Cognitive Impairment in Multiple Sclerosis
Jeff Canar, PhD
Prevalence rates for dementing illnesses in the US ranges from 2.8% for adults aged 65 to 74 up to 28% for adults greater than 65 years of age (Desai, Zhang and Hennessy, 1999). Dementia, broadly defined, involves impairment in memory plus impairment in at least one other cognitive domain including language; apraxia––the inability to carry out motor functions despite in-tact motor functioning; agnosia––the inability to recognize objects despite intact sensory function; problems with attention and concentration; or disturbance in any of a wide array of executive tasks such as planning, organizing, or abstract thinking. Memory impairment can involve either short-term memory or long-term memory, and can further be evaluated in terms of one’s ability to freely recall information versus the ability to “recognize” information that has been previously presented.
A more detailed explanation of neuropsychological functioning, its assessment, and the different types of dementing illness from which one may suffer are beyond the scope of this brief article. An excellent overview for clinicians can be found in the book, The Mental Status Examination in Neurology (2000) by R. Strub and F. W. Black, Philadelphia: F. A. Davis Publishing Co.. ISBN: 0803604270.
Because multiple sclerosis (MS) involves systemic demyelization of nerve cells, the disease can affect not only nerve cells in the periphery, but the central nervous system as well, including the brain. Thus, MS can be a risk factor for a dementing illness, and clinicians should be aware of this. The British Society of Rehabilitation Medicine ranked cognitive dysfunction as the fourth highest priority in disability associated with MS, behind locomotion, continence, and emotional distress (Langdon & Thompsom, 1996).
Prevalence rates for cognitive impairment in MS range from 30–70% of cases, depending on the study (Amato et al. 1995; Rao, 1995; Marrie, Chelune, Miller & Cohen, 2005). Just as MS can affect individuals in a number of ways, both in terms of course and level of disability, so there is variability in the type of cognitive impairment a person with MS may suffer. Associated deficits include delayed recall of verbal information, a decline in abstract reasoning, problems with initiation and planning, decreased verbal fluency, problems with attention and concentration and slowed mental processing (Amato et al, 1995; Langdon and Thompson, 1996; Marrie et al.). It is also important to note that cognitive deficits in MS may be associated with the sub-type of MS with which a person is diagnosed. For example, Wachowius, Talley, Silver, Heinze and Sailer (2005) found a higher incidence of cognitive impairment in patients with primary progressive MS as compared to secondary progressive MS. Those with secondary progressive MS had a longer duration of disease, suggesting that the rapidity with which the disease progresses is more important that the length of the disease, per se.
Three important confounding factors need to be considered when assessing cognitive functioning in an MS patient: (a) motor impairment, (b) fatigue and (c) depression. Given the motor deficits associated with MS, evaluating a person with MS can be difficult. Specifically, timed tasks involving motor involvement such as the Coding and Block Design subtests from the Wechsler Adult Intelligence Scale, the Rey-Osterrieth Complex Figure and other visual reproduction tasks, and the Trailmaking Test need to be interpreted with caution. Is the patient’s performance reflective of actual cognitive impairment or an artifact of slowed motor function? Also, over the course of testing, fatigue (one of the most common disabling conditions of MS) may result in decreasing performance capacity. Fatigue is not itself a symptom of a dementing illness and may contribute to a false positive diagnosis of cognitive impairment (Schwid et al. 2003). It has been demonstrated that patients can over-attribute the effect that fatigue has on their cognitive performance (Beatty et al. 2003), consequently reporting cognitive difficulties which may not represent true cognitive decline. Nevertheless, fatigue is something that the clinician needs to consider.
Regarding depression, up to one-half of patients with MS will have symptoms of sufficient duration and intensity to warrant a diagnosis of depression (Groom, Lincoln, Francis & Stephan, 2003). Symptoms associated with depression which may overlap with the symptom constellation associated with MS include: (a) fatigue, (b) psychomotor retardation and (c) diminished ability to think or concentrate. In this case, depression itself can exacerbate symptoms associated with MS, leading the clinician to assume that cognitive impairment is more significant than it would be otherwise. Consequently, if a patient is judged to be depressed, treating the depressive illness can lead to significant improvements in cognitive functioning (Demaree, Gaudino & DeLuca, 2003).
Overall, treatment for cognitive impairment is more compensatory than remedial (Feinstein, 2004). Some results suggest that the early use of Aricept (Donepezil) may be beneficial, as are some of the treatments for MS (Interfereon B). More rigorous investigation, however, will be necessary before it can be concluded that there is a direct, positive association between medication interventions and improved cognitive functioning in MS. Other possible compensatory strategies include improving the structure and organization of the patient’s environment and enlisting the help of care-givers to prompt, remind, and help the patient stay on task.
In summary, providers should be aware that cognitive impairment is a risk factor associated with MS and screen and assess accordingly. While the deficits themselves may be subtle, when they overlap with the disabilities associated with MS, subtle difficulties can cause significant impairment in day-to-day activities and quality of life. Impairment is not necessarily associated with the length of the disease. Additionally, providers need to take into account confounding factors such as fatigue and depression, which may impair performance on cognitive tasks, but do not necessarily indicate cognitive impairment, per se. As previously mentioned, a patient with MS and concomitant depression may experience improvement in cognitive function simply by receiving effective treatment for the affect disorder.
References
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Demaree, H. A., Gaudino, E., & Deluca, J. (2003). The relationship between depressive symptoms and cognitive dysfunction in multiple sclerosis. Cognitive Neuropsychiatry, 8(3), 161-171.
Desai, M. M., Zhang, P., & Hennessy, C. H. (1999). Surveillance for Morbidity and Mortality Among Older Adults––United States, 1995-1996. Morbidity and Mortality Weekly Reports: Surveillance Summaries, 48(SSO8), 7-25.
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Marrie, R. A., Chelune, G. J., Miller, D. A., & Cohen, J. A. (2005). Subjective cognitive complaints relate to mild impairment of cognition in multiple sclerosis. Multiple Sclerosis, 11, 69-75.
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Schwid, S. R., Tyler, C. M., Scheid, E. A., Weinstein, A., Goodman, A. D., & McDermott, M. P. (2003). Multiple Sclerosis, 9, 503-508.
Wachowius, U., Talley, M., Silver, N., Heinze, H., & Sailer, M. (2005). Journal of Clinical and Experimental Neuropsychology, 27, 65-77.