that can occur in the neonates (thrombosis and bleeding), peritoneal dialysis can be an alternative, especially in patients with less severe forms of hyperammonemia (Schaefer et al. 1999; Arbeiter et al. 2010).

Ammonia scavengers are a therapeutic option in hyperammonemic patients with normal renal function. SP and SB are intravenously administered (250 mg/kg of each drug), while NaPB and GPB can be orally administered. SP acts increasing the renal excretion of glutamine in the form of phenylacetylglutamine, while SB combines with glycine to eliminate hippuric acid in the urine, stimulating the glycine replacement and, consequently, the nitrogen consumption. Although SP and SB are able to maintain normal levels of plasma ammonia, they can cause secondary biochemical alterations, such as potassium depletion (in case of SP use), as well as metabolic acidosis, hypernatremia, and hyperbilirubinemia (with SB infusion) (Summar 2001; Auron and Brophy 2012; Häberle et al. 2019). This way, NaPB and GPB represent a safe alternative for both adult and pediatric patients and positive results in the chronic management of UCD were related in different clinical trials (Lichter-Konecki et al. 2011; Smith et al. 2013). NaPB undergoes liver β-oxidation to phenylacetate after absorption. GPB is an ester pro-drug of phenylbutyrate which is hydrolyzed by pancreatic lipases releasing the active molecule (phenylacetate) which is absorbed more slowly when compared with NaPB. Besides ammonia control, long-term treatment (12 months) with GPB also significantly improved neurocognitive function in pediatric UCD patients (Diaz et al. 2013).

Arginine supplementation (100–250 mg/kg/day) is essential in UCD that compromise the synthesis of this amino acid (CPS1, NAGS, OTC, ASS, and ASL deficiencies). However, when administered in large amounts it can accumulate and to induce nitric oxide production, which is a potent vasodilator and can lead to symptomatic hypotension (Batshaw et al. 2001; Häberle et al. 2019). Likewise, citrulline can be provided (100–200 mg/kg/day) for CPS or OTC deficient patients in an attempt to enhance nitrogen clearance by the urea cycle (Summar 2001). Hyperammonemic patients with propionic or methylmalonic acidurias can be treated with NCG, which is a N-acetylglutamate analog that helps to increase the NAGS activity (Filippi et al. 2010). Besides, in PA and MMA patients, l-carnitine treatment (50–150 mg/kg/day) has also an essential role, not only to improve the ammonia control through stimulation of urea cycle enzymes, but mostly to prevent secondary carnitine deficiency and to stimulate the renal excretion of the accumulated organic acids (Di Donato et al. 1984; Ogier de Baulny and Saudubray 2002). Acetyl-l-carnitine, a pro-drug of l-carnitine, was able to reduce ammonia concentrations and to improve cognitive functions in patients with HE (Malaguarnera et al. 2011). In addition to all these properties, l-carnitine exerts antioxidant and anti-inflammatory effects that can possibly improve or prevent neurological damages induced by ammonia excess, needing further investigation (Ribas et al. 2014; Kazak and Yarim 2017).

The disaccharide lactulose is not metabolized in the small intestine and is converted in lactic and acetic acids into the colon. The resultant pH alteration reduces the growth of enteric bacteria that metabolize urea to ammonia and, due to the stimulation of peristalsis, fecal ammonia loss is increased (Vince and Burridge 1980; Weber et al. 1987). The aminoglycoside neomycin eliminates proteolytic bacteria that produce ammonia, therefore contributing to the hyperammonemia reduction; however, severe adverse effects like ototoxicity and renal tubular toxicity can occur, limiting its long-term therapeutic use (Leise et al. 2014; Matoori and Leroux 2015).

In this context, considering the mechanisms related with brain damage induced by ammonia, new approaches have been tested in an attempt to provide neuroprotective effects, including memantine administration (an antagonist of the NMDA receptor) to prevent ammonia-induced excitotoxicity, as well as the use of l-ornithine-l-aspartate (LOLA) to enhance the hepatic urea synthesis and the muscular glutamine synthetase activity. However, although oral and intravenous LOLA therapy reduced blood ammonia and improved cognitive function in HE patients, some works have not confirmed these benefits, therefore LOLA is not currently recommended for hyperammonemia treatment (Rodrigo et al. 2009; Acharya et al. 2009; Monfort et al. 2009; Kircheis 2016).

Liver transplantation (LT) has been indicated for patients with severe urea cycle disorders that can result in liver failure and cannot be satisfactorily controlled by conventional therapies. When performed in the first months of life, LT may increase the survival rate of the patients, as well as it reduces baseline blood ammonia levels in patients with UCD, then preventing recurrent hyperammonemia attacks. However, newborn patients with OTCD and CPS1D with initial blood ammonia levels ≥ 360 μmol/L may not achieve normal neurodevelopmental, suggesting that other factors than blood ammonia are involved in the brain damage, which are not influenced by hemodialysis or LT (Kido et al. 2021a,b). For newborns with ASSD (even with initial blood ammonia levels ≥ 360 μmol/L) or other UCD with plasma ammonia levels lower than 360 μmol/L, LT was very effective in the general condition improvement of the patients, including neurodevelopmental outcome (Kido et al. 2021b).

