Delirium and Dementia both cause severe cognitive problems affecting multiple intellectual abilities, but there are several ways to distinguish them. The most important difference is the clarity of the person’s consciousness. People with Delirium act as if they are in a cloud — it is almost impossible to sustain more than a brief conversation with them without getting the feeling that they have drifted off elsewhere. In contrast, even though the person with Dementia may not know what day it is or even recognize who you are, he is able to remain relatively alert and will stay connected with you during a conversation. Another important difference is in the way the symptoms change over time. A person with Delirium often has a fluctuating Jekyll-and-Hyde quality—whereas the cognitive problems in Dementia are more stable from day to day.
Delirium often occurs on top of a preexisting long-standing Dementia. People with Dementia are the most likely to develop Delirium when confronted with a new infection (urinary tract or respiratory); dehydration; metabolic or electrolyte imbalance; or a change in medication. Therefore, if a loved one with a relatively stable pattern of deficits suddenly becomes severely confused, agitated, and disoriented, you should not necessarily chalk it up to a bad day. A medical evaluation and review of medications may help to determine what may have precipitated the new impairments.
One must also be on the lookout for other psychiatric conditions that compromise intellectual performance. Major Depressive Disorder especially in the elderly can produce severe cognitive impairments that are virtually identical to a Dementia (in fact this is called “pseudodementia”). Successful treatment with antidepressant medications can sometimes result in a complete resolution of what appeared to be an irreversible Dementia.
There are several clues to help differentiate depression and Dementia. Are depressive symptoms present? People with pseudodementia are also much more likely to be distressed by their cognitive problems than those with Dementia, who are often blase or unaware of the extent of the difficulties. A careful history usually indicates that the person functioned well cognitively prior to the depression, as opposed to the typically gradual onset of symptoms in Dementia. Sometimes, however, the late onset of a first depression is a signal that the person may ultimately go on to have a full-blown Dementia in several years.