Sensory Thresholds of Normal Human Feet

Abstract

Hypotheses/Purpose: Although several studies in the literature have evaluated the abnormal sensory thresholds of diabetic feet to Semmes-Weinstein monofilament testing, there is very limited data on the sensory thresholds of individuals without diabetes or peripheral neuropathy. The purpose of this study was to assess the dorsal and plantar sensation of the feet from 40 healthy, college-aged volunteers using Semmes-Weinstein monofilaments. Conclusions/Significance: Semmes-Weinstein testing is a useful tool in predicting which diabetic patients may be at risk for ulceration of the feet. Several studies have determined 5.07 to be the threshold for protective sensation. Based on the normal values derived in this study, the inability to feel a Semmes-Weinstein monofilament of 5.07 (as in diabetic neuropathy) represents a sensory threshold that is more than 50 times greater than normal. This means that roughly 98% of the sensory ability has been lost. Methods: 20 male and 20 female volunteers between the ages of 18 to 22 years old were selected. None had a history of any significant injury or previous surgery to the foot or ankle. There were no known medical conditions associated with decreased foot sensation, (e.g.- diabetes, syphilis, leprosy, myelomeningocele, syringomyelia, or hereditary neuropathy). Volunteers were also questioned regarding participation in athletic activities. The subjects were blindfolded with the leg resting comfortably on a chair as 14 plantar and 5 dorsal locations were tested on each foot. The right foot was always tested first. Each site on the foot had the Semmes-Weinstein monofilaments applied to it first, in an order of increasing stiffness, then repeated in decreasing order, using all twenty monofilaments in the set. A positive threshold response was recorded when the subject could feel the filament and could accurately locate where on the foot the stimulus had been applied. The left foot was then tested in an identical fashion. Results: The mean sensitivity for all sites was 3.63 (0.0075 SEM). There were significant differences between sites, between using increasing or decreasing monofilament stiffness, between subjects, and in some instances, between right foot and left foot values. When testing was performed from the higher to lower monofilament stiffness, subjects were found to have significantly better sensitivity, which indicates the importance of a consistent testing protocol (either all up or all down). Sensation in the lesser toes and the arch were the most sensitive followed by the hallux and the plantar metatarsal heads. The least sensitive site was the heel, with 1/6th the sensitivity of the most sensitive toes.