Correlation of bite force with excitation-contraction coupling time of the masseter in myasthenia gravis.
OBJECTIVE
The aim of this study was to elucidate the relationship between the impairment of excitation-contraction (E-C) coupling of masseter and the bite force in patients with myasthenia gravis (MG).


METHODS
In 20 patients with MG, masseteric compound muscle action potential (CMAP) and mandibular movement-related potentials (MRP) were recorded simultaneously after stimulating the trigeminal motor nerve with a needle electrode. The E-C coupling time (ECCT) was calculated by the latency difference between CMAP and MRP. Bite force was measured using a pressure-sensitive sheet. Serial assessments of % decrement in masseteric repetitive nerve stimulation (RNS), ECCT, and bite force were performed before and after corticosteroid therapy alone or in various combinations with FK506, cyclosporin A, intravenous immunoglobulin and immunoabsorption.


RESULTS
Percent amplitude decrement in RNS and ECCT decreased significantly accompanying an increase in bite force after treatment. Simple regression analysis demonstrated a linear correlation among % decrement, ECCT and bite force. However, ECCT shortening accompanying bite force recovery without reduction in % decrement was observed in 4 patients.


CONCLUSIONS
Masseteric E-C coupling is impaired in some MG patients, and functional recovery of E-C coupling contributes at least in part to the increase in bite force after treatment.


CONCLUSIONS
Impaired E-C coupling contributes to muscle weakness in patients with MG.
Department of Neurology, Sapporo Medical University School of Medicine, Sapporo, Japan.
Emiko
E
Tsuda
Tomihiro
T
Imai
Takayoshi
T
Hozuki
Rika
R
Yamauchi
Masaki
M
Saitoh
Shin
S
Hisahara
Hiroaki
H
Yoshikawa
Masakatsu
M
Motomura
Shun
S
Shimohama
D000328:Adult; D000368:Aged; D001732:Bite Force; D004558:Electric Stimulation / Q000379:methods; D056966:Excitation Contraction Coupling / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000502:physiology*; D008875:Middle Aged; D009157:Myasthenia Gravis / Q000175:diagnosis* / Q000503:physiopathology*; D013997:Time Factors; D014276:Trigeminal Nerve / Q000502:physiology; D055815:Young Adult