The development of dementia is not considered a normal consequence of contracting Parkinson’s disease, but the probability of developing dementia is higher in those with the disease. Estimating what percentage of Parkinsonians might be expected to develop a diagnosis of dementia is a difficult task. Estimates vary greatly in the literature. It is difficult to know what estimate to accept since the diagnosis of dementia in Parkinsonian patients has often not been done with the same care as in those cases where dementia is suspected as the primary diagnosis. Choosing only the largest studies and the ones using the most reliable criteria, we arrive at an estimate of 20 percent. It must be noted, however, that about 10 percent of those classified as demented in these studies were considered to have a fairly mild dementia. Furthermore, not very much is known about the progressive or global nature of dementia in Parkinson’s disease. In Alzheimer’s disease the decline is progressive and global in nature, eventually touching all aspects of cognitive functioning. The same kind of information is not available for Parkinsonism.
Dementia is a clinical-behavioral syndrome that needs to be carefully diagnosed to exclude treatable disorders such as depression, and to include definite functional disability and loss of orientation to time, place, and person. The prominent clinical signs of dementia include the global impairment of higher cortical functions affecting memory, judgment, initiation of action, social functioning, control of emotion, and the ability to carry out everyday tasks. While memory loss is often a cardinal symptom of dementia, it is neither a declaration of dementia nor the sole justification for a dementia diagnosis.
Overall it is estimated that there are potentially sixty disorders that can result in dementia syndromes. About 10 to 20 percent of these disorders are treatable and involve metabolic disorders, toxins, endocrine disorders, space occupying lesions, and psychiatric problems. If dementia is suspected, a neurologist should be consulted to determine if there is a need for additional medical treatment. By far the dominant physical condition leading to dementia is Alzheimer’s disease. It is estimated that about 50 percent of those with dementia will have Alzheimer’s disease, another 10 percent will have a combination of Alzheimer’s disease and multi-infarct disease, and 10 percent will have multi-infarct disease without dementia. The remaining 30 percent can be attributed to other conditions. Typically, the diagnosis of dementia, particularly Alzheimer’s disease, is in part one of exclusion. That is, conditions need to be excluded before the diagnosis of Alzheimer’s disease can be used.
It is not clear if the diagnosis of dementia that accompanies Parkinson’s disease should be considered a separate type of dementia, or if it is actually Alzheimer’s disease. The physical changes in the brain tissue of those diagnosed with Alzheimer’s disease, such as neurofibrillary tangles, have also been found in the brain tissue of some Parkinsonian patients who have dementia. At this time, these physical manifestations of Alzheimer’s disease are not thought to occur in all individuals with Parkinson’s disease, only those of a particular subtype who develop dementia. Currently, there is no way of knowing which course Parkinson’s disease will take: none of its clinical symptoms are predictive of dementia versus nondementia. While it has been suggested that the drugs used to treat the motor symptoms of Parkinsonism might be a cause of dementia, there is no evidence to support this notion. Occasionally, treatment with anticholinergics might result in a disturbance of short-term secondary memory, but these are temporary rather than longterm effects.