Clinical manifestations

Presentation and course

Isovaleric aciduria due to isovaleryl-CoA dehydrogenase deficiency presents with two different clinical phenotypes: an acute neonatal onset and an infantile chronic intermittent form. Approximately half of the cases presented with acute neonatal onset and the other half with chronic intermittent disease (37). Both phenotypes can occur in populations or even within the same family carrying the same mutation (37; 04; 15). However, a common mutation is associated with a mild, likely asymptomatic phenotype. Patients carrying this mutation are nevertheless identified by newborn screening. Molecular genetic analysis within this group identifies around 50% of mutant alleles as the single mutation (932C-T; A282V). This mutation is also found in older, healthy siblings but not in previously identified symptomatic patients; thus, this mutation is associated with a usually asymptomatic clinical course (08).

During metabolic crises, patients develop the typical features of an organic acid disorder with acidosis, ketosis, vomiting, progressive alteration of consciousness, and, finally, without appropriate therapy, overwhelming illness, deep coma, and death (37). Children are usually born at term after an uneventful pregnancy. If clinical abnormalities develop within the first days of life, patients refuse feeding, start to vomit, and become progressively dehydrated and lethargic. Hypothermia, tremor, twitching, and seizures can occur (35). A foul odor reminiscent of “sweaty feet” caused by isovaleric acid can develop. Abnormalities of the hematopoietic system such as thrombocytopenia, neutropenia, or pancytopenia occur in metabolic decompensation (37). Hypocalcemia and hyperglycemia may be found. Hyperammonemia may occur but is usually mild compared to that of disorders of propionate degradation. Acute metabolic decompensation can quickly lead to death triggered by cerebral edema, intracerebral hemorrhage, and/or infection. Neuropathological examination shows cerebellar edema with herniation and spongiform changes in the white matter.

In the chronic intermittent form of isovaleric acidemia, children have recurrent metabolic crises because of high intake of protein or minor infections that induce a catabolic state (37). The metabolic crises are characterized by vomiting, lethargy, coma, acidosis, ketosis, and the odor of “sweaty feet.” Hematologic abnormalities develop as described above. Hyperglycemia may be found and is most likely due to stress-induced hormonal effects. The combination of ketoacidosis, dehydration, and hyperglycemia has been misjudged as diabetic ketoacidosis (09). Pancreatitis may be a complication of acute and chronic isovaleric aciduria (28). With age, children become less sensitive to minor infections. Some patients dislike food with high protein content. Older patients may have normal psychomotor development or mild to severe intellectual disability, depending on the frequency and severity of metabolic decompensations, the age at diagnosis, and beginning of specific therapy. Findings in cranial MRI examination of a patient with chronic-intermittent form of the disease have been published. After a severe metabolic decompensation, the 19-month-old girl showed signal changes in the globi pallidi and the corticospinal tracts of the mesencephalon, which were hypointense on T1-weighted and hyperintense on T2 weighted images (36). With correct treatment set in place early, most children will survive the first life-threatening episode and may then go on to have normal psychomotor development. Clinical presentation and long-term disease outcomes in adults have recently been reviewed (40). Clinical and neurocognitive outcomes in isovaleric acidemia patients depend on early diagnosis and treatment, and the majority of adolescent and adult patients had a normal clinical and neurocognitive outcome (20). In contrast to other organoacidopathies, such as propionic acidemia or methylmalonic academia, there is no apparent disease progression or multisystemic organ dysfunction. However, a case of a first acute metabolic decompensation of isovaleric acidemia in an adult has been described, and the authors emphasized that internists and adult neurologists also need to be aware of organic acidemias (11).

Prognosis and complications

In the first reports, isovaleric acidemia was associated with a poor prognosis; more than a half of the patients with acute neonatal onset did not survive the first episode, but with improvement in therapy by supplementation of glycine and L-carnitine as well as earlier diagnosis, the outcome has become much more favorable (37). If treatment can be initiated before a first severe metabolic decompensation, the patient’s prognosis is significantly improved. Individuals who receive treatment before becoming symptomatic usually have normal long-term psychomotor development (24). Therefore, early diagnosis is crucial. This is supported by a study by Grunert and colleagues, who published details on clinical and neurocognitive outcome in 21 symptomatic isovaleric acidemia patients diagnosed between 1976 and 1999 and results of a systematic review of 155 published cases (13). They found mild motor deficits in 44% and cognitive deficits in 19% of study patients. The data revealed that the patients’ intelligence quotient was not related to the number of metabolic decompensations, but inversely related to the age at diagnosis. In contrast to the high mortality (33%) in literature, only one patient of the study cohort died due to metabolic decompensation in the neonatal period. The authors, therefore, strongly advocate newborn screening for isovaleric acidemia. Results from the European registry and network for intoxication-type metabolic diseases (E-IMD) confirm that clinical and neurocognitive outcomes depend on early diagnosis and treatment (14).

