Identifying patients with diabetes mellitus who are at risk for lower extremity complications: use of Semmes-Weinstein monofilaments.

Plantar ulcers and lower-extremity amputation are frequent, severe, and; costly problems in patients with diabetes mellitus (DM).[1] Although there are many risk factors for developing foot problems, peripheral neuropathy is an important factor that seems to predispose patients to plantar ulcers and amputation.[2]

Patients with DM and peripheral neuropathy typically show a gradual loss of sensation on the plantar surface of their feet in a stocking-glove pattern.[2] They seem to reach a threshold of insensitivity that puts them at risk for developing unnoticed ulcers on the plantar surface of the foot. This threshold has been referred to as the level of "protective sensation."[3] Clinicians need a method of easily and reliably testing for protective sensation to identify patients who are at risk for foot problems. Identification of those patients may allow preventive intervention.

Studies have indicated that Semmes-Weinstein (SW) monofilaments can be an effective, reliable, and useful tool for identifying patients with DM who are at risk for foot problems.

Description of SW Monofilaments

Semmes-weinstein monofilaments are calibrated nylon monofilaments (Figure). Monofilaments generate a reproducible buckling stress and are identified by manufacturer-assigned numbers that range from 1.65 to 6.65. The higher the value of the monofilament, the stiffer and more difficult it is to bend, according to the formula: nominal value = [log.sub.10][force (in milligrams) X 10].(*) Three monofilaments commonly used to screen patients at risk for peripheral neuropathy are the 4.17, the 5.07, and the 6.10.[3,4] Forces required to bend these monofilaments are 1, 10, and 75 g of force, respectively.[3]

Key Words: Ankle and foot, Diabetes, Peripheral neuropathy, Ulcers. [Mueller MJ. Identifying patients with diabetes mellitus who are at risk for lower-extremity complications: use of Semmes-Weinstein monofilaments. Phys Ther. 1996;76:68-71.1

The monofilaments can be used to measure the patient's ability to sense a point of pressure. The rationale for testing the ability of a patient to sense pressure is that repeated bouts of moderate amounts of unnoticed pressures are thought to be the primary mechanism for development of plantar ulcers in patients with DM and peripheral neuropathy.[2]

Ability to Determine Levels of Pr-otective Sensation

Birke and Sims[3] were the first to define the level of protective sensation using 4.17, 5.07, and 6.10 monofilaments in a group of 72 patients with Hansen's disease and 28 patients with DM.[3] The lowest threshold of pressure sensation in patients with a history of an ulcer was SW monofilament 6. 10. No patient with a neuropathic ulcer was able to sense the 5.07 monofilament. The authors concluded that the 5.07 monofilament was the best indicator of protective sensation.[3] Other studies,[4-8] all retrospective, have confirmed these results.

Sosenko et al[9] used 17 different monofilaments and concluded that the 4.21 monofilament was the threshold that best discriminated between patients with and without foot ulcers. The 4.21 monofilament lies between the 4.17 and the 5.07 monofilaments reported in the other studies and may represent a more specific level of protective sensation. Additional studies are needed to verify this level.

Rith-Najarian et al[10] conducted a prospective evaluation of simple screening criteria, which included the 5.07 SW monofilament. Patients with DM in a well-defined American Indian population were stratified into risk categories based on ability to sense the 5.07 SW monofilament, the presence of foot deformity, and a history of lower-extremity ulceration or amputation. Over a 32-month prospective period, 41 of the 358 patients developed ulcers and 14 patients underwent a lower-extremity amputation. Insensitivity to the 5.07 SW monofilament occurred in 19% of the patients screened. Among this group, the odds ratio of subsequent ulceration was 9,9 and the odds ratio of amputation was 17 compared with the odds ratios among those who retained sensation.[10]

Preliminary results from another prospective study also support us of the 5.07 SW monofilament as a screening tool. Boyko et al[11] are following 778 subjects with DM in an internal medicine outpatient clinic. They report having observed 88 foot ulcers and 20 lower-extremity amputations over a cumulative 1,375 person-years of follow-up. Inability to sense the 5.07 SW monofilament was an independent predictor for higher risk of foot ulceration (odds ratio=18.42).[11] These prospective studies,[10,11] along with retrospective studies,[3-8] provide considerable evidence to support the use of the 5.07 SW monofilament as a valid predictor of patients at risk for foot problems.

