The epidemiology of myasthenia gravis,Lambert-Eaton myasthenic syndrome and their associated tumours in the northern part of the province of South Holland

We studied the epidemiology of myasthenia gravis (MG) and the Lambert-Eaton myasthenic syndrome (LEMS), and their association with small cell lung carcinoma (SCLC) and thymoma, in a well defined region of the Netherlands. Available data on all the patients with MG, LEMS, thymoma or SCLC living between 1 January 1990 and 31 December 1999 in the northern region of South Holland, with a population of 1.7 million inhabitants, were evaluated. A total of 202 patients with MG (20 with thymoma) and ten patients with LEMS (seven with SCLC) were identified. LEMS was 46 times less prevalent (2.32 x 10(-6)) than MG (106.1 x 10(-6)), whereas the annual incidence rate of LEMS was 14 times lower (0.48 x 10(-6)) than of MG (6.48 x 10(-6)), reflecting the poor survival of LEMS patients with SCLC. SCLC was diagnosed in 1593 patients, seven (0.44 %) of whom developed LEMS. Mean age at diagnosis of SCLC was significantly lower in SCLC patients with LEMS (p = 0.006). A thymoma was diagnosed in 32 patients, of whom the ten patients with MG (31 %) had a younger age at diagnosis of thymoma than the patients without MG (p = 0.27). This study confirms the increasing prevalence of MG over the last few decades as reported by others, and underscores the relative rarity of LEMS. The frequency of LEMS in our patients with SCLC was lower than reported in previous studies. In patients with a SCLC or thymoma, the tumour was diagnosed at younger age in those who had the associated myasthenic syndrome.