[2] A feature of these diseases is that they are caused by defects in the mitochondrial genome which is inherited purely from the female parent.

Though less common, infantile onset may occur and may present as failure to thrive, growth retardation and progressive deafness.

Onset in older children typically presents as recurrent attacks of a migraine-like headache, anorexia, vomiting, and seizures.

Increased acidity in the blood can lead to vomiting, abdominal pain, extreme tiredness (fatigue), muscle weakness, loss of bowel control, and difficulty breathing.

MERRF patients may also have hearing loss, visual disturbance secondary to optic atrophy, and short stature.

The characteristic myoclonic seizure in MERRF may help to narrow diagnosis, but genetic testing should be considered to distinguish the two conditions.

[1] Leigh syndrome may also present with progressive neurological deterioration, seizures, and vomiting, mainly in young children.

[5] Some of the genes (MT-ND1, MT-ND5) affected in MELAS encode proteins that are part of NADH dehydrogenase (also called complex I) in mitochondria, that helps convert oxygen and simple sugars to energy.

Less commonly, the disorder results from a new mutation in a mitochondrial gene and occurs in people with no family history of MELAS.

Initial lesions often occur in the occipital or parietal lobes with eventual involvement of the cerebellum, cerebral cortex, basal ganglia, and thalamus.

MR spectroscopy may show an elevated lactate peak in affected and even unaffected brain areas.