Increased Intramuscular Pressure in Lumbar Paraspinal Muscles and Low Back Pain: Model Development and Expression of Substance P in the Dorsal Root Ganglion

The association between intramuscular pressure and low back pain was investigated by measuring intramuscular pressure and blood flow, assessing histologic appearance, and performing immunohistochemical testing in rats. To develop an experimental rat model of increased intramuscular pressure (IMP) in the lumbar paraspinal muscles accompanied by reduced intramuscular blood flow (IMBF). The expression of neuropeptides in the dorsal root ganglion of the experimental model was also investigated. Studies have reported that IMP in the lumbar paraspinal muscles is one of the causes of chronic low back pain. However, the pathology of low back pain accompanied by IMP has not been sufficiently clarified. A balloon was inflated below the vertebral fascia of rats (balloon group) and intramuscular pressure and blood flow in the lumbar paraspinal muscles were measured. Intramuscular pressure was measured using a pressure transducer, whereas IMBF was measured using a contact-type laser Doppler flowmeter. Compared with the sham operation group, intramuscular pressure was higher and IMBF was lower for the balloon group at 1 hour and 1 day after insertion. In addition, at 1 hour and 1 day after insertion, IMBF and pressure were continuously measured while rats were positioned in flexion for 1 hour. Intramuscular pressure was significantly higher and IMBF was significantly lower in the balloon group at 1 day after insertion (P < 0.05). Expression of substance P, a neuropeptide, was also observed in the dorsal root ganglion of the first lumbar vertebra. These findings suggest that IMP and decreased IMBF in the lumbar paraspinal muscles induce inflammation and pain in the lower back.