The co-occurrence of multiple sclerosis and type 1 diabetes: Shared aetiologic features and clinical implication for MS aetiology

• MS and T1D considered as autoimmune diseases may share several aetiological features.
• We identified shared genetic and environmental factors in the aetiology of the two diseases.
• Evidence on whether T1D will affect the disease course and severity of MS is lacking.
• Further studies are therefore needed to ascertain the effect of T1D on clinical disability in MS.

We reviewed the evidence for the co-occurrence of type 1 diabetes mellitus (T1D) and multiple sclerosis (MS), and assessed the clinical significance of this association and the shared aetiological features of the two diseases. T1D and MS contribute considerably to the burden of autoimmune diseases in young adults. The co-occurrence of MS and T1D has been reported by a number of studies, suggesting that the two conditions share one or more aetiological components. Both conditions have been associated with distinct human leukocyte antigen (HLA) haplotypes but share a number of similarities in clinical, epidemiological and immunological features, leading to suggestions of possible common mechanisms of development. While underlying genetic factors may be important for the co-occurrence of both conditions, some evidence suggests that environmental factors such as vitamin D deficiency may also modulate an individual's risk for the development of both conditions. Evidence on whether the co-occurrence of the two autoimmune conditions will affect the disease course and severity of MS is merely absent. Further studies need to be conducted to ascertain whether the neuropathology associated with T1D might influence the disease course and contribute to the severity of MS.