Clinical manifestations

Presentation and course

Ethylmalonic encephalopathy is typically a devastating, autosomal recessive metabolic disorder affecting the brain, gastrointestinal tract, and peripheral vessels of infants. The metabolic disorder was originally described in the Mediterranean (34) and Italian families (08). The patients presented with neonatal hypotonia followed by severe progressive encephalopathy with intellectual disability, pyramidal dysfunction, development of dystonia, orthostatic acrocyanosis with distal swelling, chronic diarrhea, recurrent petechiae, elevated ethylmalonic acid in the urine, and abnormalities on brain MRI.

MRI images showed areas of increased signal in the cerebellar white matter and in the caudate and lenticular nuclei. Some patients had an increased lactate resonance intensity on proton MR spectroscopy in the basal ganglia of both hemispheres, indicating an abnormality in oxidative metabolism (30). More patients from Spain (26), Saudi Arabia (56), Egypt (56), and Canada (50) have been described with individual variations. Overall, the disease course is variable. Most patients have severe symptoms with a fatal disease course in early infancy, but occasionally it can occur with a late onset and mild clinical symptoms (21). The clinical course is characterized by clinical heterogeneity, and phenotypic heterogeneity may occur even in monochorionic twins (62).

The Spanish case was a 20-month-old boy with encephalopathy, petechiae, chronic diarrhea, and acrocyanosis. The brain MRI of this patient demonstrated bilateral lesions in the caudate and lenticular nuclei, the periaqueductal region, subcortical white matter, and brainstem (26).

Five patients from Saudi Arabia had similar clinical features but did not have chronic diarrhea, and failure to thrive was not prominent (56). All these patients had retinal lesions characterized by tortuous veins. Three of them died following the sudden appearance of severe lesions in the basal ganglia. In one patient, the changes on the brain CT were suggestive of infarction. All of the patients had mild-to-moderate hematuria, and one had a mild hemoperitoneum at the terminal stage. Affected individuals developed pulmonary congestion or edema and secondary respiratory difficulties during the terminal stage of the disease.

Two Canadian siblings with ethylmalonic encephalopathy had CNS malformations (one with tethered cord and the other with cerebellar tonsillar ectopia or Chiari I malformation) (50).

Of three siblings in an American Hispanic family, a girl with ethylmalonic encephalopathy had dysmorphic features and hepatomegaly in addition to the previously described clinical features of this disease (10).

One case with the typical clinical picture and biochemical abnormalities also had microcephaly, horizontal nystagmus, mild hepatomegaly, a small penis, and undescended testes (32). Brain MRI revealed multiple areas of abnormal signal in the basal ganglia and prominent frontal and temporal subarachnoid spaces. A new homozygous mutation in exon 3 of the ETHE1 gene was found.

Milder phenotypes can also occur. One case had only mild speech and gross motor delays, subtle biochemical abnormalities, and normal brain imaging; the child was homozygous for a pathogenic ETHE1 variant (c.586G> A) (37).

Early-onset epilepsy presenting with West syndrome has been reported in one patient (57). Ethylmalonic encephalopathy may also masquerade as a hematological disorder with leukocytosis and thrombocytosis (58) or fatal progressive pancytopenia (34) and may be associated with rapidly progressive glomerulonephritis (19).

Ethylmalonic aciduria is not necessarily a constant biochemical marker of this disease, and normal excretion of this metabolite, at least between metabolic decompensations, does not exclude this metabolic disorder. A child with ethylmalonic encephalopathy due to a homozygous R163W mutation in the ETHE1 gene presented atypically with a clinical picture suggestive of a connective tissue disorder (ie, vascular fragility, joint hyperlaxity, delayed motor development, and normal cognitive development) (17); no pathologic excretion of ethylmalonic acid was found.

SCAD deficiency. Although most patients with SCAD deficiency are clinically asymptomatic, two clinical phenotypes have been delineated. The first phenotype occurs in infants and small children who variably present with metabolic acidosis, failure to thrive, developmental delay, seizures, hypotonia, and myopathy. The second phenotype occurs in middle-aged patients who present with an adult-onset chronic myopathy (74).

SCAD deficiency may be diagnosed coincidently during metabolic workup of hypoglycemia (75).

