Table 3.

Revisited mitochondrial syndromes. Possible definitions for some of the most typical mitochondrial syndromes.

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) Patients with histological, biochemical, and/or molecular evidence of mitochondrial disease who experience stroke-like episodes. Specifically associated features (at least in MELAS due to the m.3243A>G mutation) include lactic acidosis, generalized seizures, cognitive involvement, and hearing loss.

Myoclonic encephalomyopathy with ragged red fibers (MERRF) A mitochondrial syndrome where myoclonus is the prominent clinical feature, and which does not meet the criteria of other well-defined mitochondrial encephalopathic syndromes, including MELAS, Leigh, and Alpers syndromes. Ataxia is a specifically associated feature, differently from epileptic seizures.

Kearns-Sayre Syndrome (KSS) spectrum Ophthalmoparesis and/or ptosis due to a mtDNA single large-scale deletion and at least one of the following features: Ataxia Cardiac conduction defects Cardiomyopathy Cognitive involvement Failure to thrive/short stature Hearing loss Retinopathy

Progressive external ophthalmoplegia (PEO) Ophthalmoparesis and/or ptosis, not fulfilling the criteria for Pearson syndrome nor “KSS spectrum” criteria or other encephalopathic syndromes. “Pure PEO”: isolated ocular myopathy. “PEO plus”: ocular myopathy with other features of neuromuscular involvement.

Primary mitochondrial myopathies (PMM) Genetically defined disorders leading to defects in oxidative phosphorylation affecting predominantly skeletal muscles (not fulfilling the criteria for other more complex syndromes).