Carnitine supplementation for inborn errors of metabolism

Abstract

Background

Inborn errors of metabolism are genetic conditions which can lead to abnormalities in the synthesis and metabolism of proteins, carbohydrates, or fats. It has been proposed that in some instances carnitine supplementation should be provided to infants with a suspected metabolic disease as an interim measure, particularly whilst awaiting test results. Carnitine supplementation is used in the treatment of primary carnitine deficiency, and also where the deficiency is a secondary complication of several inborn errors of metabolism, such as organic acidaemias and fatty acid oxidation defects in children and adults.

Objectives

To assess the effectiveness and safety of carnitine supplementation in the treatment of inborn errors of metabolism.

Search methods

We searched the Cystic Fibrosis and Genetic Disorders Group's Inborn Errors of Metabolism Trials Register, the Cochrane Central Register of Controlled Trials (The Cochrane Library 2007, Issue 4) and MEDLINE via Ovid (1950 to July week 4 2007), LILACS (15/05/2008) and Iranmedex (15/05/2008) and also the reference lists of retrieved articles.

Date of most recent search of the Group's Inborn Errors of Metabolism Register: 27 October 2011.

Selection criteria

Randomised controlled trials and quasi‐randomised controlled trials comparing carnitine supplementation (in different dose, frequency, or duration) versus placebo in children and adults diagnosed with an inborn error of metabolism.

Data collection and analysis

Two authors independently screened and assessed the eligibility of the identified trials.

Main results

No trials were included in the review.

Authors' conclusions

There are no published or ongoing randomised controlled clinical trials relevant to this review question. Therefore, in the absence of any high level evidence, clinicians should base their decisions on clinical experience and in conjunction with preferences of the individual where appropriate. This does not mean that carnitine is ineffective or should not be used in any inborn error of metabolism. However, given the lack of evidence both on the effectiveness and safety of carnitine and on the necessary dose and frequency to be prescribed, the current prescribing practice should continue to be observed and monitored with care until further evidence is available. Methodologically sound trials, reported according to the Consolidated Standards of Reporting Trials (CONSORT) statement, are required. It should be considered whether placebo‐controlled trials in potentially lethal diseases, e.g. carnitine transporter disorder or glutaric aciduria type I, are ethical.

Plain language summary

Carnitine supplements for treating people with inborn errors of metabolism

Inborn errors of metabolism are genetic disorders which have a wide range of symptoms. These often start at or soon after birth but may appear first at any time during adulthood. Affected individuals may need to deal with symptoms of the disease throughout their lifetime. Symptoms are often non‐specific and may affect any organ. It can be difficult to diagnose an inborn error of metabolism. However, early detection is important and screening of infants for some disorders, such as phenylketonuria, is routine in several countries. It is recommended that carnitine supplements are prescribed in the diet of individuals with certain inborn errors of metabolism, along with other standard treatments. Carnitine supplements take the form of tablet, oral liquid, paediatric liquid and injection and might be taken with food for ease of administration. Unfortunately, we did not find any good quality trials to include in the review. This does not mean that carnitine is ineffective or should not be used in treating inborn errors of metabolism; however, individuals receiving carnitine should be carefully observed and monitored. Therefore, we recommend that clinicians base their decision to prescribe carnitine on clinical experience together with individual preferences. Future trials should include patient‐reported outcomes using validated and internationally recognised scales. Any adverse events associated with the treatment should be reported. It should be carefully considered whether placebo‐controlled trials in potentially lethal diseases, e.g. carnitine transporter disorder or glutaric aciduria type I, are ethical.

Background

Inborn errors of metabolism (IEMs) are genetic conditions caused by defects in enzymes or faulty transport proteins. They can lead to abnormalities in the synthesis and metabolism (biochemical procedures) of proteins, carbohydrates, or fats.

These disorders usually involve multiple organ systems; they can be life‐threatening or may include intervals without apparent symptoms, but with slow and progressive degeneration in health over extended periods of time.

Aetiology and prevalence

There are over 500 inherited biochemical disorders, which vary in age of onset, clinical severity, and often also mode of inheritance. A number of these are quite rare (1 in 500,000), whilst the most common disorders occur in 1 in 100 individuals (Winter 1998).

Inborn errors of metabolism are mainly caused by the lack of a specific enzyme, or a transmembrane transporter, or similar protein; this results in a blockage of the metabolic pathway (biochemical procedure) with a consequent build up of toxic metabolites (Pollitt 1997; Seymour 1997). Carnitine helps in the consumption and disposal of fat in the body and is an essential requirement in the delivery of long‐chain fatty acids for the oxidation process which takes place in the mitochondria of the cell. Carnitine is also involved in the removal of toxic waste products of this oxidation process (Seymour 1997).

To limit the possibility of any serious permanent neurological damage in children, the early identification of high risk individuals is essential and should be followed by the provision of prompt and appropriate treatment aimed at metabolic stabilization (Leonard 2006; Pollitt 1997).

Diagnosis and symptoms

Inborn errors of metabolism have unique biochemical phenotypes (patterns), which permit diagnoses by biochemical laboratory tests. However, and quite problematically, their clinical symptoms may often be indistinguishable from other infancy disorders and diseases (Lepage 2006) and may even vary quite markedly between individuals (Pollitt 1997).

Symptoms of IEMs can often be observed at or soon after birth, or there may have been indications (complications) during pregnancy. However, symptoms may also first appear at any time during life. Several disorders may be present at birth, such as early‐onset seizures, severe hypotonia (low muscle tone), ascites (accumulation of fluid in the peritoneal cavity) or hydrops fetalis (severe swelling in the fetus and newborn), and dysmorphic syndromes (facial malformations). Even though many of the affected babies appear reasonably healthy at birth, deterioration can occur quite rapidly and may lead to coma and even death after a seemingly symptom‐free period (Leonard 2006). As a result of this wide variation in presentation, the differential diagnosis of IEMs can be quite complex (Wilcken 2003). Early detection of these metabolic abnormalities is paramount and screening of infants for some IEMs, such as phenylketonuria, is a routine procedure in a number of countries (Wilcken 2003).

A variety of tests may be required to make a definitive diagnosis, potentially involving a number of time‐consuming and highly specialised metabolic investigations; therefore, some form of interim management may need to be instituted whilst awaiting test results.

