Diagnostic Process

A number of medical and psychiatric conditions may cause or contribute to cognitive impairment in older adults. It is vital to identify and aggressively treat any potentially reversible causes of cognitive impairment. The table below lists a number of reversible causes of cognitive impairment.

Potentially Reversible Causes of Cognitive Impairment in the Elderly

  • Nutritional deficiency states (B12 thiamine)
  • Infectious (encephalitis, HIV)
  • Metabolic (hypothyroidism, hypo/hyperglycemia, uremia, anoxia)
  • Psychiatric (depression, psychosis)
  • Malignancy
  • Alcoholism
  • Subdural hematoma
  • Hydrocephalus
  • Delirium
  • Heavy metal intoxication
  • Drug toxicity (anticholinergics, benzodiazepines, barbiturates, narcotics, anticonvulsants)

adapted from reference 2

Because of considerable symptom overlap, it can be difficult to differentiate between dementia, depression, and delirium.

   

Dementia Depression Delirium
Level of consciousness Alert Alert Waxes/wanes
Course Chronic Chronic or acute Acute
Other Features   Neuro-vegetative state Medical causes

It is not unusual for delirium to be superimposed on a dementing process, further complicating diagnosis. Clinicians should maintain a high index of suspicion for delirium when assessing significant cognitive decline in a patient with pre-existing dementia. Dementia appears to predispose toward development of delirium often as a result of seemingly inconsequential insults such as urinary tract infections or use of CNS active medications.

Although AD is the most common diagnosis associated with cognitive impairment in the elderly, the etiology of other types of dementing conditions must also be considered during the assessment process. Among the dementing disorders, vascular dementia follows AD as the second most common form. From the Cache county study that identified 324 community dwelling individuals with cognitive impairment, vascular dementia was diagnosed in 19%, in contrast to 65% of subjects identified with AD.

  Alzheimer's Disease
  Vacular Dementia
  Other Dementia
     

source: Cache County Study on Memory and Aging: Prevalence of major types of dementia.4

Vascular dementia is generally characterized by a relatively acute onset that is related to an event such as a CVA or TIA and appears to have a more "step-wise" progression as compared with the gradual decline associated with AD.The Hachinski scale can be used to assess for the likelihood of vascular dementia. Cerebral ischemic pathology may also coexist with AD, resulting in what is referred to as "mixed dementia". This type of diagnosis was thought to be relatively uncommon until the publication of the 1997 Nun Study.6

Other Less Common Dementias
  • Frontal lobe dementia (Picks's Disease)
  • Creutzfeldt-Jakob disease
  • Progressive supranuclear palsy
  • Binswanger's disease
  • Parkinson's disease
  • Huntington's disease

It is often stated that a definitive diagnosis of AD can be made only with a brain biopsy or an autopsy - this statement tends to instill an unnecessary sense of diagnostic nihilism. While it is true that the neuropathological changes associated with AD can only be identified by brain tissue examination, researchers have made significant progress in developing accurate diagnostic tests and criteria for the clinical diagnosis of AD. A growing number of guidelines and diagnostic algorithms are available for use by clinicians; use of standardized criteria is thought to greatly improve diagnostic accuracy. One study demonstrated up to 90% diagnostic accuracy for AD utilizing specialized cognitive testing in combination with medical and neurological evaluation.7

The "gold standard" of diagnostic criteria is the National Institute of Neurological and Related and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association for diagnosis of possible and probable AD.

The Diagnostic and Statistical Manual (DSM IV) criteria are also commonly used in clinical practice.

DSM IV Diagnostic criteria for dementia of the Alzheimer's type
  1. The development of multiple cognitive deficits manifested by both
    1. memory impairment (impaired ability to learn new information or to recall previously learned information) and
    2. one (or more) of the following cognitive disturbances:
        1. aphasia (language disturbance);
        2. apraxia (impaired ability to carry out motor activities despite intact motor function);
        3. agnosia (failure to recognize or identify objects despite intact sensory function);
        4. disturbance in executive functioning (i.e., planning, organizing, sequencing, abstracting)
  2. The cognitive deficits in Criteria A1 and A2 each cause significant impairment in social or occupational functioning and represent a significant decline from a previous level of functioning.
  3. The course is characterized by gradual onset and continuing cognitive decline.
  4. The cognitive deficits in Criteria A1 and A2 are not due to any of the following:
    1. other central nervous system conditions that cause progressive deficits in memory and cognition (e.g., cerebrovascular disease, Parkinson's disease, Huntington's disease, subdural hematoma, normal-pressure hydrocephalus, brain tumor);
    2. systemic conditions that are known to cause dementia (e.g., hypothyroidism, vitamin B 12 or folic acid deficiency, niacin deficiency, hypercalcemia, neurosyphilis, HIV infection);
    3. substance-induced conditions
  5. The deficits do not occur exclusively during the course of a delirium.
  6. The disturbance is not better accounted for by another Axis 1 disorder
    (e.g., Major Depressive Disorder, Schizophrenia)

The diagnostic evaluation for dementia includes the following components: