MS clusters: chance or design? - multiple sclerosis

Inside MS, Summer, 1989 by Phyllis Shaw

Since the plant used zinc as a principal raw material and workers might have inhaled or swallowed zinc dust, the scientists measured zinc levels in the blood of employees with multiple sclerosis, a group of MS patients who were not plant employees, and a group of healthy people in the plant. There were no significant differences in zinc levels among the groups studied, though all people working in the plant (MS and non-MS) showed higher levels of the metal.

Zinc seemed of possible interest to the Rochester investigators because it is known to affect the body's immune regulation. Obviously, some of the plant workers are genetically predisposed to MS. Dr. Byron Waksman, the Society's vice president for research, points out that people with genetic susceptibility do not all get the disease. "It may be," he says, "that zinc 'resets their thermostat' so that a larger proportion gets MS."

However, if zinc had that effect on the workers at the plant, presumably other immunologically-based disorders would also have increased. Since they did not, the Society's Medical Advisory Board decided the study showed no particular risk factor associated with the development of MS.

To get a general perspective on cluster research, INSIDE MS contacted Dr. Leonard Kurland, senior consultant and former chairman of medical statistics and epidemiology at the Mayo Clinic in Rochester, Minnesota, who has published widely on the subject.

"There are two things you have to be sure of before you start talking clusters," he said. "First you must be certain that people in the cluster really have the disease. As we all know, many other illnesses mimic multiple sclerosis. Secondly, it's essential to find out where these people were residing at the time of onset of their MS and during the years before the onset of symptoms.

"The cluster may simply mean that these people assembled in the community at some later date. This happened with an apparent cluster of eight cases of MS in Duxbury, Massachusetts in 1959. When we examined the patients, indeed they all had multiple sclerosis but only one had developed it locally; the other seven had had MS before their arrival in the town. They all came to Duxbury because it was known to be a charming retirement community. So this washed out that cluster."

Other factors, too, may dim the luster of a cluster. Says Dr. Carmel Armon, research fellow in neuroepidemiology working with Dr. Kurland at Mayo, "Remember that in the past many mild cases of multiple sclerosis used to go undiagnosed. Now we are using more sensitive diagnostic techniques. It is therefore easier to identify MS cases, including milder ones. So the use of improved technology helps create an apparent cluster.

"Moreover," he adds, "you might have a community which has never had a neurologist in residence before. Then a neurologist comes in to set up his practice. More cases of multiple sclerosis will be identified in that community because there is someone there now to spot them."