Cognitive Dysfunction is one of the more scary symptoms of Multiple Sclerosis. It used to be thought that cognitive dysfunction was relatively rare symptom of the disease but it is now understood to be quite a common feature.
Among the reasons that it's frequency was formerly underestimated is that the dysfunction is often mild, cognition is a very complex subject and, in the past, physicians have felt more comfortable denying its existence to their patients. Things have changed over the past few years. Rather than lumping cognitive dysfunction in the same bracket as fatigue and depression, it is now studied on its own. Cognitive evaluation techniques have also improved greatly and now proper studies into cognitive dysfunction in MS are beginning to be done.
It is important to understand the nature of the range of cognitive problems associated with MS. Dementia, as we understand it from diseases like Alzheimer's disease, CJD and Huntington's Corea is really quite rarely seen in multiple sclerosis. When it does occur, it is known as Cerebral Multiple Sclerosis and occurs in around three per cent of people with MS and usually only in the later stages of the disease. Less severe dysfunction is estimated to occur in 54 to 65% of people with MS.
As with other dysfunctions in MS, cognitive ones tend to be seen within specific intellectual functions. A person with short-term memory deficits may or may not have problems generating concepts - just as a person with spasticity in their right leg may or may not have double vision.
A very commonly observed feature across the range cognitive dysfunctions seen in MS is that performance accuracy is rarely affected. What tends to be damaged is the speed of performance. For example, in memory tests, people with MS may remember fewer numbers than other people but the numbers that they do remember are correct.
They may be slower at doing certain tests but they get them right eventually. This is, perhaps, to be expected because multiple sclerosis is a white matter disease and rarely affects the grey matter. As a gross simplification, we can say that the grey matter is responsible for the processing and the white matter for passing the results of that processing to other parts of the brain.
Cognitive dysfunction is most severe in people with the Secondary Progressive form of the disease. Although some studies show that as many as 50% of people with MS have some form of impairment from disease onset, it is usually mild and often recovers with remission. Perhaps because the Primary Progressive form prefers to attack the spinal cord, people with this form have been shown to be more rarely affected by cognitive impairment.
It is worth noting that a recent comparative study of cognitive dysfunction in depression, MS and chronic fatigue syndrome found that people with depression performed less well on most of the tests than people with MS. Medications used to treat some of the other symptoms of MS also affect cognitive performance.
One study found that people who had mild cognitive deficits early in their disease course were more likely to have more marked dysfunction later on. Another study found that cognitive dysfunction was unrelated to Expanded Disability Status Score which is essentially a measure of physical disability. Total lesion load was found to be correlated with level of cognitive dysfunction but not with lesions in specific brain areas. This is not surprising given the complexity and plasticity of the human brain.
Some of the specific cognitive deficits observed in people with MS are: