Lipid Myopathies & Fatty Acid Oxidation Defects

Fatty acid oxidation (FAO) disorders are a group of autosomal recessive metabolic myopathies resulting from enzymatic defects in the mitochondrial beta-oxidation pathway or the carnitine transport system. Because long-chain fatty acids are the primary fuel for skeletal muscle during prolonged, low-to-moderate intensity exercise and fasting states, impaired FAO leads to energy failure manifesting as exercise intolerance, myalgia, recurrent rhabdomyolysis, and, in severe forms, cardiomyopathy and hepatic dysfunction. Carnitine palmitoyltransferase II (CPT II) deficiency is the most common FAO disorder presenting to neurologists and the most frequent cause of hereditary myoglobinuria. These disorders are collectively estimated to affect approximately 1 in 9,000–15,000 live births. The recognition of lipid myopathies is critical because many are treatable with dietary modification, supplementation, or targeted pharmacotherapy, and delayed diagnosis risks life-threatening rhabdomyolysis and renal failure.

Fatty Acid Oxidation Pathway in Muscle

Skeletal muscle depends on fatty acid oxidation for approximately 60–65% of its energy needs during sustained, low-to-moderate intensity exercise. The pathway can be divided into three sequential stages:

Carnitine Shuttle

Long-chain fatty acids (C14–C20) cannot cross the inner mitochondrial membrane directly. They require the carnitine shuttle system:

• Step 1: Long-chain fatty acids are activated to acyl-CoA esters in the cytoplasm by acyl-CoA synthetase
• Step 2: CPT I (carnitine palmitoyltransferase I), located on the outer mitochondrial membrane, converts acyl-CoA to acylcarnitine
• Step 3: CACT (carnitine-acylcarnitine translocase) transports acylcarnitine across the inner mitochondrial membrane in exchange for free carnitine
• Step 4: CPT II (carnitine palmitoyltransferase II), on the inner surface of the inner mitochondrial membrane, reconverts acylcarnitine to acyl-CoA for beta-oxidation

Mitochondrial Beta-Oxidation

Once inside the mitochondrial matrix, long-chain acyl-CoA undergoes repeated cycles of four enzymatic reactions (oxidation, hydration, oxidation, thiolysis), each cycle shortening the chain by two carbons and generating one acetyl-CoA, one FADH2, and one NADH. Chain-length-specific enzymes catalyze the first oxidation step:

• VLCAD (very long-chain acyl-CoA dehydrogenase): C14–C20 substrates
• MCAD (medium-chain acyl-CoA dehydrogenase): C6–C12 substrates
• SCAD (short-chain acyl-CoA dehydrogenase): C4–C6 substrates

The mitochondrial trifunctional protein (MTP) is a hetero-octamer that catalyzes the last three steps of long-chain beta-oxidation. Its alpha subunit contains LCHAD (long-chain 3-hydroxyacyl-CoA dehydrogenase) and enoyl-CoA hydratase activities; the beta subunit contains long-chain 3-ketoacyl-CoA thiolase activity.

Electron Transfer to the Respiratory Chain

FADH2 generated by the acyl-CoA dehydrogenases is transferred to the respiratory chain via electron transfer flavoprotein (ETF) and ETF dehydrogenase (ETFDH). Defects in ETF or ETFDH cause multiple acyl-CoA dehydrogenase deficiency (MADD), affecting all FAO enzymes simultaneously.

CPT II Deficiency

CPT II deficiency is the most common inherited disorder of lipid metabolism affecting skeletal muscle and the most frequent cause of hereditary myoglobinuria. The CPT2 gene is located on chromosome 1p32, and pathogenic variants are inherited in an autosomal recessive pattern. Three clinical forms exist:

Myopathic (Adult) Form

The myopathic form accounts for the vast majority of cases presenting to neurologists. Onset is typically in the second or third decade but ranges from the first to sixth decade. Between episodes, patients are entirely normal with no fixed weakness or muscle atrophy.

Severe Infantile and Neonatal Forms

The severe infantile hepatocardiomuscular form presents before age 1 year with hypoketotic hypoglycemia, hepatomegaly, cardiomyopathy, and seizures. The lethal neonatal form manifests within hours to days of birth with hepatic failure, cardiomyopathy, seizures, dysmorphic features (including renal cystic dysplasia), and is usually fatal in the neonatal period. These severe forms are caused by variants that abolish nearly all CPT II enzyme activity, in contrast to the myopathic form where ~20–25% residual activity is preserved.

