When should MERRF (myoclonus epilepsy associated with ragged-red fibers) be the diagnosis? / Quando o diagnóstico deveria ser MERRF (epilepsia mioclônica associada com fibras vermelhas rasgadas)?

Myoclonic epilepsy associated with ragged red fibers (MERRF) is a rare mitochondrial disorder. Diagnostic criteria for MERRF include typical manifestations of the disease: myoclonus, generalized epilepsy, cerebellar ataxia and ragged red fibers (RRF) on muscle biopsy. Clinical features of MERRF are not necessarily uniform in the early stages of the disease, and correlations between clinical manifestations and physiopathology have not been fully elucidated. It is estimated that point mutations in the tRNALys gene of the DNAmt, mainly A8344G, are responsible for almost 90% of MERRF cases. Morphological changes seen upon muscle biopsy in MERRF include a substantive proportion of RRF, muscle fibers showing a deficient activity of cytochrome c oxidase (COX) and the presence of vessels with a strong reaction for succinate dehydrogenase and COX deficiency. In this review, we discuss mainly clinical and laboratory manifestations, brain images, electrophysiological patterns, histology and molecular findings as well as some differential diagnoses and treatments.

Epilepsia mioclônica associada com fibras vermelhas rasgadas (MERRF) é uma rara doença mitocondrial. O critério diagnóstico para MERRF inclui as manifestações típicas da doença: mioclonia, epilepsia generalizada, ataxia cerebelar e fibras vermelhas rasgadas (RRF) na biópsia de músculo. Na fase inicial da doença, as manifestações clínicas podem não ser uniformes, e correlação entre as manifestações clínicas e fisiopatologia não estão completamente elucidadas. Estima-se que as mutações de ponto no gene tRNALys do DNAmt, principalmente a A8344G, sejam responsáveis por quase 90% dos casos de MERRF. As alterações morfológicas na biópsia muscular incluem uma grande proporção de RRF, fibras musculares com deficiência de atividade da citocromo c oxidase (COX) e presença de vasos com forte reação para succinato desidrogenase e deficiência da COX. Nesta revisão, são discutidas as principais manifestações clínicas e laboratoriais, imagens cerebrais, padrões eletrofisiológicos, histológicos e alterações moleculares, bem como, alguns dos diagnósticos diferenciais e tratamentos.

Histochemical and Molecular Genetic Study of MELAS and MERRF in Korean Patients

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episode (MELAS) and myoclonic epilepsy and raggedred fibers (MERRF) are rare disorders caused by point mutation of the tRNA gene of the mitochondrial genome. To understand the pathogenetic mechanism of MELAS and MERRF, we studied four patients. Serially sectioned frozen muscle specimens with a battery of histochemical stains were reviewed under light microscope and ultrastructural changes were observed under electron microscope. The polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis was performed and the tRNA genes were sequenced to confirm mutations. In two patients with MELAS, strongly succinyl dehydrogenase positive blood vessels (SSVs) and many cytochrome oxidase (COX) positive raggedred fibers (RRFs) were observed, and A3243G mutations were found from the muscle samples. In two patients with MERRF, neither SSV nor COX positive RRFs were seen and A8344G mutations were found from both muscle and blood samples. In the two MERRF families, the identical mutation was observed among family members. The failure to detect the mutation in blood samples of the MELAS suggests a low mutant load in blood cells. The histochemical methods including COX stain are useful for the confirmation and differentiation of mitochondrial diseases. Also, molecular biological study using muscle sample seems essential for the confirmation of the mtDNA mutation.