31P MR spectroscopic assessment of muscle in patients with myasthenia gravis before and after thymectomy: initial experience.
OBJECTIVE
To prospectively assess muscle metabolism in myasthenia gravis (MG) patients before and after thymectomy by using phosphorus 31 (31P) magnetic resonance (MR) spectroscopy.


METHODS
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).


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


CONCLUSIONS
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.
Ko
Sheung-Fat
SF
Department of Radiology, Chang Gung University, College of Medicine, Chang Gung Memorial Hospital-Kaohsiung Medical Center, Kaohsiung, Taiwan. sfatko@adm.cgmh.org.tw
Huang
Chung-Cheng
CC
Hsieh
Ming-Jang
MJ
Ng
Shu-Hang
SH
Lee
Chen-Chang
CC
Lee
Chih-Chia
CC
Lin
Tsu-Kung
TK
Chen
Min-Chi
MC
Lee
Liangshiu
L
D000255:Adenosine Triphosphate / Q000032:analysis; D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D007551:Isotonic Contraction; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000737:chemistry*; D009157:Myasthenia Gravis / Q000378:metabolism* / Q000601:surgery; D010710:Phosphates / Q000032:analysis; D010725:Phosphocreatine / Q000032:analysis; D013934:Thymectomy
D010710:Phosphates; D010725:Phosphocreatine; D000255:Adenosine Triphosphate