Due to the substantial risks for peri- and post-operative complications, as well as the limited availability of liver donors, hepatocyte transplantation was introduced in 1997 in an attempt to prevent metabolic alterations in patients with different inborn errors of metabolism (Strom et al. 1997; Meyburg et al. 2010). Although a reduction of hyperammonemic crises and an improvement of neurodevelopment have been observed with this procedure (Kido et al. 2018; Meyburg and Hoffman 2010; Leonard and McKiernan 2004), several factors limit its use in the clinical practice. The need for invasive procedures to perform portal infusions, as well as the risk of extrahepatic spread of transplanted hepatocytes (mostly to the lung) led to an increased interest in the use of liver stem-like cells (HLSCs) with hepatic differentiation capability (Spada et al. 2020). In 2017, it was reported that HLSC-derived extracellular vesicles were able to correct ureagenesis in an in vitro model of ASS1D (Herrera Sanchez et al. 2017). More recently, a phase I clinical study (Spada et al. 2020) showed that HLSCs administered percutaneously into liver parenchyma of 3 patients with neonatal-onset hyperammonemia due to ASLD or MMA was safe and able to prevent ammonia decompensation. However, more studies are still necessary to better evaluate the long-term effectiveness of this treatment.

Finally, gene therapy has emerged as a promising approach for patients with inherited disorders, since the introduction of a gene capable of encoding a functional enzyme of the urea cycle, can restore the cycle activity avoiding long-term hyperammonemia. Preclinical studies showed that the intravenous administration of adeno-associated virus2/8 coding for OTC was able to restore the enzymatic activity and to control ammonia levels in a mouse model of OTC deficiency (Ye et al. 1996; Wang et al. 2012; Moscioni et al. 2006; Brunetti-Pierri et al. 2008). In 2002, a pilot study in patients with OTCD showed that the infusion of E1- and E4-deleted vector based on adenovirus type 5 and containing human OTC cDNA resulted in transgene expression in hepatocytes of 7 of 17 subjects, although metabolic correction has not been observed at the tested doses (Raper et al. 2002). Besides, the 18th patient recruited in this study died due to a systemic inflammatory response that resulted in disseminated intravascular coagulation and multi-organ failure.

In relation to ASS1D, it was shown that mice treated with adenoviral vectors expressing ASS1 lived significantly longer than untreated animals, although they have not presented an improved clinical outcome (Patejunas et al. 1998). Besides, measurements of the flux of nitrogen from orally administered 15NH₄ to [15N]urea revealed that the systemic administration of a adenoviral vector expressing human ASS resulted in a partial correction of the enzyme defect (Lee et al. 1999). Promising findings were also described for argininosuccinic aciduria, since the neonatal administration of a codon-optimized human ASL gene packaged within adeno-associated virus serotype 8 led to an increased survival and metabolic correction in a mouse model of this disease (Ashley et al. 2018). In mice knockout for arginase 1, gene therapy improved hyperammonemia and prolonged the animal’s survival (Truong et al. 2019). Taken together, these preliminary findings show that significant therapeutic effects can be obtained with minimal enzymatic activity; however, the safety and clinical efficacy of gene therapy still needs to be better investigated so that we can conclude about the cost–benefit of this treatment.

Final Considerations

A considerable amount of evidence demonstrates the seriousness of inherited metabolic diseases that course with hyperammonemia, such as urea cycle diseases, mitochondrial fatty acid beta-oxidation defects, and some organic acidemias, especially propionic, methylmalonic, and isovaleric aciduria. Among these, the diseases of the urea cycle are certainly the ones that are more severe and can quickly lead to death if not diagnosed and treated early. Furthermore, in metabolic diseases that progress with hyperammonemia, adherence to treatment is essential for a good prognosis, considering that high brain levels of ammonia lead to tissue damage and severe neurological and cognitive manifestations. The therapy used to reduce ammonia and the mechanisms involved in its toxicity (altered brain electrolytic homeostasis, accumulation of glutamine in astrocytes, free radical production, mitochondrial energy metabolism, altered NMDA and GABA neurotransmission) which can act synergistically to explain the neurological manifestations induced by hyperammonemia are discussed in this review. The diagnosis of inborn errors of metabolism that lead to hyperammonemia presupposes the measurement of metabolites in biological fluids and, sometimes, enzymatic assays and molecular tests, which must be carried out quickly. Ammonia levels are critical to diagnosis and should nevertheless precede this laboratory investigation. It should be noted that in addition to the current therapies of protein dietary restriction and supplementation with SB, SP, or NaPBA, the development of new therapeutic strategies aiming to decrease the high ammonia levels may be beneficial in the future to ameliorate disease progression and improve neurological symptomatology in inherited metabolic disorders.

Funding

No specific source of funding is associated with this work.

Data Availability

Not applicable.

Code Availability

Not applicable.

Declarations

Conflict of interest

The authors declare there are no conflicts of interest.

Ethical Approval

Not applicable.

Consent to Participate

Not applicable.

Consent for Publication

Not applicable.