Experts continue to debate the relative merits of the Semmes-Weinstein monofilament for assessing diabetic foot ulcer risk, including how many sites on the foot should be tested, and which ones. But it’s tough to argue with the technique’s cost-effectiveness.
By Cary Groner
As clinicians continue to refine protocols for assessing ulcer risk in the feet of diabetes patients, some researchers have questioned the utility of one old standby—the 10-g Semmes-Weinstein monofilament. Others have quickly responded that the monofilament remains the easiest and most sensitive of screening devices, though they acknowledge it is not as specific they’d like. The issue remained largely a matter of opinion until recent research began to shed more light on the role of monofilament testing in detecting the loss of sensation that puts feet at risk.
“Monofilament testing has been around a long time, and many people are familiar with it,” said David Armstrong, DPM, MD, PhD, professor of surgery and director of the Southern Arizona Limb Salvage Alliance at the University of Arizona College of Medicine in Tucson. “The tool itself may not be very accurate, but after study it does seem useful in indentifying clinically significant loss of sensation.”
Michael Pinzur, MD, professor of orthopedic surgery and rehabilitation at Loyola University Medical Center in Maywood, IL, concurred.
“There’s no question that there are more accurate methods than the monofilament, but they’re also more expensive and labor intensive,” he pointed out. “Primary care physicians are under tremendous stress; they have to see so many patients a day, and every specialist is telling them to screen for various conditions. They can’t do everything.”
Research supports the perception that first-line providers don’t screen enough, and that care quality suffers as a result. In 2008, for example, researchers reported on 565 patients with diabetes who were either referred to a chronic illness management (CIM) practice (n=288) or treated with usual care (n=277). Medical assistants were trained to perform an annual monofilament exam, per American Diabetes Association (ADA) guidelines. In the retrospective analysis, researchers discovered that only 30% of the control-group patients had received the exams annually. More troubling, however, was that only 65% of those in the CIM cohort got the test.1
Such findings are not unusual, and the consequences of clinical failure—ulceration, amputation, and death—are well known to those who treat patients with diabetes. Early detection is critical not only because of the severity of sequelae, but because up to half of diabetic peripheral neuropathy (DPN) is asymptomatic. Insensate patients are at greater risk of foot injury than those without neuropathy, and more than 80% of amputations follow a foot injury or ulcer.2,3
The Semmes-Weinstein device is simplicity itself: a plastic grip that holds a heavy nylon monofilament. With a couple dozen yards of the nylon line you could land a catfish; it’s basically the same stuff. When pressed against the foot, the line bends, exerting 10-g of force. If a patient can’t feel that force, the clinician is alerted that protective sensation may be subnormal. Research suggests that patients who can’t feel the monofilament have a 7.7-fold increase in ulceration risk.4
One problem, however, has been that not all monofilaments are created equal. Fortunately, there’s less variation than there used to be when they were made of horsehair. But even then there was dispute: was the mane better, or the tail?
A study published in Diabetes Care in 2000 reported that of 160 new 10-g nylon monofilaments, only those from a couple of manufacturers actually buckled within a gram of 10g. Worse, the devices—even the accurate ones—experienced material fatigue, bending too easily and giving false positives after testing about 10 patients. They had to be rested for 24 hours before regaining their firmness.5
As Armstrong pointed out in an editorial in that issue of Diabetes Care, “Cycles of applied stress may make these devices inaccurate, rendering them potentially hypersensitive in identifying loss of protective sensation.” Clinics should, he suggested, have multiple monofilaments available so clinicians don’t overdiagnose as the day wears on. “We can no longer afford to look at any tool, even one as useful and important as the monofilament, as an infallible diagnostic divining rod,” he concluded.6
As noted earlier, reports published in the past few years have questioned whether monofilament testing is the optimal screening approach. For example, researchers at the University of Miami reported in 2005 that both a biothesiometer (which detects a patient’s vibration perception threshold [VPT]), and a modified neuropathy disability score (NDS) based on four clinical tests, were more sensitive than a 10-g monofilament for assessing individuals at risk of foot ulcers.7 Moreover, a 2008 study reported that a pin-prick test more accurately delineated the extent of lower-limb neuropathy than a monofilament test in patients with diabetes who did not have a history of foot problems.8
Not all clinicians agree with such findings, of course.
