WASHINGTON, DC—Among patients with anti-muscle-specific kinase (MuSK) myasthenia gravis, thymectomy is not associated with greater likelihood of clinical improvement, according to an analysis of data from a multicenter cohort study. The results were presented at the 2018 Annual Meeting of the American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM).
Although a randomized trial has demonstrated benefit from thymectomy in nonthymomatous antiacetylcholine receptor (AChR) antibody positive generalized myasthenia gravis, observational studies suggest that thymectomy may not be efficacious in anti-MuSK myasthenia gravis. Histologic studies have found that patients with anti-MuSK myasthenia gravis have less hyperplastic thymic tissue, compared with patients with anti-AChR myasthenia gravis.
To evaluate the therapeutic impact of thymectomy in patients with anti-MuSK myasthenia gravis, Katherine Clifford, a medical student at the University of Vermont Larner College of Medicine in Burlington, and colleagues analyzed data from a multicenter, retrospective, blinded review of rituximab treatment in patients with anti-MuSK myasthenia gravis. The primary outcome was favorable outcome on the Myasthenia Gravis Foundation of America (MGFA) Post-Intervention Status (PIS). The researchers defined a favorable outcome as an MGFA PIS score of minimal manifestations or better.
Secondary outcomes included prednisone dose; use of other immunosuppressant medications, IV immunoglobulin (IVIG), or plasma exchange (PLEX) treatment; and Myasthenia Gravis Status and Treatment Intensity (MGSTI).
Baseline characteristics were similar between patients with anti-MuSK myasthenia gravis who underwent thymectomy (n = 26) and those who did not (n = 29), including treatment with rituximab (42% vs 45%). Median follow-up was more than three years.
At last visit, 35% (nine of 26) of patients who underwent thymectomy had a favorable outcome, compared with 55% (16 of 29) of patients who did not undergo thymectomy. In addition, 69% of patients who underwent thymectomy were taking prednisone, compared with 41% of patients who did not undergo thymectomy (median dose, 10 mg/day vs 0 mg/day).
“After controlling for rituximab, baseline prednisone, and final IVIG/PLEX treatment, thymectomy was not associated with greater likelihood of favorable clinical outcome, but broad confidence intervals cannot exclude therapeutic effect (odds ratio, 0.43),” the investigators reported.
A. Gordon Smith, MD
“The recent MGTX trial clearly demonstrated the benefit of thymectomy for patients with AChR antibody positive myasthenia gravis,” said A. Gordon Smith, MD, Cochair of the AANEM Annual Meeting Program Committee. “Ms. Clifford and her colleagues now provide compelling data suggesting thymectomy may not be effective in MuSK-positive myasthenia gravis.”
The study’s follow-up is long enough for the findings to be clinically “relevant to all physicians treating myasthenia gravis,” said Robert W. Irwin, MD, Cochair of the AANEM Annual Meeting Program Committee.