Electrophysiological and Functional Effects of Shock Waves on the Sciatic Nerve of Rats

Abstract

Extracorporeal shockwave therapy (ESWT) has been applied in lithotripsy and treatments of musculoskeletal disorders over the past decade, but its effects on peripheral nerves remain unclear. This study investigated the short-term effects of shockwaves on the sciatic nerve of rats. The nerves were surgically exposed and then stimulated with shockwaves at three intensities. We evaluated the motor nerve conduction velocity (MNCV) of treated sciatic nerves before, immediately after (day 0) and at 1, 4, 7 and 14 d after shockwave treatment. Two functional tests—the sciatic functional index and the withdrawal reflex latency—were evaluated before and at 1, 4, 7 and 14 d after shockwave application. The rats were sacrificed on days 0, 1, 4, 7 and 14 for morphologic observation. The degassed treatment group received high-intensity shockwave treatment using degassed normal saline as the contact medium, and MNCV was measured before and on days 0, 1, 4, 7 and 14. The sham group received the same procedure as the treatment groups (i.e., the surgical operation to expose the sciatic nerve) but with no shockwave treatment. The control group received no surgical operation or shockwave treatment. The results showed moderate decrease in the MNCV after shockwave treatment and damage to the myelin sheath of large-diameter myelinated fibers. The effect was largest (reduction to 60.9% of baseline MNCV) and of longest duration (7 to 14 d) in the high-intensity group. There were no significant changes in functional tests. These results indicated that direct application of shockwaves can induce reversible segmental demyelination in large-diameter fibers, with the electrophysiological changes being positively correlated with the intensity of the shockwaves. (E-mail: [email protected])

Introduction

Shockwaves are characterized by high positive pressures (∼100 MPa) and negative pressures (5 to 10 MPa), a rapid rise time (30 to 120 ns) and a short pulse duration (5 μs) (Sturtevant 1996). They were first used in lithotripsy to treat kidney stones (Chaussy et al. 1982) and subsequently to treat musculoskeletal disorders (Loew et al 1995, Rompe et al 1996a, Rompe et al 1996b).

The effects of extracorporeal shockwave therapy (ESWT) on bone, cartilage, connective tissue and vessels have been studied in human and rabbits, with the aim of understanding mechanisms that underlie treatments for musculoskeletal disorders (Durst et al 2002, Wang et al 2003). The effects of ESWT on peripheral nerves have also been reported recently. Applying ESWT to normal rat skin can induce degeneration of intracutaneous nerve fibers, with this effect reversing within two weeks (Ohtori et al 2001, Takahashi et al 2006). It was postulated that this effect is responsible for the immediate pain relief after ESWT. However, the myelin sheath can be damaged histologically by ESWT in horses and dogs (Bolt et al 2004, Wang et al 2002), and further functional damage induced by such histologic changes would challenge the safety of ESWT. The range of clinical indications has widened, with ESWT being applied to different body regions. However, the possibility of damage to the nerve remains unknown because the nerves cannot be seen from the body surface. If damage to peripheral nerves does occur, it is not known whether these changes are long lasting, cause functional damage or relate to the intensity of ESWT.

The purpose of this study was to evaluate the short-term effects of shockwaves on the electrophysiological, histologic and functional properties of mixed peripheral nerves. We hypothesized that shockwaves could induce reversible segmental demyelination of peripheral nerves, with this effect being correlated with the shockwave intensity.