Treatment options

The severity of symptoms will generally dictate the management strategy of IEMs. Due to the national screening programs for IEMs in several countries, more individuals are being diagnosed, but unfortunately some of these disorders do not have useful treatment options and supportive care is indicated. They often require changes in diet which is dependant on the specific metabolic disease. However, the majority of small molecule disorders (defects of amino acid, organic acid and fatty acid metabolism) can be treated by a combination of firstly removing toxic metabolites using dialysis in an acute situation; secondly, reducing catabolism; and finally, by using diet to reduce the accumulation of toxic metabolites, restricting and substituting precursors (the primary component which enters a biochemical procedure) such as proteins, or specific carbohydrates or fatty acids (Leonard 2006). For some disorders, other therapies may be useful, of which one of these could be carnitine medication.

Carnitine

Primary carnitine deficiency, due to mutations in the carnitine transporter, is a rare disorder, but secondary carnitine deficiency is a recognised secondary complication of several IEMs, especially the organic acidaemia and fatty acid oxidation defects. Carnitine is an amino acid derivative and has multiple physiological functions in several major metabolic pathways. It plays a critical role in the intermediary metabolism of fatty acids and the transport across mitochondrial membranes, and in the ultimate removal of their accumulated metabolites (Evangeliou 2003).

Even though it is estimated that only half of all IEMs can be treated biochemically, it has been proposed that carnitine supplementation should, under some circumstances, be provided to infants with a suspected metabolic disease as an interim measure, particularly whilst awaiting test results (Chakrapani 2001). The primary defect in many of these disorders of organic acid metabolism is enzymatic, meaning supplementation is unlikely to correct the enzyme abnormality; nevertheless, L‐carnitine may still represent a useful therapeutic pathway specifically for the removal of accumulated toxic metabolites in IEMs.

A Cochrane Review did not find any evidence to support the routine supplementation of parenterally fed neonates with L‐carnitine (Cairns 2000). However, L‐carnitine has been used in the acute and long‐term treatment of people with IEMs, such as disorders of organic acid and fatty acid metabolism, i.e. propionic acidaemia (PA); methylmalonic acidaemia (MMA); isovaleric acidaemia (IVA); and medium chain acyl‐CoA (MCAD) deficiency. The drug was approved by the Food and Drug Administration (FDA) in the USA in 1992, and is generally considered to be a cheap, safe and apparently effective therapeutic measure for treating IEMs (Itoh 1996; Walter 2003; Winter 2003).

Objectives

To evaluate the effectiveness and safety of carnitine supplementation in the treatment of IEMs.

Methods

Criteria for considering studies for this review

Types of studies

Randomised controlled trials (RCTs). Trials with quasi‐randomised methods, such as alternation, were included if there was sufficient evidence that the treatment and control groups were similar at baseline.

Types of participants

Children and adults diagnosed with an IEM.

Types of interventions

Intravenous or oral carnitine supplementation (in different dose, frequency, or duration) versus placebo or no supplementation.

Studies which had permitted concomitant therapy were included provided this therapy was distributed similarly between the two groups.

Types of outcome measures

Primary Outcomes

1. Changes in measures of growth (weight gain, body mass index (BMI), growth velocity z score, height z score or other indices of nutritional status and growth)

2. Measures of neuropsychological performance (e.g. attention, verbal fluency, verbal learning and memory) by validated instruments

3. Number of deaths or age at death in each group

Secondary Outcomes

1. Changes of eating behaviour

2. Measures of neurodevelopment by validated instruments (e.g. measures of intelligence (intelligence quotient (IQ))

3. Muscle strength

4. Measures of endurance

5. Quality of life as assessed by any validated disease specific or generic instrument

6. Number of hospital admissions

7. Biochemical outcomes

   1. level of plasma carnitine (levels of acyl carnitine, i.e. levels of octanoylcarnitine for medium‐chain acyl‐CoA dehydrogenase deficiency) relative to age‐related normal concentrations

   2. liver function tests (change in aspartate and alanine aminotransferase levels)

   3. changes in serum high density lipoprotein (HDL) cholesterol levels

   4. changes in serum triglyceride concentration

   5. changes in serum low density lipoprotein (LDL) cholesterol levels

Adverse effects 
 We considered any clinically diagnosed toxicity and any reported unacceptable adverse events associated with this medication.

Search methods for identification of studies

There were no language restrictions on included studies and we have arranged to translate and report any relevant non‐English papers.

Electronic searches

We searched the Cystic Fibrosis and Genetic Disorders Group's Inborn Errors of Metabolism Trials Register using the terms: carnitine AND (other IEMs OR PKU OR hyperlip*).

The Inborn Errors of Metabolism Trials Register was compiled from electronic searches of the Cochrane Central Register of Controlled Trials (Clinical Trials) (updated each new issue of The Cochrane Library), quarterly searches of MEDLINE and the prospective handsearching of the Journal of Inherited Metabolic Disease. Unpublished work was identified by searching through the abstract books of the Society for the Study of Inborn Errors of Metabolism conference and the SHS Inborn Error Review Series. For full details of all searching activities for the register, please see the relevant section of the Cystic Fibrosis and Genetic Disorders Group Module.

We also searched the Iranmedex databases (www.iranmedex.com) which is a national database containing the articles published in Iranian health care journals and LILACS on (15/05/2008) using the term 'carnitine'.

We also searched the Cochrane Central Register of Controlled Trials (Clinical Trials) and MEDLINE, details of which are provided in Appendix 1.

Date of most recent search of the Group's Inborn Errors of Metabolism Register: 27 October 2011.

Searching other resources

The reference lists of any clinical trials identified were cross‐checked and the review authors' personal databases of trial reports were examined in an attempt to identify any other relevant trials. In the future, we also plan to contact investigators of any included trials by either conventional or electronic mail to ask for details of additional published and unpublished trials.

Data collection and analysis

Selection of studies

Three authors, Mona Nasser (MN), Hoda Javaheri (HJ) and Zaman Noorani (ZN), have independently assessed the abstracts of trials identified from the searches for the original version of the review in 2009. One person (MN) has screened the results from the searches for this update.

