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³¹P MR Spectroscopic Assessment of Muscle in Patients with Myasthenia Gravis before and after Thymectomy: Initial Experience¹

¹ From the Departments of Radiology (S.F.K., C.C.H., S.H.N., Chen-Chang Lee, Chih-Chia Lee), Thoracic and Cardiovascular Surgery (M.J.H.), Neurology (T.K.L.), and Public Health and Biostatistics (M.C.C.), Chang Gung University, College of Medicine, Chang Gung Memorial Hospital-Kaohsiung Medical Center, Kaohsiung, Taiwan; and Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan (L.L.). Received March 31, 2007; revision requested May 30; revision received June 11; accepted July 18; final version accepted September 7. S.F.K. supported by grant NSC 93-2314-B-182A-085 from the National Science Council, Taiwan. Address correspondence to S.F.K. (e-mail: sfatko{at}adm.cgmh.org.tw).

Purpose: To prospectively assess muscle metabolism in myasthenia gravis (MG) patients before and after thymectomy by using phosphorus 31 (³¹P) magnetic resonance (MR) spectroscopy.

Materials and Methods: With institutional review board approval and informed consent, resting and dynamic ³¹P 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 ³¹P 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).

Results: 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 .001), but these values showed significant restoration to normal after thymectomy (P = .018).

Conclusion: 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.

© RSNA, 2008

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