Treatment has to be supervised by an experienced metabolic center and must continue for life. Special care must be taken to ensure efficient emergency procedures at all times (including travel and holidays) and to always monitor carnitine status and dietary management closely, including careful avoidance of overtreatment or malnutrition (47).

Patients identified by newborn screening who carry the common mutation (A282V, 932C> T) are usually asymptomatic and most likely do not need treatment. Siblings of these individuals who have the same genotype may remain asymptomatic during episodes of febrile illnesses without any specific treatment (08; 42; 24).

Clinical vignette

A six-year-old girl was admitted to the hospital with ongoing vomiting for the past three days without fever and diarrhea. Her general condition was poor. She was drowsy without focal neurologic deficits. She also suffered from hallucinations. She was severely dehydrated and a particular fetor like “sweaty-feet” was prominent.

The medical history revealed that she has had eight similar past episodes with vomiting and the same smell. During these episodes a prominent metabolic acidosis was present. Symptoms disappeared with glucose infusion treatment. Her diet history was remarkable as she preferred fruits and vegetables and avoided dairy products and meat. She also insisted on taking her meals regularly. Her somatic and cognitive development were normal: she attended primary school with good success. Her family history was unremarkable.

Laboratory investigations revealed a severe metabolic acidosis and ketosis. Ammonium was within normal limits, but investigation of organic acids in urine showed massive ketonuria and lactaturia and highly elevated excretion of 3-hydroxy isovaleric acid as well as isovalerylglycine. The diagnosis of isovaleric aciduria was made accordingly and the girl was treated with high-dose intravenous glucose and carnitine. The severe metabolic acidosis was treated with a single dose of sodium bicarbonate. Fortunately, the girl recovered completely and was further treated with mild protein restriction, which reflected her preferred food pattern, and carnitine supplementation. The family also received an emergency card and a sick-day nutrition plan, which contained high energy and low protein food. In case of illness her carnitine dosage was doubled.

Biological basis

Etiology and pathogenesis

Isovaleric acidemia is an inborn error of metabolism that is caused by deficiency of the mitochondrial enzyme isovaleryl-CoA dehydrogenase (MIM 243500; EC 1.3.99.10).

Genetics. Isovaleric acidemia is caused by deficiency of the mitochondrial enzyme isovaleryl-CoA dehydrogenase with an autosomal recessive inheritance. The gene coding for isovaleryl-CoA dehydrogenase was mapped by southern blot analysis to chromosome 15q14-q15 (21). The 172 kDa enzyme is a tetramer of four identical subunits. Both clinical presentations, acute-neonatal and infantile chronic-intermittent, are found within the same family and are most likely determined by nongenetic factors. Genetic complementation studies point to involvement of a single genetic locus (06). The molecular basis was investigated in detail: missense mutations (29; 44), an in-frame mutation (43), or point mutations leading to abnormal splicing and altered mitochondrial import (45) have been described.

The four identical subunits of isovaleryl-CoA dehydrogenase are synthesized on cytosolic polysomes and imported into the mitochondrial matrix where the enzyme is assembled and is situated in the inner layer of the mitochondrial membrane. It is a flavin enzyme containing ca. 1 mol FAD/subunit. The FAD transports electrons via coenzyme Q to the respiratory chain.

Pathophysiology. Due to the metabolic block, isovaleryl-CoA accumulates, and the pathognomonic metabolite isovalerylglycine is formed by conjugation of isovaleryl-CoA to the amino group of glycine. The hydrolysis product isovaleric acid is quantitatively less important. The mitochondrial enzyme that catalyzes this reaction is named glycine-N-acylase (EC 2.3.1.13). Isovalerylglycine is nontoxic and can be secreted via urine. This reaction is an important detoxification pathway augmented in therapeutic intervention.

The mechanism of isovaleric acid toxicity remains unclear. Some findings point to an inhibition of mitochondrial energy metabolism (27; 25). In line with this finding, a study demonstrated an altered thiol status with reduced glutathione levels as markers for oxidative stress in a cohort of 11 patients (34). In bone marrow cell cultures, isovaleric acid was found to be an inhibitor of granulopoietic progenitor cell proliferation, which might be an explanation for the neutropenia frequently seen in isovaleric acidemia during metabolic decompensation (37). It has been hypothesized that amino-acid depletion is induced by abnormal amino-acid conjugation combined with protein restriction in isovaleric acidemia patients, but this has not yet been demonstrated in a large cohort of patients (26).