We were not blinded to either the author or journal names. We obtained full copies of all relevant and potentially relevant trials (i.e. those appearing to meet the inclusion criteria) or for which there were insufficient data in the title and abstract to make a clear decision. We assessed the full text papers independently and resolved any disagreement on the eligibility of trials through discussion and consensus; or if necessary through a fourth party, Zbys Fedorowicz (ZF).

After assessment, the authors eliminated from further review any remaining trials that did not match the inclusion criteria and noted the reasons for their exclusion in the Characteristics of excluded studies table.

Data extraction and management

For future updates, when we include trials in the review, we will collect outcome data using a customised form. Two review authors (MN and Hoda Javaheri (HJ)) will enter the extracted data into RevMan 5.1 and sequentially and automatically check for differences (RevMan 2011). MN will hold the master copy and we will only include data when we reach a consensus. We will discuss any disagreement and if required, consult a third review author.

As there were no included trials, it was not possible to undertake any data collection for this review. However, the following methods will be used if further trials are included in a subsequent update of this review.

Each review author will enter details of included trials separately into the Characteristics of included studies table in RevMan 5.1 and cross‐check these details.

We will extract the following details:

1. Trial methods: method of allocation; masking of participants and outcomes; exclusion of participants after randomisation; and proportion of follow‐up losses.

2. Participants: country of origin; type and symptoms of IEM; sample size; age; sex; inclusion and exclusion criteria.

3. Intervention: dose; frequency; duration and length of time in follow up.

4. Control: placebo or no treatment.

5. Outcomes: primary and secondary and adverse outcomes mentioned in the section of outcome measures.

We plan to group outcome data into those measured at one month, at six months, at 12 months and annually thereafter. If data are reported at other time periods we will also consider these.

Assessment of risk of bias in included studies

For future updates, when we include trials in the review, two authors will independently assess the risk of bias according to the method described in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011a). Authors will compare evaluations and discuss and resolve any inconsistencies in their interpretation of inclusion criteria and their significance to the selected trials.

We will assess the following domains as having either a low, unclear or high risk of bias:

1. generation of the allocation sequence;

2. concealment of allocation;

3. blinding (of participants, personnel and outcome assessors);

4. incomplete outcome data;

5. selective outcome reporting.

Measures of treatment effect

Unfortunately, we have not included any relevant trials in this review and we could not undertake data extraction or data synthesis. However, we will use the following methods if further studies will be identified in subsequent updates of this review.

We will calculate risk ratios and 95% confidence intervals for all dichotomous primary and secondary outcomes. We will calculate the mean differences and 95% confidence intervals for continuous data.

Unit of analysis issues

For cross‐over trials, if possible, we will undertake a paired analysis of the data obtained to allow a within‐individual comparison of the treatment interventions as recommended by Elbourne (Elbourne 2002). If we identify cluster‐randomised trials in this review, we will analyse these data using the effective sample size approach as described in Chapter 16 of the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011b).

Dealing with missing data

We plan to contact the primary investigators of the trials to obtain any data missing from the published study reports.

Assessment of heterogeneity

Given that the review question is broad, we would expect clinical diversity in the identified studies on this topic. If we identify a sufficient number of studies for any individual comparison and outcome, we will explore and attempt to quantify the inconsistency using the I² statistic (Higgins 2003).

We will use the following guide for the interpretation of the results:

• 0% to 40%: might not be important;

• 30% to 60%: may represent moderate heterogeneity;

• 50% to 90%: may represent substantial heterogeneity;

• 75% to 100%: considerable heterogeneity.

However, we recognize that this is a general guide to interpreting the results and the importance of inconsistency depends on several factors.

Assessment of reporting biases

If we identify a sufficient number of randomised controlled trials (RCTs) (at least 10), we will attempt to assess publication bias using a funnel plot (Egger 1997) and if asymmetry is identified, we will also try to investigate other possible causes.

In order to evaluate outcome reporting bias, we will compare the trial protocol (if available) and the final published paper. If we are not able to identify the trial protocol, we aim to compare the 'Methods' and the 'Results' sections of the paper and explore whether all the outcomes that had been measured were reported in the results of the paper.

Data synthesis

If data are available from different trials on a similar comparison and outcome, we plan to analyse the data together. We will analyse the data using a fixed‐effect model. If we identify moderate, substantial or considerable heterogeneity between trials, we will analyse the data using a random‐effects model.

Subgroup analysis and investigation of heterogeneity

As there were no included trials, it was not possible to undertake any subgroup analyses for this review. However, for future updates, if a sufficient number of trials are included and if we establish that there are large numbers of participants in specific age groups, we will undertake a subgroup analyses in which participants will be categorized by age.

Sensitivity analysis

As there were no included trials, it was not possible to undertake any sensitivity analyses for this review. However, in subsequent updates of the review, if sufficient trials are available, we plan to conduct sensitivity analyses to assess the robustness of the review results by repeating the analysis with the following adjustments:

1. exclusion of trials with unclear or high risk of bias for allocation concealment;

2. unclear or a high risk of bias for blinding;

3. including and excluding quasi‐randomised trials; and

4. any missing data or data extraction problems.

In addition, we may undertake sensitivity analyses to examine the effect of completeness of follow‐up.

Results

Description of studies

See: Characteristics of excluded studies and Characteristics of studies awaiting classification.

Results of the search

The search strategy (last run in October 2008) retrieved 315 records (MEDLINE (OVID) 1950 to October Week 3 2008 [searched 28/10/08]: 227 references to studies; Cochrane Central Register of Clinical Trials (Clinical Trials) all years [searched 28/10/08]: 88 references; Cystic Fibrosis and Genetic Disorders Group's Inborn Errors of Metabolism Trials Register: 0 references). After examination of the titles and abstracts of these references, all but 30 studies were subsequently excluded from further analysis.

Full text copies of these remaining studies were obtained and were then subjected to further assessment. We also scrutinised the bibliographical references of these papers (including two reviews (Kolker 2004; Angelini 1987)) for any further potentially eligible studies. We found two references for which we attempted to obtain full text copies (Bjugstad 2000; Baric 1998).

One trial was a cross‐over placebo‐controlled clinical trial (Rodriguez 1997). We translated this study from Spanish into English but unfortunately, the paper did not include sufficient details on study design and study groups. We have attempted to contact the authors but, to date, have not received a response yet and therefore the study is currently listed in Studies awaiting classification. For further information on this trial, please refer to the Characteristics of studies awaiting classification table.

