Diagnostic and therapeutic implications of medium-chain acylcarnitines in the medium-chain acyl-coA dehydrogenase deficiency

Is carnitine essential in children?

Carnitine has a fundamental biological role as a long-chain fatty acid carrier across the mitochondrial membrane and in ketone body formation. Although the body normally synthesizes carnitine, in certain circumstances such as total parenteral nutrition and haemodialysis a dietary supplement may be needed to maintain adequate levels. Several considerations suggest that carnitine is a truly essential nutrient in infancy and in other situations where the energy requirement is particularly high, e.g. pregnancy and breast feeding. There are, for example, congenital deficit syndromes due to enzymatic inadequacies. There is also the possible role of carnitine in serious metabolic disorders such as organic acidaemias and, above all, it has multiple physiological functions in major metabolic pathways which are essential for development and growth.

Carnitine in dried blood spots: a method suitable for neonatal screening

A method is described which enables the quantitative determination of both free and total carnitine levels in dried blood spots. This method is suitable for neonatal screening for either primary or secondary carnitine deficiency. The 95% confidence interval for free carnitine was 26-76 mumol/l (median = 44) and for total carnitine was 35-102 mumol/l (median = 60).

L-Carnitine prevents mitochondrial damage induced by octanoic acid in the rat choroid plexus

Cytochrome oxidase activity was investigated histochemically in the choroid plexus epithelium. Intense staining for the enzyme was exclusively limited to the mitochondria. Rats treated with octanoic acid displayed extensive ultrastructural disruptions in the epithelial cells of the choroid plexus. Mitochondria were fewer in number and more disrupted compared to the control. The enzyme activity was greatly reduced. However, pretreatment with an equimolar dose of L-carnitine followed by octanoic acid injection produced little alteration of either ultrastructure or enzyme staining. This study suggests that L-carnitine supplementation may restore mitochondrial function of the choroid plexus subjected to toxic organic anions in metabolic disorders, and may be useful in the prevention of metabolic encephalopathy.

Medium-chain acyl CoA dehydrogenase deficiency: electron microscopic differentiation from Reye syndrome

Inborn errors involving the oxidative metabolism of fatty acids may present clinically with a Reye syndrome-like picture. This case report of a patient with medium-chain acyl CoA dehydrogenase (MCAD) deficiency illustrates that electron microscopy may help to differentiate this disorder from Reye syndrome even if a liver biopsy is performed in a patient who recovered from an acute metabolic decompensation. Together with this case, a review of the few reports in the literature of pathological findings in MCAD deficiency is given. Changes uncharacteristic for Reye syndrome are a large-droplet steatosis and the presence of distinctive mitochondrial abnormalities on electron microscopy. The detection of an electron dense mitochondrial matrix and a widened space of inner mitochondrial membranes rules out Reye syndrome and is suggestive of a disorder of mitochondrial fatty acid oxidation.

Primary lipid cardiomyopathy

In this communication, we describe an isolated, apparently congenital cardiomyopathy (CMP) characterized by the accumulation of stainable lipid in mitochondria of cardiomyocytes. This lesion, which we term primary lipid cardiomyopathy, has not been reported so far. The structural alteration was associated with progressive heart failure, leading to death at the age of 3 years, and with massive hypertrophy of myocardium. Lipid storage in heart muscle cells resulted in an impressive yellow to orange color of the myocardium. We suggest that this type of primary CMP may represent a new member within the group of mitochondrial CMPs. Possible pathogenic mechanisms are discussed.

Early diagnosis and treatment of neonatal medium-chain acyl-CoA dehydrogenase deficiency: report of two siblings

Two siblings are reported who were symptomatic in the neonatal period. The first died suddenly at 4 days of age after regurgitating a meal. The postmortem examination showed steatosis of the liver, kidney and muscle. In the second, medium-chain acyl-CoA dehydrogenase (MCAD) deficiency was diagnosed at 3 days of age with muscular hypotonia, vomiting, hyperammonaemia and mild acidosis. Thus disorders of fatty acid oxidation should also be considered in newborns. The biochemical work up indicates that in neonates, analysis of serum medium-chain fatty acids and of acyl and free carnitine are more likely to lead to a diagnosis than determining dicarboxylic acids alone in urine. Long-term treatment was effective and monitored by the acyl/free carnitine ratio.

