Although it is debatable today, the Semmes-Weinstein Monofilament Examination (SWME) is considered to be the gold standard for first-line screening of diabetic peripheral neuropathy in the feet.^1,2 Testing the feet of patients with diabetes for loss of sensation can help clinicians evaluate if a patient is at a high risk for developing wounds and ulcers as well as potentially needing a lower extremity amputation.^3,4 The test itself involves applying a monofilament wire (5.07 g) to the test site perpendicularly until it bends, then holding for one second (with a buckling force of 10 g). If patients sense the monofilament, they should say “yes”; if patients do not sense the monofilament after it bends, they are considered to be insensate.^3,4 This noninvasive, low-cost, and easy-to-perform test can easily be done in an office and clinic setting with a relatively lower user error.²

Although there are other exams to test for diabetic peripheral neuropathy (eg, pinprick, vibration perception), the SWME is still arguably the most used. With its place in wound care in mind as well as considering the large number of patients with diabetes being treated in wound clinics, this installment of the History of Wound Care column looks into the progression and history of this widely used screening tool.

From Horsehair to Nylon

Prior to the development of the SWME, the area of cutaneous sensation was underway in the 1880s.^5,6 There is published literature dating back to the early 1880s on the study of the sensitivity of the human skin; these publications originated in Sweden, Germany, and the United States.⁵ The discovery of sensory spots influenced research from others during this time, including Maximillian von Frey.

In the 1890s, von Frey studied discrete pain points on the human skin when probed with a horsehair at threshold intensity for sensation.⁵ Von Frey varied the size of the hair to determine the threshold force needed to produce the sensation.⁵ His investigation into this area led him to conclude that the skin is a mosaic of four types of sensory spots: touch, cold, warmth, and pain.^5,6 Thus, the birth of the von Frey filament test over 120 years ago.

Inspired by the von Frey horsehair instrument, Sidney Weinstein, PhD, along with Josephine Semmes, PhD, developed the Semmes-Weinstein Pressure Aesthesiometer in 1958 but swapped the horsehair out for nylon.^1,7 They published their research in a book, Somatosensory Changes After Penetrating Brain Wounds in Man, in 1960.⁸ However, like most medical advancements, its initial use is not the same as its use today. For Semmes and Weinstein, their research was conducted to test sensory loss in the hands of patients with brain injury, which then gained recognition from its use in leprosy research.^8,9 Due to its increased recognition, the SWME was used as a standard assessment tool in patients with diabetic peripheral neuropathy.⁹

Defining the Threshold

Naturally, research into the SWME was conducted. Birke and Sims were the first to define the sensory threshold of protective sensation on the plantar surface of the foot—ie, 5.07-gauge monofilament.⁹ Their study was conducted on 72 patients with leprosy and 28 patients with diabetes who had lower extremity ulcers. Birke and Sims used Semmes-Weinstein filaments in the gauge sizes of 4.17, 5.07, and 6.10 for pressure measurements. Among the inclusion criteria was that patients in the study had to have had prior foot ulceration, which is a limitation the researchers noted in their published report.⁹ However, their test concluded that the lowest pressure threshold of patients with leprosy and diabetes with a history of plantar foot ulcers was the Semmes-Weinstein filament 6.10 gauge. Therefore, the level of protective sensation was determined to be the Semmes-Weinstein filament 5.07 gauge with 10 g of force to buckle.⁹

Semmes-Weinstein in Foot Ulcer Screening

In a review article evaluating the results of 30 studies on the SWME, Feng et al determined there was a wide variation in the number and location of the sites for testing, with most of the selected articles lacking detailed information on this part of the technique.⁴ The evaluated studies did show that there was consensus on the test being performed on the plantar aspect of the first toe, which aligns with the report by Birke and Sims.^4,9 Feng et al also found that the advantages of SWME include the constant pressure as a more objective examination when diagnosing diabetic peripheral neuropathy.⁴

