The term dementia has an intellectually constricting effect on differential diagnostic considerations and should be applied with care. A confused elderly person seen for the first time might be described as demented but may in fact be confused because of toxicity from a narcotic analgesic received postoperatively. A more generic term that is less diagnostically constricting is encephalopathy. Encephalopathy can be acute, subacute, or chronic, and there are reversible and irreversible etiologies. Dementia is a type (or types) of chronic encephalopathy that can have many causes, including irreversible degenerative and potentially reversible nondegenerative causes.

The most important diagnostic step in evaluating dementias is to determine whether a chronic encephalopathy results from a degenerative or other potentially reversible cause. Clues may exist at any level of the diagnostic ladder, although typically only a subset of clues is found in any one patient. Historical clues suggesting a reversible process include fluctuating severity, altered level of consciousness or hypersomnolence, and visual hallucinations. Clues on mental status testing include finding the patient to be inattentive, disoriented, and somnolent but not particularly amnesic. Clues on physical examination include a variety of findings that may be common in elderly patients but are not part of the typical picture of Alzheimer's disease (AD) such as ataxia, hyperreflexia, and tremulousness.

Laboratory evaluation should be directed toward common medical ailments, and a reasonably recent complete physical evaluation should be available or obtained in all patients suspected of having a significant brain-related disease. Typical evaluations include thyroid function studies, vitamin B 12 or related tests, and serological tests for syphilis, even though the latter two have become relatively uncommon in the present day as causes of chronic progressive encephalopathy. When a nondegenerative-based chronic progressive encephalopathy is suspected, electroencephalography (EEG) may be helpful in demonstrating severe dysrhythmic slowing. Cerebrospinal fluid (CSF) examination may show an elevated IgG index and synthesis rate, and occasionally oligoclonal bands, suggesting an intrathecal inflammatory reaction. Autoimmune and paraneoplastic serologies may be helpful in such patients as well. Ultimately, if suspicion remains high based on noninvasive tests for a nondegenerative cause, cerebral angiography and meningeal and brain biopsy should be considered. In the absence of such invasive testing, an empirical therapeutic trial of prednisone could be considered in order to be certain that the steroid-responsive type of chronic inflammatory meningoencephalitis (CIME) does not exist.

If a degenerative disease is ultimately diagnosed, then it must be realized that an incurable, invariably progressive, and ultimately fatal condition has been diagnosed.

Chronic progressive encephalopathies can be divided into potentially reversible and irreversible diseases. More important than remembering a few specific diseases is to keep pathophysiological categories in mind. Some reversible causes of subacute and chronic widespread progressive encephalopathy include

1. inflammatory: CIME, sarcoidosis, primary and secondary central nervous system (CNS) vasculitides, CNS complications of systemic lupus erythematosus, and paraneoplastic limbic encephalitis

2. infectious: chronic meningitis due to fungi, tuberculosis, Listeria monocytogenes, Lyme disease, syphilis, CNS Whipple's disease.

3. nutritional: vitamin B12 deficiency.

4. toxic: drugs (particularly prescription medications)

5. mass lesion: subdural hematoma, communicating (normal pressure) hydrocephalus, meningioma and other tumors

6. complex partial status epilepticus Some irreversible causes of chronic widespread progressive encephalopathy include

1. degenerative: AD, Pick's disease, asymmetrical cortical degeneration syndromes (ACDS), diffuse Lewy body dementia (DLBD), progressive supranuclear palsy (PSP), Huntington's disease (HD), Parkinson's disease (PD).

2. vascular: multi-infarct dementia, disseminated intravascular coagulation

3. metabolic: storage diseases, leukodystrophies

4. neoplastic: meningeal metastases, gliomatosis cerebri

With this perspective in mind, this chapter considers the subset of irreversible causes of chronic progressive encephalopathies referred to as neurodegenerative dementing diseases, which affect adults. They fall into three broad categories: cortical, subcortical, and mixed. Essentially all neurodegenerative dementias have both cortical and subcortical pathology, but those that primarily target the cerebral cortex are clinically distinguishable from those that primarily target subcortical structures, so the distinction has both heuristic and practical value. Finally, there are some diseases that have a more balanced cortical- subcortical mix pathologically and clinically.

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