Expanding Role of Proton Magnetic Resonance Spectroscopy: Timely Diagnosis and Treatment Initiation in Partial Ornithine Transcarbamylase Deficiency

Abstract

We report the case of a 3-year-old male patient who presented with a 3-day history of altered mental status, emesis, and abdominal pain in the setting of a viral illness. A rapid screening revealed a high ammonia level and after reviewing his proton magnetic resonance spectroscopy (1H MRS) which showed the classic triad of high glutamate, low choline, and myoinositol, a diagnosis of ornithine transcarbamylase deficiency (OTCD) was made within 6 hours of presentation. Therapy with sodium phenylbutyrate and sodium benzoate was initiated and patient was discharged after 3 days with no neurologic disability. Biochemical and molecular testing eventually confirmed the diagnosis. 1H MRS is a practical and fast neuroimaging modality that can aid in diagnosis of OTCD and enables faster initiation of treatment in acute settings.

Introduction

The urea cycle disorders (UCDs) represent one of the most common groups of inborn errors of metabolism (IEMs), with an estimated incidence of 1 in 35,000. ^{1 2 3} UCDs encompass eight distinct disease entities caused due to deficiencies of six enzymes (five involving the urea cycle directly and a sixth one which gives the first enzyme its cofactor) as well as two transporters. ⁴ The most common of these is ornithine transcarbamylase deficiency (OTCD), the only X-linked UCD, with an estimated incidence of 1 in 56,500. ¹ More than 500 mutations have been identified in the OTC gene. ^{5 6 7} Approximately 60% of hemizygous males will present in the newborn period, whereas 40% have later onset of symptoms. Variation in age of onset and severity of phenotype is determined by the effect of the mutation on enzyme activity, tertiary protein structure, and substrate affinity. ^{6 8} Presentation outside the newborn period can be provoked by illness, surgery, or any catabolic stress, as well as after increased protein loads. The manifestations in heterozygous females can vary widely from asymptomatic to fully symptomatic and are determined by the extent of X-inactivation in hepatocytes. Some may have nonspecific symptoms including aversion to proteins, attention deficit hyperactivity disorder, cyclic vomiting, and unexplained liver disease. ^{8 9} Due to the vague nature of symptoms, heterozygous females and males with partial OTCD can often go undiagnosed for several years. ¹⁰ Ammonia levels can be drawn in the acute setting, but plasma amino acids and orotic acid can take 3 to 4 days to result. In addition, molecular confirmation can take several weeks. The National Institutes of Health, Rare Diseases Clinical Research Network, Urea Cycle Disorders Consortium (UCDC) has been studying magnetic resonance imaging (MRI) modalities as a means to identify early biomarkers that can be used to facilitate swift diagnosis in both asymptomatic and symptomatic OTCD patients. Herein, we discuss our first case of late-onset OTCD in which proton magnetic resonance spectroscopy (1H MRS) was utilized in a clinical setting to emergently identify the diagnosis.

Case Report

Our patient was a 3-year-old Ashkenazi Jewish boy who presented in the emergency room at our institute with abdominal pain, vomiting, and altered mental status. He had been experiencing generalized abdominal pain and had two episodes of nonbilious, nonbloody emesis during the past 3 days. He had been seen at an outside emergency department and was discharged with a diagnosis of viral gastroenteritis. During 24 hours prior to admission at our institute, his parents noted several irregularities in his behavior. He was sleepier the evening prior to admission and on the day, he was irritable and threw a temper tantrum, which was very uncharacteristic for him. Later he was seen running around in circles in the living room and tried to bite his mother. He had a similar episode of aggressive behavior 3 months previously in the setting of a streptococcal infection which resolved after a couple of days.

The patient was born full term after an uneventful pregnancy. He acquired his early motor and language skills appropriately on time. He consumes a wide variety of food, including proteins, and has been growing normally. Family history is significant for mother's aversion to protein-containing food. She described feeling “sick and foggy” after eating protein and became a vegetarian during her teenage years.

Initial laboratory tests including electrolytes, transaminases, complete blood count, urine analysis, and urine toxin studies were unrevealing. An ammonia level was subsequently requested which was abnormally elevated at 176 µmol/L (normal range: 29–54 µmol/L). A lumbar puncture had been completed which showed no evidence of infectious or autoimmune encephalitis. Neuroimaging studies included a head computed tomography and MRI which did not reveal any acute intracranial pathology. Single voxel 1H MRS was acquired with 2 × 2 × 2 cm voxel of interest placed over the left basal ganglia using Point RESolved Spectroscopy sequence (repetition time 1,500; echo time 35, 144, and/or 288 milliseconds). Results showed an elevated glutamine peak with decreased myoinositol and choline peaks ( Fig. 1 ). Specialized biochemical and genetic testing was ordered by the genetics team. However, given the abnormally elevated ammonia and characteristic evidence on 1H MRS, suspicion for OTCD was high and treatment was subsequently initiated. Dietary intake was halted and the patient was started on intravenous sodium phenylbutyrate and sodium benzoate in addition to supplementation with 10% dextrose and normal saline. Electroencephalogram showed diffuse encephalopathy but no evidence of epileptiform discharges. Neurological exam and ammonia level normalized 48 hours after admission. Plasma amino acid results were abnormal with an elevated glutamine level of 1,096 µmol/L (normal range: 254–736 µmol/L), arginine level was on lower side of normal at 9 µmol/L (normal range: 10–111µmol/L), and urine orotic acid was elevated at 168.4 mmol/mol UCr (normal range: 0–4mg/g). Mutational analysis ultimately identified one variant in the OTC gene (c.829C > T, p.Arg277Trp) which has been previously reported as pathogenic, thereby confirming the diagnosis of OTCD. ⁵ The patient was discharged on oral ammonia scavengers, supplemental citrulline, and a protein-restricted diet. Due to the identification of the pathogenic variant in the proband and the mother's aversion to protein and reported symptoms, molecular testing was ordered on her and is pending at the time of writing this report.