2,4-Dienoyl-coenzyme A reductase deficiency: a possible new disorder of fatty acid oxidation

Several inherited disorders of fatty acid beta-oxidation have been described that relate mainly to saturated precursors. This study is the first report of an enzyme defect related only to unsaturated fatty acid oxidation and provides the first in vivo evidence that fat oxidation in humans proceeds by the reductase-dependent pathway. The patient was a black female, presenting in the neonatal period with persistent hypotonia. Biochemical studies revealed hyperlysinemia, hypocarnitinemia, normal organic acid profile, and an unusual acylcarnitine species in both urine and blood. The new metabolite was positively identified by mass spectrometry as 2-trans,4-cis-decadienoylcarnitine, derived from incomplete oxidation of linoleic acid. In spite of dietary therapy, the patient died of respiratory acidosis at four months of age. Samples of liver and muscle from the autopsy were assayed for 2,4-dienoyl-coenzyme A reductase activity. Using the substrate 2-trans,4-cis-decadienoylcoenzyme A, the reductase activity was 40% of the control value in liver and only 17% of that found in normal muscle. It is suggested that unsaturated substrates should be used for in vitro testing to cover the full range of potential beta-oxidation defects and that acylcarnitine species identification be used for in vivo detection of this disorder.

Secondary carnitine deficiency in handicapped patients receiving valproic acid and/or elemental diet

We examined serum-free carnitine (SFC) concentrations and serum acylcarnitine (SAC)/SFC ratios in 40 severely handicapped patients, aged 2 to 36 years, and 69 age-matched control subjects. SFC levels in the patients treated with valproic acid (VPA) and/or receiving carnitine-deficient elemental diets (ED) were significantly lower, and their SAC/SFC ratios were significantly higher than in the other patients or in control subjects. There were 6 patients whose SFC levels were less than the -2SD level (15.8 +/- 6.7 microM, range 6.3-25.5) of those in control subjects (52.1 +/- 11.5 microM). They had no clinical symptoms of carnitine deficiency such as non-ketotic hypoglycemia, hepatomegaly, muscle weakness or cardiac function impairment, and showed normal transaminase, lipid and ammonia levels. In two cases (SFC = 11.0, 13.4 microM), the ketogenic responses to intravenous administration of fat-emulsion were impaired, but they were restored after D-,L-carnitine supplementation (30 mg/kg/day, po) for 1 month. However, in one case with the lowest SFC level (6.3 microM), the ketogenic responses to fat-emulsion infusion or fasting were normal, and dicarboxylic aciduria was not detected. These results indicate that 1) SFC levels are reduced in handicapped patients receiving VPA and/or ED, although clinical symptoms of carnitine deficiency do not easily develop, 2) some of these hypocarnitinemic cases show a subclinical impairment of hepatic fatty acid metabolism, not always correlated with the degree of SFC reduction, which can be restored by exogenous carnitine supplements, and therefore 3) in patients with acquired hypocarnitinemia, carnitine therapy should be considered, although a low SFC level alone may not imply an immediate indication.

Carnitine deficiency syndromes

Carnitine deficiency syndromes manifest as metabolic encephalopathy, lipid storage myopathy, or cardiomyopathy. Impairment of long-chain fatty acid metabolism and failure of energy production affect tissues reliant on oxidative metabolism. The accumulation of toxic fatty acyl derivatives impedes gluconeogenesis and urea cycle function which, in turn, causes hypoketotic hypoglycemia, transaminase elevations, and hyperammonemia. Oxidation of accumulated fatty acids through an alternative pathway, omega-oxidation, produces dicarboxylic aciduria. Carnitine must be transported into skeletal muscle. Myopathic carnitine deficiency occurs when this transport mechanism is defective. Most systemic carnitine deficiencies are secondary to other disorders that promote excretion of carnitine as acylcarnitine; however, primary systemic carnitine deficiency, likely due to impaired renal conservation of carnitine, also occurs.