The updated searches on 27 October 2011 retrieved 113 records from the Cochrane Central Register of Clinical Trials (Clinical Trials) and 15 records from MEDLINE (OVID). None of these matched the inclusion criteria of the review.

Included studies

No trials are included in this review.

Excluded studies

From these studies, six studies were excluded as they were not controlled clinical trials (Bohles 1991;Gillingham 2003; Lee 2005; Schulpis 1990; Vilaseca 1993; Wolff 1986); a further ten because the included participants did not match our inclusion criteria (Bhuiyan 1992; Bowyer 1989; Chazot 2003; Cruciani 2004; Huidekoper 2006; Mayer 1989; Schmidt‐S 1983; Sirtori 2000; Yeh 1985; Zilleruelo 1989); and one because it included valproic acid as one of the interventions in the trial (Igarashi 1990). A further study had two parts, the first part was an RCT which included healthy people as participants and the second part evaluated the therapeutic effect of DL‐carnitine chloride in people with hyperlipoproteinaemia. Unfortunately, the second part of the study was uncontrolled and was therefore excluded (Maebashi 1978). Nine reports were literature reviews and did not contain any reports of randomised controlled trials relevant to this review (Angelini 1987; Baric 1998; Bjugstad 2000; Goa 1987; Kolker 2004; Krähenbühl 1995; Morris 1998; Muller 2004; Winter 2003); and one was an editorial and did not describe any relevant RCTs (Kelley 1994).

During the peer review process, an RCT was identified by the peer reviewers as a potential trial to be included in our review (Ellaway 1999). However, we excluded the trial from our review as we do not regard Rett syndrome as an IEM (this does not match our IEM criteria) and listed it in Characteristics of excluded studies.

Risk of bias in included studies

No trials are included in this review.

Effects of interventions

No trials are included in this review.

Discussion

Suitable trials were not found in the literature for our objectives to be achieved at present.

A number of studies tried to address the effectiveness and safety of carnitine supplementation for people with an IEM. A guideline on 'Diagnosis and management of glutaryl‐CoA dehydrogenase deficiency (GDHD)' partially addressed this question and states that lysine restricted diet and carnitine supplementation had a beneficial effect in people with GDHD. It can prevent secondary carnitine depletion, influences the progression of neurological disease and decreases the mortality rate in these patients. However, these results were only based on a multicenter cross‐sectional study and a synthesis of a number of case reports of people with Glutaric acidemia type 1 and not on interventional trials (Kolker 2007; Kolker 2006; Bjugstad 2000). The diverse nature of inborn errors of metabolism patients also raises questions on the applicability of results on the effectiveness of carnitine for one of the disorders to other individuals with different disorders. A possible solution could be grouping some of the disorders with similar characteristics e.g. long chain fatty acid oxidation defects in a clinical trial.
 
 There is a large body of research exploring and discussing the role of carnitine in metabolic pathways and a limited number of observational and interventional studies examining the effectiveness of dietary interventions in people with an IEM. This was also obvious in a number of other Cochrane Reviews evaluating dietary interventions and nutrition supplements for people with phenylketonuria (Poustie 2010; Webster 2010; Yi 2008).

For over 20 years carnitine has been used to treat fatty acid oxidation defects and organic acid disorders, and has provided moderate benefits in patients. Since then, carnitine has been added to the treatment of many IEMs based on those early clinical observations without any evidence from controlled clinical trials. Recently, a number of initiatives have led to improvements in research on IEMs (Kolker 2007; Seymour 1997) along with calls for more RCTs in these areas (Kruer 2008; Steiner 2005). On the other hand, doubts have been raised on the safety of carnitine supplementation in some people with certain IEMs (e.g. people with long‐chain 3‐hydroxyacyl‐CoA dehydrogenase deficiency (LCHAD)) (Winter 1998). Unfortunately, the effectiveness of these interventions has not been addressed by high quality RCTs, thus both the effectiveness and safety of carnitine in IEM remains an important question to be answered. The lack of RCTs undertaken in this area could be partially due to concerns raised on the ethical and practical aspects of undertaking a double‐blind RCT and the variability in characteristics of people with different IEMs (Winter 1998). In this respect it would be important that future RCTs are robust, well‐designed and reported according to the CONSORT statement.  Ethical issues should be carefully taken into consideration to decide whether it is possible to conduct placebo‐controlled trials in potentially lethal diseases, e.g. carnitine transporter disorder or glutaric aciduria type I. For other IEMs, such as MCAD deficiency, in which some clinicians recommend the use of carnitine and others do not, we strongly recommend that RCTs are conducted as soon as possible.

Authors' conclusions

Implications for practice.

There are no published or ongoing randomised controlled clinical trials relevant to this review question. Therefore, in the absence of any high‐level reliable evidence in the form of well‐conducted RCTs, no firm conclusions can be made about the effectiveness of carnitine for IEMs. This does not mean that carnitine is ineffective or should not be used in treating IEMs. However, there is a lack of evidence on the effectiveness and safety of carnitine and the necessary dose and frequency of carnitine to be prescribed. Hence, current practice should continue to be observed and monitored with care until further evidence is available, preferably, in a controlled environment or in the context of a clinical trial. This could be an individual based N‐of‐1 RCT or a multi‐center RCT. Similar to previous Cochrane Reviews on people with phenylketonuria (Poustie 2010; Webster 2010; Yi 2008), we recommend that people with an IEM should preferably enter larger, rather than smaller trials. Clinicians should base their treatment decisions on clinical experience and in conjunction with patient preference where appropriate. Moreover, clinicians should comprehensively inform patients on the advantages and disadvantages of carnitine in IEMs and existing uncertainties of its effectiveness and safety.

Implications for research.

The results of this systematic review confirm the necessity for further methodologically sound trials that are reported according to the Consolidated Standards of Reporting Trials (CONSORT) statement. Trialists should include patient‐reported outcomes using validated and internationally recognised scales and ensure that any adverse events associated with the treatment are reported. Trialists should also involve the patients (or their carers) in trial design to ensure that important and relevant factors are considered. We have provided a research recommendation based on the EPICOT format (Table 1) (Brown 2006). Careful consideration should be given when deciding whether it is ethical to conduct placebo‐controlled trials in potentially lethal diseases, e.g. carnitine transporter disorder or glutaric aciduria type I. In other disorders, such as MCAD deficiency, where some clinicians recommend the use of carnitine and others do not, it is strongly recommended that RCTs are undertaken. Due to the low incidence of each type of IEM, we recommend that researchers consider grouping similar IEMs within international multi‐center clinical trials.

