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