P.377Myasthenia gravis (MG) anti-musk in Argentina: demographic, clinical findings and therapeutic response in a cohort of 56 patients

Abstract

MuSK antibody positive Myasthenia Gravis (MuSK-MG) is diagnosed between 0-49% cases with generalized seronegative MG in different populations. Segmental muscular atrophy, frequent bulbar symptoms and a high incidence of myasthenic crisis characterize the disease. The aim of this study is to present a large cohort of MuSK-MG patients from Argentina. We performed a retrospective chart review on 56 muscle-specific kinase antibody (MuSK-Ab)-positive myasthenia gravis (MG) patients in nine neuromuscular centers in Argentina. All of them were identified to have MuSK-Ab on commercial laboratory testing. MG severity and response to treatment were assessed according to MGFA classification. We identified 56 patients with diagnosis of Musk-MG. Of them, 66% were female. Mean age at diagnosis was 45,6 and 44,9 years-old in women and men respectively. Initial symptoms were: oculo-bulbar (34,3%), pure ocular (21,8%), pure bulbar (21,8%). 2 patients initiated with ocular and limb weakness, 2 with only limb weakness, 2 showed respiratory symptoms an 1 presented with dropped head. Initial MGFA status was: IIB (62%), I (13,7 %), IIIB (7%), IIA 6,9 % and 27,5% had myasthenic crisis at diagnosis. Peak MGFA was: IIb (20%), IIIB 26,6%, IVB 30 %. 6 (20%) patients had myasthenic crisis Three clinical phenotypes were found: the oculo-bulbar, the respiratory and the pseudo-myopathic. We did not find a clearly female predominance as described in previous reports. The presence of thymoma or thymic pathology was not detected by CT scan or MRI. Most patients respond favorably to immunotherapy. The best clinical response was to rituximab ,corticosteroids and plasma exchange, and the poorest response was to IVIG and anticholinesterase. Long-term outcome is favorable in about 80% of cases. This is the first series of patients with MuSK-MG described in South America and one of the largest worldwide.