Cognitive Impairment: Characterization and Management

Eric B. Larson

BACKGROUND

Formal structured assessment characterizes cognitive impairment relative to defined objective standards. In clinical settings, formal assessment identifies specific needs of the patient, which informs management of care, including both treatment planning and patient education. This chapter will focus on implications for providers who work with patients who have moderate-to-severe TBI.

Classification of Cognitive Impairment

• Rationale: Results of standardized assessment are reported in many different formats that vary by clinical setting. Although in some situations it is sufficient to limit reports to a terse qualitative summary that concludes with a diagnosis, in most settings it is necessary to document the details of test findings in a transparent format that explicitly states the quantitative basis of any conclusions and recommendations. Moreover, while citing test scores in documentation is helpful, it is also essential to recognize those quantitative data are useless and potentially harmful unless that documentation also includes interpretation in light of other clinical findings, as well as the patient’s background and history. The following is a brief breakdown of parameters for reporting assessment results:

Approach

Dichotomous:

An aspect of cognition can be described with a brief qualitative label that indicates whether a problem is present or absent (e.g., “the patient was anomic” or “the patient remains in posttraumatic amnesia”).

An advantage of this approach is that it does not require the reader of a report to know measurement theory to understand a finding.

Continuous:

The same aspect of cognition can be described with a quantitative score along a continuum of severity (e.g., “the patient scored 35 on the Boston Naming Test [<1st percentile for age and education], which indicates a severe anomia” or “the patient’s Orientation Log score is 25, which indicates she is emerging from posttraumatic amnesia.”

An advantage of this approach is it communicates more information about the extent of the impairment and it specifies a criterion against which that impairment is measured.

Other parameters: Clinicians who choose to report assessment results in the form of continuous data will also need to consider the following when reporting those results: direction of scaling, selection of test norms, and range labels.

Direction of scaling

Positive scaling: large values indicate degree of strength.

Negative scaling: large values indicate severity of impairment.

Selection of test norms

Neurologically intact normative sample

• Reference sample: all examinees are measured against the same healthy reference group.

• Demographic-defined sample: each examinee is measured against healthy individuals with similar backgrounds (e.g., the same age and education).

Clinical normative sample:

• To determine severity of a clinical condition, examinees are measured against others with that condition. For example, severity of aphasia in TBI may be characterized with percentiles relative to aphasics.

• Note: A score corresponding to the 50th percentile relative to aphasics is much worse than a score that corresponds to the 50th percentile relative to a neurologically intact sample.

Range labels

Deviation-based labels: describe how much a score differs from the mean

• Extremely low

• Borderline

• Low average

• Average

• High average

• Superior

• Very superior

Impairment-based labels: describe a score relative to a defined standard of impairment (which may or may not be based on deviation from a population mean)

• Normal

• Mildly impaired

• Moderately impaired

• Severely impaired

Classification of Other Relevant Characteristics

• Severity of injury is rated by the Glasgow Coma Scale [1], duration of loss of consciousness, and the duration of posttraumatic amnesia (PTA; reviewed elsewhere in the present text).

• Stage of recovery is described using the Rancho Los Amigos Scale of Cognitive Functioning [2] (see Chapter 3, pages 18–19).

Etiology of Observed Impairments

• Mechanism of injury—Blunt head trauma can result in diffuse impairment while penetrating wounds may cause circumscribed deficits.

• Risk factors—Factors that predict TBI also predict reduced cognitive performance in individuals both before and after injury. Consequently, they should be considered in interpretation of assessment data and subsequent treatment planning.

Low socioeconomic status can result in educational and cultural disadvantages that affect outcome. A high school education in an affluent suburb may be a greater asset than the same number of years of education in a low-income school system.

Substance abuse is associated with an increased prevalence of TBI. Resumption of drinking after injury complicates cognitive recovery and increases risk of reinjury.

Role of Neuropsychological Assessment

• Typical time to refer inpatients for formal neuropsychological evaluation is after emergence from PTA.

• Examples of referral questions: Identify distinct areas of cognitive strengths and weaknesses. Assess severity of cognitive impairment. Determine if patient has independent decision-making ability. Assess need for supervision. Determine if patient is ready to attempt a return to work or school. Assess need for further cognitive rehabilitation. Determine impact of psychiatric factors on independent functioning.

DIAGNOSIS

Most cognitive constructs are complex and consequently the factors that cause a score to fall in the impaired range are very complicated. For example, interpretation of a score on a memory test could take the following into consideration.

• Age and education: This can usually be controlled by selecting the proper norm set (see earlier section). Most memory tests will allow you to compare a patient to others from the same demographic background.

• Premorbid intelligence: This can be estimated through a number of procedures. Demographic variables (e.g., years of education) are sometimes used by clinicians to form a rough estimate of cognitive ability. However, this is subject to substantial error. More precise estimates are provided through a combination of demographic variables and performance on standardized tests such as the Wechsler Test of Adult Reading [3]. The effects of intelligence on memory can then be controlled through analysis of the discrepancy of an actual memory score versus an expected memory score.

• Level of effort: Symptom validity testing can help assess whether patients may be exhibiting symptom magnification.

• Fatigue: Observation of a patient’s level of arousal is an essential component of formal evaluation. When a patient shows evidence of somnolence during a particular test, the resulting score should be interpreted with caution.

• Sensory impairment or loss of motor control: Patients who are cognitively intact can appear impaired when physical impairment interferes with testing. For example, patients with intact capacity to learn and retain new information might appear impaired on a visual memory task if they do not wear corrective lenses that they need during assessment.

• Other cognitive abilities: Impairment in one domain often exerts secondary effects in other domains. For example, attention deficits can affect initial encoding of information on memory testing. Language impairment can confound memory assessment when it interferes with comprehension of test items or expression of responses.

• Clinical interpretation of a test score can reliably address the construct it was intended to measure only after all the aforementioned factors have been controlled or taken into consideration.

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