Despite distinct clinical features, SBMA is often misdiag-nosed due to the heterogeneity of manifestations and lack of full expression in some family members. Many of the cases are sporadic, and some patients have only mild signs of motor neuron disease and mild gynecomastia. Amytrophic lateral sclerosis, type III SMA, hereditary motor and sensory neuropathy, limb-girdle muscular dystrophy, and facioscapulohumeral muscular dystrophy are included in the differential diagnosis for some patients affected with SBMA.43 The PCR amplification of the repeat sequence within the first exon of the AR gene provides an accurate confirmation of the diagnosis of SBMA, distinct from other neuromuscular disorders. In addition, carrier females and young asymptomatic males may be identified by molecular testing for a repeat expansion with implications for genetic counseling and potential early treatment.
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