Severe infantile hypotonia can be the only manifestation of ethylmalonic aciduria spectrum disorders. Two unrelated infants with SCAD deficiency and axonal neuropathy presented with generalized peripheral hypotonia, profound generalized weakness preferentially affecting the upper limbs, and absent deep tendon reflexes (43). Another 8-month-old girl with SCAD deficiency presented with significant hypotonia and weakness (54); ethylmalonic acid was increased in urine, and a variant SCAD gene polymorphism was found.

Other reported abnormalities in cases of SCAD deficiency include the following:

The clinical impact of SCAD deficiency is questionable. In a U.S. study of the medical and neurodevelopmental characteristics of 14 children with SCAD deficiency, eight were detected as neonates by newborn screening, three had symptoms but normal newborn screening results, and three were born in states that did not screen for SCAD deficiency (79). All children identified by newborn screening demonstrated normal medical and neuropsychological development, except for one child who exhibited mild speech delay. Of the three clinically identified children with newborn screening results below the cut-off value, two were healthy and performed within the normal range on cognitive and motor tests at follow-up. Although four clinically identified children with SCAD deficiency experienced persistent symptoms or developmental delay, there were supplementary or alternative explanations for the deficits. No genotype-phenotype correlation was evident. Another study in California examined a large series of SCAD-deficient patients and provided evidence that SCAD deficiency diagnosed by newborn screening represents a benign condition (24). A follow-up study of 16 children with SCAD deficiency showed that most of these patients had normal growth and development (59). It is unclear if clinical symptoms previously ascribed to short-chain acyl-CoA dehydrogenase (SCAD) deficiency are due to ascertainment bias, or if early identification and treatment prevent complications that may have occurred due to interaction between genetic susceptibility and environmental stressors. Likely, SCAD deficiency is mostly a benign biochemical phenotype without clinical manifestations (79; 77; 24; 59).

In addition to severe mutations, several common polymorphisms in the SCAD gene, especially 625G/A and 511C->T, may cause variable SCAD deficiency depending on additional genetic or environmental factors. The combination of these complex factors may result in borderline to mild ethylmalonic aciduria. Again, the clinical significance is unknown.

Prognosis and complications

Ethylmalonic encephalopathy is usually lethal in infancy or early childhood. In early-diagnosed patients, such as those detected by newborn screening, liver transplantation seems to improve the clinical course.

Most individuals with SCAD deficiency remain asymptomatic. Thus, SCAD is now viewed as a biochemical phenotype rather than a disease. Earlier reported clinical findings in a few patients included ketotic hypoglycemia, dysmorphic facial features, failure to thrive, lethargy, developmental delay, seizures, muscle hypotonia, dystonia, and myopathy.

Clinical vignette

An infant girl with ethylmalonic encephalopathy presented with progressive pancytopenia and psychomotor retardation (34). Hematomas and petechiae, irregular nystagmoid eye movements, and convergent strabismus were noted at birth. She subsequently developed progressive psychomotor retardation. From the age of 11 months, gross motor abilities declined.

Diagnostic tests. Routine investigations initially showed hemoglobin of 146 g/L and severe thrombocytopenia (19,000/µl). All three hematopoietic lineages eventually became involved, resulting in a progressive impairment of hematopoiesis. After the first year of life, thrombocytes fell below 10,000/µl. Because of increasing urinary and intestinal bleeding, the patient had to be transfused regularly. Severe pancytopenia developed by the age of 2. Thereafter, the patient gradually developed hypersensitivity against HLA identical thrombocytes.

The first urinary specimen for organic acid analysis was done at the age of 4 months. Urinary organic acid analyses consistently showed severely elevated ethylmalonic acid levels up to 113 mmol/mol creatinine (normal range < 5 mmol/mol creatinine) accompanied by a 10-fold increase in excretion of methylsuccinate up to 29 mmol/mol creatinine. There was no dicarboxylic aciduria. Ethylmalonic acid in plasma was quantified by stable isotope dilution GC-MS with selected ion monitoring and found to be elevated to 5.2 µmol/L (control mean 0.45 µmol/L). Complex 1 and 4 enzyme activities in fibroblasts were significantly reduced. No deletions or mutations were found in mitochondrial DNA.