“A pinprick is a waste of time,” said Pinzur, with characteristic candor. “How insensate do you have to be to not feel a pinprick? The point is to establish the patients with whom you’re going to intervene. The monofilament is the simplest, easiest method, and it can be used by any healthcare professional.”
Even so, clinicians diverge on many points—especially which points they test. A common approach—and that recommended by the ADA—is to test the plantar surface of the distal hallux, then the first, third, and fifth metatarsals on each foot.9 Needless to say, there are variants.
In a 1998 study assessing screening methods, Armstrong and his colleagues tested at 10 sites per foot: the plantar aspects of the first, third, and fifth digits; the plantar medial and lateral sides of the midfoot; the plantar area of the heel; and the dorsal aspect of the midfoot.10
This approach derived from monofilament testing’s origins assessing neuropathy associated with leprosy, as pioneered in the late 1960s by Paul Brand, MD, at the National Hansen’s Disease Center in Louisiana.
“Leprosy neuropathy doesn’t occur symmetrically in a distal-to-proximal fashion,” explained Lee C. Rogers, DPM, codirector of the Amputation Prevention Center at Valley Presbyterian Hospital in Los Angeles. “It can affect your heel before it affects your toes, and that’s the reason there were originally ten testing sites. You don’t need to do all those points in diabetes patients.”
A 1999 study published in the Journal of General Internal Medicine11 found that monofilament testing using only four sites per foot detected abnormal sensation in 90% of patients diagnosed using eight sites per foot. Those four sites have subsequently been recommended by the ADA protocol, which Armstrong said that he and his colleagues now follow.
“We believe it to be more efficient, and we are likely not overdiagnosing too many patients,” he said.
The field trends toward minimalism, in fact. In a review published in the Journal of Vascular Surgery in 2009,12 Yale researchers concluded that to maximize the monofilament’s accuracy, clinicians should use a three-site test involving the plantar aspects of the great toe, the third metatarsal, and the fifth metatarsal based on the findings of three previous studies.11,13,14 There are other permutations; some clinicians don’t like testing callused metatarsals, for example, because the calluses interfere with the skin’s elasticity and sensation.
There’s also debate about whether 10 g of pressure is the best threshold to detect sensation loss. Minimalism applies here, too. For example, a study from the University of Iowa published in the Journal of Bone & Joint Surgery in 2004 reported that if a patient cannot feel a 4.5-g monofilament under either first-metatarsal head, he has probably lost protective sensation.15 A 2005 study from Japan found that a 2-g monofilament tested at the great toe or the plantar aspect of the fifth metatarsal was the most useful diagnostic test for DPN.16 Research published in the Journal of Foot and Ankle Research in 2008 concluded that the 6-g monofilament identified the threshold of normal sensory perception when detected at six sites on the plantar aspect of the feet.17
Some of this professional disagreement is related to different ideas about what condition, exactly, clinicians are screening for. Often, as noted above, research has evaluated the monofilament as a test for DPN, and results have varied widely. But increasingly, clinicians are carefully specifying that monofilament testing is not for diagnosing neuropathy, but rather for identifying the high-risk foot. This is not a new idea, but lately it’s gained traction.
“It’s a subtle distinction,” Armstrong said. “My perspective is that neuropathy is a spectrum; loss of protective sensation is a point along that sensory spectrum beyond which one is at greater risk for getting a sore. If the patient can’t feel the monofilament at certain areas, then we say they’ve lost protective sensation.”
The monofilament is not good at detecting early neuropathy, acknowledged Andrew Boulton, MD, a professor of medicine at the University of Manchester, U.K., and a visiting professor at the University of Miami. But it may be very good for detecting clinically significant neuropathy.
“We’re talking about identifying the patient with neuropathy of sufficient severity that they are at risk of developing foot ulcers,” Boulton said.
