Lethal infantile mitochondrial myopathy

Lethal infantile mitochondrial myopathy (LIMM) is an extremely rare and severe mitochondrial disorder characterized by progressive skeletal muscle weakness and dysfunction that presents in the neonatal or early infantile period and leads to death typically within the first year of life. The condition primarily affects skeletal muscle, though other organ systems dependent on mitochondrial oxidative phosphorylation may also be involved. LIMM results from defects in mitochondrial respiratory chain function, leading to impaired cellular energy production in muscle tissue. The disease can be caused by mutations in nuclear-encoded genes that affect mitochondrial function, or in some cases by mitochondrial DNA (mtDNA) depletion or deletions. Key clinical features include severe generalized hypotonia (floppy infant), progressive muscle weakness, respiratory insufficiency requiring ventilatory support, feeding difficulties, and failure to thrive. Lactic acidosis is a common laboratory finding reflecting impaired aerobic metabolism. Muscle biopsy typically reveals ragged-red fibers and cytochrome c oxidase (COX)-negative fibers, along with ultrastructural mitochondrial abnormalities. Some patients may also develop cardiomyopathy or hepatic dysfunction depending on the underlying genetic cause. There is currently no curative treatment for lethal infantile mitochondrial myopathy. Management is supportive and palliative, focusing on respiratory support, nutritional optimization through tube feeding, and management of metabolic acidosis. Supplementation with cofactors such as coenzyme Q10, riboflavin, and carnitine has been attempted, though evidence of efficacy in this severe form remains limited. The prognosis is extremely poor, with most affected infants succumbing to respiratory failure within the first months to year of life. Genetic counseling is essential for affected families to understand recurrence risks.

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