Review Question - QID 213482

This patient likely has myoclonic epilepsy with ragged red fibers (MERRF syndrome) as demonstrated by his seizure episode, myoclonus (e.g., sudden jerking of his feet), and the presence of ragged red fibers seen in Figure A. As MERRF is a mitochondrial disease, the transmission of the disease is only through the mother.

MERRF syndrome is an extremely rare mitochondrial disease that has varying degrees of expressivity due to heteroplasmy. The pathogenic variant is present in mitochondrial DNA, which is only passed down through the mother. The signs and symptoms often present during childhood or early adolescence and include myoclonus, seizures, cerebellar ataxia, myopathy, dementia, optic atrophy, and spasticity. Diagnosis is based on the individual and may include evaluation of the clinical phenotype, genetic studies, and muscle biopsy. The characteristic finding on muscle biopsy is the presence of “ragged red fibers.”

Figure/Illustration A is a muscle biopsy specimen demonstrating “ragged red fibers” (arrow) suggestive of MERRF syndrome.

Incorrect Answers:
Answer 1: Father is incorrect as MERRF is a mitochondrial disorder and mitochondrial DNA is transmitted through maternal inheritance.

Answer 2: Maternal grandfather is incorrect as MERRF is transmitted through maternal inheritance. The mother likely inherited the pathogenic variant via her mother (maternal grandmother).

Answer 4: Paternal grandfather is incorrect as MERRF is transmitted through maternal inheritance.

Answer 5: Paternal grandmother is incorrect as MERRF is transmitted through maternal inheritance. Even if the pathogenic variant is present in the paternal grandmother and is passed to the father, it is irrelevant as it will not be passed to the child.

Bullet Summary:
Myoclonic epilepsy with ragged red fibers (MERRF syndrome) is a mitochondrial disease, which is passed through maternal inheritance.