Alzheimer’s disease (AD) has long been recognized as a heterogeneous illness, with a common clinical presentation of progressive amnesia and less common “atypical” clinical presentations, including syndromes dominated by visual, aphasic, “frontal,” or apraxic symptoms. Our knowledge of atypical clinical phenotypes of AD comes from clinicopathologic studies, but with the growing use of in vivo molecular biomarkers of amyloid and tau pathology, we are beginning to recognize that these syndromes may not be as rare as once thought. When a clinician is evaluating a patient whose clinical phenotype is dominated by progressive aphasia, complex visual impairment, or other neuropsychiatric symptoms with relative sparing of memory, the differential diagnosis may be broader and a confident diagnosis of an atypical form of AD may require the use of molecular biomarkers. Despite the evolving sophistication in our diagnostic tools, and the acknowledgment of atypical AD syndromes in the 2011 revised diagnostic criteria for AD, the assessment of such patients still poses substantial challenges. We use a case-based approach to review the clinical and imaging phenotypes of a series of patients with typical and atypical AD, and discuss our current approach to their evaluation. One day, we hope that regardless of whether a patient exhibits typical or atypical symptoms of AD pathology, we will be able to identify the condition at a prodromal phase and institute a combination of symptomatic and disease-modifying therapies to support cognitive processes, function, and behavior, and slow or halt progression to dementia.

A group of neurodegenerative diseases is outlined that affect cortical and subcortical areas of the brain. These diseases give rise to atypical forms of dementia and, unlike Alzheimer's disease (AD), are often associated with neurological symptoms. Clinical symptoms reflect the localization of the degenerative process rather than the nature of the underlying histopathology. Degeneration of the frontal and anterior temporal lobe presents initially with behavioral alterations, but later in the course, impairment of cognition and activities of daily living develops. Posterior cortical atrophy affects the parietal and occipital association cortices and causes complex visual disturbances. In corticobasal degeneration (CBD) the focus of pathology includes the frontoparietal cortex and several subcortical nuclei, causing symmetrical rigidity, bradykinesia, myoclonus and dystonia. Progressive supranuclear palsy (PSP) involves the frontal, temporal and parietal cortex as well as parts of the brain stem. Clinical features include a hypokinetic rigid syndrome with nuchal dystonia and vertical gaze palsy. Huntington's disease is a prototypical autosomal dominant disorder that affects the extrapyramidal system and causes choreatic movements in combination with personality changes and cognitive deterioration. Amyotrophic lateral sclerosis (ALS) with dementia is a neurodegeneration of the frontotemporal cortex and of the anterior horn of the spinal cord. Behavioral change similar to frontotemporal dementia (FTD) is paralleled or followed by the classic features of motor neuron disease.

A patient with progressive posterior cortical atrophy (PCA) was examined on several tests of visual cognition. The patient displayed multiple visual cognitive deficits, which included problems identifying degraded stimuli, attending to two or more stimuli simultaneously, recognizing faces, tracing simple visual stimuli, matching simple shapes, and copying objects. The patient was also impaired in identifying visual targets contained at the global level within global–local stimuli (i.e., smaller letters that compose a larger letter). Although the patient denied any conscious awareness of the global form, he nevertheless displayed a normal pattern of global interference when asked to identify local level targets. Thus, the patient processed the global information despite not being consciously aware of such information. These results suggest that global–local processing can take place in the absence of awareness. Possible neurocognitive mechanisms explaining this dissociation are discussed. (JINS, 2002, 8, 461–472.)