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  • Updated 06.10.2021
  • Released 09.06.1993
  • Expires For CME 06.10.2024

Myoclonus epilepsy with ragged-red fibers

Introduction

Overview

Myoclonus epilepsy with ragged-red fibers (MERRF) is a multisystem mitochondrial disorder defined by myoclonus, generalized epilepsy, ataxia, and myopathy with ragged-red fibers detected in muscle biopsy. MERRF is a clinical syndrome that has been associated with at least six different point mutations of mitochondrial DNA; however, about 80% of MERRF patients harbor an A-to-G transition at nucleotide 8344 (m.8344A> G) of the MT-TK gene that is a hot spot for MERRF mutations. The pathogenesis of the disease is incompletely understood. In this article, the authors refer to reports from Italian and German cohorts that the great majority of 8344A>G patients do not have myoclonus and that myoclonus, if present, is not inextricably linked to epilepsy but, at least in the Italian population, is linked with cerebellar ataxia. Hence, the term “myoclonic epilepsy” seems inadequate, and the acronym MERRF could better be read as myoclonic encephalomyopathy with ragged-red fibers.

Key points

• Myoclonus epilepsy with ragged-red fibers (MERRF) is clinically defined by: myoclonus, generalized epilepsy, ataxia, and myopathy with ragged-red fibers.

• In addition, MERRF patients often have sensorineural hearing loss, cognitive impairment, multiple lipomatosis, peripheral neuropathy, exercise intolerance, ptosis, ophthalmoparesis, optic atrophy, cardiomyopathy, muscle wasting, respiratory impairment, diabetes, muscle pain, tremor, and migraine.

• About 80% of patients with MERRF have a pathogenic m.8344A> G mutation in the MT-TK gene encoding tRNALys.

• The term “myoclonic epilepsy” seems inadequate, and the acronym MERRF could better be read as myoclonic encephalomyopathy with ragged-red fibers.

Historical note and terminology

In 1921 Ramsay Hunt described six patients with a disorder resembling Friedreich ataxia characterized by ataxia, myoclonus, and epilepsy, which he called "dyssynergia cerebellaris myoclonica" (28). Several different disorders have been associated with this clinical triad; however, over 50 years passed before mitochondrial abnormalities were described in one family with these clinical features (58). In 1980 Fukuhara and colleagues reported two patients with a syndrome that they named "myoclonus epilepsy associated with ragged-red fibers" (21). Eight years later, Shoffner and colleagues identified a mitochondrial DNA point mutation in myoclonus epilepsy with ragged-red fibers pedigrees (51).

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