To describe a novel case of lipid metabolism myopathy with chronic motor neuron disorder.

Lipid metabolism myopathy can result in recurrent episodes of rhabdomyolysis triggered by prolonged exercise or illness with many patients undiagnosed until their teenage years. These disorders show distinctive muscle pathology, are difficult to characterize genetically, and generally spare motor neurons and their axons.

A 19-year-old man presented to a tertiary academic hospital with myalgias and hyperCKemia. He had normal developmental and motor milestones; however, since childhood he experienced episodes of myalgias and “heat stroke” after intense physical activity. Initial workup revealed elevated CK, free fatty acids, uric acid, serum and urine ketones, and urine 4-hydroxybutryic and acetoacetic acid, with low free carnitine. Muscle biopsy showed myofiber lipid accumulation with concurrent grouped atrophic muscle fibers suggestive of superimposed neurogenic process. EMG/NCS findings suggested a chronic > active disorder of motor neurons, their axons, or both at the cervical and lumbosacral levels with concurrent bilateral ulnar and right radial neuropathies. Further workup in collaboration with the NIH included normal whole genome and mitochondrial genome sequencing. He is presumed to have mixed neurologic syndrome with abnormal myofiber lipid accumulation and a disorder of motor neurons or their axons. Over the next 2 years he improved after decreased dietary fat intake and riboflavin and ubiquinone supplementation, with recurrent proximal myalgias after high-fat meals. During his initial hospitalization he was wheelchair dependent and at best on treatment he had 5/5 strength in all groups.

Lipid metabolism disorders associated with myopathy typically cause recurrent episodes of rhabdomyolysis. We present a case of lipid metabolism myopathy with concurrent evidence for a disorder of motor neurons or their axons. Extensive testing has yet to reveal a definitive etiology for his symptoms. He responded to restricted fat intake with riboflavin and ubiquinone supplementation.