When considering the sensitivity of the SWME, Kumar et al compared 182 patients tested with vibration perception threshold by biothesiometry and Semmes-Weinstein monofilaments.¹⁰ They found that the Semmes-Weinstein monofilaments were more sensitive than biothesiometry (100% vs. 78.6%, respectively), but they were less specific than biothesiometry (77.7% vs. 93.4%, respectively).¹⁰ The authors concluded that the SWME is a reliable indicator in screening for patients at risk of developing foot ulcers; they may even be superior to biothesiometry since sensitivity is more important.¹⁰ Also of note, Kumar et al commented on the benefit of the inexpensiveness of the Semmes-Weinstein monofilament versus the more expensive biothesiometer.¹⁰

In another review of 9 studies, the authors further confirmed the SWME’s significance as an independent predictor of future foot ulcers in patients with diabetes.¹¹ The authors argued that if a patient with diabetes has a positive SWME result (compared with those having a negative result), their chances of developing an ulcer increase by 10%–20%. Further, the risk of undergoing a lower extremity amputation increase by 5%–15% in comparison to those with negative SWME results.¹¹

Like all screening tools used in medicine (including wound care), not everyone is a supporter and avid user of the SWME. There is a great variation in the literature regarding SWME’s diagnostic value as a result of different methodologies, but most agree the SWME should be used on the plantar aspect of the great toe, third metatarsal head, and fifth metatarsal head.^1,4,10,11

Conclusions

Although considered to be the gold standard in detecting diabetic peripheral neuropathy, the SWME was not originally intended for that purpose. Over the years, and through rigorous research in primates and humans, the von Frey horsehair was replaced with nylon in various gradients first to assess peripheral neuropathy in people with brain injuries, then in those with leprosy, and then further in those with diabetes.^4,8-11 Regardless of whether a clinician is a believer in the SWME, all should be able to agree that it is vital to detect diabetic peripheral neuropathy early in order to start earlier intervention and management to reduce the risk of patients developing ulcers and potentially requiring lower extremity amputations.

Jaclyn Gaydos is Managing Editor of Wounds: A Compendium of Clinical Research and Practice.

1. Craig AB, Strauss MB, Daniller A, Miller SS. Foot sensation testing in the patient with diabetes: introduction of the Quick & Easy assessment tool. Wounds. 2014;26(8):221-231.
2. Young M. A perfect 10? Why the accuracy of your monofilament matters. Diabet Foot J. 2008;11(3):106-111.
3. MedlinePlus. Diabetic foot exam. U.S. National Library of Medicine. Accessed June 13, 2020. Available at: https://medlineplus.gov/lab-tests/diabetic-foot-exam/
4. Feng Y, Schlösser FJ, Sumpio BE. The Semmes Weinstein monofilament examination as a screening tool for diabetic peripheral neuropathy. J Vasc Surg. 2009;50(3):675-682. doi:10.1016/j.jvs.2009.05.017
5. Norrsell U, Finger S, Lajonchere C. Cutaneous sensory spots and the “law of specific nerve energies”: history and development of ideas. Brain Res Bulletin. 1999;48(5):457-465. doi:10.1016/S0361-9230(98)00067-7
6. Pearce JMS. Von Frey’s pain spots. J Neurol Neurosurg Psychiatry. 2006;77(12):1317. doi:10.1136/jnnp.2006.098970
7. Cadogan M. Sidney Weinstein. Life in the Fast Lane. Updated June 6, 2019. Accessed on June 15, 2020. Available at: https://litfl.com/sidney-weinstein/
8. Semmes F, Weinstein S, Ghent L, Teuber HL. Somatosensory Changes After Penetrating Brain Wounds in Man. Harvard University Press; 1960.
9. Birke JA, Sims DS. Plantar sensory threshold in the ulcerated foot. Lepr Rev. 1986;57(3):261-267.
10. Kumar S, Fernando DJ, Veves A, Knowles EA, Young MJ, Boulton AJ. Semmes-Weinstein monofilaments: a simple, effective and inexpensive screening device for identifying diabetic patients at risk of foot ulceration. Diabetes Res Clin Pract. 1991;13(1-2):63-67. doi:10.1016/0168-8227(91)90034-b
11. Feng Y, Schlösser FJ, Sumpio BE. The Semmes Weinstein monofilament examination is a significant predictor of the risk of foot ulceration and amputation in patients with diabetes mellitus. J Vasc Surg. 2011;53(1):220-226. doi:10.1016/j.jvs.2010.06.100