High-performance liquid chromatographic separation of acylcarnitines following derivatization with 4'-bromophenacyl trifluoromethanesulfonate

A high-performance liquid chromatographic method for the separation of acylcarnitines after derivatization with 4'-bromophenacyl trifluoromethanesulfonate is presented. Derivatization of acylcarnitines was achieved at room temperature within 10 min. Separation of the acylcarnitine 4'-bromophenacyl esters was accomplished by high-performance liquid chromatography using as the analytical column a Resolve-PAK 5-microns C18 radially compressed cartridge eluted with a tertiary gradient containing varying proportions of water, acetonitrile, tetrahydrofuran, triethylamine, potassium phosphate, and phosphoric acid. Acylcarnitine 4'-bromophenacyl esters were detected spectrophotometrically at 254 nm. Baseline separation was obtained for a standard mixture (5 nmol of each injected) containing carnitine, acetyl-, propionyl-, butyryl-, valeryl-, hexanoyl-, heptanoyl-, octanoyl-, nonanoyl-, decanoyl-, lauroyl-, myristroyl-, palmitoyl-, and stearoylcarnitine. Nearly complete separation was obtained for a standard mixture containing butyryl-, isobutyryl-, isovaleryl-, and 2-methylbutyrylcarnitine. The method was applied to a normal human urine and then to this same urine spiked with the acylcarnitine standards. Urinary acylcarnitine profiles from patients having propionic acidemia, isovaleric acidemia, and medium-chain acyl-CoA dehydrogenase deficiency were performed. Urinary isovalerylcarnitine was quantified in the patient with isovaleric acidemia using heptanoylcarnitine as an internal standard.

Analysis of abnormal urinary metabolites in the newborn period in medium-chain acyl-CoA dehydrogenase deficiency

In order to determine which are useful early diagnostic markers for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, we have analysed urine from an asymptomatic neonate. Profiling of urinary organic acids followed by peak confirmation by electron impact mass spectrometry revealed a high suberate/adipate ratio (greater than 1.0) and the presence of n-hexanoylglycine (HG). Acylcarnitine analysis by fast atom bombardment mass spectrometry (FAB-MS) was inconclusive, but FAB-MS/MS (tandem mass spectrometry) revealed diagnostic amounts of octanoylcarnitine and hexanoylcarnitine. Quantitative analysis of acylglycines by stable isotope dilution and chemical ionization mass spectrometry revealed a 30-fold increase in HG and increased suberylglycine, but no increase in 3-phenylpropionylglycine.

Quantitation of urinary carnitine esters in a patient with medium-chain acyl-coenzyme A dehydrogenase deficiency: effect of metabolic state and L-carnitine therapy

Urinary carnitine esters were quantitated in an infant with medium-chain acylcoenzyme A dehydrogenase deficiency by means of a highly sensitive and specific radioisotopic exchange high-pressure liquid chromatography method. During fasting, the excretion of free carnitine and of acetylcarnitine, octanoylcarnitine, and hexanoylcarnitine was increased. The fractional tubular reabsorption of free carnitine was decreased, suggesting a renal leak of free carnitine. In the symptom-free, fed state, only minor amounts of free carnitine and of short-chain acylcarnitine, octanoylcarnitine, and hexanoylcarnitine were present in urine, and carnitine loss occurred in the form of "other" carnitine esters not exceeding that of control subjects. During L-carnitine therapy, the excretion of free carnitine, short-chain acylcarnitine, octanoylcarnitine, and hexanoylcarnitine, and particularly of "other" carnitine esters, was increased, suggesting a possible detoxifying effect of administered carnitine that is not confined to the elimination of octanoic and hexanoic acids. The employed method detects very low urinary concentrations of octanoylcarnitine and hexanoylcarnitine (less than 1 mumol/L) characteristic of medium-chain acyl-coenzyme A dehydrogenase deficiency and may be useful in screening for this disease, which has been associated with sudden infant death.