A CT scan at the age of 11 months showed generalized atrophy. Muscle biopsy revealed a depletion of glycogen and paracrystalline inclusion bodies on electron microscopy.

Management and course. At the age of 20 months, dietary therapy with a low-fat diet enriched with carbohydrates and protein was attempted with no significant clinical or biochemical improvement. A therapeutic trial of carnitine and riboflavin supplementation was also ineffective. She died of cardiovascular arrest secondary to severe anemia at the age of 27 months.

Biological basis

Etiology and pathogenesis

Ethylmalonic encephalopathy. Ethylmalonic encephalopathy is a devastating, infantile, autosomal recessive metabolic disorder caused by defects in the mitochondrial sulfur dioxygenase, ETHE1 (also represented as hETHE1, indicating the human enzyme).

Biochemistry. To investigate if the underlying biochemical consequences in ethylmalonic encephalopathy arise pathogenetically from an abnormal isoleucine metabolism, Nowaczyk and colleagues determined the response to oral L-isoleucine load (150 mg/kg) in a 5-year-old girl with this disease and in three healthy controls (51). Following the isoleucine load, there was an accumulation of 2-methylbutyrylglycine and a delayed and lower peak urinary excretion of tiglylglycine, suggesting a partial defect in 2-methyl-branched-chain acyl-CoA dehydrogenase. In vitro determination of enzyme activity in cultured skin fibroblasts from patients with ethylmalonic encephalopathy was normal. The authors concluded that isoleucine is a source of elevated ethylmalonic and methylsuccinic acids in affected patients. They suggested a functional, possibly secondary, deficiency of 2-methyl-branched-chain acyl-CoA dehydrogenase activity in vivo.

McGowan and colleagues determined the influence of candidate amino acids (isoleucine and methionine) on the urinary excretion of ethylmalonic acid in patients with ethylmalonic aciduria (46). Loading with methionine increased the excretion of ethylmalonic acid, whereas loading with isoleucine did not. Restriction of the dietary intake of methionine decreased ethylmalonic acid excretion. The authors conclude that methionine is a precursor of ethylmalonic acid in ethylmalonic encephalopathy.

It remains unclear how these findings are related to the defective gene’s function.

Genetics. At least 27 mutations in the ETHE1 gene causing ethylmalonic encephalopathy have been reported (73). Most of these cause loss of function through a premature stop codon, a frameshift mutation, or aberrant splicing; others are entire gene deletions and missense mutations in highly conserved portions.

Pathophysiology. Tiranti and colleagues identified the ETHE1 gene on chromosome 19 (MIM 608451) as containing the pathologic mutations in ethylmalonic encephalopathy (71; 70). The authors demonstrated that the D83198 protein product is targeted to mitochondria and internalized into the matrix after energy-dependent cleavage of a short leader peptide. The protein is required for metabolic homeostasis and energy metabolism. They discovered that ETHE1 protein works as a supramolecular, presumably homodimeric complex. A 3-dimensional model of the protein suggested that it is likely to be a mitochondrial matrix thioesterase acting on a still unknown substrate. Finally, they ruled out a pathogenic role of the 625G/A single nucleotide polymorphism in the SCAD gene in ethylmalonic encephalopathy.

Tiranti and colleagues created an ETHE1(-/-) mouse that showed the cardinal features of ethylmalonic encephalopathy (72). They found high thiosulfate and sulfide concentrations in ETHE1(-/-) mouse tissues. Sulfide is a powerful inhibitor of COX and short-chain fatty acid oxidation, with vasoactive and vasotoxic effects that may explain the microangiopathy in ethylmalonic encephalopathy. Sulfide is detoxified by a mitochondrial pathway that includes sulfur dioxygenase. Sulfur dioxygenase activity was absent in ETHE1(-/-) mice, whereas it was markedly increased by ETHE1 overexpression in HeLa cells and Escherichia coli. The authors concluded that ETHE1 is a mitochondrial sulfur dioxygenase involved in catabolism of sulfide that accumulates to toxic levels in ethylmalonic encephalopathy. After the crystalline structure of ETHE1 has been identified, further functional and mechanistic studies on ETHE1 can be done (61). ETHE1 deficiency seems to influence several important cellular functions, such as enzyme function and gene expression as demonstrated by quantitative proteomics (64).