Interestingly, with this simple change in language—and the shift in diagnostic perspective it implies—the studies often look much more positive. For example, a literature review of threshold tests for “preventing foot ulceration” concluded in 2000 that the monofilament “is currently the best choice for screening for clinically significant neuropathy because it is portable, inexpensive, painless, easy to administer, acceptable to patients, and provides good predictive ability for the risk of ulceration and amputation.”18
Similarly, a paper published in 2011 reported that the monofilament was a reliable clinical tool to assess changes in protective sensation of the feet in 28 older patients between ages 50 and 78 years, of whom six had been diagnosed with diabetes and three with neuropathy.19 And a literature review from the same year concluded that in nine studies comprising 11,007 patients, all found monofilament testing “a significant and independent predictor of future foot ulceration” in patients with diabetes.20
It’s also important to establish what constitutes a positive test. In the 1998 paper by Armstrong et al, loss of sensation at more than four sites out of 10 was considered positive. A 2010 study from Canada concluded that having fewer than five of eight sensate stimuli discriminated four-year risk of DPN.21 Rogers, who, like Armstrong, uses the ADA guidelines, said that he considers the loss of sensation at one site out of the recommended four to be positive.
Of course, such tests are only as good as the person administering them, so it’s important to pay attention to the nuances.
“You have to do it in the appropriate manner,” Rogers said. “The test takes active participation from the patient, which can confound your results.”
He has gotten good results by using a “forced choice.”
“You don’t just ask if they feel the monofilament, because you’ll get a lot of false positives, particularly if they have their eyes open,” he said.
He has patients close their eyes, then tests the same site twice, with the same pressure. He asks, “Do you feel it now? Or now?” The results of this approach, he’s found, are much more accurate.
Practical considerations arise in relation to the condition of the foot, as well. As noted earlier, calluses can complicate matters.
“I have an issue with testing plantar areas because of the variability of plantar callus,” said Matthew Dilnot, BPod, a podiatrist in Melbourne, Australia. “For a monofilament to work, there has to be some deformation of the skin, which means relatively normal thickness and elasticity.”
Dilnot believes that monofilament testing can be thrown off by the level of protein glycosylation in the skin, as well, since that also affects elasticity. To compensate, he tests eight sites: the tip of the first and third toes, the sides of the first and fifth metatarsals, the styloid process, the navicular tuberosity, and the medial and lateral malleoli.
Rogers agreed that calluses complicate the picture.
“If you look at the plantar surface of someone’s foot and there’s a lot of callus, you need to choose a different site or a different test,” he said. “That’s a case when you might want to use a biothesiometer.”
The biothesiometer employs a plastic pad that vibrates and a voltmeter in a console. The clinician gradually increases the voltage so the pad vibrates faster, and the patient reports when he or she feels the vibration. Most physicians use 25 V as the threshold for abnormal sensitivity; in other words, if the patient doesn’t feel anything until that line is crossed, there’s likely a problem.10
Rogers pointed out another advantage to the biothesiometer: it provides more information than a yes-or-no answer.
“With the monofilament, they either feel it or they don’t,” he said. “But the biothesiometer is semiquantitative because you get a number. Say they feel the vibration at fifteen volts, and the next year they feel it at twenty volts. That gives me an idea of the progression that I don’t get with a monofilament.”
The device has its limitations, however. It’s expensive—around $1500—and it requires a power supply. For these reasons alone it’s impractical as a quick screening method in some settings, particularly in developing nations.
“There’s a large concentration of diabetes patients in sub-Saharan Africa, where you may not have the electricity to power a biothesiometer,” Rogers pointed out. “If you’re doing an exam in that part of the world, the monofilament is great.”
