Cognitive impairment is important to detect in ICU survivors because it predicts poor outcomes from rehabilitation. A study of stroke patients found that those with cognitive alterations immediately after the stroke were less likely to be discharged home or to be living at home 6 months after discharge.33
A possible explanation may be that affected patients cannot fully participate in rehabilitation activities, owing to impairment in executive function, inability to remember therapy instructions, or disruption of implicit and explicit learning. Indeed, some authors consider cognitive impairment after acquired brain injury to be the most relevant surrogate marker of rehabilitation potential. Consequently, manipulation or enhancement of cognition may directly affect rehabilitation outcomes.34
Disagreement about terminology and diagnostic criteria creates a problem for health care providers working with patients with potential cognitive impairment. Numerous systems have been proposed to define this condition; in fact, Stephan et al35 reviewed the literature and found no fewer than 17. None of them is specific for cognitive impairment after an ICU stay.
Petersen et al36 in 1999 proposed initial criteria for mild cognitive impairment that included the following:
- A memory complaint
- Normal general cognitive functioning
- Normal activities of daily living
- Memory impairment in relation to age and education
- No dementia.
Later, other areas of impairment besides memory were recognized, such as language, attention, perception, reasoning, and motor planning.37 Therefore, mild cognitive impairment is currently classified into subtypes, which include amnestic (affecting single or multiple domains) and nonamnestic (also affecting single or multiple domains).38
In clinical practice, impairment of specific cognitive domains may be challenging to detect, and neuropsychological testing is often needed. Cognitive screening tests can detect impairment across a restricted range of cognitive abilities, while more comprehensive assessments address each of the primary domains of cognition.39 Formal testing provides normative and validated data on cognition performance and severity.
The Montreal Cognitive Assessment40 is popular, comprehensive, used in a variety of professions in diverse types of facilities (acute care, rehabilitation, and skilled care facilities), and brief (taking 11 minutes to administer). It evaluates orientation, memory, language, attention, reasoning, and visual-constructional abilities. The maximum score is 30; cognitive impairment is defined as a score of less than 26. It has a sensitivity of 90% and a specificity of 87%.
The Folstein Mini-Mental State Examination (MMSE) is the most commonly used of the noncomprehensive tests in clinical practice.41 It assesses orientation, memory, language, attention, and praxis. It has a maximum score of 30 points; the cutoff score for cognitive impairment is 24 points or less.
A limitation of the MMSE is that its sensitivity is very low, ranging from 1% to 49%.42,43 The MMSE scores of patients with cognitive impairment overlap considerably with those of age-matched healthy controls.39 Conversely, the MMSE’s specificity is usually high, ranging from 85% to 100%.42
Moreover, the MMSE poses copyright issues, an important consideration when selecting a test. In 2001, the authors of the MMSE transferred all intellectual property rights to Psychological Assessment Resources, which has exclusive rights to publish, license, and manage all intellectual property rights in all media and languages. Photocopying and using the MMSE without applying for permission from and paying this company ($1.23 per use) constitutes copyright infringement. Therefore, health care providers and researchers have been using other tests to evaluate cognition.
Other tests of cognition assess individual domains. Interestingly, studies of long-term cognitive impairment after ICU admission used these tests to define outcomes.25 Specific tests include:
- The Digit Span and the Trailmaking Test A (used to assess attention and orientation)25
- The Rey Auditory Verbal Learning Test (used to evaluate verbal memory)
- The Complex Figure Test (helpful in defining visual-spatial construction and delayed visual memory)
- The Trailmaking Test B (also included in the Montreal Cognitive Assessment; assesses executive functioning).
Besides formal testing, an informal battery is often recommended to provide additional information. An informal evaluation includes word definition, reading and verbal fluency, reading comprehension, and performance of instrumental activities of daily living. Observing as patients perform tasks of daily living provides therapists with a vast amount of information, as these tasks require using multiple cognitive processes. Therefore, if a functional breakdown occurs during this assessment, the clinician needs to identify the domain or specific level of cognitive dysfunction involved in that deficit.44