1. Research Recommendation based on a gap in the evidence on carnitine supplementation for Inborn errors of metabolism.

Core Elements	Issues to Consider	Status of research for this review
Evidence (E)	What is the current state of evidence	A systematic review that did not identify any RCT or CCT
Population (P)	Diagnosis, disease stage, comorbidity, risk factor, sex, age, ethnic group, specific inclusion or exclusion criteria, clinical setting	Patients with IEM (either focused on a specific IEM or similar IEMs would be grouped e.g. long‐chain fatty acid oxidation defects), Sex: men and Female
Intervention (I)	Type, frequency, dose, duration, prognostic factor	Intravenous or oral carnitine supplementation (in different dose, frequency, or duration), Compliance would be recorded
Comparison (C)	Type, frequency, dose, duration, prognostic factor	Placebo or no supplementation
Outcome (O)	Which clinical or patient related outcomes will the researcher need to measure, improve, influence or accomplish? Which methods of measurement should be used?	Changes in measures of growth, Measures of neuropsychological performance, Number of deaths or age at death in each group would be reported along with the adverse effects associated with the administration of Carnitine.
Time Stamp (T)	Date of literature search or recommendation	10 Oct 2008
Study Type	What is the most appropriate study design to address the proposed question?	Randomised controlled trial (parallel or cross over), Methods: Concealment Clear. Blindness: Patients, therapist, trialists, outcomes assessors blind. Setting: in hospital or outpatient care with follow up

CCT: controlled clinical trial

IEM: inborn error of metabolism

RCT: randomised controlled trial

What's new

Date	Event	Description
13 April 2015	Amended	Contact details updated.

History

Protocol first published: Issue 3, 2007
 Review first published: Issue 2, 2009

Date	Event	Description
3 January 2012	New search has been performed	The search retrieved 113 records from the Cochrane Central Register of Clinical Trials (Clinical Trials) and 15 records from MEDLINE (OVID). None of these matched the inclusion criteria of the review.
3 January 2012	New citation required but conclusions have not changed	The methods section has been updated.