The physiological role and pathogenic effects of sulfide focusing on ethylmalonic encephalopathy has been summarized and discussed by Tiranti and Di Meo (73; 15).

Bioenergetics. Sulfide induces COX deficiency through heme A inhibition and accelerated long-term degradation of COX subunits (14). Di Meo and colleagues conclude that the devastating effects of sulfide accumulation in ethylmalonic encephalopathy cannot be fully explained solely by COX deficiency in critical tissues but are likely secondary to several toxic effects on a number of enzymatic activities in different tissues leading to progressive multiorgan failure. The induction of bioenergetic dysfunction and lipid peroxidation along with mitochondrial pore opening by hydrogen sulfide has been described in rat brain tissue by Cardoso and colleagues (09). Pathological hallmarks in an affected patient as well as in ETHE1(-/-) mice were COX-depleted cells and widespread endothelial lesions of arterioles, capillaries of the brain, and the gastrointestinal tract (28). Di Meo and colleagues observed that AAV2/8-mediated, ETHE1-gene transfer to the liver of a ETHE1(-/-) mouse model resulted in full restoration of sulfur dioxygenase activity, correction of plasma thiosulfate, a biomarker reflecting the accumulation of hydrogen sulfide, and clinical improvement (13). The involvement of sulfide in redox regulation and cytoskeleton dynamics has been shown in an ETHE1(-/-) mouse model by Hildebrandt and colleagues (33). The authors hypothesized that sulfide signaling specifically regulates mitochondrial catabolism of fatty acids and branched-chain amino acids.

After identifying the disease-causing gene of ethylmalonic encephalopathy (71), Tiranti and colleagues hypothesized that the severe consequences of the malfunctioning of this protein may indicate an important role of the ETHE1 gene product in mitochondrial energy metabolism and homeostasis. This notion of impaired mitochondrial energy metabolism is further supported by the fact that in several affected patients, a selective vulnerability of the basal ganglia with atrophy and infarction on conventional MRI brain scans has been described (30).

SCAD deficiency. SCAD deficiency is a defect in the mitochondrial beta-oxidation pathway and is caused by a deficiency of the first enzyme of the intramitochondrial beta-oxidation spiral called short-chain acyl-CoA dehydrogenase. Potential pathophysiological mechanisms of SCAD deficiency, such as oxidative stress and energy deficiency, have been reviewed by Nochi and colleagues (49).

Biochemistry. Defective short-chain acyl-CoA dehydrogenation results in the accumulation of butyryl-CoA, which is then carboxylated by propionyl-CoA carboxylase to form ethylmalonyl-CoA. This metabolite is either hydrolyzed to ethylmalonic acid or converted to methylsuccinyl-CoA by methylmalonyl-CoA mutase, yielding methylsuccinic acid on hydrolysis.

Genetics. At least 35 inactivating mutations and two polymorphic variants have been reported in the SCAD gene (36).

The variant allele (625A) was found in homozygous form in 60% of 135 patients with elevated ethylmalonic acid excretion compared to 7% of individuals in the general population (12). The authors suggested that the 625A variant allele is a susceptibility allele of the SCAD gene, which causes variable elevation of ethylmalonic acid in the urine and, in combination with other genetic or environmental factors, may lead to a functional impairment of the enzyme’s catalytic activity. Subsequently, another possibly disease-associated susceptibility polymorphism (511C-> T) has been reported (29). However, the 511T allele was found in 9.2% of 98 control individuals and in 5.6% of 266 patients with elevated ethylmalonic acid excretion, according to Gregersen and colleagues (29). Several novel mutations of SCAD deficiency (p.L93I, p.E228K, p.P377L, and p.R386H) have been identified in Korean patients (40).

