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# MERRF (myoclonic epilepsy with ragged-red fibres)

A maternally inherited mitochondrial disease of myoclonic jerks, seizures, unsteadiness and muscle weakness.

Updated Nov 13, 2026

On this page

- The basics What it is
- Who gets it
- Causes
- Pathophysiology

- Clinical Symptoms and signs
- Red flags
- Diagnosis
- Complications
- Prognosis

- Treatment Treatment

- Study Test yourself

- Connections Connections

## What it is

A mitochondrial DNA disorder, most often caused by the m.8344A>G variant in MT-TK, the mitochondrial transfer RNA for lysine.

See it in 3D

- [Cerebellum](https://durgical.com/en/anatomy/anterior-quadrangular-lobule/)
- [Cochlea](https://durgical.com/en/anatomy/cochlea/)
- [Optic nerve](https://durgical.com/en/anatomy/optic-nerve/)

## Who gets it

Rare; estimated at a few per 100 000 people.

## Causes

- The m.8344A>G variant in MT-TK in about 80 % of cases; other MT-TK variants in a few
- Maternal (mitochondrial) inheritance with heteroplasmy

## Pathophysiology

The faulty transfer RNA impairs translation of all 13 mtDNA-encoded respiratory-chain proteins, so oxidative phosphorylation fails in tissues with high energy needs once the mutant load passes their threshold. Abnormal mitochondria accumulate under the muscle membrane, giving «ragged-red» fibres on biopsy.

## Symptoms and signs

- Myoclonus (sudden muscle jerks), often the first sign
- Generalised seizures
- Ataxia
- Muscle weakness and exercise intolerance
- Hearing loss, short stature, optic atrophy, multiple lipomas or cardiomyopathy in some

## Red flags

- Prolonged seizures
- Heart failure or arrhythmia

## Diagnosis

**Clinical**
Myoclonus with epilepsy, ataxia and myopathy, especially with similar problems along the maternal line.

**Tests**
High blood and cerebrospinal fluid lactate; mtDNA testing for m.8344A>G in blood, urine or muscle; muscle biopsy showing ragged-red fibres and cytochrome c oxidase-deficient fibres.

**Imaging**
Brain MRI may show cerebellar atrophy.

## Treatment

**Medical**
Antiseizure treatment, choosing drugs suitable for mitochondrial disease.

**Rehabilitation**
Physical therapy and hearing aids.

**Follow-up**
Cardiac and audiological follow-up; genetic counselling for maternal relatives.

## Complications

- Progressive neurological disability
- Cardiomyopathy and conduction problems

## Prognosis

Variable and usually progressive; severity depends largely on the mutant load in each tissue.

## Test yourself

Think of the answer, then tap the question to see it.

## Which gene and variant cause most cases of MERRF?

m.8344A>G in MT-TK, the mitochondrial transfer RNA for lysine.

## What are ragged-red fibres?

Muscle fibres with accumulated abnormal mitochondria beneath the membrane, seen with a special stain on biopsy.

## Why do affected relatives of the same mother differ in severity?

Heteroplasmy: they carry different proportions of mutant mtDNA in their tissues.

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## Connections

## Related conditions 1

- [MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes)](https://durgical.com/en/conditions/melas/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Myoclonic epilepsy with ragged-red fibers: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/myoclonic-epilepsy-with-ragged-red-fibers/) (MedlinePlus, US National Library of Medicine, 2026); [MT-TK gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/mt-tk/) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/conditions/merrf/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/merrf/