H-Reflex Plasticity Following Cast Immobilization Of Distal Radius Wrist Fractures

Abstract

The Hoffmann (H)-reflex has been used as a recovery marker in disuse-immobilization models. The data is limited on the effects of forearm immobilization on the H-reflex, especially in clinical populations. PURPOSE: To determine H-reflex changes following a distal radius wrist fracture requiring cast immobilization. METHODS: Five fracture (FX) patients (4 W; 1 M; age: 45.2±18.9 yrs; ht: 164.1±7.0 cm; wt: 64.4±4.9 kg) and five uninjured age-matched controls (CON) (4 W; 1 M; age: 44.6±17.3 yrs; ht: 171.5±5.5 cm; wt: 68.9±8.8 kg) were enrolled. FX group was measured four times up to 12-weeks post fracture; the CON group was tracked over the same period, with measures taken at least three weeks apart. Hmax and Mmax were elicited in the flexor carpi radialis (FCR) of each arm via stimulation of the median nerve (0.5 ms pulse) under a 10% maximal voluntary contraction (MVC). A recruitment curve was built using 0.5 mA increments. Measures included wrist flexion (WF) torque (dynamometer), peak electromyography (EMG) amplitude normalized to Mmax, grip strength, range of motion (ROM; goniometry), muscle thickness (MT; ultrasound), and function via Patient-Rated Wrist Evaluation (PRWE). RESULTS: In the fractured limb, Hmax:Mmax amplitude ratio (23.0±16.1 to 23.1±12.0%) did not change over time (p > 0.05), but Hmax:Mmax stimulation ratio significantly decreased: 70.5±2.9% to 63.1±4.5% (p < 0.05). WF increased during recovery (6.4±1.2 to 8.5±2.3 Nm; p < 0.05), but normalized EMG during MVC showed a decreasing trend from 18.3±6.4 to 13.8±9.6% Mmax (p = 0.054). Time main effects were also evident for MT (2.7±0.5 to 3.2±0.5cm; p < 0.05), grip strength (6.0±6.8 to 19.6±8.5kg; p < 0.001), wrist ROM (87.9±18.7 to 119.4±20.5°; p < 0.05), and PRWE (84±24 to 28±32 score; p < 0.001), all reflecting incomplete recovery up to 12-weeks post fracture. No significant differences were shown for the uninjured arm of FX group or in any measures for CON, except WF MVC (24.7±7.2 to 22.0±6.9Nm; p < 0.05). CONCLUSION: Although peak amplitude of the H-reflex did not change during recovery, decreased intensity needed to evoke Hmax could reflect changes to the excitability of spinal circuitry after wrist fracture. These data have important therapeutic implications for understanding how the nervous system is impacted after wrist fracture and cast immobilization.