SCAD deficiency may be divided into at least two separate clinical and genetic phenotypes. One group is comprised of patients with conventional SCAD deficiency caused by disruptive mutations in the SCAD gene, and the second group consists of patients with polygenic or multifactorial conditions caused by the presence of a number of susceptibility alleles, located either in the SCAD locus, as are the 625A or 511T variants, or elsewhere, that may cause SCAD deficiency depending on the synergistic effect of additional genetic factors or cellular conditions. The different temperature profiles of the variant enzymes R147W (511T) and G185S(625A) in comparison to normal enzymes raised the notion that a decreased stability of the tetrameric structure or an instability to intermediates of the folding pathway of the protein may be responsible for reduced enzymatic activity in these patients (29).

Pathophysiology. Acyl-CoA dehydrogenase deficiencies constitute an important group of inborn errors of metabolism. A reduced or absent enzymatic activity of short-chain acyl-CoA dehydrogenase (SCAD, EC 1.3.99.2), the first enzyme of the intramitochondrial short-chain beta-oxidation spiral catalyzing the dehydrogenation of short-chain fatty acids (C4-C6), is the cause of SCAD deficiency. Like the other four enzymes belonging to the acyl-CoA dehydrogenase gene family, SCAD is a homotetrameric mitochondrial flavoenzyme.

Wood and colleagues discovered a subline of BALB/c mice (BALB/cByJ) that excreted abnormally high concentrations of ethylmalonate, methylsuccinate, n-butyrylglycine, and n-butyrylcarnitine (82). These metabolites suggested a defect in short-chain acyl-CoA dehydrogenase. Amendt and colleagues, who authenticated the SCAD deficiency of the BALB/cByJ mouse as a model of human SCAD deficiency, reported that SCAD-deficient mice and affected humans share similar clinical characteristics, including a tendency toward hypoglycemia, hepatic lipid deposition, and similar organic acidurias (01). In contrast to humans, the SCAD-deficient mice excreted large amounts of n-butyrylglycine and n-butyrylcarnitine, probably resulting from differences between rodents and humans in glycine conjugation of various acyl-CoA derivatives, particularly butyryl-CoA. As the BALB/cByJ mice had a lack of SCAD activity in all investigated tissues (liver, muscle, fibroblasts), Schuck and colleagues reported that in the rat cerebral cortex, ethylmalonic acid increases lipoperoxidation and protein oxidative damage and decreases nonenzymatic antioxidant defenses (66). The authors postulated that oxidative stress might be involved in the neuropathogenesis of ethylmalonic acidurias. Vulnerability to oxidative stress in SCAD-deficient fibroblast cultures is exacerbated by hyperthermia and can be rescued by antioxidants and bezafibrate (86). The authors suggested that impaired mitochondrial energy metabolism in muscle contributes to the pathogenesis of hypotonia, myopathy, and lactic acidosis in affected patients. In contrast, in vivo administration of ethylmalonic acid to young rats does not affect mitochondrial energy metabolism (23). The administration of a short-term high-fat diet in ACADs -/- knock-out mice results in perturbation of mitochondrial energy metabolism (27).

Because SCAD deficiency does not impair long-chain fatty acid oxidation, it is unlikely that a defect in SCAD would have a major effect on the yield of energy from fatty acid oxidation. This notion is supported by the finding that SCAD-deficient patients are capable of mounting a ketogenic response. The fact that patients display such different clinical symptoms further indicates that the pathophysiology involves more than a simple energy deficit. Shirao and colleagues suggest the induction of mitochondrial fragmentation and autophagy by a mutant SCAD protein as possible pathophysiologic mechanisms in SCAD deficiency (68). Furthermore, misfolded mutant SCAD protein elicits a toxic reaction in mitochondria, including mitochondrial fission and the production of oxidative stress in transiently transfected astrocytes (65). An alteration of mitochondrial proteins and a relation to mitochondrial dysfunction has been identified by proteomic studies in cultured human skin fibroblasts (20) and in the mouse model of SCAD deficiency (80).

Epidemiology

Since the initial report, only about 40 cases of ethylmalonic encephalopathy have been described worldwide, suggesting that this condition is an ultra-rare autosomal recessive disorder. With a few exceptions (84), most patients with ethylmalonic encephalopathy have been of Mediterranean descent (34; 08; 25; 26; 31) or Arabic (56). However, the actual incidence of this condition could have been significantly underestimated because the biochemical phenotype may be incorrectly attributed to other metabolic disorders, particularly defects of the mitochondrial electron-transfer flavoprotein pathway. Several cases of ethylmalonic aciduria were initially diagnosed as glutaric aciduria type II, but this diagnosis was not confirmed by in vitro enzyme assays or molecular studies. Some of these cases could have been caused by ethylmalonic encephalopathy.