New approaches have recently arisen to address perceived shortfalls in existing protocols, in any case. One of these takes minimalism to its logical extreme: U.K. clinicians have developed a monofilament test that doesn’t require a monofilament. In the Ipswich Touch Test, the clinician simply touches his or her fingertips for a second or two to the targeted points on the patient’s feet (in this case, the tips of the first, third, and fifth toes; in one protocol the dorsum of the hallux is added as well). A positive test, indicating a foot at risk, consists of more than two insensate sites.4
The Ipswich test was developed because its inventors determined that fewer than a third of diabetes patients in U.K. hospitals had received a foot exam while there, and 3% developed new ulcers during their stays. Admitting doctors said that they didn’t screen because they often couldn’t find a monofilament tester. One might wonder why they’re so hard to come by, but in any case the Ipswich test was a success: compared against a gold standard of VPT greater than 25 V with a biothesiometer, the touch test had sensitivity and specificity values similar to 10-g monofilament testing. When used at six sites (the authors’ eventual recommendation), the touch test missed just 12 of 147 patients diagnosed as at risk by monofilament testing, and gave only four false positives.
“The Ipswich is almost identical to monofilament in its predictive value,” said Boulton, who is friends with the clinicians who developed it. “It shows that you don’t need equipment to identify the high-risk foot. In this case, the equipment is free and, so to speak, readily at hand.”
It seems likely that the monofilament will persist in its present form for awhile, however, so researchers and clinicians are developing testing protocols that leverage its qualities. The ADA suggests combining tests such as monofilament, vibration perception, and pinprick, noting that combinations have better than 87% sensitivity, and that loss of monofilament perception, coupled with reduced vibration perception, predicts foot ulcers.22
The combination approach has a robust history: even in his 1998 paper, Armstrong evaluated monofilament testing in tandem with both vibration perception using a biothesiometer and a four-question verbal neuropathy score. He and his colleagues reported that combining modalities increased specificity with minimal loss of sensitivity. For example, monofilament and biothesiometer testing each had a sensitivity of 100% and a specificity of 76.5%. When used together, sensitivity and specificity were both 88.2%. The verbal survey offered improvements as well; it wasn’t tested alone, but when it was combined with the monofilament test, sensitivity was 96.7% and specificity was 85.9%.10
In 2000, researchers tested 248 patients from three large diabetic foot centers for risk factors using a battery of tests: neuropathy symptom score, NDS, VPT, monofilament, joint mobility, peak plantar pressures, and vascular status.23 The NDS included VPT, temperature perception, pinprick test, and Achilles reflex test. Subjects were evaluated at baseline, then every six months for a mean of 30 months. Seventy-three (29%) developed a total of 95 foot ulcers during the study.
The authors concluded that clinical examination (NDS) and the monofilament test were the most sensitive for examining patient at risk for ulceration, especially when both were used together; the combination of NDS and monofilament tests identified all but one of the 95 ulcerated feet.
The authors noted, moreover, that “these are screening tests in which a high sensitivity level is of paramount importance because the most important aspect of these tests is not to miss at-risk patients. One could argue that such methods may lead to overdiagnosis and overtreatment, but this is not a major problem for the at-risk diabetes patient, for whom foot care, education, and proper footwear are the cornerstones of prevention.”
One issue that has dogged monofilament testing is that once a patient loses sensation, repeated pricks from a nylon line aren’t going to tell the clinician anything new.
“Clinicians lack guidance on what to do after a patient loses sensation,” wrote the authors of a review published last year in Diabetes Care. “Many keep testing repeatedly with a monofilament, at scheduled intervals, although sensation rarely ever returns.”24
Those patients may benefit more from diligent follow up on the part of the practitioner, Andrew Boulton said.
“The best way to reduce amputation is not with expensive tests, but by observing the feet every time you see the patient,” Boulton said. “I can tell a high-risk foot by looking at it—clawing of the toes, pes cavus, small muscle wasting, dry skin from autonomic neuropathy. The power of clinical observation is the take-home message.”
Regardless of the form it takes, good clinical follow up decreases reulceration rates by 60%, and amputations by 85%.25-27 David Armstrong thinks clinicians need to change the way they think about the problem, and his terminology reflects this.
“After the initial healing of an index wound, we are now using the term ‘remission,’ because it communicates the severity of the problem better,” he said. “One-, three-, and five-year mortality from these complications is equivalent to aggressive forms of cancer. We think this powerfully connotes the necessity for frequent follow up and rapid intervention for complications.”
Cary Groner is a freelance writer in the San Francisco Bay Area.
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