Foot Sensation Testing in the Patient With Diabetes: Introduction of the Quick & Easy Assessment Tool

Introduction. Sensory testing of patients with diabetes is an integral part of preventing new and recurrent wounds. The Semmes-Weinstein monofilament (SWM) test is considered the gold standard to screen for loss of protective sensation; however, the authors’ experience has shown that it is not only time consuming, but is of negligible value for a patient with a diabetic foot ulcer (DFU). Methods. This article discusses the shortfalls with regard to the SWM test and reviews other techniques for sensory evaluation. In addition, the Quick & Easy system is introduced, which combines sensory assessment with guidance for anesthesia requirements during wound debridements or other surgical interventions. Results. A scale ranging from grade 2 (normal sensation) to grade 0 (absent sensation) reflects the patient’s responses to wound manipulation, palpation of an underlying deformity, and/or evaluation of the difference between light touch sensation with the patient’s hands compared to the feet. For patients with total loss of sensation (grade 0), no anesthesia is needed for surgical procedures. If there is diminished sensation (grade 1), surgical intervention can be performed following administration of either topical or local anesthesia. For patients with normal sensation (grade 2), complete anesthesia of the surgical site will be required. A preliminary observation was conducted on 50 patients with DFUs using the Quick & Easy system. Anesthetic requirements were accurately predicted in all cases without the need to modify the type of anesthesia during the procedure. Conclusion. The Quick & Easy system serves as a simple sensory evaluation for a patient with a DFU and provides valuable anesthesia guidance for wound care procedures.

In the United States, foot infections are the most common reason for the hospitalization of patients with diabetes.¹ These infections are a major risk factor for lower extremity amputation.² The loss of sensation in the foot is a predominant finding in these patients. Consequently, sensory testing of the foot in a patient with diabetes is an integral component of the clinical evaluation and its management.

  The International Neuropathy Guidelines define diabetic peripheral neuropathy (DPN) as the presence of symptoms and/or signs of peripheral nerve dysfunction in patients with diabetes after exclusion of other causes.³ This condition affects 30%-50% of the patient population with diabetes and this prevalence tends to increase proportionally with the duration of diabetes.⁴ Neuropathy often presents with a loss of protective sensation, defined as a level of sensory deficit, where a patient can sustain an injury without recognizing any inciting trauma.³ The progression from minor injury into ulceration and, ultimately, evolution into a nonhealing wound with underlying infection has been documented as the most common series of events preceding lower extremity amputation.²

  Invariably, diabetic neuropathy is a critical independent risk factor for foot ulceration and lower extremity amputation.⁵ Results from the Diabetes Control and Complications Trial showed that strict glucose control reduces the development of neuropathy in patients with type 1 diabetes.⁶ However, there is no definitive treatment that completely restores sensation in patients with chronic DPN, unless there is a component of nerve compression that can be treated with surgical intervention.⁷ This paper presents an overview of the commonly used clinical techniques to screen for sensory DPN with a predominant focus on the Semmes-Weinstein monofilament (SWM) test. In addition, the Quick and Easy assessment tool, which can help detect both sensory impairment as well as guide anesthesia requirements for management of wounds, is introduced.

  The history and physical examination provides essential information for detecting peripheral neuropathy. Typically, the early symptoms of DPN include the gradual loss of sensation and/or development of pain. Other neurological symptoms, such as paresthesia, burning, or an aching sensation, may be secondary to DPN or nerve compression. If a patient relates a past history of lower extremity ulceration or nontraumatic amputation, then there is high clinical suspicion for underlying DPN. Patients taking medication to mitigate neuropathic pain such as gabapentin or pregabalin (ie, Neurontin and Lyrica, Pfizer, New York, NY) often have some level of sensory deficit that is usually more distal to the site of their pain symptoms. Ischemic symptoms consistent with peripheral arterial disease should be ruled out accordingly, as they can mimic sensory neuropathy and may be a potential cause of a chronic wound. Motor symptoms often appear in conjunction with sensory neuropathy as a late finding in the spectrum of DPN. Clinical manifestations include weakness in the distal legs, muscle imbalances, and/or joint contractures. These findings may be evident by observing structural deformities, or alterations in gait including imbalance and calcaneal gait patterns. In chronic cases, a muscle imbalance can evolve into gross deformities such as toe contractures, equinus, and angular/rotational abnormalities (eg, hallux valgus, metatarsus adductus, excessive pronation, hindfoot varus, or bunionettes).

  Screening techniques are used to detect early signs of sensory neuropathy. This recognition is essential to prevent foot ulceration in patients with diabetes. Once sensory neuropathy has been identified, the clinician can initiate early interventions by educating patients on the do’s and don’ts (Figure 1) for wound prevention and recommend protective footwear (Figure 2).

  Origins. In 1899, Max Von Frey introduced variable diameters of monofilament to evaluate cutaneous sensation.⁹ The testing apparatus was constructed from horsehair mounted inside a tube. The filament was applied perpendicularly to the skin until there was an obvious bend of the fiber. The patient then provided verbal feedback when the sensation was perceived. This device was further refined in 1960 by psychologists Florence Semmes and Sidney Weinstein.¹⁰ They developed a set of 20 nylon monofilaments to evaluate gross sensory loss in the hands of patients with brain injury. After gaining increased recognition from its use in leprosy research at the Gillis W. Long Hansen’s Disease Center in Carville, LA, the SWM test was integrated as the standard assessment tool for patients with DPN.¹¹

  The SWM is a noninvasive screening instrument that is not only simple to learn, but is also relatively inexpensive and readily accessible. These characteristics render it the single most frequently used test to detect sensory neuropathy; however, the SWM test appears to be more of an academic-based technique with limited practical use. In order for the test to be performed properly, the patient may require repeated assessments at the same site or callus debridement to detect the stimulus. Also, a series of random applications may be needed to minimize any guessing by the patient. This test often requires 10 to 15 minutes to complete, which is often not practical in a busy clinic. Aside from time considerations, the reliability of the monofilament has inherent flaws. This makes interpretation of the testing results questionable and, therefore, limits its clinical application.

