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Multiple sclerosis (MS) is a progressive condition in which the immune system mistakes myelin, the substance responsible for protecting the nerve fibers in the central nervous system, for a foreign body.
The immune system removes some or all of the myelin sheath protecting the nerve fibers, leaving them damaged with lesions or plaques. This damage disrupts the smooth transition of messages from the brain through the central nervous system.
Messages travel slowly and are sometimes blocked either partly or completely. Increasing levels of disability can be experienced as the condition progresses because the nerve fibers themselves are damaged by the attacks from the immune system.
The exact cause of MS is currently unknown, but immunological, genetic and viral factors have been identified as possible sources for the disease.
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The symptoms of the disease can include weakness, fatigue, chronic pain, muscle spasms and both cognitive and vision problems. However, the symptom profile is different for every patient, depending on which parts of the nervous system have been damaged and what the function of the damaged area is.
MS is more commonly seen in women than in men, the reason for which is still unknown. It is because of the wide range of different symptoms that the understanding of the identification, progression, and treatment of MS is lacking.
An estimated 2.3 million people are living with MS worldwide, reports the National Multiple Sclerosis Society. However, the exact number and prevalence is still unclear, especially in the United States. MS cases are not consistently reported or tracked in the country.
Cognitive impairment is an area of MS that is often overlooked, as the common deficits caused by the condition concern motor skills and vision, despite cognitive impairment showing prevalence rates of 43% to 70% at earlier and later stages of the condition. Cognitive problems can appear independently of the physical problems caused by MS, which further complicates their identification.
Cognitive functions affected by MS include:
Processing speed, visual learning, and memory are thought to be the most commonly affected areas in MS.
Long-term memory is most commonly impaired cognitive function in MS with rates of 40% to 65%. Recent research has shown that the main cause for long-term memory impairment is less to do with difficulties in the retrieval of information, and more to do with the initial learning of the information. Patients with MS require information to be repeated more, but once the information has been learned, they can recall the information at the same levels as patients without MS. Other factors related to learning abilities affected by MS include slow processing speeds and difficulty to disregard irrelevant information or stimuli.
The processes identified under ‘attention’ are very varied, and as such concrete conclusions about the effect of MS on attention, processes have been difficult to make. Maintaining sustained attention is typically affected, as well as the ability to multitask.
Fatigue is also a factor that can affect attention processes, in particular, the ability to hold attention for long periods of time, but is often overlooked.
Problems with information processing have been seen across the disease course and are one of the most commonly seen cognitive symptom of MS. The ability to maintain information for a short period of time, also known as working memory, is negatively affected by MS. Impairment in information processing is usually seen concurrently with the other cognitive deficits in MS. Processing speed can be used to predict a patient’s long-term cognitive decline and also predict their performance in everyday tasks.
Defined as the cognitive abilities necessary for goal-directed tasks and complex decision making and adaptation to changes in the environment, executive functioning is one of the less frequent deficits seen in patients with MS.
Executive tasks include:
Deficits in fluency see patients struggle with both phonemic (letter and word fluency) and semantic fluency (word category fluency). Depression affects the measurement of executive functioning and should be taken into account when examining and interpreting a patient’s performance in executive functioning.
A reduction in processing speed is the most common cognitive area affected by MS, and deficits are seen concurrently with other areas of decline. The effect on a patient’s working memory is less than on their processing speed there is a positive correlation between increased demand on working memory and deficits in processing speed.
A sensitive test of cognitive dysfunction is a paced auditory serial addition test (PASAT), which tests working memory that puts a high demand on a patient’s processing speed.
Cognitive Rehabilitation for MS
Cognitive rehabilitation programs concentrate on improving attention and memory deficits and communication skills. Their effect has not been consistent, however, the programs available to provide a starting point from which to build further research to improve future rehabilitation.
Pharmacological Treatment for MS
Disease-modifying therapies, drugs that change the MS disease course, have been explored to improve the cognitive effects of MS. Studies on disease-modifying therapies have not focused on cognitive improvement and therefore their benefits are difficult to determine.
Although MS mainly connected with a decline in motor skills, cognitive functions are commonly adversely affected by the disease, with long-term memory and information processing being the most affected areas.
There are currently few studies exploring the treatment of the cognitive deficits of MS. There are cognitive rehabilitation programs available, but their results have been varied with some studies showing no improvements at all. Pharmacological treatments have been explored but studies in this area had significant limitations, meaning their results are not concrete.
As a whole, it is believed that further extensive research needs to be conducted in this area to develop the understanding of the disease course and development of effective treatments.