Stable isotope dilution analysis of n-hexanoylglycine, 3-phenylpropionylglycine and suberylglycine in human urine using chemical ionization gas chromatography/mass spectrometry selected ion monitoring

We describe a gas chromatographic/mass spectrometric method for the accurate determination of n-hexanoylglycine, 3-phenylpropionylglycine and suberylglycine in urine for the diagnosis of hereditary medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. These acylglycines had previously been detected in urine from patients with MCAD deficiency, but their diagnostic values were unknown because of a lack of appropriate analytical methods. n-Hexanoyl(1,2-13C)glycine, 3-phenylpropionyl(2-13C,15N)glycine and suberyl(2-13C,15N)glycine were synthesized and used as internal standards. Ammonia chemical ionization was utilized to generate intense [M + H]+ ions for selected-ion monitoring quantification. The whole procedure is fast and can be performed by a low-resolution gas chromatographic/mass spectrometric system, giving accurate results over a range of three orders of magnitude (0.0167-16.7 micrograms/ml). The results from the analyses of 54 urine samples from 21 MCAD-deficient patients and various control samples using this method established that n-hexanoyglycine and 3-phenylpropionylglycine were highly diagnostic for this disease, while suberylglycine was found less specific.

Medium-chain acyl-CoA dehydrogenase deficiency: metabolic effects and therapeutic efficacy of long-term L-carnitine supplementation

Medium-chain acyl-CoA dehydrogenase deficiency is a recently described inborn error of metabolism characterized by episodes of coma and hypoketotic hypoglycaemia in response to prolonged fasting. Secondary carnitine deficiency has been documented in these patients as well as the excretion in the urine of medium-chain-length acyl carnitine esters, such as octanoylcarnitine. Based on the potential toxicity of medium-chain fatty acid metabolites and the beneficial responses of patients with other inborn errors of metabolism and secondary carnitine deficiency, oral carnitine has been proposed as treatment for children with medium-chain acyl-CoA dehydrogenase deficiency. We report the results of carefully monitored fasting challenges of an infant with this deficiency both before and after 3 months of oral carnitine therapy. Carnitine supplementation failed to prevent lethargy, vomiting, hypoglycaemia and accumulation of free fatty acids in response to fasting despite normalization of plasma carnitine levels and a marked increase in urinary excretion of acyl-carnitine esters. Potentially toxic medium-chain fatty acids accumulated in the plasma in spite of therapy. Based on this study of one patient, we stress that avoidance of fasting and prompt institution of glucose supplementation in situations when oral intake is interrupted remain the mainstays of therapy for medium-chain acyl-CoA dehydrogenase deficient patients.

A clinical biochemist's view of the investigation of suspected inherited metabolic disease

The necessity for a multi-disciplinary approach to the study of genetic disease is discussed. The progress of laboratory investigation programmes made it not feasible and inefficient to run a full metabolic investigation programme in every new patient suspected of inherited metabolic disease. An application form for metabolic investigation is described, which can be used to collect clinical information relevant to metabolic disease. On the basis of the patient's clinical information, selection criteria are given to decide which laboratory investigation programme has to be performed in the individual patient. A full metabolic laboratory investigation programme is described and illustrated with some examples of abnormal metabolite patterns. Diagnostic results over a 2-year period are presented.

Medium-chain acyl-CoA dehydrogenase deficiency. Diagnosis by stable-isotope dilution measurement of urinary n-hexanoylglycine and 3-phenylpropionylglycine

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, one of the most common inherited metabolic disorders, is often mistaken for the sudden infant death syndrome or Reye's syndrome. Diagnosing it has been difficult because of a lack of fast and reliable diagnostic methods. We developed a stable-isotope dilution method to measure urinary n-hexanoylglycine, 3-phenylpropionylglycine, and suberylglycine, and we retrospectively tested its accuracy in diagnosing MCAD deficiency. We measured the concentrations of these three acylglycines in 54 urine samples from 21 patients with confirmed MCAD deficiency during the acute and asymptomatic phases of the illness and compared the results with the concentrations in 98 samples from healthy controls and patient controls with various diseases. The levels of urinary hexanoylglycine and phenylpropionylglycine were significantly increased in all samples from the patients with MCAD deficiency, clearly distinguishing them from both groups of controls. Although urinary suberylglycine was increased in the patients, the range of values in the normal controls who were receiving formula containing medium-chain triglycerides was very wide, overlapping somewhat with the values in the patients with asymptomatic MCAD deficiency. These results indicate that the measurement of urinary hexanoylglycine and phenylpropionylglycine by our method is highly specific for the diagnosis of MCAD deficiency. The method is fast and can be applied to random urine specimens, without any pretreatment of patients.

Urinary C6-C12 dicarboxylic acylcarnitines in Reye's syndrome

C6-C12 dicarboxylic acylcarnitines have been identified for the first time in urine from a 2-year-old girl presenting with Reye's syndrome. The acylcarnitines were extracted by ion-exchange chromatography and analysed, both underivatised and as methyl esters using high-resolution fast-atom-bombardment mass spectrometry and B/E-linked scanning. The acylcarnitines were quantified by capillary gas chromatography of the acids extracted after hydrolysis of the acylcarnitine esters. Dodecandioylcarnitine was present in the highest concentration (35.9 mmol/mol creatinine) which exceeded the urinary free dodecandioic acid concentration. The adipic, suberic and sebacic acylcarnitine concentrations were less than 10% of the respective free acid concentrations. It is possible that beta-oxidation of dicarboxylic acids is partially inhibited in Reye's syndrome leading to accumulation of precursor dodecandioyl CoA which is metabolised to dodecandioylcarnitine. The accumulation of these metabolic intermediates may be significant in the pathogenesis of Reye's syndrome.

2,4-Dichlorophenoxyacetic acid intoxication increases its accumulation within the brain

Exposure to the phenoxyacetic acid herbicides has been shown to produce neurotoxicity. Therefore, adult mice (pregnant) and rabbits were used to examine the accumulation and regional distribution of 2,4-dichlorophenoxyacetic acid (2,4-D) within the brain following intraperitoneal injection of a low dose (0.2-0.4 mg/kg) of [14C]2,4-D. Controls, i.e. animals not previously exposed to 2,4-D, were compared to animals acutely pretreated with higher doses (40-160 mg/kg) of unlabeled 2,4-D. Both autoradiography and direct tissue analysis showed that in control animals brain levels were much lower than plasma in both adult (approximately 4%) and fetus (approximately 8%). In both species, small variations were seen between the brain regions, with brainstem and cerebellum somewhat higher than other regions. Pretreatment with unlabeled 2,4-D caused a 5- to 10-fold increase in accumulation of [14C]2,4-D in both mice and rabbits. On the other hand, 2-deoxyglucose entry into the brain was not altered by 2,4-D pretreatment. Thus, there was no generalized increase in blood-brain barrier permeability. Instead, increased 2,4-D accumulation appeared to be caused by its decreased elimination from the brain. Pretreatment with 40 mg/kg led to a CSF 2,4-D concentration of 10 microM, a concentration sufficient to inhibit choroid plexus transport of [14C]2,4-D by nearly 50% in vitro. These results suggest that exposure to organic anions like 2,4-D may lead to the retention of potentially toxic anions within the CNS via competitive inhibition of the organic anion transport system which normally reduces their brain and CSF concentrations to very low levels.

Lipid storage myopathy associated with recurrent Reye syndrome-like attacks, but with a normal carnitine level

A 7-year-old girl developed recurrent episodes of hepatic and cerebral dysfunction which mimicked those in Reye syndrome (RS). Because of mild muscle weakness, she had repeated muscle biopsies which showed markedly increased amounts of lipid droplets, predominantly in type 1 fibers. Liver histological examination showed widespread hepatocellular steatosis. However, diffuse microvesicular fat, seen in RS, was not found in the cytoplasm. The concentrations of free- and acylcarnitine in serum and muscle were within normal ranges. Normal ketogenesis was induced by fasting. Based on the clinical, laboratory, and histopathological findings, our patient was initially thought to have systemic carnitine deficiency. However, the serum and muscle carnitine levels were within normal limits. Although the primary metabolic defect has yet to be elucidated, the present study indicates that lipid storage myopathy in the absence of carnitine deficiency can be complicated with RS-like episodes.

