C.	Complicated hereditary and sporadic cerebellar ataxias (later-onset ataxia with brainstem and other neurologic disorders) 1. Multiple system atrophies (MSA-C) 2. Dentatorubral degeneration (Ramsay Hunt type) 3. Dentatorubropallidoluysian atrophy 4. Machado-Joseph disease; SCA-3 (ataxia, basal ganglia features) 5. Other complicated late-onset, autosomal dominant ataxias with pigmentary retinopathy, ophthalmoplegia, slow eye movements, polyneuropathy, optic atrophy, deafness, extrapyramidal features, and dementia
V. Syndrome of slowly developing muscular weakness and atrophy
A.	Motor disorders with amyotrophy 1. Amyotrophic lateral sclerosis 2. Progressive spinal muscular atrophy 3. Progressive bulbar palsy 4. Kennedy syndrome and other hereditary forms of progressive muscular atrophy and spastic paraplegia 5. Motor neuron disease with frontotemporal dementia
B.	Spastic paraplegia without amyotrophy 1. Primary lateral sclerosis 2. Hereditary spastic paraplegia (Strümpell
VI. Sensory and sensorimotor disorders (neuropathies; see Chap. 43)
A.	Hereditary sensorimotor neuropathies—
B.	Pure or predominantly sensory or motor neuropathic
C.	Riley-Day autonomic degeneration
VII. Syndrome of progressive blindness with or without other neurologic disorders (see Chap. 12)
A. Pigmentary degeneration of retina (retinitis pigmentosa)
B. Stargardt disease
C. Age-related macular degeneration (ARMD)
VIII. Syndromes characterized by degenerative neurosensory deafness (see Chap. 14)
A. Pure neurosensory deafness
B. Hereditary hearing loss with retinal diseases
C. Hereditary hearing loss with system atrophies of the nervous system
This is the most common and important degenerative disease of the brain, having an immense societal impact. Some aspects of the intellectual deterioration that characterize this disease were described in Chap. 20, under “The Neurology of Dementia,” and the still ambiguous relationship of this disease to the aging process is mentioned above and in Chap. 28. There it was pointed out that some degree of shrinkage in size and weight of the brain, that is “atrophy,” is an inevitable accompaniment of advancing age, but that these changes alone are of relatively slight clinical significance and uncertain structural basis (e.g., whether the loss of brain weight aging is the result of a simple depletion of neurons). By contrast, severe degrees of diffuse cerebral atrophy that evolve over a few years are associated with dementia, and the underlying pathologic changes in these cases most often prove to be those of Alzheimer disease. As also commented on in Chap. 28, the rate of cerebral atrophy, specifically of the hippocampus and medial parts of the temporal lobes, is accelerated in the early stages of Alzheimer disease, and longitudinal studies by magnetic resonance imaging can identify individuals who will subsequently develop the disease (Rusinick). Nevertheless, there is not a continuous increase in the deposition of plaques and tangles, the pathologic markers of Alzheimer disease, with increasing age.