Appendices

Appendix 1. Details of the Search Strategies for each database

Database	Date of search	Search Strategy
Clinical Trials	28/10/ 2008	#13‐Hydroxy‐3‐methylglutaric aciduria #2Abetalipoproteinemia #3Acatalasia #4Renal Tubular Acidosis #5(Adrenal Hyperplasia NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #6Adrenoleukodystrophy #7Albinism #8Alkaptonuria #9alpha‐Mannosidosis #10Amino acidopath* #11Amyloid Neuropath* #12argininaemia #13beta‐Mannosidosis #14Biotinidase deficiency #15Ketothiolase deficiency #16Carbamoyl‐Phosphate Synthase I Deficiency #17Carbohydrate‐Deficient Glycoprotein Syndrome #18(Cerebral Amyloid Angiopathy NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #19Cholesterol Ester Storage Disease #20Rhizomelic Chondrodysplasia Punctata #21Citrullinemia #22adrenal hyperplasia #23Crigler‐Najjar Syndrome #24Cystinuria #25Cytochrome‐c Oxidase Deficiency #26Fabry* Disease #27hypercholesterolaemia #28Fanconi Syndrome #29fatty acid metabolism #30fatty acid oxidation #31Fructose Intolerance #32Fructose NEAR/10 Metabolism #33Fructose‐1,6‐Diphosphatase Deficiency #34Fucosidosis #35Galactosaemia #36Galactosemia #37Gangliosidoses #38Gangliosidosis #39Gaucher #40gilbert NEAR/5 disease #41Glucosephosphate Dehydrogenase Deficiency #42Glutaric aciduria #43Glycogen Storage Disease #44Renal Glycosuria #45Gout #46Hartnup NEAR/5 Disease #47Hemochromatosis #48Hepatolenticular Degeneration #49Histidinaemia #50Holocarboxylase Synthetase Deficiency #51Homocystinuria #52Hyperargininemia #53(Hyperbilirubinemia NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #54(Hyperglycinemia NEAR/10 Nonketotic) #55Hyperhomocysteinemia #56Hyperlipidemia #57Hyperlipoproteinemia #58Hyperlysinemia #59(Hyperoxaluria NEAR/10 Primary) #60hyperprolinaemia #61Hypoalphalipoproteinemia #62Hypobetalipoproteinemia #63Hypokalemic Periodic Paralysis #64Hypolipoproteinemia #65(Hypophosphatemia NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #66Hypophosphatasia #67Hypophosphatemic Rickets #68X‐Linked Ichthyosis #69Isovaleric acidaemia #70Chronic Idiopathic Jaundice #71Lactose Intolerance #72Lecithin Acyltransferase Deficiency #73Leigh Disease #74Lesch‐Nyhan Syndrome #75Leukodystrophy #76Lipidoses #77Lysosomal Storage Disease* #78Mannosidase Deficiency Disease* #79Mannosidosis #80Maple Syrup Urine #81Medium‐chain acyl‐CoA dehydrogenase #82MELAS #83MERRF #84Methylmalonic aciduria #85Mineralocorticoid Excess Syndrome NEAR/10 apparent #86mitochondrial b‐oxidation #87mitochondrial long‐chain #88Mucolipidoses #89Mucopolysaccharidoses #90Mucopolysaccharidosis #91Multicarboxylase deficienc* #92Multiple acyl CoA dehydrogenase deficiency #93Multiple Carboxylase Deficiency #94Multiple Sulfatase Deficiency #95Neuronal Ceroid‐Lipofuscinoses #96Niemann‐Pick #97Oculocerebrorenal Syndrome #98organic acidaemia* #99organic aciduria* #100Ornithine Carbamoyltransferase Deficiency #101(Peroxisomal NEAR/10 (disease or disorder)) #102Phenylketonuria #103Porphyria #104Progeria #105Propionic acidaemia #106Pseudohypoaldosteronism #107Pseudohypoparathyroidism #108Purine‐Pyrimidine Metabolism #109(Pyruvate Carboxylase Deficiency NEAR/10 Disease) #110(Pyruvate Dehydrogenase Complex Deficiency NEAR/10 Disease) #111(Pyruvate Metabolism NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #112Refsum Disease #113Renal Aminoaciduria* #114(Renal Tubular Transport NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #115Sandhoff Disease #116SCAD deficiency #117Sea‐Blue Histiocyte Syndrome #118Sialic Acid Storage Disease #119Sjogren‐Larsson Syndrome #120Smith‐Lemli‐Opitz Syndrome #121Sphingolipidoses #122(Steroid Metabolism NEAR/10 (hereditary or congenital or familial or inborn or inherited)) #123Sulfatidosis #124Tangier Disease #125Tyrosinaemia #126Wolman Disease #127(Xanthomatosis NEAR/10 Cerebrotendinous) #128Zellweger Syndrome #129MeSH descriptor Metabolism, Inborn Errors explode all trees #130(#1 OR #2 OR #3 OR #4 OR #5 OR #6 OR #7 OR #8 OR #9 OR #10 OR #11 OR #12 OR #13 OR #14 OR #15 OR #16 OR #17 OR #18 OR #19 OR #20 OR #21 OR #22 OR #23 OR #24 OR #25 OR #26 OR #27 OR #28 OR #29 OR #30 OR #31 OR #32 OR #33 OR #34 OR #35 OR #36 OR #37 OR #38 OR #39 OR #40 OR #41 OR #42 OR #43 OR #44 OR #45 OR #46 OR #47 OR #48 OR #49 OR #50 OR #51 OR #52 OR #53 OR #54 OR #55 OR #56 OR #57 OR #58 OR #59 OR #60 OR #61 OR #62 OR #63 OR #64 OR #65 OR #66 OR #67 OR #68 OR #69 OR #70 OR #71 OR #72 OR #73 OR #74 OR #75 OR #76 OR #77 OR #78 OR #79 OR #80 OR #81 OR #82 OR #83 OR #84 OR #85 OR #86 OR #87 OR #88 OR #89 OR #90 OR #91 OR #92 OR #93 OR #94 OR #95 OR #96 OR #97 OR #98 OR #99 OR #100 OR #101 OR #102 OR #103 OR #104 OR #105 OR #106 OR #107 OR #108 OR #109 OR #110 OR #111 OR #112 OR #113 OR #114 OR #115 OR #116 OR #117 OR #118 OR #119 OR #120 OR #121 OR #122 OR #123 OR #124 OR #125 OR #126 OR #127 OR #128 OR #129) #131levocarnitine #132carnitine #133L‐carnitine #134(#131 OR #132 OR #133) #135(#130 AND #134)
MEDLINE via Ovid (1950 to 28/10/2008)	28/10/2008	1 randomized controlled trial.pt. (267719) 2 controlled clinical trial.pt. (80501) 3 randomized.ab. (175733) 4 placebo.ab. (110727) 5 drug therapy.fs. (1311086) 6 randomly.ab. (127547) 7 trial.ab. (183149) 8 groups.ab. (883669) 9 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 (2370962) 10 (animals not (humans and animals)).sh. (3279583) 11 9 not 10 (2011516) 12 3‐Hydroxy‐3‐methylglutaric aciduria.tw. (46) 13 Abetalipoproteinemia.tw. (285) 14 Acatalasia.tw. (23) 15 Renal Tubular Acidosis.tw. (1737) 16 (Adrenal Hyperplasia adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (2938) 17 Adrenoleukodystrophy.tw. (1405) 18 Albinism.tw. (1524) 19 Alkaptonuria.tw. (479) 20 alpha‐Mannosidosis.tw. (128) 21 Amino acidopath$.tw. (35) 22 Amyloid Neuropath$.tw. (131) 23 argininaemia.tw. (7) 24 beta‐Mannosidosis.tw. (97) 25 Biotinidase deficiency.tw. (236) 26 Ketothiolase deficiency.tw. (60) 27 Carbamoyl‐Phosphate Synthase I Deficiency.tw. (2) 28 Carbohydrate‐Deficient Glycoprotein Syndrome.tw. (208) 29 (Cerebral Amyloid Angiopathy adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (73) 30 Cholesterol Ester Storage Disease.tw. (54) 31 Rhizomelic Chondrodysplasia