The importance of recognizing secondary carnitine deficiency in organic acidaemias: case report in glutaric acidaemia type II

Secondary carnitine deficiency in a patient with glutaric acidaemia type II, due to deficient ETF-dehydrogenase activity, is described. The patient responded clinically to a pharmacological dose of riboflavin and a restricted protein diet. In the second year of her life she developed more frequent and severe exacerbations during intercurrent infections from which she did not fully recover. Hypotonia and marked ataxia persisted. Plasma carnitine was entirely complexed as acylcarnitine with no free carnitine detected. Retrospective evaluation of several frozen urine specimens obtained since the age of 10 months revealed undetectable free carnitine with elevated acylcarnitine levels. Marked clinical improvement was observed following L-carnitine supplementation. The hypotonia and ataxia disappeared. The frequency and the severity of the exacerbations were noticeably decreased. The role of L-carnitine in preventing the accumulation of acyl-CoA compounds in inborn errors of organic acid metabolism is further emphasized by this patient. The necessity to evaluate free carnitine, acylcarnitine and acyl/free ratio in the assessment, follow-up and management of patients with inborn errors of organic acid metabolism is discussed.

Neonatal onset of medium-chain acyl-CoA dehydrogenase deficiency in two siblings

Two male siblings with medium-chain acyl-CoA dehydrogenase deficiency were reported, in whom the enzyme activity was essentially undetectable and the symptoms and signs, including cyanosis, apnea, low body temperature, hypoglycemia and hyperammonemia, appeared within 48 hours of life. Muscle weakness and cardiomegaly in association with morphological abnormalities of mitochondria in skeletal and cardiac muscles, respectively, were found on electron microscopic examination in one of them. These observations suggest that the patients suffered from the most severe form of the disease, which has not been described in the literature.

Genetic deficiency of short-chain acyl-coenzyme A dehydrogenase in cultured fibroblasts from a patient with muscle carnitine deficiency and severe skeletal muscle weakness

Genetic deficiency of short-chain acyl-coenzyme A (CoA) dehydrogenase activity was demonstrated in cultured fibroblasts from a 2-yr-old female whose early postnatal life was complicated by poor feeding, emesis, and failure to thrive. She demonstrated progressive skeletal muscle weakness and developmental delay. Her plasma total carnitine level (35 nmol/ml) was low-normal, but was esterified to an abnormal degree (55% vs. control of less than 10%). Her skeletal muscle total carnitine level was low (7.6 nmol/mg protein vs. control of 14 +/- 2 nmol/mg protein) and was 75% esterified. Mild lipid deposition was noted in type I muscle fibers. Fibroblasts from this patient had 50% of control levels of acyl-CoA dehydrogenase activity towards butyryl-CoA as substrate at a concentration of 50 muM in a fluorometric assay based on the reduction of electron transfer flavoprotein. All of this residual activity was inhibited by an antibody against medium-chain acyl-CoA dehydrogenase. These data demonstrated that medium-chain acyl-CoA dehydrogenase accounted for 50% of the activity towards the short-chain substrate, butyryl-CoA, under these conditions, but that antibody against that enzyme could be used to unmask the specific and virtually complete deficiency of short-chain acyl-CoA dehydrogenase in this patient. Fibroblasts from her parents had intermediate levels of activity towards butyryl-CoA, consistent with the autosomal recessive inheritance of this metabolic defect.

