Incidence of multiple sclerosis in the Republic of Ireland: A prospective population-based study

Multiple sclerosis (MS) incidence and prevalence is increasing worldwide, with a disproportionally higher rate in women. Recent studies have questioned the presence of a latitudinal gradient in Europe. Ireland is a high prevalence country for MS with a previously reported North-South gradient making it ideal to further explore this concept.

In this study we prospectively determined the incidence rate of newly diagnosed MS in Ireland over a 12-month period and demonstrated the presence of a North-South gradient.

A national prospective population-based observational study was performed to ascertain all new cases of MS diagnosed from 1st March 2014 - 28th February 2015 in the Ireland. Within the main study there was a smaller nested cohort study to explore clinical outcomes with a view to future prospective follow-up of this cohort. Sources of case ascertainment included neurologists, MS nurse specialists and MS support services. The Irish census 2011 was used to obtain population statistics and the incidence rate was age-standardized to a European Standardised Population (ESP 2011). The North-South gradient was assessed, by comparing incidence rates between northern and southern counties.

292 patients fulfilled the inclusion criteria equating to an age-standardised incidence rate (A-SIR) of 6/100,000 (95% CI: 5.3-6.6); for women the rate was 8.7/100,000 (95% CI: 7.7-9.6) and for men 3.3/100,000 (95% CI: 3.0-3.7). The female to male sex ratio was 2.7:1. Mean age at diagnosis amongst the RRMS group was 37 years (SD: 9.6) and 55 years (SD: 7.7) in the PPMS group; there were no gender differences associated with age of diagnosis. Onset was progressive in 10% of cases. A significant difference was seen in incidence rates between the northern region (A-SIR: 9.6×10⁵, CI: 6.9-12.3) and the southern region (A-SIR: 5.1×10⁵, CI: 3.8-6.3) (Z-score =3.34, p<0.05). Amongst the nested cohort (n=113) mean age at symptom onset in the RRMS group (n=106) was 34 years (SD: 8.7) and 50 years (SD: 11.8) in the PPMS group (n=7). The female to male sex ratio was 3.5:1. Eighty percent had started or were due to start disease modifying therapy at time of review and 77% were taking supplemental vitamin D. Using the hospital depression and anxiety scale (HADS) mild to severe depressive symptoms were reported in 34% with no prior history of depression. Seventy-five percent were in full or part-time employment with 8% not working due to disability arising from their MS.

This is the first study to prospectively assess the incidence rate of MS in Ireland and shows that Ireland has a high incidence rate, comparable with the rest of the British Isles, with a persistent North-South gradient. The age of onset of relapsing remitting multiple sclerosis appears to be increasing over the last 20 years. It will be of interest to re-assess this population over time to see if increasing incidence rates, as well as improved survival, are driving the reported increases in MS prevalence.