The frequency of SCAD deficiency is unknown, but results from newborn screening suggest frequencies varying between 1:33,000 and 1:340,000 (79; 45).

Prevention

Because an effective treatment for ethylmalonic encephalopathy is not known, prevention of the disease is not possible. However, genetic counseling or even prenatal diagnostics should be done when a disease-causing mutation has been identified in a family.

Prevention of metabolic decompensation in fatty acid oxidation disorders like SCAD deficiency consists of avoiding prolonged fasting periods, maintaining a continuous dietary treatment, and managing intercurrent illness. Fasting periods should be limited to 12 hours; during intercurrent illness, carbohydrate feeding should be provided every 4 to 6 hours. With gastroenteritis or the development of early signs of unusual lethargy, prompt intervention with intravenous glucose infusion is necessary. As many subjects with SCAD deficiency are completely asymptomatic, it is difficult to decide who would benefit from these measures.

Differential diagnosis

Confusing conditions

The differential diagnosis of persistent ethylmalonic aciduria includes not only ethylmalonic encephalopathy and SCAD deficiency but also the common SCAD variants: glutaric acidemia type II (sometimes described as ethylmalonic adipic aciduria), some forms of respiratory chain deficiencies, and Jamaican vomiting sickness. A diagnosis of ethylmalonic encephalopathy can be guided by rather specific clinical features, such as orthostatic acrocyanosis with distal swelling and recurrent petechiae. Clinically, ethylmalonic encephalopathy has some similarities with myo-, neuro-, gastrointestinal encephalopathy (MNGIE syndrome). Since 2004, the definitive diagnosis of ethylmalonic encephalopathy is possible by mutation analysis.

The typical clinical presentation of fatty acid oxidation disorders with episodes of fasting-induced vomiting, lethargy, and coma with hypoketotic hypoglycemia is rarely seen in patients with SCAD deficiency. If these features are present, the following diseases must be considered in the differential diagnosis: plasma membrane carnitine transporter defect, carnitine palmitoyltransferase I/II deficiencies, acylcarnitine translocase deficiency, MCAD deficiency, very long-chain acyl-CoA dehydrogenase deficiency (VLCADD), long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD), trifunctional protein deficiency, 2,4-dienoyl-CoA-reductase deficiency, HMG synthase deficiency, HMG lyase deficiency, and glutaric aciduria type II. Other inborn errors of metabolism presenting with hypoglycemia and coma are organic acidurias (usually associated with more severe acidemia than fatty acid oxidation disorders) and disorders of galactose or fructose metabolism (galactosemia, hereditary fructose intolerance, fructose-1,6-bisphosphatase deficiency). Most, but not all, of these diagnoses can be differentiated by quantitative organic acid analysis.

Diagnostic workup

Ethylmalonic encephalopathy. The main biochemical features of ethylmalonic encephalopathy are increased urinary ethylmalonic and methylsuccinic acids associated with abnormal excretion of C4-C5 (n-butyryl-, isobutyryl-, isovaleryl-, and 2-methylbutyryl-) acylglycines and acylcarnitines and intermittent lactic acidosis. Urine organic acid analysis is performed by gas chromatography-mass spectrometry, the gold standard for the identification of metabolic disorders from urine specimens (04), and stable isotope dilution techniques for quantification, if necessary. Liquid chromatography with tandem mass spectrometry can also be used for plasma and urine analysis (60). Short- and branched-chain acylcarnitines may also be elevated in dry blood spots, plasma, or whole blood samples (67). Usually, 2-ethylmalonic aciduria can indicate a defect in fatty acid oxidation, so fatty acid oxidation disorders should always be investigated. Because respiratory chain deficiencies are an important differential diagnostic consideration, the enzyme activities in different tissues (skin fibroblasts or muscle) may be determined. A partial COX deficiency has been described in muscle for several affected patients. An MRI scan of the brain is informative in searching for the main neuropathological features, like symmetrical lesions in the deep gray matter structures. Mutation analysis of the ETHE1 gene in available tissues will provide a definitive diagnosis.

