When many cases of multiple sclerosis (MS) occur in a certain area or within a distinct amount of time, it is sometimes referred to as a cluster of MS. Scientists have studied these clusters in an attempt to identify new causes or risk factors for MS. However, these studies have failed to produce any clear evidence for new risk factors associated with the disease.
One major problem with the analysis of MS clusters is the difficulty of comparing expected and reported incidence. Incidence is the number of new cases in a set period of time. To report increased incidence, or a cluster, experts need to know the expected incidence for that area. Understanding this number can be challenging because MS incidence can change with geography, gender, and ethnic group. When studying a cluster, there may not be a uniform population of other people with MS outside of the cluster for comparison.
Another problem with cluster analysis involves the diagnostic criteria for MS. It can be difficult to diagnose MS, as it shares symptoms with some other diseases. Cases of MS within a cluster may be inaccurately diagnosed and reported. For example, clusters of MS have been reported in the United States, thought to be associated with increased heavy metals in the environment. However, it is unclear whether heavy metal exposure can cause MS, although heavy metal exposure can cause neurological symptoms that are similar to those seen in MS. Without proper diagnosis by a neurologist, exposure to heavy metals may be misdiagnosed as MS.
Additionally, there is often a lag time between the first symptoms of MS and the diagnosis of MS. This lag creates the possibility of someone developing MS in one region but being diagnosed and included in a cluster study in a new region.
It is unclear whether clusters of MS are a valid phenomenon that occurs with the disease — what people may perceive as excessive case numbers may be normal rates of MS. There are some well-studied patterns associated with MS, including environmental factors and genetic factors associated with disease development.
Scientists and epidemiologists have found a clear pattern with geography and the prevalence of MS. There is an observed gradient in MS prevalence with latitude, and more temperate regions further from the equator. For example, Northern Europe, the northern United States, Canada, Southern Australia, and New Zealand tend to have increased cases of MS.
The geographical differences in MS incidence may be due to environmental risk factors in these areas, such as pathogens, the amount of sunlight received in those parts of the world, and dietary habits of people in various cultures. The risk of MS is increased with environmental risk factors such as lack of vitamin D, cigarette smoke, and infection with Epstein-Barr virus (EBV). Some studies also suggest obesity may be a risk factor, especially for children with MS.
Scientists have performed genome-wide association studies (GWAS) in people with MS to help determine the genetic risk factors associated with MS. MS is not a hereditary disease because it is not predictably passed to family members. However, certain genes may affect a person’s susceptibility to MS when combined with other risk factors.
Much of the genetic susceptibility for MS is related to the immune system and its destruction of the myelin sheath that covers nerve cells in the central nervous system. One genetic risk factor is the human leukocyte antigen (HLA) gene HLA-DRB1*15:01. Individuals carrying this gene are three times more likely to develop MS.
If someone thinks they have observed a high prevalence of MS in their community, they should report this observation to public health officials. These clusters are of interest because they may provide clues to environmental or genetic risk factors that might cause or trigger the disease.
A seemingly increased number of cases may be due to certain demographics like gender distribution and familial association, and it may be surprising how many cases of MS are predicted for a given area. Regardless, if you have concerns about the prevalence of MS in your community, public health officials may be able to determine if a cluster exists.
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