BACKGROUND: Spinal cord stimulation (SCS) is an effective treatment for neuropathic pain, but its effect on chronic muscle pain is unclear. We designed this study to test the effect of SCS in an animal model of noninflammatory muscle pain.
METHODS: Male Sprague-Dawley rats were implanted with an epidural SCS lead on the upper lumbar spinal cord (L3-L4) under isoflurane anesthesia (4%). Ten days after implantation, chronic muscle pain was induced by giving 2 injections of pH 4 saline into the left gastrocnemius muscle, 5 days apart. In experiment 1, SCS was delivered daily (6-hour duration/day) for 4 days at one of 4 different frequencies (0 (sham), 4, 60, and 100 Hz) from day 6 to day 9. Paw withdrawal threshold and muscle withdrawal threshold were measured before the first injection, and before and during SCS daily. Physical activity (distance, crossing, stand, and grooming) was assessed before the first injection, before SCS on day 6 and during SCS on day 9. In experiment 2, SCS was delivered (6 hours) on day 6 at either 60 or 100 Hz. Paw withdrawal threshold and muscle withdrawal threshold were assessed before the first injection, before and during SCS on day 6, and daily for the following 3 days (day 7–day 9).
RESULTS: Paw withdrawal threshold and muscle withdrawal threshold significantly decreased bilaterally after the second injection of acidic saline. SCS delivered at 60 or 100 Hz significantly reversed the decreased paw withdrawal threshold and muscle withdrawal threshold bilaterally when compared with that of sham SCS, but 4 Hz SCS had no effect on paw withdrawal threshold and muscle withdrawal threshold. SCS (60 or 100 Hz) delivered daily provided a persistently reversed effect, and SCS delivered singly provided a carryover effect for 24 hours. During 60 Hz SCS, the distance traveled and the number of crossings increased significantly when compared with that of sham SCS.
CONCLUSIONS: The current study shows that higher frequencies of SCS (60 and 100 Hz) significantly reduce mechanical hyperalgesia of the paw and muscle in an animal model of noninflammatory muscle pain, and 60 Hz SCS restores physical activity levels of animals, not 4 Hz.