There are two major forms:
1. Encephalomyopathy: These infants are typically normal for the first 6 to 12 months of life, then start showing features of Leigh syndrome, including developmental regression, ataxia, lactic acidosis, optic atrophy, ophthalmoplegia, nystagmus, dystonia, pyramidal signs, and respiratory problems.
2. Myopathy: Two main variants:
1. Fatal infantile myopathy: may begin soon after birth and is accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory failure, and kidney problems.
2. Reversible infantile myopathy: may begin soon after birth and is accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory problems, but (if the child survives) is followed by spontaneous improvement and complete recovery.
Cause: The fatal infantile form is probably autosomal recessive. The reversible form is maternally inherited and due to a mutation in the tRNAGlu of mtDNA. The mutation is homoplasmic, i.e. it affects all copies of mtDNA.