Clinical Features

Mitochondrial diseases occur from infancy to childhood, adolescence, and early adulthood. In children, the incidence is estimated at 1 per 11,000. Most diseases are multiple-organ disorders: the brain, retina, skeletal muscles, and heart are affected because of their high energy demand. Diseases with concurrent involvement of the brain and skeletal muscles constitute the group of encephalomyopathies. Commonly encountered manifestations are:

• Ocular—Optic atrophy, pigmentary retinopathy, bilateral extraocular muscle palsy progressing to ophthalmoplegia

• Neurologic—Combinations of movement disorders, cerebellar ataxia, hearing impairment, mental decline, seizures, episodic encephalopathy, and polyneuropathy

• Myopathic—Presence of ragged-red fibers

• Visceral/endocrine—Cardiomyopathy, liver and kidney dysfunctions, diabetes mellitus, and thyroid, parathyroid, and gonadal dysfunctions

• Metabolic—Elevated lactate and pyruvate levels in serum and CSF

Diagnosing mitochondrial diseases requires a panel of metabolic tests, neuroimaging, muscle biopsy, and genetic testing. Basic metabolic tests are for lactate and pyruvate in blood and CSF, amino acids in blood and urine, organic acids in urine, and carnitine in blood. Computed tomography (CT) scans and MRI are helpful in demonstrating calcifications and lesions in gray and white matter. The presence of ragged-red fibers (RRF) in muscle biopsy supports the diagnosis of a mitochon-drial disease, but its absence does not rule it out. Testing for genetic mutations is carried out on blood leucocytes and on DNA in muscle fibers.

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