Initial laboratory studies in the investigation of ethylmalonic aciduria should include blood glucose, lactate, ammonium, electrolytes, blood gases, a complete blood count, blood acylcarnitine profile, and quantitative urinary organic acid analysis by gas chromatography-mass spectrometry. Indicators of defective SCAD activity may be acidosis, an increased anion gap, elevated blood urea nitrogen, liver transaminases, and abnormal coagulation tests. Plasma carnitine levels, which can be normal or low, may be determined to exclude carnitine deficiency (either secondary or primary). Hypoketotic hypoglycemia, the main feature of fatty acid oxidation disorders, is usually not present in SCAD-deficient patients. The biochemical hallmark of SCAD deficiency is increased urinary excretion of ethylmalonic acid, the carboxylation product of butyryl-CoA with normal to slightly elevated excretion of other dicarboxylic acids. Methylsuccinic acid, butyrylglycine, and butyrylcarnitine may be elevated in the urine of patients with SCAD deficiency, although these metabolites are also excreted by patients with multiple acyl-CoA dehydrogenation defects and ethylmalonic encephalopathy. Plasma C4-carnitine may be moderately increased.

SCAD deficiency. Because affected patients do not consistently excrete characteristic metabolites, the diagnosis of SCAD deficiency may be difficult. The abnormal metabolic pattern is often amplified during illness or metabolic stress. A persistent increase of plasma butyryl-/isobutyryl-carnitine with ethylmalonic aciduria can be due either to ethylmalonic encephalopathy or to short-chain acyl-CoA dehydrogenase deficiency, sometimes leading to misdiagnosis (07). Consequently, further investigations are required to differentiate the underlying defect with this abnormal metabolic pattern. Enzymatic or molecular studies are indispensable for obtaining a definitive diagnosis. For direct enzyme measurement or for probing the metabolic pathway in vitro by using radiolabeled (16-²H₃-palmitate, [1-¹⁴C]-butyrate) or unlabeled (palmitic/butyric acid) fatty acids for measurement of C4-acylcarnitine production, a skin biopsy may be performed (53). SCAD activity can be measured in cultured fibroblasts, muscle, or liver using the electron-transfer flavoprotein-linked dye-reduction assay with immune-inactivation of the MCAD activity to confirm the diagnosis. Complete absence of activity toward butyryl-CoA in cultured fibroblasts after incubation with an anti-MCAD antibody proves SCAD deficiency. Immunoblot or electrophoretic analysis of the SCAD protein in different tissues (skin fibroblasts, liver, and muscle) may be performed to verify the diagnosis.

Mutation analysis of the two most common mutations (C511T and G625A), both conferring susceptibility to ethylmalonic aciduria, as well as mutation analysis of the whole gene, may be investigated in DNA obtained from fibroblasts and leukocytes (04). SCAD is known to be labile and easily affected secondarily. Because some patients thought to have SCAD deficiency due to increased ethylmalonate excretion also may have persistent elevation of lactate, the possibility of a primary lesion in the respiratory chain should also be considered.

Management

Ethylmalonic encephalopathy

No specific effective treatment for ethylmalonic encephalopathy is known.

Treatment with cofactors and vitamins. Some cases improve with riboflavin or coenzyme Q10 (84; 83). In other cases, riboflavin, carnitine, ascorbic acid, vitamin E, and glycine supplementation were tried without benefit (56; 63).

Medical treatment. Combined treatment with oral metronidazole and N-acetylcysteine produced "marked clinical improvement" in five patients (78). N-acetylcysteine may also be given intravenously (39). In case of metabolic decompensation during chronic treatment with N-acetylcysteine and metronidazole, continuous renal replacement therapy may help reestablish metabolic control (42). Anecdotally, one patient with ethylmalonic encephalopathy remained stable on methylprednisolone (56).

Dietary treatment. A diet restricted in selected sulfur-containing amino acids (ie, methionine and cysteine) may improve clinical outcome (06). A diet low in branched-chain amino acids (ie, leucine, isoleucine, and valine) has not been shown to be beneficial and may induce malnutrition and metabolic imbalance.

