Alzheimer's disease is the single most common cause of dementia, a chronically progressive brain condition that impairs intellect and behavior to the point where customary activities of daily living become compromised. Over 4 million Americans have Alzheimer's disease. Its high prevalence may lead people to believe that dementia is always due to Alzheimer's disease and that memory loss is a feature of all dementias.
However, an article by Alzheimer's disease expert M.-Marsel Mesulam, M.D., in the Oct. 16 issue of The New England Journal of Medicine reports that nearly a quarter of all dementias, especially those of presenile onset, may be caused by diseases other than Alzheimer's disease and that some of these so-called atypical dementias involve cognitive abnormalities in areas other than memory.
Mesulam is Ruth and Evelyn Dunbar Professor of Psychiatry and Behavioral Sciences and professor of neurology at the Feinberg School of Medicine and director of the Cognitive Neurology and Alzheimer's Disease Center at Northwestern University.
Mesulam described, for example, primary progressive aphasia, an unusual dementia of unknown cause that is characterized by a relentless loss of language but with memory relatively preserved. Once considered a rare condition, primary progressive aphasia is now commonly included among dementia syndromes and has been reported in several hundred individuals.
Alzheimer's disease patients have forgetfulness, usually accompanied by apathy. They misplace personal objects, repeat questions and forget recent events. However, while these patients may forget people's names, word-finding during conversation is not a major problem.
In contrast, patients with primary progressive aphasia come to medical attention because of the onset of word-finding difficulties, abnormal speech patterns and glaring spelling errors. Some patients cannot find the right words to express their thoughts. Others cannot understand the meaning of words either heard or seen. Still others cannot name objects in their environment.
In some patients with primary progressive aphasia, the ability to write language may be less impaired than the ability to speak it. Others develop agrammatism, using inappropriate word order and misusing word endings, prepositions, pronouns, conjunctions and verb tenses.
Language is the only area of prominent dysfunction for at least the first two years of primary progressive aphasia. In these patients, structural brain imaging studies do not reveal a specific lesion, other than atrophy, that can account for the language deficit, Mesulam said. Language difficulties may be the patient's only symptoms for 10 to 14 years. Other cognitive impairments may emerge, but the language deficit remains the primary feature throughout the illness and progresses more rapidly than deficits in other areas.
Also in contrast to many patients with Alzheimer's disease, who tend to lose interest in recreational and social activities, some individuals with primary progressive aphasia maintain and even intensify their involvement in complex hobbies such as gardening, carpentry, sculpting and painting. One patient Mesulam described continued to fly his airplane until aphasia prevented him from communicating with ground control.
In patients with suspected primary progressive aphasia, evaluation by a speech therapist is useful for exploring alternative communication strategies, Mesulam said. Unlike patients with Alzheimer's disease, who cannot retain new information in memory, patients with primary progressive aphasia can recall and evaluate recent events even though they may not be able to express their knowledge verbally.
Currently, there is no effective pharmacologic treatment for primary progressive aphasia. However, from the vantage point of research, the condition provides a rare opportunity for investigating the molecular mechanisms of focal neurodegeneration and the neuropsychological organization of language function, Mesulam said.