Effects of 2,4,5-trichlorophenoxyacetic acid and quinolinic acid on 5-hydroxy-3-indoleacetic acid transport by the rabbit choroid plexus: pharmacology and electron microscopic cytochemistry

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) reduced the uptake of 5-hydroxy-3-indoleacetic acid (5-HIAA) by the choroid plexus in a dose-related manner, while treatment with quinolinic acid at comparable concentrations did not inhibit 5-HIAA uptake. The role of carrier-mediated transport in the clearance of 5-HIAA from cerebrospinal fluid (CSF) was also evaluated in vivo by ventriculocisternal perfusion. Steady-state clearance of 5-HIAA from CSF exceeded that of inulin and was reduced competitively in the presence of 2,4,5-T. However, the clearance was not affected by quinolinic acid. The effect of 2,4,5-T on transport enzyme systems was also studied by electron microscopic cytochemistry. Na+-K+-ATPase and cytochrome oxidase activities in the choroid plexus were reduced by 2,4,5-T. Since this transport system in the choroid plexus is normally responsible for the excretion of the serotonin metabolite from the brain to the plasma, accumulation of endogenously produced organic acids in the CSF and the brain, secondary to reduced clearance by the choroid plexus, could be a contributing factor in the development of neurotoxicity.

The inborn errors of mitochondrial fatty acid oxidation

To date, seven inborn errors of mitochondrial fatty acid oxidation have been identified. A total of about 100 patients in the world have been reported. Clinically the beta-oxidation defects are more often characterized by episodic hypoglycaemia leading to a coma mimicking Reye's syndrome. The hypoglycaemia is non-ketotic since the synthesis of ketone bodies is deficient. Periods of decompensation occur when carbohydrate supply is poor, e.g. prolonged fasting, vomiting, or increased caloric requirements, as and when lipid stores are used. Defects in beta-oxidation have also been reported to be one cause of sudden infant death syndrome. The diagnosis of these inborn errors is by biochemical investigation since where symptoms suggest such a defect, the precise aetiology cannot be assessed. The biochemical diagnosis is based firstly on identification of abnormal plasma and of urinary metabolites during acute attacks. Derivatives of the omega-oxidation and omega-1-oxidation of medium chain fatty acids have been identified, as well as acylglycine and acylcarnitine conjugates. These metabolites are nearly always absent when patients are in good clinical condition. Secondly, the diagnosis must be based on the identification of the enzymatic defects: this involves global assays which allow a localization of the 'level' of the defect (i.e. the oxidation of long, medium or short chain fatty acids) and specific measurement of enzyme activities (acyl-CoA dehydrogenases and electron carriers: ETF and ETF-DH). The diagnosis of these disorders is of prime importance because of the severity of the clinical symptoms. These can be prevented, in some cases, by an appropriate diet (a high carbohydrate, low fat diet, sometimes supplemented with L-carnitine). In other cases, genetic counselling can be offered.

Therapy of mitochondrial disorders

Mitochondrial disorders, namely defects of fatty acid oxidation, defects of pyruvate metabolism and defects of the respiratory chain are heterogenous in clinical picture and in response to therapeutic attempts. Defects of fatty acid metabolism are amenable to therapy by dietary means, carnitine substitution and in some cases with vitamins. Defects in pyruvate metabolism do not respond to therapy except in some special cases. Therapeutic attempts include dietary measures, vitamins as coenzyme precursors. Defects in the respiratory chain appear to respond to treatment only in exceptional cases. Evaluation of treatment effects appears to be singularly difficult. General measures that can be of benefit to different defects are discussed.

Medium-chain and long-chain acyl CoA dehydrogenase deficiency: clinical, pathologic and ultrastructural differentiation from Reye's syndrome