Methods for Testing With SW Monofilaments

All studies agreed that the SW monofilaments are easy to use. In general, the patient is instructed to lie supine in a quiet area. Subjects are blindfolded or told to close their eyes to eliminate visual input. The filament is applied perpendicular to the skin (Figure) with enough force to cause the monofilament to buckle for approximately I second.[3]

Sensory testing with SW monofilaments can be applied to any anatomical region, but when used as a screen for risk of foot ulcers, testing usually is per-formed in those regions at highest risk for skin breakdown. Those regions on the plantar surface of the foot at highest risk for skin breakdown are the metatarsal heads and the great toe.[3,6,10] Mueller et al[4] tested the first, third, and fifth metatarsal heads and toes; the medial and lateral midfoot; and the heel. Holewski et al5 also advised testing the dorsal surface of the foot between the base of the first and second toes, the base of the third digit, and the base of the fifth metatarsal. Their rationale was that, in addition to testing the function of the plantar ner-ves, use of these sites provides a representation of different peripheral nerves and dermatomes of the foot.[5]

Patient instruction

Two methods of patient instruction have been described., The "yes-no" method instructs the patient to say "Yes" each time he or she senses the application of a SW monofilament.[3,4,12] Five to 10 trials are to be taken at each site, and the patient needs to respond to 80% of the trials to be graded a given value at that site. If the patient is unable to sense at least 80% of the trials, he or she is tested with the next highest monofilament. Using this method, Birke and Sims[3] reported reliability "at the 95% confidence interval." Diamond et al[2] reported Kappa values for intrarater and interrater reliability ranging from .72 to .83.

Other authors[5,6,9,10] reported using a "forced-choice" procedure. Using this procedure, the monofilament is applied to each testing site during only one of two time intervals, with the count of either "one" or "two." The patient is asked whether he or she felt any sensation after the first or second interval, but the patient is allowed to report that he or she could not determine the interval during which the stimulus was applied.[5] Holewski et al[5] argued that this method forces the patient to choose when the stimulus was applied during one of the two possible intervals, eliminating the need for the patient to decide when a stimulus was sufficiently above the background level to justify a response. The authors tested a group of patients three times and found that the responses agreed in 89% of the trials.[5] Gerr and Letz[13] compared the yes-no method with the forced-choice method when using a test of peripheral cutaneous vibration sensitivity. They concluded that the yes-no method was more reliable and less time consuming than the forced-choice method.[13]

Criteria for Determining Level of Protective Sensation Authors have differed slightly in their criteria for determining protective sensation. The more conservative approach advocates that the foot be considered "at risk" if any portion of the foot is insensitive to the 5.07 monofilament.[3,4,8,10] Others[5] have suggested that three of six sites should be insensitive to the 5.07 monofilament to be considered "at risk." The one completed prospective study that identified a strong correlation between ability to sense the 5.07 SW monofilament and foot problems graded the foot as 'insensitive' if one or more areas failed to sense the monofilament.[10] These results are strong evidence to support the more conservative approach.

Authors have agreed that testing with SW monofilaments is inexpensive and easy when compared with other quantitative tests of sensation. Studies[6-9] have demonstrated that SW monofilament testing is as effective as or superior to more time-consuming tests of vibration, temperature, and peroneal nerve current perception in identifying patients at risk for ulceration.

Limitations of This Review

Other methods are available to indicate the presence of peripheral neuropathies, and many factors other than peripheral neuropathy may contribute to foot problems in patients with DM. The focus of this update, however, is to review the use of SW monofilaments in measuring pressure sensation to screen patients at risk for foot problems. The interested reader is referred to other references that provide a thorough overview of the factors that can contribute to foot disease in patients with diabetes and appropriate assessment and management strategies for this patient population.[2,14,15]

Summary

Research indicates that the SW monofilament is an inexpensive, reliable, valid, and easy-to-use clinical' indicator for identifying patients who are at risk for developing foot ulcers and subsequent amputations. Those patients unable to sense the 5.07 SW monofilament on any part of their foot should be provided preventive care, including patient education and prescription of appropriate therapeutic footwear.

