The American Diabetes Association recommends the use of the 10-g Semmes-Weinstein monofilament for the early identification of diabetic patients at risk for foot ulceration(1). However, this 10-g monofilament can only detect advanced diabetic peripheral neuropathy (DPN). Thus, we evaluated whether other monofilaments could be useful for the detection of early DPN with high sensitivity and high specificity, and compared the results of monofilament tests with the results of microneurography.
We recruited 65 patients (aged 61.0 ± 1.3 years, diabetes duration 9.8 ± 1.0 years [range 1-35]) with type 2 diabetes to participate in this study. Of these patients, 24 were treated with oral hypoglycemic agents,32 were treated with insulin, and 9 were kept on a controlled diet only. In addition, 19 patients had background retinopathy, 2 had proliferative retinopathy, and 21 had microalbuminuria (>30 mg/day) defined as diabetic nephropathy. Patients with at least two of the following criteria were diagnosed with DNP: 1) numbness in the toes, 2) loss of ankle jerk, and 3) decreased vibratory sensation assessed by a 128-Hz tuning fork (<10 s). Participants were tested for their ability to sense three kinds of Semmes-Weinstein monofilament, with target forces of 2, 4, and 10 g, respectively, at three sites of the foot: the great toe, the plantar aspect of the first metatarsal, and the plantar aspect of the fifth metatarsal(2,3). Individual cut off monofilament was defined as the lowest target force to sense. Microneurography was performed as previously described(4).
The sensitivity and specificity of the 2-g monofilament in the detection of DPN were 0.48 and 0.86, respectively. On the other hand, the corresponding values for the 4-g monofilament, 0.85 and 0.73, respectively, were quite close to those for the 10-g monofilament (0.88 and 0.68, respectively). Thus, we concluded that the 4-g monofilament was clinically useful for detecting DPN. Participants were divided into two groups based on a cut off target force: the 2-g monofilament group (group A, n = 52) and the 4- and 10-g monofilaments group (group B, n = 13). The maximal nerve velocity of group B was significantly lower than that of group A (50.4 ± 1.5 vs. 57.3 ± 0.6 m/s, P < 0.0001). The amplitude of group B was also significantly lower than that of group A (119.9 ± 31.8 vs. 184.1± 11.5 μV, P < 0.05).
The 10-g monofilament has typically been considered the easiest tool to use for detecting the loss of protective sensation(5,6). However, the 10-g monofilament is only capable of detecting severe DPN, and there is no information on the usefulness of other monofilaments for detecting early DPN. Consistent with studies on micro-neurography, our data show that the 4-g monofilament is clinically useful in the detection of relatively early DPN. Although the sensitivity and specificity of the 4-g monofilament were not sufficient for detecting DPN, this monofilament might be the easiest to apply to the entire diabetic population when factors such as cost, ease of application, and portability are taken into consideration. Because the number of participants was limited, further study is needed to clarify the usefulness of the 4-g monofilament for detecting early DPN.