The clinical and pathologic findings in 12 patients with medium-chain acyl CoA dehydrogenase deficiency and three patients with long-chain acyl CoA dehydrogenase deficiency are summarized. Although these inborn errors of intramitochondrial beta-oxidation of fatty acids present with similar findings to Reye's syndrome, there are clinical, laboratory and hepatic histologic differences. Younger age at presentation, history of unexplained sibling death, a previous episode of lethargy, hypoglycemia or acidosis precipitated by fasting stress and only mildly elevated serum transaminases with normal or only mildly prolonged prothrombin time may all suggest an acyl CoA dehydrogenase deficiency. Long-chain acyl CoA dehydrogenase deficiency is differentiated from medium-chain acyl CoA dehydrogenase deficiency by younger age at presentation, more profound cardiorespiratory depression, evidence of cardiomyopathy, and sequelae of muscle weakness, hypotonia and developmental delay. Definitive diagnosis is made by assay of medium-chain or long-chain enzyme activity in cultured skin fibroblasts or in leukocytes. Hepatic light microscopic alterations are essentially limited to steatosis, which may be either macro- or microvesicular. The cases with microvesicular steatosis can be differentiated morphologically from Reye's syndrome by electron microscopy, showing the absence of the mitochondrial changes characteristic of Reye's. Four of seven cases of acyl CoA dehydrogenase deficiency showed some variations from normal in the appearance of the hepatocyte mitochondria. The relationship of these variations to the basic metabolic defect(s) remains to be determined.

Serum dicarboxylic acids in patients with Reye syndrome

Reye syndrome resembles disorders of fatty acid metabolism. Analysis of serum free fatty acids from 18 patients with Reye syndrome revealed that dicarboxylic acids comprise as much as 55% (range 4% to 55%) of the patients' total free fatty acids; both medium- (6 to 12 carbon lengths) and long-chain (14 to 18 carbon lengths) dicarboxylic acids were identified. Long-chain dicarboxylic acids were not found in any control samples, whereas 86% +/- 4% of the serum dicarboxylic acids were long chain in 10 patients with Reye syndrome in state 3 to 4 coma and 31% +/- 8% in eight patients with a milder illness. The serum concentration of dicarboxylic acids correlated with the clinical state (P less than 0.001) and with the elevation in blood ammonia concentration (r2 = 0.8767). No long-chain dicarboxylic acids were found in the urine. The dicarboxylic acidemia in Reye syndrome may be secondary to the general mitochondrial dysfunction or could indicate that an insult to fatty acid metabolism or the stimulation of omega-oxidation is important in the pathogenesis of the illness. Measurement of serum dicarboxylic acids, especially long chain, may be important in assessing Reye syndrome and may prove useful in distinguishing this from other diseases.

Identification of 3-methylglutarylcarnitine. A new diagnostic metabolite of 3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency

Deficiency of 3-hydroxy-3-methylglutaryl-coenzyme A (CoA) lyase affects the metabolism of leucine as well as ketogenesis. This disorder is one of an increasing list of inborn errors of metabolism that presents clinically like Reye's Syndrome or nonketotic hypoglycemia. Four patients with proven 3-hydroxy-3-methylglutaryl-CoA lyase deficiency were shown to excrete a new diagnostically specific metabolite. The technique of fast atom bombardment and tandem mass spectrometry revealed that only 3-methylglutaryl-CoA is a substrate for acylcarnitine formation. Neither 3-methylglutaconyl-CoA nor 3-hydroxy-3-methylglutaryl-CoA are excreted as acylcarnitines. The excretion of 3-methylglutarylcarnitine may explain, in part, the apparent secondary carnitine deficiency in this disorder. Carnitine supplementation with moderate dietary restrictions may be a useful treatment strategy for this disorder.

Recognition of medium-chain acyl-CoA dehydrogenase deficiency in asymptomatic siblings of children dying of sudden infant death or Reye-like syndromes

The medium-chain acyl-CoA dehydrogenase (MCAD) deficiency of mitochondrial beta oxidation has been identified in two asymptomatic siblings in a family in which two previous deaths had been recorded, one attributed to sudden infant death syndrome and the other to Reye syndrome. Recognition of this disorder in one of the deceased and in the surviving siblings was accomplished by detection of a diagnostic metabolite, octanoylcarnitine, using a new mass spectrometric technique. This resulted in early treatment with L-carnitine supplement in the survivors, which should prevent metabolic deterioration. Further studies suggest that breast-feeding may be protective for infants with MCAD deficiency. Families with children who have had Reye syndrome or in which sudden infant death has occurred are at risk for MCAD deficiency. We suggest that survivors and asymptomatic siblings should be tested for this treatable disorder.