31P MR spectroscopic assessment of muscle in patients with myasthenia gravis before and after thymectomy: initial experience.

Abstract

To prospectively assess muscle metabolism in myasthenia gravis (MG) patients before and after thymectomy by using phosphorus 31 (31P) magnetic resonance (MR) spectroscopy. With institutional review board approval and informed consent, resting and dynamic (31)P MR spectroscopy were performed in 14 healthy volunteers (five men, nine women; mean age, 33 years; range, 23-48 years) and 16 MG patients (six men, 10 women; mean age, 37 years; range 18-50 years) before and after thymectomy. Patients were stratified into groups according to the modified Osserman classification: mild-MG group (classes I-IIA) and moderate-to-severe-MG group (classes IIB-IV). Variables compared among the three groups (Kruskal-Wallis test) included the inorganic phosphate (P(i))-adenosine triphosphate (ATP) (P(i)/ATP) ratio, phosphocreatine (PCr)-ATP (PCr/ATP) ratio, P(i)/PCr ratio, muscle pH at resting and at end-exercise ( 31)P MR spectroscopy, rate constant for PCr recovery (k(PCr)), and maximum oxidative capacity (V(max)). These variables were also compared in MG patients before and after thymectomy (Wilcoxon signed rank test). There were no significant differences in resting P(i)/ATP, PCr/ATP, and P(i)/PCr ratios and resting muscle pH among the three groups (control group, 14; mild-MG group, nine; moderate-to-severe-MG group, seven). Comparison of the control group with the mild-MG group and comparison of the mild-MG group before thymectomy with the mild-MG group after thymectomy showed no significant differences in end-exercise P(i)/ATP, PCr/ATP, and P(i)/PCr ratios; end-exercise muscle pH; k(PCr); and V(max). Compared with the control and mild-MG groups, the moderate-to-severe-MG group had significantly higher end-exercise P(i)/ATP and P(i)/PCr ratios and significantly lower end-exercise muscle pH, k(PCr), and V(max) before thymectomy (P < or = .001), but these values showed significant restoration to normal after thymectomy (P = .018). Mild-MG group patients have muscle oxidative metabolism similar to that of healthy control subjects, whereas moderate-to-severe-MG group patients have impaired V(max) during exercise and a noticeable shift to glycolytic metabolism, but these abnormalities are reversible after thymectomy.