Punctata.tw. (176) 32 Citrullinemia.tw. (264) 33 adrenal hyperplasia.tw. (3803) 34 Crigler‐Najjar Syndrome.tw. (244) 35 Cystinuria.tw. (1002) 36 Cytochrome‐c Oxidase Deficiency.tw. (266) 37 Fabry$ Disease.tw. (1856) 38 hypercholesterolaemia.tw. (3316) 39 Fanconi Syndrome.tw. (857) 40 fatty acid metabolism.tw. (2973) 41 fatty acid oxidation.tw. (3917) 42 Fructose Intolerance.tw. (341) 43 (Fructose adj10 Metabolism).tw. (914) 44 Fructose‐1,6‐Diphosphatase Deficiency.tw. (56) 45 Fucosidosis.tw. (256) 46 Galactosaemia.tw. (320) 47 Galactosemia.tw. (1077) 48 Gangliosidoses.tw. (165) 49 Gangliosidosis.tw. (1055) 50 Gaucher.tw. (1837) 51 (gilbert adj5 disease).tw. (19) 52 Glucosephosphate Dehydrogenase Deficiency.tw. (23) 53 Glutaric aciduria.tw. (343) 54 Glycogen Storage Disease.tw. (1606) 55 Renal Glycosuria.tw. (162) 56 Gout.tw. (6106) 57 (Hartnup adj5 Disease).tw. (98) 58 Hemochromatosis.tw. (4667) 59 Hepatolenticular Degeneration.tw. (493) 60 Histidinaemia.tw. (60) 61 Holocarboxylase Synthetase Deficiency.tw. (51) 62 Homocystinuria.tw. (1231) 63 Hyperargininemia.tw. (71) 64 (Hyperbilirubinemia adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (221) 65 (Hyperglycinemia adj10 Nonketotic).tw. (188) 66 Hyperhomocysteinemia.tw. (2910) 67 Hyperlipidemia.tw. (10393) 68 Hyperlipoproteinemia.tw. (2873) 69 Hyperlysinemia.tw. (43) 70 (Hyperoxaluria adj10 Primary).tw. (751) 71 hyperprolinaemia.tw. (35) 72 Hypoalphalipoproteinemia.tw. (200) 73 Hypobetalipoproteinemia.tw. (234) 74 Hypokalemic Periodic Paralysis.tw. (375) 75 Hypolipoproteinemia.tw. (23) 76 (Hypophosphatemia adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (97) 77 Hypophosphatasia.tw. (530) 78 Hypophosphatemic Rickets.tw. (508) 79 X‐Linked Ichthyosis.tw. (282) 80 Isovaleric acidaemia.tw. (31) 81 Chronic Idiopathic Jaundice.tw. (65) 82 Lactose Intolerance.tw. (841) 83 Lecithin Acyltransferase Deficiency.tw. (0) 84 Leigh Disease.tw. (117) 85 Lesch‐Nyhan Syndrome.tw. (623) 86 Leukodystrophy.tw. (1782) 87 Lipidoses.tw. (143) 88 Lysosomal Storage Disease$.tw. (1398) 89 Mannosidase Deficiency Disease$.tw. (0) 90 Mannosidosis.tw. (447) 91 Maple Syrup Urine.tw. (723) 92 Medium‐chain acyl‐CoA dehydrogenase.tw. (624) 93 MELAS.tw. (1105) 94 MERRF.tw. (343) 95 Methylmalonic aciduria.tw. (319) 96 (Mineralocorticoid Excess Syndrome adj10 apparent).tw. (32) 97 mitochondrial b‐oxidation.tw. (2) 98 mitochondrial long‐chain.tw. (62) 99 Mucolipidoses.tw. (90) 100 Mucopolysaccharidoses.tw. (679) 101 Mucopolysaccharidosis.tw. (1919) 102 Multicarboxylase deficienc$.tw. (1) 103 Multiple acyl CoA dehydrogenase deficiency.tw. (49) 104 Multiple Carboxylase Deficiency.tw. (121) 105 Multiple Sulfatase Deficiency.tw. (92) 106 Neuronal Ceroid‐Lipofuscinoses.tw. (363) 107 Niemann‐Pick.tw. (1564) 108 Oculocerebrorenal Syndrome.tw. (106) 109 organic acidaemia$.tw. (39) 110 organic aciduria$.tw. (311) 111 Ornithine Carbamoyltransferase Deficiency.tw. (42) 112 (Peroxisomal adj10 (disease or disorder)).tw. (428) 113 Phenylketonuria.tw. (4275) 114 Porphyria.tw. (6321) 115 Progeria.tw. (636) 116 Propionic acidaemia.tw. (111) 117 Pseudohypoaldosteronism.tw. (401) 118 Pseudohypoparathyroidism.tw. (1013) 119 Purine‐Pyrimidine Metabolism.tw. (9) 120 (Pyruvate Carboxylase Deficiency adj10 Disease).tw. (7) 121 (Pyruvate Dehydrogenase Complex Deficiency adj10 Disease).tw. (1) 122 (Pyruvate Metabolism adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (8) 123 Refsum Disease.tw. (212) 124 Renal Aminoaciduria$.tw. (9) 125 (Renal Tubular Transport adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (14) 126 Sandhoff Disease.tw. (245) 127 SCAD deficiency.tw. (42) 128 Sea‐Blue Histiocyte Syndrome.tw. (41) 129 Sialic Acid Storage Disease.tw. (77) 130 Sjogren‐Larsson Syndrome.tw. (257) 131 Smith‐Lemli‐Opitz Syndrome.tw. (534) 132 Sphingolipidoses.tw. (138) 133 (Steroid Metabolism adj10 (hereditary or congenital or familial or inborn or inherited)).tw. (13) 134 Sulfatidosis.tw. (13) 135 Tangier Disease.tw. (419) 136 Tyrosinaemia.tw. (187) 137 Wolman Disease.tw. (84) 138 (Xanthomatosis adj10 Cerebrotendinous).tw. (430) 139 Zellweger Syndrome.tw. (599) 140 exp metabolism, inborn errors/ or exp amino acid metabolism, inborn errors/ or exp amino acid transport disorders, inborn/ or exp amyloidosis, familial/ or exp brain diseases, metabolic, inborn/ or exp carbohydrate metabolism, inborn errors/ or exp cytochrome‐c oxidase deficiency/ or exp hyperbilirubinemia, hereditary/ or exp lipid metabolism, inborn errors/ or exp lysosomal storage diseases/ or exp metal metabolism, inborn errors/ or exp peroxisomal disorders/ or exp porphyrias/ or exp progeria/ or exp purine‐pyrimidine metabolism, inborn errors/ or exp renal tubular transport, inborn errors/ or exp steroid metabolism, inborn errors/ (124515) 141 or/12‐140 (159671) 142 levocarnitine.tw. (47) 143 carnitine.tw. (8489) 144 L‐carnitine.tw. (2595) 145 exp Carnitine/ (6706) 146 or/142‐145 (9950) 147 11 and 141 and 146 (305)
MEDLINE via Ovid (1950 to July week 4 2007)	10 Oct 2008	1 RANDOMIZED CONTROLLED TRIAL.pt. (267369) 2 CONTROLLED CLINICAL TRIAL.pt. (80428) 3 RANDOMIZED CONTROLLED TRIALS.sh. (0) 4 RANDOM ALLOCATION.sh. (63132) 5 DOUBLE BLIND METHOD.sh. (100865) 6 SINGLE BLIND METHOD.sh. (12612) 7 or/1‐6 (401918) 8 (ANIMALS not HUMAN).sh. (4360195) 9 7 not 8 (364590) 10 CLINICAL TRIAL.pt. (459949) 11 exp Clinical Trial/ (567966) 12 (clin$ adj25 trial$).ti,ab. (167797) 13 ((singl$ or doubl$ or trebl$ or tripl$) adj25 (blind$ or mask$)).ti,ab. (105799) 14 PLACEBOS.sh. (28238) 15 placebo$.ti,ab. (121110) 16 random$.ti,ab. (473183) 17 RESEARCH DESIGN.sh. (54761) 18 or/10‐17 (1001547) 19 18 not 8 (890483) 20 19 not 9 (538260) 21 COMPARATIVE STUDY.sh. (1439860) 22 exp EVALUATION STUDIES/ (111837) 23 FOLLOW UP STUDIES.sh. (381719) 24 PROSPECTIVE STUDIES.sh. (254647) 25 (control$ or prospectiv$ or volunteer$).ti,ab. (2166497) 26 or/21‐25 (3666472) 27 26 not 8 (2610816) 28 27 not (9 or 20) (2109014) 29 9 or 20 or 28 (3011864) 30 exp Rett Syndrome/ (1363) 31 Rett syndrome.tw. (1609) 32 (levocarnitine or carnitine or L‐carnitine).tw. (8879) 33 30 or 31 (1801) 34 33 and 32 and 29 (7) 35 from 34 keep 1‐7 (7)