Epidemiology

The incidence of isovaleric acidemia is difficult to estimate in absence of a general registry of patients. Newborn screening results indicate 1:100,000 in Caucasian populations (24). In countries where isovaleric acidemia is not part of population newborn screening, the diagnosis can be missed. The cases reported so far do not seem to cluster in certain ethnic groups. Selective screening by analysis of organic acids in urine or tandem mass spectrometry in Oman, Thailand, and Hong Kong demonstrated the presence of isovaleric acidemia in different populations around the world (17; 46; 22; 41). Moorthie and coworkers provide an estimate of by meta-analysis of studies in western populations and worldwide (30).

Prevention

Isovaleric acidemia can be easily diagnosed in newborn screening programs with electrospray tandem mass spectrometry. If treatment is initiated before the development of severe metabolic decompensation, the patient’s prognosis can be significantly improved. Patients who are diagnosed by newborn screening usually have normal psychomotor development (24).

Differential diagnosis

The acute metabolic decompensation is similar to that in other organic acid disorders, fatty acid oxidation disorders, or urea cycle disorders. Specifically, the catabolic pathway of leucine comprises six enzymes, for all of which inherited deficiencies have been described.

Organoacidopathies can best be differentiated by their specific pattern in gas chromatography-mass spectrometry analysis of nonvolatile urinary organic acids (16).

If vomiting in infancy becomes prominent, hypertrophic pyloric stenosis may be suspected, and patients may undergo surgery (03; 23). The combination of the symptoms ketoacidosis and hyperglycemia may be misinterpreted as diabetes mellitus (09).

Diagnostic workup

The clinical symptoms of isovaleric acidemia are similar to other organic acidemias, even the suggestive “odor of sweaty feet” is shared by some other disorders. The best way to distinguish the organic acidemias is through analysis of the urinary nonvolatile organic acid pattern by gas chromatography-mass spectrometry (16; 47). Another complementary and rapid diagnostic technique is the analysis of acylcarnitine profiles by tandem mass spectrometry. The accumulating CoA esters are in equilibrium with their corresponding acylcarnitines, which are easy to analyze in dried blood spots by tandem mass spectrometry. This technique has been adapted to perform newborn screening, leading to early diagnosis and appropriate therapy (07; 24). False-positive results in newborn screening can arise from pivalic acid containing antibiotics or pivalic acid derivatives, which are used in the cosmetic industry under the term “neopentanoate” (02).

During metabolic decompensation, the urinary organic acid profile reveals high excretion of isovalerylglycine (2000 to 9000 mmol/mol creatinine), which remains markedly elevated after recompensation (1000 to 3000 mmol/mol creatinine). 3-Hydroxyisovaleric acid is only found elevated during metabolic decompensation. 4-Hydroxyisovaleric, mesaconic acid, methylsuccinic acid, 3-hydroxyisoheptanoic acid, isovalerylglutamic acid, isovalerylglucoronide, isovalerylalanine, and isovalerylsarcosine are minor pathological metabolites detectable in smaller amounts (20 to 300 mmol/mol creatinine) (37). Other organic acidemias can be differentiated by their specific profiles of urinary organic acids. The acylcarnitine profile in isovaleric aciduria is characterized by high levels of isovalerylcarnitine, depending on the quantity of oral carnitine administration. The diagnosis of isovaleric acidemia can be confirmed by enzymatic assay or mutation analysis (19). Several methods have been successfully used for prenatal diagnosis: stable isotope dilution analysis of amniotic fluid (elevated isovalerylglycine at 12 weeks of gestation in quantities of 3.5 to 6 µM), macromolecular labeling from (1-14C)-isovaleric acid in cultured amniocytes, and electrospray ionization tandem mass spectrometry analysis of acylcarnitines in amniotic fluid (highly elevated levels of isovalerylcarnitine from 3.12 to 12 µM compared to control values from 0.59 to 0.99 µM) (19).

Management

Isovaleric acid is toxic (37); therefore, the primary aim in treating isovaleric acidemia is to prevent the formation and to lower the levels of accumulating metabolites. As leucine is an essential amino acid, the catabolic pathway is challenged by increased protein intake or increased endogenous protein degradation (37). Total natural protein intake is restricted according to the patient’s leucine tolerance and is adjusted to age-specific requirements. The reduction of leucine intake must be carefully monitored to prevent over-restriction. A diet with inadequate leucine intake can impair protein synthesis and lead to catabolic metabolic decompensation or failure to thrive (47; 10), and additional nutritional insufficiencies may occur (05). To provide a complementary source of the other amino acids, a leucine-free formula is available (37; 19). Dietary practice in Europe has been systematically studied and has been found to vary considerably among centers (32).Universal treatment recommendations are urgently required; in 2014, web-based, preliminary guidelines (quick reference guide) were published by the European registry and network for intoxication-type metabolic diseases (EIMD Recommendations), but more extensive guidelines are needed.

