Smoking is an established risk factor for the onset and progression of multiple sclerosis (MS). This article describes the evidence for this and how smoking contributes to the symptoms of MS.
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Smoking increases the incidence of MS
Smokers have a higher risk of MS compared to those who have never smoked, according to a study conducted in a Norwegian city. It is now thought that tobacco smoking interacts with a separate risk factor, namely, the Epstein-Barr virus, to increase the risk to almost twice that of non-smokers.
There is a definite correlation between dosage and MS incidence, with people who have smoked for 25 or more years showing the greatest risk compared to people who have never smoked.
Smoking can exacerbate the clinical course of MS
Smoking is known to be a risk factor for cancer of lung, throat, and oral cavity, among others. But smoking can also affect MS, by changing a relapsing-remitting disease course to the more severe secondary progression form. The more a person smokes, the greater the brain damage shown on MRI for patients with MS.
The degree of disability due to MS is also greater in smokers, but this may be arrested, at least partially, be quitting smoking.
Smoking can also hinder MS treatment, as it promotes the development of antibodies to the interferon beta molecules that are used to treat the condition.
Thus, many epidemiologic studies have concluded that smoking is among the chief environmental triggers of MS. The average interval between the time a person begins smoking and develops MS clinical disease is about 15 years.
Why does smoking exacerbate the symptoms of MS?
Several suggestions have been made for the pathophysiology behind smoking and MS. These are:
Effect of smoking on the immune system
Nicotine is known to cause immunosuppression, while tobacco glycoprotein stimulates the immune system. This could explain increased autoimmune phenomena in smokers, such as rheumatoid arthritis, systemic lupus erythematosus, and Crohn’s disease. However, not all autoimmune diseases show this link.
Detrimental impact of smoking on the blood-brain barrier
Nicotine increases the blood flow to the microcirculation to the brain, and may boost the influx of permeable solutes across the blood-brain barrier, as seen in rodent studies. This development of leakiness in the blood-brain barrier has been thought to be an important trigger in initiating MS.
Toxicity of cigarette smoke on the brain
Cigarette smoke contains cyanide and other metabolites that have been shown to be associated with demyelination in the brains of animals exposed to these chemicals. These are found in increased amounts in the blood of smokers and repeated smaller exposures to these toxins may be even more effective in inducing demyelinating changes than a single large dose.
Smoking is also known to increase the number and persistence of respiratory infections, and this may be another risk factor in MS.
Research is currently being carried out to confirm these findings, so that appropriate preventive measures can be recommended and healthcare steps implemented to reduce the incidence of MS.
Cessation of smoking is, however, a very important preventive measure beyond doubt. This habit should be targeted with intensive follow-up, especially for vulnerable age groups when smoking is likely to begin.