Potential treatments of ethylmalonic encephalopathy. Neurologic improvement and reversal of biochemical abnormalities several months after liver transplantation suggest that this may be an alternative therapeutic option in affected patients (16; 69). Dionisi-Vici and colleagues reported an infant with ethylmalonic encephalopathy who received a living-donor liver transplantation (16). Clinical improvement after orthotopic liver transplantation in ethylmalonic aciduria has been observed in several more patients (69; 85). Follow-up reports show that liver transplantation in pediatric patients with ethylmalonic encephalopathy improves or stabilizes the clinical course and should, therefore, be considered as an elective therapeutic option early in the disease course before the occurrence of irreversible neurologic damage (85; 55).

In the mouse model, an AAV2/8-mediated liver gene therapy has been established for ethylmalonic encephalopathy, but it has not yet been tested in patients (13; 15).

SCAD deficiency. Most individuals with SCAD deficiency are asymptomatic, so there is no need for treatment. Some symptomatic patients have been treated similarly to those with fatty acid oxidation disorders, including avoiding fasting by frequent feeding to prevent using fatty acids as a fuel, maintaining high carbohydrate and low-fat intake, and treating intercurrent illness with intravenous glucose.

Some patients with mild variants of MADD and SCAD deficiencies have responded to supplementation with high doses of riboflavin (100 to 300 mg per day), the cofactor for these enzymes (76). In cases of low plasma free carnitine concentrations, oral supplementation of carnitine (50 to 100 mg/kg per day) may be considered to prevent deficiency and facilitate detoxification (04).

Outcomes

Different treatment strategies, such as the supplementation of cofactors, vitamins, medical and dietary treatment, and even liver transplantation, may have positive effects on the clinical course in ethylmalonic encephalopathy. The outcome of individuals with SCAD deficiency is usually excellent, especially when detected incidentally by population newborn screening.

Special considerations

Pregnancy

Maternal illness attributed to a SCAD-deficient fetus during pregnancy has been reported (04). The mother of a patient with SCAD deficiency developed a hemolysis-elevated liver enzyme-low platelet (HELLP) syndrome while pregnant with the affected child (05); the patient was homozygous for the inactivating 1138C>T mutation. This is the first report that SCAD deficiency may be related to maternal HELLP syndrome, as it is reported for long-chain 3-hydroxyacyl-dehydrogenase (LCHAD) deficiency.

Prenatal diagnosis can be performed by reverse transcription polymerase chain reaction (RT-PCR) for ethylmalonic encephalopathy (18).

Anesthesia

Ethylmalonic encephalopathy. In patients with ethylmalonic encephalopathy, it is important to maintain metabolic homeostasis during the perioperative period, as in mitochondriopathies or fatty acid oxidation defects. Precautions should be taken before anesthesia to avoid perioperative prolonged fasting periods (02; 35). Intravenous glucose infusion (8 to 12 mg/kg per minute) should be given to prevent activation of fatty acid oxidation. Metabolic acidosis should be corrected. Ringer’s lactate should be avoided because of lactic acidosis. All anesthetic agents interfering with mitochondrial function (eg, barbiturates, etomidate, propofol, and benzodiazepines) should be avoided (81).

In a case report and review of the anesthetic management of patients with mitochondrial diseases undergoing cardiopulmonary bypass, recommendations included avoidance of inhalation anesthetics, maintenance of blood sugar within normal limits, and avoidance of hypothermia (22). The anesthesia was performed with ketamine and fentanyl, and relaxation was accomplished with rocuronium, although muscle relaxants are often avoided in mitochondrial cytopathies due to reports of prolonged recovery time (48). Volatile anesthetics have been used uneventfully in several case reports (44). However, in one case, malignant hyperthermia occurred when inhalation anesthetics and succinylcholine were used (52). Vigilant monitoring of respiratory function should be maintained because of a decreased ventilatory response to hypoxia and hypercarbia in patients with mitochondrial cytopathies (41).

SCAD deficiency. Because SCAD deficiency is benign, no specific precautions are necessary before anesthesia.