A comparison of long-term post-thymectomy outcome of anti-AChR-positive, anti-AChR-negative and anti-MuSK-positive patients with non-thymomatous myasthenia gravis

José M Ponseti, Norma Caritg, Josep Gamez, Manuel López-Cano, Ramón Vilallonga, Manuel Armengol
Expert Opinion on Biological Therapy 2009, 9 (1): 1-8

OBJECTIVE: A single-centre, non-randomized, non-controlled study was designed to compare the long-term post-thymectomy clinical outcome in anti-AChR-positive, anti-AChR-negative and anti-MuSK-positive patients with non-thymomatous myasthenia gravis (MG).

METHODS: A total of 331 consecutive patients with seropositive MG, 55 with seronegative MG and 10 with anti-MuSK-positive MG underwent extended transsternal thymectomy (T-3b according to Myasthenia Gravis Foundation of America). The primary endpoint was to assess differences in the rate of complete stable remission (CSR) in patients with and without anti-AChR and anti-MuSK antibodies.

RESULTS: The mean follow-up was 218.3 (SD 128.1) months in the seropositive MG group, 149.8 (SD 131.1) in the seronegative group and 169.9 (SD 116) in the anti-MuSK-positive group. In the seropositive MG group, the probability of obtaining CSR at 5 years post-thymectomy was 51.1% for the seropositive group compared with 40 for the seronegative group (p = 0.05) and 20 for the anti-MuSK-positive group (p = 0.03). Differences between the seronegative and anti-MuSK-positive groups were not observed. The estimated median follow-up to obtain a CSR was 17.8 months (95% confidence interval [CI] 15.7 - 19.8 months) in seropositive MG patients, 22.1 (95% CI 16.7 - 27.4 months) in seronegative MG patients and 20.6 (95% CI 13.3 - 27.9 months) in anti-MuSK-positive MG patients (long-rank test, p = 0.07).

CONCLUSIONS: Long-term post- thymectomy clinical outcome was better in patients with conventional anti-AChR antibodies than in those with seronegative disease. In seronegative anti-MuSK-positive MG, thymectomy seems to be less effective than in anti-MuSK-negative MG but this study cannot answer the question of whether thymectomy should be undertaken in anti-MuSK-positive patients.

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