Fig. 1.

Single voxel magnetic resonance spectroscopy (left basal ganglia) shows an elevated glutamine and glutamate peak complex and reduction of myoinositol, and choline was mildly depressed (purple: control, black: ornithine transcarbamylase deficiency patient).

Discussion

Classic OTCD presents in newborn period with hyperammonemia (HA) characterized by progressive symptoms including irritability, vomiting, difficulty feeding, lethargy, polypnea, seizures, and if untreated, coma, multiorgan failure, and death. ⁹ Symptoms may mimic sepsis. However, not all patients with OTCD present as newborns; partial enzyme defects may present in later childhood with symptoms triggered by metabolic stressors. Recurrent or mild HA after a viral or bacterial infection, surgery, medications (such as steroids, valproate), or trauma is well described in the literature. ^{10 11} Patients with partial enzyme defects typically have less dramatic presentations in the form of recurrent vomiting, failure to thrive, protein aversion, cognitive, or attention deficit. ⁸ Behavioral irregularities, out of proportion to discomfort caused by the illness, are often the sole red flag and present a window of opportunity to diagnose these individuals by obtaining an ammonia level and 1H MRS.

In addition to UCDs, the differential diagnosis of HA in children includes other IEMs such as organic acidemias, maple syrup urine disease (MSUD), fatty acid oxidation disorders, and disorders of pyruvate metabolism. ¹² Simplistically, HA in these diseases represents a secondary inhibition of the urea cycle. The management strategies vary greatly, and therefore, a rapid confirmation of diagnosis can be lifesaving. The diagnostic profile of OTCD includes high plasma glutamine, low citrulline, and elevated urine orotic acid. ⁹ The typical turnaround time for plasma biochemical testing can range between 3 and 4 days and even longer at institutes that rely on sending these tests to outside laboratories. Molecular confirmation in the form of mutation analysis can take up to several weeks, and may depend on insurance approval. HA is a medical emergency and treatment initiation for partial OTCD can be challenging. In this case, we faced a similar dilemma prior to starting treatment with ammonia scavenger therapy. However, clinical clues in conjunction with the classic results on 1H MRS provided enough evidence to initiate crucial treatment for this patient who had remained undiagnosed for the first 3 years of his life. A case of neonatal MSUD is described where diagnosis was made based on large methyl resonance group prior to results of biochemical testing, providing another example of diagnostic efficacy of 1H MRS in the acute setting. ¹³ This case demonstrates the practicality of obtaining 1H MRS that can aid in expediting diagnosis and should be considered in any patient with unexplained encephalopathy.

Our group in the past has described the utility of multimodal imaging and especially 1H MRS in a research setting, in understanding the long-term effects of HA on the structure and biochemistry of brain. ¹⁴ The pathognomonic spectroscopic findings in proximal UCD (OTCD and carbamoyl phosphate synthetase I deficiency) consist of elevated glutamine peak with reduced choline and myoinositol peaks. ¹⁵ In arginase deficiency, glutamine is elevated with normal myoinositol and normal or increased choline. Our unpublished data suggest increased guanidino peaks may be seen in argininosuccinate synthetase and argininosuccinate lyase deficiency. ¹⁶ Differential diagnosis of elevated glutamine peak alone on 1H MRS is broad and includes UCD, hepatic encephalopathy, secondary to valproic acid, and in some mitochondrial disorders; however, the trio of elevated glutamine with decreased choline and myoinositol is the hallmark of proximal UCD. ^{17 18} OTCD disease severity has been shown to directly correlate with glutamine and inversely correlate with myoinositol concentrations. Choline is decreased, a finding that has been noted in chronic as well as acute states of HA, reflective of aberrations in phospholipid metabolism, membrane alterations, and membrane fluidity. ¹⁶ Myoinositol is released from astrocytes as an osmotic buffering mechanism to compensate for accumulation of glutamine. The role of 1H MRS in diagnosing neurometabolic disorders is irrefutable. Increased lactate peak in mitochondrial diseases, reduced creatine peak in cerebral creatine deficiency syndromes, and increased N-acetyl aspartate peak in Canavan's disease have been diagnostic hallmarks for these respective disorders for decades prior to the advent of specialized genetic testing. ¹⁹ The ethos of 1H MRS lies in its ability to provide insights into the biochemistry of the central nervous system since measurement of metabolites in the plasma is seldom a true representation of their levels in the cerebrospinal fluid.

While biochemical and molecular testing will always be the mainstay for accurate diagnosis, 1H MRS has the untapped potential to support the diagnosis of a milder/atypical UCD, which can help speed treatment initiation with ammonia scavengers and ultimately improve outcome. We studied spectroscopic findings 6 months after an episode acute HA in a different patient, which suggest that the peaks and troughs are more persistent than previously hypothesized. ²⁰

Conclusion

We reported a patient with partial OTCD who presented with altered mental status and vomiting in conjunction with a mild infection. In this case, rapid diagnosis and treatment was possible due to high index of suspicion and classic findings on 1H MRS. This report demonstrates the practicality of 1H MRS in differentiating the diagnosis of OTCD from other IEMs causing HA, and the unique opportunity it provides to make a crucial impact on improving treatment and outcome.