Characteristics of studies

Characteristics of excluded studies [ordered by study ID]

Study	Reason for exclusion
Angelini 1987	This study is a review and did not describe any relevant RCT or CCT.
Baric 1998	This study is a review and did not describe any relevant and additional RCT or CCT.
Bhuiyan 1992	This study is evaluating a method for the estimation of individual carnitine.
Bjugstad 2000	This study is a review undertaking a multiple regression analysis on a number of cases, however, it did not describe any relevant and additional RCT or CCT.
Bohles 1991	This study is not controlled.
Bowyer 1989	The participants were not diagnosed with an IEM. They had low plasma carnitine concentration due to long‐term parenteral nutrition.
Chazot 2003	The participants underwent hemodyalisis, and did not have an IEM.
Cruciani 2004	This study included patients with cancer not individuals affected with IEM.
Ellaway 1999	Rett syndrome is not an IEM.
Gillingham 2003	This study is not a controlled trial. It evaluates the effects of contemporary dietary therapy upon various biochemical parameters of metabolic control and clinical outcome.
Goa 1987	This study is a review and did not describe any relevant and additional  RCT or CCT.
Huidekoper 2006	Five participants were compared with healthy volunteers.
Igarashi 1990	The study includes valproic acid as an intervention arm.
Kelley 1994	This paper is an editorial and did not describe any relevant new RCT.
Kolker 2004	This study is a narrative review and did not describe any relevant RCT or CCT.
Krähenbühl 1995	This is a review and did not describe any relevant and additional RCT or CCT.
Lee 2005	This study is not a controlled trial.
Maebashi 1978	The first part of the study included healthy participants and the second part of study did not have a control group.
Mayer 1989	This study included participants with chronic haemodialysis with hyperlipidaemia.
Morris 1998	This study is a narrative review and did not describe any relevant RCT.
Muller 2004	This study is a narrative review and did not describe any relevant and additional  RCT or CCT.
Schmidt‐S 1983	The participants are premature infants and do not match our inclusion criteria.
Schulpis 1990	This is an observational study and not a RCT or CCT.
Sirtori 2000	The study included people with hyperlipidaemia but they did not necessarily have an IEM.
Vilaseca 1993	This study evaluated the level of serum, free, total carnitine and acylcarnitine in phenylketonuria and is not a controlled trial.
Winter 2003	This study is a narrative review and did not describe any relevant and additional  RCT or CCT.
Wolff 1986	This study is not a RCT or CCT.
Yeh 1985	This study is a prospective trial on premature infants and not on individuals with IEM.
Zilleruelo 1989	This study did not include individuals with an IEM and only included 9 participants with end‐stage renal disease on maintenance hemodialysis treatment.

CCT: controlled clinical trial
 IEM: inborn error of metabolism
 RCT: randomised controlled trial

Characteristics of studies awaiting assessment [ordered by study ID]

Rodriguez 1997.

Methods	The cross‐over trial is a CCT but it is not clear whether the method of randomisation was adequate.
Participants	25 Children with the average age of 9.76 years old (6 to 12 years) with primary dyslipidaemia and excluded people with secondary dyslipidaemia.
Interventions	Oral carnitine (1g (10cc), 3 times a day) (Group A) versus placebo (Group B) for 8 weeks, at which point the participants had a 15 days washout period and another 8 weeks of treatment: oral carnine (Group B) versus placebo (Group A).
Outcomes	Biochemical variables (serum LDL, total cholesterol, cholestrol‐HDL, glycaemia, uric acid, creatinine, triglycerides, hemoglobin, level of plasma carnitine). Anthropometric variables (weight, height, body mass index (BMI), measures of circumferences waist/thigh, the cutaneous pleats (sub scapular, brachial pleat)). Changes in eating behaviour, treatment tolerances (patient satisfaction).
Notes	The number of participants allocated in each group and some methodological details are not clear. We have contacted the trialists but have not yet received a response.

BMI: body mass index
 CCT: controlled clinical trial
 HDL: high‐density lipoprotein
 LDL: low‐density lipoprotein

Differences between protocol and review

The section of 'Assessment of risk of bias in included studies' has been modified in light of the release of the new RevMan 5.1 software and the publication of the new Cochrane Handbook for Systematic Review of Interventions 5.0.0 produced by the Cochrane Collaboration.

In the update of the review, we deleted the following sentence from 'Data extraction and management' as we were concerned that it might result in losing important data:

If appropriate, we will transform data obtained from visual analogue scales and any categorical outcomes into dichotomous data prior to analysis in order to pool data from other trials with already dichotomous data or other categorizations.

Contributions of authors

Mona Nasser (MN) is responsible for co‐ordinating the review.
 MN and Hoda Javaheri (HJ) and Zaman Noorani (ZN) are responsible for screening search results and screening retrieved papers against inclusion criteria.
 MN and Hoda Javaheri (HJ) are responsible for appraising the quality of papers.
 MN is responsible for organising the retrieval of papers and writing to authors of papers for additional information.
 MN and HJ are responsible for data management of the review including extracting data from papers and entering data into RevMan 5.1.
 MN and Zbys Fedorowicz (ZF) are responsible for obtaining and screening data on unpublished studies.
 MN and ZF are responsible for the interpretation and analysis of data.
 MN, ZM, HJ and ZF are responsible for writing the protocol and the review.
 MN conceived the idea for the review and will be the guarantor for the review.

Declarations of interest

There are no financial conflicts of interest and the review authors declare that they do not have any associations with any parties who may have vested interests in the results of this review.

Edited (no change to conclusions)