Myasthenia gravis with thymoma is more common in the Maori and Pacific Island populations in New Zealand

Background: The association of myasthenia gravis (MG) with thymoma is well recognized. Our clinical impression has been that MG associated with thymom a may be more common in patients of Polynesian descent than in other races. Aim: To determine the influence of ethnicity on the association of MG with thymoma in our population. Method: Review of all cases of thymectomy performed at Greenlane Hospital i n Auckland for the 20-year period from June 1978 to June 1998. Results: There were 103 thymectomies performed in the study period. Fifty-f ive thymomas were identified, 15 in subjects of Maori or Pacific Island eth nicity and 40 in subjects of other races, predominantly Caucasian. Ten of 1 5 Maori or Pacific Island subjects with thymoma had MG (67%), compared with 15 of 40 subjects of other races (37.5%, P = 0.05). The mean age of Maori or Pacific Island subjects with thymoma and MG was 42.5 years, compared wit h 56.3 years in subjects from other races (P = 0.06). All five Maori and Pa cific Island subjects with invasive thymoma had MG, whereas only four of 15 subjects (27%) from other races with invasive rumours had MG (P < 0.01). T he overall incidence of thymoma and the proportion of thymomas that were in vasive did not differ between the ethnic groups. Conclusions: Myasthenia gravis with thymoma occurs more frequently among Ma ori or Pacific Island people than in other racial groups in our population. This is due to an increase in the proportion of cases with thymoma who hav e MG in this group, while the overall frequency of cases of thymoma is simi lar between groups. MG with thymoma in the Maori or Pacific Island populati ons also presents at a younger age and is more often associated with tumour invasion.