The widespread availability of genetic testing will provide a more accurate estimate in the next several years. Onset of the disease is variable from 15 to 59 years of age, although most commonly these patients seek medical attention between the fourth and fifth decade of life. Most authors agree that the clinical features include muscle weakness, atrophy, and fasciculations. The proximal muscle groups are affected at an earlier stage, subsequently followed by atrophy and weakness of the intrinsic muscles of the hands and peroneal muscles. Weakness and fasciculations of the oro-mandibular musculature, as well as perioral fasciculations, are present early and are subsequently followed by facial weakness, atrophy, and fibrillations of the tongue.
Some authors also describe a nasal component to the speech, most likely due to facial weakness.
Deep tendon reflexes are usually absent. Sensation may be abnormal. Gynecomastia is present in many of the cases, but can be quite variable, as are testicular atrophy and reduced fertility. EMG and muscle biopsy typically show chronic neurogenic atrophy with reinnervation. PCR analysis revealed amplification of CAG triplet nucleotide repeats.