Diagnosis of CPT II Deficiency

• Acylcarnitine profile (plasma): Elevated long-chain acylcarnitines (C16, C18, C18:1) with reduced free carnitine; plasma analysis is preferred over dried blood spots for sensitivity
• Total and free carnitine: Low total carnitine with reduced free-to-acylcarnitine ratio
• Genetic testing: CPT2 gene sequencing; the p.Ser113Leu variant accounts for ~60% of mutant alleles in the myopathic form
• Enzyme assay: CPT II activity in fibroblasts, leukocytes, or muscle; reduced to ~20–25% of normal in the myopathic form
• Muscle biopsy: Often normal between episodes; may show lipid accumulation in type 1 fibers on Oil Red O staining

VLCAD Deficiency

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, caused by biallelic pathogenic variants in the ACADVL gene (17p13.1), is the second most common long-chain FAO disorder. It presents with three phenotypes: severe infantile (cardiomyopathy, hypoglycemia, high mortality), intermediate childhood (hypoketotic hypoglycemia, hepatopathy), and late-onset myopathic (exercise intolerance and rhabdomyolysis in adolescents and adults).

LCHAD and Trifunctional Protein Deficiency

Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency and complete mitochondrial trifunctional protein (MTP) deficiency are caused by biallelic pathogenic variants in HADHA (alpha subunit) or HADHB (beta subunit), respectively. These disorders are unique among FAO defects for their association with progressive peripheral neuropathy and pigmentary retinopathy—features not seen in other lipid myopathies.

Clinical Features

• Myopathy: Episodic rhabdomyolysis and progressive limb-girdle weakness; exercise intolerance
• Peripheral neuropathy: Progressive sensorimotor axonal neuropathy (median onset ~7 years); can present as pure sensory, pure motor, or mixed; electrophysiology shows axonal pattern with possible secondary demyelination
• Retinopathy: Pigmentary retinopathy progressing to visual impairment; more common in isolated LCHAD deficiency than in complete MTP deficiency; may be related to DHA (docosahexaenoic acid) deficiency
• Cardiomyopathy: Hypertrophic or dilated cardiomyopathy; cardiac arrhythmias
• Hepatopathy: Hypoketotic hypoglycemia, Reye-like episodes, cholestasis
• Maternal complications: Mothers heterozygous for LCHAD/MTP variants are at increased risk of HELLP syndrome and acute fatty liver of pregnancy

Diagnosis

Elevated long-chain 3-hydroxyacylcarnitine species (C16-OH, C18-OH, C18:1-OH) on acylcarnitine profile and increased 3-hydroxy-dicarboxylic acids in urine. The common LCHAD variant p.Glu510Gln in HADHA accounts for ~90% of mutant alleles. Genetic testing of both HADHA and HADHB confirms the diagnosis.

Primary Carnitine Deficiency

Primary carnitine deficiency (PCD) results from biallelic loss-of-function variants in SLC22A5, encoding the high-affinity carnitine transporter OCTN2. OCTN2 mediates carnitine reabsorption in the renal tubules and uptake into cardiac and skeletal muscle. Without functional OCTN2, urinary carnitine wasting leads to profoundly low plasma and tissue carnitine levels, effectively starving the mitochondria of their long-chain fatty acid fuel.

Neutral Lipid Storage Diseases

Neutral lipid storage diseases (NLSDs) result from defects in cytoplasmic triglyceride (triacylglycerol) metabolism, leading to accumulation of lipid droplets in multiple tissues. Unlike FAO disorders, the defect is in triglyceride lipolysis rather than mitochondrial beta-oxidation itself.

Chanarin-Dorfman Syndrome (NLSD with Ichthyosis)

Caused by biallelic pathogenic variants in ABHD5 (CGI-58), which encodes an activator of adipose triglyceride lipase (ATGL). The hallmark features are:

• Ichthyosis: Congenital nonbullous ichthyosiform erythroderma is the most prominent clinical feature, present from birth
• Jordan anomaly: Lipid droplets visible in peripheral blood leukocytes (neutrophils, eosinophils, monocytes) on routine peripheral smear—a pathognomonic and easily obtained diagnostic clue
• Myopathy: Mild proximal weakness; lipid vacuoles on muscle biopsy
• Hepatosteatosis: Fatty liver with elevated transaminases
• Other: Sensorineural hearing loss, cataracts, intellectual disability (variable)

