Assessing mechanical allodynia in the rat paw with a new electronic algometer

Von Frey filaments used for testing mechanical thresholds are mechanically unstable and their use is difficult to standardize. We have therefore const ructed a hand-held electronic pressure algometer. The pressure algometer is connected to a computerized data collection system, allowing on-line displ ay of the applied force as well as the application rate. Data stored on the computer can be replayed and further analyzed. Using this apparatus, we ha ve measured the pressure-induced withdrawal thresholds in rats with surgica lly induced neuropathy. The probe, with a circular tip of 1.0 mm diameter, was applied manually with a pressure increasing by approximately 0.05 N/s. Presurgical thresholds were normally distributed with a mean of 0.415 N, sh owing no significant difference between paws. During 2 weeks after surgery, the thresholds of the operated side were significantly reduced (range, 0.2 09-0.318 N), while the thresholds of the non-operated side remained at high er values (range, 0.432-0.491 N). Thresholds of control rats without surger y were in the 0.380-0.520 N range, with no significant difference between p aws. In an additional experiment it was shown that interobserver reliabilit y was high, both between withdrawal threshold values obtained and between r ates of application used. In conclusion, the electronic algometer allows st andardization of testing, detailed documentation of each experiment and pro vides an objective and accurate method for measuring the reactions of test animals to mechanical stimuli. (C) 1998 Elsevier Science B.V. All rights re served.