To examine the correlation between muscular pain and bladder hypersensitivity in order to clarify the pathogenesis of comorbidity of bladder pain syndrome/interstitial cystitis with other chronic pain conditions such as fibromyalgia syndrome (FMS).
Under isoflurane anaesthesia, 0.2 mL of hydrochloric acid (HCl) solution (pH 4.0) was injected into the bilateral gluteus muscles of female Sprague-Dawley rats to produce an FMS model, as the gluteus is one of the specific tender points in patients with FMS. Control rats received saline injection (0.2 mL). The mechanical sensitivity of the plantar was evaluated using the mean number of bilateral hindlimb withdrawals in response to tactile stimulation with a 2.0-g von Frey filament at 1, 2 and 3 weeks after the HCl injection. In a separate rat group, cystometry was performed with the rats awake during saline infusion (0.06 mL/min) into the bladder before and after 1% lidocaine injection (0.2 mL) into the bilateral gluteus 1, 2 and 3 weeks after the HCl injection.
The mean number of hindlimb withdrawals was significantly higher in FMS rats than in controls at 1 and 2 weeks. Using cystometry, we found that the intercontraction interval (ICI) and voided volume (VV) were significantly lower in FMS rats than in controls at 1 and 2 weeks. In addition, the voiding threshold pressure, ICI and VV were significantly higher after lidocaine injection in FMS rats, but not in controls, at 1 and 2 weeks.
HCl injection (pH 4.0) into the gluteus can induce plantar hypersensitivity and urinary frequency for up to 2 weeks after the injection, suggesting that somatic (gluteus)-to-visceral (bladder) cross-sensitization might underlie bladder hypersensitivity in patients with FMS. Moreover, intervention at specific tender points outside the bladder could be effective in treating urinary frequency because lidocaine injection into the gluteus normalized bladder function in FMS rats for up to 2 weeks.