Muscle force, motor nerve conduction velocity and compound muscle action potentials after parathyroidectomy for secondary hyperparathyroidism

Objective - We studied the motor nerve conduction velocity (MNCV) and the amplitude of compound muscle action potentials (CMAP) in patients with symptomatic secondary hyperparathyroidism at preoperative and 3 months post-operative period, to find the factors affecting muscle force after parathyroidectomy. Methods - Twenty-six patients with symptomatic secondary hyperparathyroidism with levels of intact parathyroid hormone (iPTH) over 6.8 pmol/l who underwent total parathyroidectomy and autotransplantation of 60 mg of tissue were included in this research. Extension force of the quadriceps muscle was measured at 60° of right knee flexion and expressed as Newtons (N) in peak force and average force. Nerve conduction studies of four limbs were checked and the MNCV and CMAP of right femoral and tibial nerves were analyzed and correlated with the muscle force. Results - Three months after operation, the peak force increased from 272 ± 108 to 315 ± 123 N (P = 0.015) and the average force from 215 ± 94 to 253 ± 103 N (P = 0.006). MNCV and latencies of femoral and tibial nerves did not show definite change, but the amplitude of CMAP increased significantly from 7.1 ± 4.1 to 10.7 ± 3.2 mV (P = 0.005) at femoral stimulation and from 9.8 ± 4.6 to 11.7 ± 4.3 (P = 0.007) and 13.2 ± 5.9 to 14.9 ± 6.0 (P = 0.011) at proximal and distal tibial stimulation. Conclusion - The improvement of muscle force after surgery was found to be parallel to the increment of the amplitude of CMAP but not to nerve conduction velocity or latency. We propose that the weakness of the patients with secondary hyperparathyroidism is probably related to alteration of muscle fiber contraction.