MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is a multisystem disorder with protean manifestations. The vast majority of affected individuals develop signs and symptoms of MELAS between ages two and 40 years. Common clinical manifestations include stroke-like episodes, encephalopathy with seizures and/or dementia, muscle weakness and exercise intolerance, normal early psychomotor development, recurrent headaches, recurrent vomiting, hearing impairment, peripheral neuropathy, learning disability, and short stature. During the stroke-like episodes neuroimaging shows increased T2-weighted signal areas that do not correspond to the classic vascular distribution (hence the term "stroke-like"). Lactic acidemia is very common and muscle biopsies typically show ragged red fibers.

An elevation of the level of the enzyme creatine kinase (also known as creatine phosphokinase (CK; EC 2.7.3.2) in the blood. CK levels can be elevated in a number of clinical disorders such as myocardial infarction, rhabdomyolysis, and muscular dystrophy.

Mitochondrial DNA-associated Leigh syndrome spectrum (mtDNA-LSS) is part of a continuum of progressive neurodegenerative disorders caused by abnormalities of mitochondrial energy generation, which includes the overlapping phenotypes mtDNA-associated Leigh syndrome and mtDNA-associated Leigh-like syndrome. Mitochondrial DNA-LSS is characterized by onset of manifestations typically between ages three and 12 months, often following an intercurrent illness (usually viral) or metabolic challenge (vaccinations, surgery, prolonged fasting). Decompensation (often with elevated lactate levels in blood and/or cerebrospinal fluid) is typically associated with developmental delay and/or regression. Neurologic features include hypotonia, spasticity, seizures, movement disorders, cerebellar ataxia, and peripheral neuropathy. Brain stem dysfunction may manifest with respiratory symptoms, swallowing difficulties, ophthalmoparesis, and abnormalities in thermoregulation. Extraneurologic manifestations may include poor weight gain, cardiomyopathy, and conduction defects. Up to 50% of individuals die by age three years, most often from respiratory or cardiac failure.

A rare, genetic, mitochondrial DNA-related mitochondrial myopathy disorder characterized by slowly progressive muscular weakness (proximal greater than distal), predominantly involving the facial muscles and scapular girdle, associated with insulin-dependent diabetes mellitus. Neurological involvement and congenital myopathy may be variably observed.

Hereditary myopathy with lactic acidosis (HML) is an autosomal recessive muscular disorder characterized by childhood onset of exercise intolerance with muscle tenderness, cramping, dyspnea, and palpitations. Biochemical features include lactic acidosis and, rarely, rhabdomyolysis. It is a chronic disorder with remission and exacerbation of the muscle phenotype (summary by Sanaker et al., 2010).

A rare metabolic myopathy presenting during childhood, and characterized clinically by growth failure, severe muscle weakness, and moderate sensorineural deafness and biochemically by metabolic acidosis, elevated serum pyruvate concentration, hyperalaninemia and hyperalaninuria. There have been no further descriptions in the literature since 1973.

Sengers syndrome is an autosomal recessive mitochondrial disorder characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, exercise intolerance, and lactic acidosis. Mental development is normal, but affected individuals may die early from cardiomyopathy (summary by Mayr et al., 2012). Skeletal muscle biopsies of 2 affected individuals showed severe mtDNA depletion (Calvo et al., 2012).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Infantile mitochondrial myopathy due to reversible COX deficiency is a rare mitochondrial disorder characterized by onset in infancy of severe hypotonia and generalized muscle weakness associated with lactic acidosis, but is distinguished from other mitochondrial disorders in that affected individuals recover spontaneously after 1 year of age (summary by Mimaki et al., 2010). See also transient infantile liver failure (LFIT; 613070), which is a similar disorder.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Any autosomal recessive progressive external ophthalmoplegia in which the cause of the disease is a mutation in the TK2 gene.

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease is characterized by progressive gastrointestinal dysmotility (manifesting as early satiety, nausea, dysphagia, gastroesophageal reflux, postprandial emesis, episodic abdominal pain and/or distention, and diarrhea); cachexia; ptosis/ophthalmoplegia or ophthalmoparesis; leukoencephalopathy; and demyelinating peripheral neuropathy (manifesting as paresthesias (tingling, numbness, and pain) and symmetric and distal weakness more prominently affecting the lower extremities). The order in which manifestations appear is unpredictable. Onset is usually between the first and fifth decades; in about 60% of individuals, symptoms begin before age 20 years.

Episodic mitochondrial myopathy with or without optic atrophy and reversible leukoencephalopathy (MEOAL) is an autosomal recessive neuromuscular disorder characterized mainly by childhood onset of progressive muscle weakness and exercise intolerance. Patients have episodic exacerbation, which may be associated with increased serum creatine kinase or lactic acid. Additional more variable features may include optic atrophy, reversible leukoencephalopathy, and later onset of a sensorimotor polyneuropathy. The disorder results from impaired formation of Fe-S clusters, which are essential cofactors for proper mitochondrial function (summary by Gurgel-Giannetti et al., 2018)