NLSD with Myopathy (NLSD-M)

Caused by biallelic pathogenic variants in PNPLA2, encoding adipose triglyceride lipase (ATGL). Unlike Chanarin-Dorfman syndrome, ichthyosis is absent:

• Progressive myopathy: Proximal and distal skeletal muscle weakness, often asymmetric; onset typically in early adulthood (20s–40s)
• Cardiomyopathy: Dilated cardiomyopathy develops in approximately 40–50% of patients; may require cardiac transplantation
• Jordan anomaly: Also present; lipid droplets in peripheral blood leukocytes
• CK elevation: Persistently elevated (2–10× normal)
• Muscle biopsy: Marked lipid accumulation predominantly in type 1 fibers on Oil Red O staining
• No ichthyosis: Key distinguishing feature from Chanarin-Dorfman syndrome

Multiple Acyl-CoA Dehydrogenase Deficiency (MADD)

MADD, also known as glutaric aciduria type II, results from biallelic pathogenic variants in genes encoding ETF alpha subunit (ETFA), ETF beta subunit (ETFB), or ETF dehydrogenase (ETFDH). Because ETF/ETFDH accepts electrons from multiple acyl-CoA dehydrogenases involved in fatty acid, amino acid, and choline metabolism, deficiency produces a broad metabolic derangement.

Clinical Forms

• Type I (neonatal with congenital anomalies): Severe, often fatal; dysmorphic features, renal cystic dysplasia, nonketotic hypoglycemia
• Type II (neonatal without anomalies): Severe metabolic decompensation, cardiomyopathy, hypoglycemia; high mortality
• Type III (late-onset): Most common form presenting to neurologists; episodic or progressive proximal myopathy, exercise intolerance, lipid storage myopathy on biopsy; episodes of metabolic crisis with rhabdomyolysis, vomiting, and hepatopathy triggered by fasting or illness

Comparison of Fatty Acid Oxidation Disorders

Diagnostic Approach

The evaluation of a patient with suspected lipid myopathy should be systematic, combining clinical history, biochemical testing, and genetic confirmation.

Clinical Clues Suggesting a Lipid Myopathy

• Recurrent rhabdomyolysis triggered by prolonged exercise rather than brief intense exertion (unlike glycogen storage disorders)
• Exacerbation by fasting, intercurrent illness, or cold exposure
• Absence of fixed contractures or second-wind phenomenon (contrast with McArdle disease)
• Normal neurologic examination between episodes
• Family history consistent with autosomal recessive inheritance

Biochemical Testing

Muscle Biopsy

While genetic testing has increasingly supplanted biopsy for diagnosis, muscle pathology remains informative when genetic results are inconclusive:

• Oil Red O staining: Reveals lipid droplet accumulation, predominantly in type 1 (oxidative) muscle fibers
• Modified Gomori trichrome: May show vacuolar change; ragged-red fibers suggest concomitant mitochondrial dysfunction (as in some MADD cases)
• Electron microscopy: Excessive lipid droplets between myofibrils and beneath the sarcolemma; increased mitochondrial number with abnormal cristae in some disorders
• Limitations: Biopsy between episodes in CPT II deficiency may be entirely normal; timing relative to clinical episodes affects sensitivity

Genetic Testing

Next-generation sequencing panels that include genes for metabolic myopathies, FAO disorders, and muscular dystrophies are the preferred approach. When panels are negative and clinical suspicion remains high, whole-exome or whole-genome sequencing should be pursued. Functional enzyme studies in fibroblasts or leukocytes can help resolve variants of uncertain significance.

Treatment

General Principles

• Avoid prolonged fasting: Regular meals every 3–4 hours; complex carbohydrate snack before bed; raw cornstarch at bedtime in children to provide sustained glucose release overnight
• Dietary modification: Low-fat, high-carbohydrate diet for long-chain FAO disorders; fat restriction to 20–25% of total calories with emphasis on medium-chain triglycerides (MCT) that bypass the carnitine shuttle and VLCAD/LCHAD steps
• Avoid trigger combinations: Prolonged exercise during fasting or illness is particularly dangerous; patients should consume carbohydrate-rich snacks before and during sustained exercise
• Sick-day protocols: Increased carbohydrate intake during febrile illness; low threshold for IV dextrose infusion in the emergency department; avoid fasting for medical procedures

