The neuropathy, ataxia, retinitis pigmentosa (NARP) syndrome is due to a point mutation at bp8993 in the ATPase 6 gene (MTATP6*NARP8993), which changes a highly conserved leucine at position 156 to an arginine. The exact incidence is unknown, but this is a rare, functionally recessive, with variable systemic expression, adult-onset progressive disease. Presentation is with neuropathy or neurogenic weakness, ataxia, and pigmentary retinopathy. It can also be associated with dementia, generalized seizures, cerebellar and brain stem atrophy, and corticospinal degeneration. The first presentation is usually nyctalopia, followed by loss of peripheral vision. The differential diagnosis includes multisystem atrophy, familial ataxias in mid to late life, any other causes of retinitis pigmentosa, such as adult-onset neuronal ceroid lipofuscinosis, and peroxisomal disorders such as Refsum's disease. The evaluation includes specific analysis for MTATP6*NARP8993 and MTTL1* MELAS3243. Results of metabolic and muscle investigations are usually normal or slightly abnormal, with lactic acidemia only in the more severe cases.
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