The other important principle of therapy is to increase the excretion of isovaleric acid as nontoxic glycine and carnitine conjugates (37). Glycine is conjugated to isovaleric acid to form isovalerylglycine by glycine-N-acylase. Normal tissue concentrations of glycine are already lower than the Km concentrations for optimal enzyme functioning and tend to decrease further during metabolic decompensation (37). Therefore, it is important to ensure sufficient glycine levels to allow optimal detoxification. Therapeutic guidelines recommend a dosage of 150 to 300 mg/kg per day of glycine while the patient is stable (01). The dosage can be augmented during metabolic crisis.

It is a common finding that many patients with isovaleric acidemia show low levels of total carnitine in plasma and a high percentage of esterified carnitines in plasma and urine (37; 19). These findings are due to the enzymatic production of isovalerylcarnitine by carnitine acetyltransferase and the subsequent loss of isovalerylcarnitine in the urine. As a consequence, carnitine stores may become exhausted, resulting in severe secondary metabolic consequences. L-carnitine is supplemented in doses of 40 to 100 mg/kg to ensure high normal free carnitine levels in plasma.

It is hard to determine whether treatment with glycine is more effective than treatment with carnitine because all reports lack a common study protocol, do not have adequate sample size, and do not control for the patients’ age, diet, and medication dosage. As a first estimation, excretion of isovaleric acid via glycine is quantitatively more important, together with a moderate restriction of leucine intake (37). Treatment with L-carnitine prevents carnitine deficiency and provides a second detoxification pathway (37). A recent treatment recommendation from the European registry and network for intoxication-type metabolic diseases (E-IMD) recommends L-carnitine in combination with L-glycine in metabolically severe phenotypes (EIMD Recommendations).

During acute decompensation, isovaleric acidemia must be treated like other organic acidemias. Measures include increased provision of energy via oral, nasogastric, or intravenous routes by 20% to 100% above the recommended daily requirements using carbohydrate (such as glucose or dextrose 20% orally or glucose intravenously) and fat (intralipid 20%). Soluble insulin is provided to avoid hyperglycemia and to support intracellular glucose uptake. The intake of natural protein is stopped for 24 to 48 hours and is then reintroduced gradually as tolerated (16). Additional specific treatment with augmented doses of glycine and high-dose L-carnitine is recommended (37). Kasapkara and colleagues described the use of N-carbamylglutamate to overcome neonatal hyperammonemia due to inhibition of N-acetylglutamate synthetase by isovaleryl-CoA in analogy to the use of this substance in other organic acidemias (18).

Patients should be supplied with an emergency card, letter, or bracelet containing instructions for emergency measures and phone numbers. Logistics of rational therapeutic measures should be repeatedly evaluated by the specialist team with the family and the primary care physicians.

It is important to realize that aspirin is contraindicated in patients with isovaleric acidemia because salicylic acid is a competing substrate for glycine-N-acylase, interfering with isovalerylglycine synthesis.

It remains to be elucidated whether and to what extent patients carrying the common mutation (A282V, 932C> T) associated with a biochemically and clinically mild phenotype require any treatment. Close observation during periods of metabolic stress and carnitine supplementation if free carnitine concentrations are very low seem reasonable (42).

Outcomes

If treatment can be initiated before a first severe metabolic decompensation, the patient’s prognosis is significantly improved. Individuals who receive treatment before becoming symptomatic usually have normal long-term psychomotor development (24; 13). Therefore, early diagnosis is crucial.

Treatment has to be supervised by an experienced metabolic center and must continue for life. Special care must be taken to ensure efficient emergency procedures at all times (including travel and holidays) and to always monitor carnitine status and dietary management closely, including careful avoidance of overtreatment and malnutrition (16).

Patients identified by newborn screening who carry the common mutation (A282V, 932C> T) are usually asymptomatic and most likely do not need treatment. Siblings of these individuals who have the same genotype remain asymptomatic during episodes of febrile illnesses without any specific treatment (08; 42; 24).

Special considerations

Pregnancy

Some pregnancies in women with isovaleric acidemia have been reported and outcome for mother and child seems to be good (40). However, close monitoring of sufficient protein and energy intake is important. Catabolic stress has to be anticipated during labor and involution of the uterus and may require intravenous glucose treatment (40). Prenatal diagnosis is possible in families at risk.

Anesthesia

Patients are at risk for metabolic decompensation by catabolism induced by fasting, anesthesia, or a surgical procedure. It is of utmost importance to meet increased energy requirements by intravenous administration of dextrose (16). Medication with glycine and carnitine should be provided. No information is available about particular side effects of drugs normally used to induce anesthesia in patients with isovaleric acidemia.