Specific Therapies

Prognosis and Long-Term Management

The prognosis of lipid myopathies varies substantially by disorder. Myopathic CPT II deficiency carries a favorable long-term prognosis when patients learn to avoid triggers and manage acute episodes, with most maintaining normal strength and function. VLCAD deficiency requires ongoing cardiac surveillance due to the risk of cardiomyopathy even in the late-onset form. LCHAD/MTP deficiency has the most guarded prognosis, with progressive neuropathy and retinopathy causing cumulative disability despite dietary management. Primary carnitine deficiency has an excellent prognosis with lifelong carnitine supplementation. Late-onset MADD is among the most treatable metabolic myopathies, with sustained improvement on riboflavin therapy.

A 2024 European study examining the long-term prognosis of FAO disorders in adults (n=44 patients) found that rhabdomyolysis was the most frequent muscle symptom (84%), triggered by exercise (75%), fasting (50%), fever (32%), or infection (25%). Despite limited specific therapies, most patients with adult-onset forms maintained functional independence with appropriate dietary and lifestyle modifications.

All patients with lipid myopathies should carry medical alert identification specifying their diagnosis and the need for IV dextrose during illness. Genetic counseling is recommended for family planning given the autosomal recessive inheritance of all FAO disorders.

References

1. Kushlaf H. Muscle channelopathies and rhabdomyolysis. Continuum (Minneap Minn) 2025;31(5):1409–1436.
2. Longo N, Amat di San Filippo C, Pasquali M. Disorders of carnitine transport and the carnitine cycle. Am J Med Genet C Semin Med Genet 2006;142C(2):77–85.
3. Bonnefont JP, Djouadi F, Prip-Buus C, et al. Carnitine palmitoyltransferase deficiencies. Mol Genet Metab 2004;82(2):81–99.
4. Joshi PR, Deschauer M, Zierz S. Carnitine palmitoyltransferase II (CPT II) deficiency: genotype-phenotype analysis of 50 patients. J Neurol Sci 2014;338(1-2):107–111.
5. Rouyer A, Romero NB, Maisonobe T, et al. Long-term prognosis of fatty-acid oxidation disorders in adults: optimism despite the limited effective therapies available. Eur J Neurol 2024;31(3):e16138.
6. Leslie ND, Saenz-Ayala S. Very long-chain acyl-coenzyme A dehydrogenase deficiency. In: Adam MP, Feldman J, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993–2025.
7. Spiekerkoetter U, Lindner M, Santer R, et al. Management and outcome in 75 individuals with long-chain fatty acid oxidation defects: results from a workshop. J Inherit Metab Dis 2009;32(4):488–497.
8. den Boer ME, Wanders RJ, Morris AA, et al. Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency: clinical presentation and follow-up of 50 patients. Pediatrics 2002;109(1):99–104.
9. Matern D, Rinaldo P. Long-chain hydroxyacyl-CoA dehydrogenase deficiency/trifunctional protein deficiency. In: Adam MP, Feldman J, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993–2025.
10. Longo N. Primary carnitine deficiency. In: Adam MP, Feldman J, Mirzaa GM, et al, eds. GeneReviews. University of Washington, Seattle; 1993–2025.
11. Fischer J, Lefevre C, Morava E, et al. The gene encoding adipose triglyceride lipase (PNPLA2) is mutated in neutral lipid storage disease with myopathy. Nat Genet 2007;39(1):28–30.
12. Lefevre C, Jobard F, Caux F, et al. Mutations in CGI-58, the gene encoding a new protein of the esterase/lipase/thioesterase subfamily, in Chanarin-Dorfman syndrome. Am J Hum Genet 2001;69(5):1002–1012.
13. Olsen RK, Olpin SE, Andresen BS, et al. ETFDH mutations as a major cause of riboflavin-responsive multiple acyl-CoA dehydrogenation deficiency. Brain 2007;130(Pt 8):2045–2054.
14. Roe CR, Sweetman L, Roe DS, et al. Treatment of cardiomyopathy and rhabdomyolysis in long-chain fat oxidation disorders using an anaplerotic odd-chain triglyceride. J Clin Invest 2002;110(2):259–269.
15. Bonnefont JP, Bastin J, Laforet P, et al. Long-term follow-up of bezafibrate treatment in patients with the myopathic form of carnitine palmitoyltransferase 2 deficiency. Clin Pharmacol Ther 2010;88(1):101–108.