For reasons that are unclear, patients with multiple sclerosis appear to be less prone to develop cancer in general than other individuals.

Multiple sclerosis is a disease of the nervous system caused by damage to the sheaths that protect nerve cells. It affects 2.5 million people globally and can cause mild illness in some people and permanent disability in others. Symptoms may include numbness or weakness in the limbs, loss of vision and an unsteady gait.

“We speculate that the lower risk for cancer among people with multiple sclerosis could be a result of lifestyle changes or treatment following diagnosis,” lead author Dr. Shahram Bahmanyar, of the Karolinska Institute in Sweden, said in a statement.

However, for brain tumors and cancers of the bladder and other urinary organs, multiple sclerosis is associated with elevated risks.

“The increase in brain tumor diagnoses may be due to brain inflammation,” Bahmanyar added, “but this finding may not reflect a real increase in cancer risk, as there is some evidence that more frequent (neurologic exams) in these patients mean that brain tumors are more likely to be found sooner. There may also be reasons related to the disease that could increase the risk for urinary organ cancers, resulting from chronic irritation to those organs as a result of multiple sclerosis.”

The authors found no increased or decreased cancer risk in the parents of the patients, suggesting that simple inheritance is unlikely to explain the association seen in the study group.

The findings, reported in the journal Neurology, stem from a study of 20,276 patients with multiple sclerosis and 203,951 subjects from the Swedish general population. Data from the parents of subjects in both groups were also analyzed. On average, subjects were followed for 35 years.

Multiple sclerosis patients were 9 percent less likely to develop cancer overall than the general population. The patients were, however, 44 percent more likely to develop brain tumors and 27 percent more likely to develop urinary organ cancers.

Although at increased risk for these cancers, the chance that a patient will develop one is still very small. For instance, with urinary cancer, “less than 0.2 percent of people with multiple sclerosis developed this cancer for every 10 years of follow-up,” Bahmanyar said.

SOURCE: Neurology, March 31, 2009.