References

[1] Most RS, Sinnock P. The epidemiology of lower-extremity amputation in diabetic individuals. Diabetes Care. 1983;6:87-91.

[2] Bild DE, Selby JV, Sinnock P, et al. Lower-extremity amputation in people with diabetes: epidemiology and prevention. Diabetes Care. 1989;12:24-31.

[3] Birke JA, Sims DS. Plantar sensory threshold in the Hansen's disease ulcerative foot. Read at the Proceedings of the International Conference on Biomechanics and Clinical Kinesiology of Hand and Foot; Madras, India; December 16-18,1985.

[4] Mueller MJ, Diamond JE, Delitto A, Sinacore DR. Insensitivity, limited joint mobility, and plantar ulcers in patients with diabetes mellitus. Phys Ther. 1989;69:45,9-462.

[5] Holewski JJ, Stress RM, Graf PM, Grunfeld C. Aesthesiometry: quantification of cutaneous pressure sensation in diabetic peripheral neuropathy. J Rehabil Res Dev. 1988;25:1-10.

[6] Kumar S, Fernando DJS, Veves A, et al. Semmes-weinstein monofilaments: a simple, effective and inexpensive screening device for identifying diabetic patients at risk of foot ulceration. Diabetes Res Clin Pract. 1995;13: 63-68.

[7] Olmos PR, Cataland S, O'Dorisio TM, et al. The Semmes-weinstein monofilament as a potential predictor of foot ulceration in patients with noninsulin-dependent diabetes. Am J Med Sci. 1995;309:76-82.

[8] McNeely MJ, Boyko EJ, Ahroni JH, et al. The independent contributions of diabetic neuropathy and vasculopathy in foot ulceration: How great are the risks? Diabetes Care. 1995;18:216-219.

[9] Sosenko JM, Kato M, Soto R, Bild DE. Comparison of quantitative sensor),threshold measures for their association with foot ulceration in diabetic patients. Diabetes Care. 1990;13:1057-1061.

[10] Rith-Najarian SJ, Stolusky T, Gohdes DM. Identifying diabetic patients at high risk for lower-extremity amputation in a primary health care setting: a prospective evaluation of simple screening criteria. Diabetes Care. 1992;15: 1386-1389.

[11] Boyko EJ, Smith DG, Ahroni JH. A prospective study of risk factors for diabetic foot ulcer. Rehabilitation R & D Progress Reports. 1994-1995;32:318-319.

[12] Diamond JE, Mueller MJ, Delitto A, Sinacore DR. Reliability of a diabetic foot evaluation. Phys Ther. 1989;69:797-802.

[13] Gerr FE, it. R. Reliability of a widely used test of peripheral cutaneous vibration sensitivity and a comparison of two testing protocols. Br J Ind Med. 1988;45:635-639.

[14] Caputo GM, Cavanagh PR, Ulbrecht JS, et al. Assessment and management of foot disease in patients with diabetes. N Engl J Med. 1994;331:854-860.

[15] Levin ME. Pathogenesis and management of diabetic foot lesions. In: Levin ME, O'Neal LW, Bowker JH, eds. The Diabetic Fool. 5th ed. St Louis, Mo: Mosby-Year Book; 1993:17-60.

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*Research Designs Inc, 7320 Ashcroft, Houston, TX 77081.

MJ Mueller, Phd, PT, is Assistant Professor, Program in Physical Therapy, Washington University School of Medicine, Box 8502, 4444 Forest Park Blvd, St Louis, MO 63110 (USA) ([e-mail]).

This work was supported in part by a grant (R01-HD 31486-01) from the National Institutes of Health, National Center for Medical Rehabilitation