Alzheimer's disease and its focal variants.

Alzheimer's disease (AD) is the most common form of dementia in elderly individuals. Although the diagnosis of AD requires pathological confirmation, several common clinical features of AD have been identified. These include insidious onset, gradual progression, medial temporal atrophy, temporoparietal hypoperfusion, early and prominent deficits in episodic memory and mental tracking, and later deficits in semantic memory. These characteristic features of AD have enabled excellent in vivo diagnostic sensitivity. It has become increasingly clear, however, that AD is a heterogeneous disorder. The left and right cerebral hemispheres can be affected to varying degrees, and variability on the anterior-posterior axis has also been reported. AD can also present as a focal degenerative disease, and clinicians must be aware that there may be diagnostic overlap between AD, primary progressive aphasia, posterior cortical atrophy, corticobasal degeneration, and frontotemporal dementia.