Von Frey filaments used for testing mechanical thresholds are mechanically unstable and their use is difficult to standardize. We have therefore constructed a hand-held electronic pressure algometer which corrects for these problems.

The pressure algometer is connected to a computerized data collection system, allowing on-line display 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 have measured the pressure induced withdrawal thresholds in rats with surgically induced neuropathy and compared the results with data obtained in the same animals by conventional use of Von Frey filaments. Five applications were made to each hind paw. The probe, with a circular tip of 1.0 mm diameter, was applied manually with a pressure increasing by approximately 0.05 N/second. Before surgery the withdrawal thresholds of the paws were normally distributed with a mean of 0.415 N, showing no significant difference between paws. During the two weeks of measurement after surgery, the mean withdrawal thresholds of the operated side were significantly reduced (range: 0.209-0.318 N), while the mean thresholds of the non-operated side remained at higher values (range: 0.432-0.491 N). Mean withdrawal thresholds of control rats without surgery were in the 0.380-0.520 N range, with no significant difference between paws or over time. The overall pattern of results were similar when Von Frey filaments were used but the variability was greater.

The visual feedback during application of the electronic algometer provides a means for standardized testing. Results obtained by two independent investigators aiming at the same application rate showed high inter-observer reliability, both between withdrawal threshold values obtained and between rates of application used.

In conclusion, the electronic algometer allows detailed documentation of each experiment and provides an objective and accurate method for measuring the reactions of test animals to mechanical stimuli.