This page contains information about the nine most common mitochondrial myopathies and encephalomyopathies. They are:

• Kearns-Sayre syndrome (KSS)
• Leigh syndrome and maternally-inherited Leigh syndrome (MILS)
• Mitochondrial DNA depletion syndrome (MDS)
• Mitochondrial encephalomyopathy, lactic acidosis and strokelike episodes (MELAS)
• Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)
• Myoclonus epilepsy with ragged red fibers (MERRF)
• Neuropathy, ataxia and retinitis pigmentosa (NARP)
• Pearson syndrome
• Progressive external ophthalmoplegia (PEO)

Kearns-Sayre syndrome (KSS)

Onset: Before age 20

Symptoms: This disorder is defined by PEO and pigmentary retinopathy, a “salt-and-pepper” pigmentation in the retina that can affect vision, but often leaves it intact. Other common symptoms include conduction block (in the heart) and ataxia. Less typical symptoms are mental retardation or deterioration, delayed sexual maturation and short stature.

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Leigh syndrome (subacute necrotizing encephalomyopathy) and maternally inherited Leigh syndrome (MILS)

Onset: Infancy

Symptoms: Leigh syndrome causes brain abnormalities that can result in ataxia, seizures, impaired vision and hearing, developmental delays and altered control over breathing. It also causes muscle weakness, with prominent effects on swallowing, speech and eye movements.

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Mitochondrial DNA depletion syndrome (MDS)

Onset: Infancy

Symptoms: This disorder typically causes muscle weakness and/or liver failure, and more rarely, brain abnormalities. “Floppiness,” feeding difficulties and developmental delays are common symptoms; PEO and seizures are less common.

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Mitochondrial encephalomyopathy, lactic acidosis and strokelike episodes (MELAS)

Onset: Childhood to early adulthood

Symptoms: MELAS causes recurrent strokelike episodes in the brain, migraine-type headaches, vomiting and seizures, and can lead to permanent brain damage. Other common symptoms include PEO, general muscle weakness, exercise intolerance, hearing loss, diabetes and short stature.

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Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)

Onset: Usually before age 20

Symptoms: This disorder causes PEO, ptosis (droopy eyelids), limb weakness and gastrointestinal (digestive) problems, including chronic diarrhea and abdominal pain. Another common symptom is peripheral neuropathy (a malfunction of the nerves that can lead to sensory impairment and muscle weakness).

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Myoclonus epilepsy with ragged red fibers (MERRF)

Onset: Late childhood to adolescence

Symptoms: The most prominent symptoms are myoclonus (muscle jerks), seizures, ataxia and muscle weakness. The disease also can cause hearing impairment and short stature.

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Neuropathy, ataxia and retinitis pigmentosa (NARP)

Onset: Infancy to adulthood

Symptoms: NARP causes neuropathy (a malfunction of the nerves that can lead to sensory impairment and muscle weakness), ataxia and retinitis pigmentosa (degeneration of the retina in the eye, with resulting loss of vision). It also can cause developmental delay, seizures and dementia.

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Pearson syndrome

Onset: Infancy

Symptoms: This syndrome causes severe anemia and malfunction of the pancreas. Children who survive the disease usually go on to develop Kearns-Sayre syndrome.

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Progressive external ophthalmoplegia (PEO)

Onset: Usually in adolescence or early adulthood

Symptoms: PEO — the gradual paralysis of eye movements — is often a symptom of mitochondrial disease, but sometimes it stands out as a distinct syndrome. It’s frequently associated with exercise intolerance.

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