The number of neurodegenerative diseases is currently estimated to be a few hundred, and, among these, many appear to overlap with one another clinically and pathologically, rendering their practical classification quite challenging. The issue is further complicated by the fact that, in diseases such as multisystem atrophy in which several areas of the brain are affected, different combinations of lesions can give rise to different clinical pictures (2). Furthermore, the same neurodegenerative process, especially at the beginning, can affect different areas of the brain, making a given disease appear very different from a symptomatic standpoint. Despite these difficulties, the most popular categorization of neurodegenerative disorders is still based on the predominant clinical feature or the topography of the predominant lesion, or often on a combination of both. Accordingly, neurodegenerative disorders of the CNS may, for example, be first grouped into diseases of the cerebral cortex, the basal ganglia, the brainstem and cerebellum, or the spinal cord. Then, within each group, a given disease may be further classified based on its main clinical features. For instance, the group of diseases that predominantly affect the cerebral cortex may be divided into dementing (eg, AD) and nondementing conditions. Of note, while AD is by far the most frequently cited cause of dementing cerebral cortex pathology (3), dementia can apparently be observed in at least 50 different diseases (4). Moreover, dementia is not exclusively observed in neurodegenerative disorders; it is also frequently observed in ischemic, metabolic, toxic, infectious, and traumatic insults of the brain.

Diseases that predominantly involve the basal ganglia (a series of deep nuclei situated at the base of the forebrain, including the caudate nucleus putamen, globus pallidus, substantia nigra, subthalamic nucleus, red nucleus, and some thalamic and brainstem nuclei) are essentially characterized by abnormal movements. Yet, based on the phenomenology of the abnormal movements, diseases of the basal ganglia can be classified as hypokinetic or hyperkinetic. Hypokinetic basal ganglia disorders are epitomized by PD, in which the amplitude and velocity of voluntary movements are diminished or, in extreme cases, even nonexistent, causing the patient to become a prisoner within his or her own body. Aside from PD, parkinsonism—which refers to an association of at least two of the following clinical signs: resting tremor, slowness of movements, stiffness, and postural instability—is found in a variety of other diseases of the basal ganglia as well. In some (eg, striatonigral degeneration), there is only parkinsonism, but in others, often called parkinson-plus syndromes, there is parkinsonism plus signs of cerebellar ataxia (eg, olivopontocerebellar atrophy), orthostatic hypotension (eg, Shy-Drager syndrome), or paralysis of vertical eye movements (eg, progressive supranuclear palsy). Because, early on, parkinsonism may be the only