Clinical spectrum is proportional to the amount
of normal and mutant DNA. Onset is in the neonatal period, with mental
deterioration, intention tremor, myoclonic epilepsy, spasticity, ataxia,
muscle weakness and atrophy, myopathy, and sensory neural hearing loss. The
neuropathologic findings are (a) degeneration of dentate nucleus, red
nucleus, globus pallidus, subthalamic nucleus, and pontine tegmentum; (b)
degeneration of the Clarke column (the dorsal nucleus of the spinal cord), spinocerebellar tract, posterior column
and corticospinal tract, and posterior spinal nerve root and sural nerve;
and (c) degeneration of substantia nigra, locus caeruleus, cerebellar
cortex, and inferior olivary nucleus.