Chronic progressive external ophthalmoplegia (cPEO)
Onset: usually in adolescence or early adulthood
Features: PEO is often a symptom of mitochondrial disease. In some people, it is a chronic, slowly progressive condition associated with instability to move the eyes and general weakness and exercise intolerance.
Inheritance pattern: autosomal, but may occur sporadically
Kearns-Sayre syndrome (KSS)
Onset: before age 20
Features: PEO (usually as the initial symptom) and pigmentary retinopathy, a “salt-and-pepper” pigmentation in the retina that can affect vision. Other common symptoms include cardiomyopathy, conduction block (a type of cardiac arrhythmia) ataxia, short stature, neuropathy, and deafness.
Inheritance pattern: autosomal (mostly sporadic)
Leigh syndrome
Onset: infancy (3-24 months) or early childhood
Features: Brain abnormalities that can result in abnormal muscle tone, ataxia, seizures, impaired vision and hearing, developmental delays, and respiratory problems. Infants with the disease have a poor prognosis.
Inheritance pattern: maternal, autosomal recessive, X-linked
Note: when inherited through mtDNA, it may be called MILS or Maternally Inherited Leigh Syndrome
Mitochondrial DNA depletion syndromes (MDDS)
Onset: infancy to adulthood
Features: A myopathic form of MDDS is characterized by weakness that eventually affects the respiratory muscles. Some forms of MDDS, such as Alpers syndrome, are marked by brain abnormalities and progressive liver disease. The anticonvulsant sodium valproate should be used with caution in children with Alpers syndrome because it can increase the risk of liver failure.
Inheritance pattern: autosomal
Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS)
Onset: childhood to early adulthood – Sometimes onset may be as late as 40 years old
Features: The hallmarks of MELAS are encephalomyopathy with seizures and/or dementia, lactic acidosis, and recurrent stroke-like episodes. These episodes are not typical strokes, which are interruptions in the brain’s blood supply that cause sudden neurological symptoms. However, the episodes can produce stroke-like symptoms in the short term (such as temporary vision loss, difficulty speaking, or difficulty understanding speech) and lead to progressive brain injury. The cause of the stroke-like episodes is unclear.
Inheritance pattern: maternal
Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)
Onset: infancy to adulthood, usually before age 20
Features: This disorder is characterized by PEO, ptosis, limb weakness, and gastrointestinal (digestive) problems, including vomiting, 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).
Inheritance pattern: autosomal recessive
Myoclonus epilepsy with ragged red fibers (MERRF)
Onset: late childhood to adolescence
Features: The most prominent symptoms of MERRF are myoclonus (muscle jerks), seizures, ataxia, and muscle weakness. The disease also can cause hearing impairment and short stature.
Inheritance pattern: maternal
Neuropathy, ataxia, and retinitis pigmentosa (NARP)
Onset: infancy to adulthood
Features: NARP is caused by an mtDNA mutation that is also linked to MILS, and the two syndromes can occur in the same family. In addition to the core symptoms for which it is named, NARP can involve developmental delay, seizures, and dementia. (Retinitis pigmentosa refers to a degeneration of the retina in the eye, with resulting loss of vision).
Inheritance pattern: maternal
Pearson syndrome
Onset: infancy
Features: This syndrome involves severe anemia and malfunction of the pancreas. Children who have the disease usually go on to develop Kearns-Sayre syndrome.
Inheritance pattern: autosomal (often sporadic)