Mental Status Dysfunction in the Intensive Care Unit: Postoperative Cognitive Impairment

Joan M. Swearer

Shashidhara Nanjundaswamy

Cognitive dysfunction following major surgery is one of the common reasons neurologists are asked to evaluate postoperative patients in the intensive care unit (ICU): patients whose memory and intellectual abilities seem impaired when they otherwise appear to have recovered from the immediate effects of surgery. It is a major concern for the family, patient, and physician when a patient is found not to be intellectually the same on awakening following surgery as he or she was before.

There has been extensive research on cognitive dysfunction following major cardiac surgery and a growing literature from noncardiac surgery. In a literature review of cognitive decline following cardiac surgery published between 1985 and 2005, Newman et al. [1] reported that the incidence of decline noted within the first perioperative week varied from 50% to 70%. The incidence fell to 30% to 50% after 6 weeks, and to 20% to 40% at 6 months and 1 year. Differences in methods between studies (e.g., patient sampling, specific tests used, testing intervals, definitions of cognitive decline) make it difficult to compare the studies in literature reviews and meta-analyses directly. Despite these differences, increased age has been the most consistent factor associated with cognitive dysfunction; prolonged cardiopulmonary bypass has also been noted as a risk factor [1,2].

In a study of major noncardiac surgery [3], 1,064 patients aged 18 years and older completed neuropsychological testing before surgery, at hospital discharge, and 3 months after surgery. At 1 year postsurgery patients were contacted to determine survival status. At hospital discharge 36.6% of the young (18 to 39 years), 30.4% of the middle aged (40 to 59 years), and 41.4% of the elderly (60 years and older) had evidence of postoperative cognitive decline. At 3 months cognitive dysfunction was present in 5.7% young, 5.6% middle aged, and 12.7% elderly patients. Increased age, lower educational level, history of premorbid cerebral vascular accident (with no residual impairment), and cognitive decline at discharge were found to be independent risk factors for postoperative dysfunction at 3 months. Patients with postoperative cognitive decline were at increased risk of death in the first year postsurgery.

Although it is clear from these and other studies that postoperative cognitive decline can occur in elderly patients undergoing both major cardiac and noncardiac surgery, the precise pathophysiologic mechanisms have yet to be elucidated.

Mental Status Examination in the Intensive Care Unit

The primary objectives of a mental status evaluation in the ICU are to screen for the presence of postoperative cognitive decline, to analyze both the nature and extent of the impairment, and to evaluate improvement or worsening over time. Cognitive changes may be obvious when there are gross deficits in learning, memory, attention, or concentration. The decline can also be subtle, with problems in initiative and planning (“executive” functions).

Many mental status screening tests are available [4,5,6,7], but none have been specifically developed for, or standardized in, the ICU. A brief screening test may provide a general impression of the patient’s mental status, but the clinician must be able to assess areas of relative strength and weakness in greater depth. The following is offered as an outline for a mental status evaluation in the ICU [8,9,10].

Behavioral Observation and Patient Variables

Determination of the patient’s level of wakefulness and arousal is the essential first step in a mental status examination: levels may range from deep coma to stupor, obtundation, normal alertness, hyperalertness, and manic states. Any further interpretation of mental status test results depends on full alertness, and is severely limited if arousal is not normal.

Test performance is also substantially influenced by the patient’s ability to sustain attention. A patient who is easily distractible will perform poorly on most cognitive tests. Lack of motivation and effort during testing can have deleterious effects on test performance, and may lead to an overestimation of cognitive impairment. Abnormalities in mood and affect, and behavioral disturbances such as psychosis, disinhibition,
hyperactivity, or impulsivity will also negatively impact the patient’s test performance.

Other patient variables that can influence test performance include demographic variables (e.g., premorbid cognitive abilities, age, gender, education, cultural background) and medical and psychosocial history (e.g., psychiatric history, social history, present life circumstances). A history from family members is extremely useful in assessing the patient’s premorbid abilities.

Finally, test performance is compromised by postoperative pain, use of analgesic and sedating medications, limitations in arm/hand mobility, and possible sensory loss (e.g., hemianopia) or motor impairment (e.g., hemiparesis). Assessment of mental status becomes challenging, and the results uncertain, if the patient is on a ventilator.

Attention

The patient’s span of attention can be assessed at the bedside using digit span, which also depends on immediate verbal recall. Repetition of digits both forward and backward should be evaluated. Both tests consist of increasingly longer strings of random number sequences that are presented aloud to the patient. The average score obtained by adults is seven digits forward and five digits backward.

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