  Challenges. Birke and Sims¹¹ defined the sensory threshold for protective sensation in a study consisting of 72 patients with leprosy and 28 patients with diabetes who had lower extremity ulcerations. The investigators used various gradients of monofilament to evaluate plantar foot sensation, including 4.17, 5.07, and 6.10 gauges. As the value of the monofilament increases, the construct becomes progressively more stiff and more difficult to bend. The designated monofilament gauges are based on a logarithmic formula: nominal value = log₁₀ [force (mg) x 10].¹² Results of this study showed that none of the patients could detect less than the 6.10 gauge monofilament; therefore, the authors concluded that the 5.07 gauge monofilament was the ideal threshold to successfully identify loss of protective sensation. However, further analysis showed a significant difference in the force required to collapse the 6.10 gauge monofilament (75 grams) compared to the 5.07 gauge (10 grams).

  The 5.07 gauge monofilament (with 10 grams of force to buckle) has been adopted as the standard SWM model for identifying loss of protective sensation.^13,14 In fact, the American Diabetes Association recommends using the 10-gram monofilament during foot screening exams.¹⁵ In Japan, however, a smaller caliber of monofilament has been recommended. The Japanese design consists of a 4.31 gauge monofilament with a buckling force of only 2 grams.¹⁶ Results from the study showed 60.0% sensitivity and 73.8% specificity for detecting sensory neuropathy. The mechanical properties of the 4.31 gauge/2 gram monofilament, essentially detects a higher sensory threshold when compared to testing with the 5.07 gauge/10 gram monofilament. This evidence suggests that the standard 5.07 monofilament may not adequately identify patients with diabetes who exhibit early signs of sensory deficit.

  Although the monofilament size and buckling criteria are well documented, manufacturers often fail to perform accurately within these parameters. Smith et al¹⁷ studied the force of 10 unused monofilaments, which were specifically calibrated to collapse with approximately 10 grams of direct force. The results revealed that the monofilaments buckled on average after only 8.4 grams of force, a 16% discrepancy, during the first application. This outcome was statistically significant as the calibrated 10 grams of buckling force was outside of the designed 5% window of standard deviation; therefore, even calibrated monofilaments may provide imprecise results.

  Another complicating factor inherent in the design of the SWM is mechanical fatigue incurred after a series of continuous applications. Once the monofilament is subjected to cyclical stress, the reproducibility of the testing device is altered. Yong et al¹⁸ tested several monofilaments with a fatigue testing device. After 500 consecutive cycles, the results showed there was an average reduction of 1.2 grams (12%) needed to bend the monofilament. Other than obvious manufacturing and/or injury -related defects in the monofilament, there are no guidelines that describe the service life of a standard SWM. Aside from mechanical flaws, there are hygiene concerns for disinfecting the equipment to minimize the risks of cross contamination between patients. Notwithstanding, the cost to replace the device per patient would increase the general expenses for the practitioner.

  Challenges of clinical mapping. To date, there has been no consensus regarding the standard technique for monofilament mapping locations and the number of testing sites required per foot. Typically, the clinician begins the exam distally at the forefoot and progresses proximally to establish a baseline sensory profile. Polyneuropathy characteristically has a glove-and-stocking distribution, with the longest nerves initially affected in the course of the disease process.¹⁹ Therefore, diminished sensation localized to the forefoot is a classic presentation of early DPN.

  The 10 most frequently tested sites on the foot are illustrated in Figure 3. The specific regions of the foot tested during the examination may affect both accuracy and precision. Smieja et al²⁰ demonstrated that assessment of the forefoot had moderate reproducibility, but anatomic sites located more proximally had only fair reproducibility. How many sites of sensory deficit are required to define a loss of protective sensation? The criteria for this have varied depending on the protocol designated by the examiner. Armstrong et al²¹ reported that failure to detect pressure in 4 out of 10 points is 97% sensitive and 83% specific for identifying loss of protective sensation.

  Structural deformities can obscure the SWM testing results. For example, patients often have concomitant findings including preulcerative lesions, callused tissue, underlying bursa, woody edema, and cicatrix formation. These associated conditions may alter the natural elasticity and inherent contour of the skin, which may lead to spurious readings. In contrast, erythema overlying an area of deformity is likely to be associated with hypersensitivity, and may therefore misrepresent the exam findings. When this occurs, an alternate testing site is required; however, this protocol not only deviates from the standard mapping sites, but also requires additional time for sensory testing.

  A single clinical test is often insufficient to rule out loss of protective sensation. There are several techniques to screen for DPN and each approach has its distinct advantages and disadvantages. The more recognized clinical methods include vibratory testing with a tuning fork, vibration perception threshold testing (VPT), reflex testing, pinprick testing, light touch sensory testing (Ipswich touch test), and 2-point discrimination testing with a device such as The Disk-Criminator (AliMed, Dedham, MA) or a pressure-specified sensory device (PSSD) (Table 1). A panel of clinicians under the direction of the American Diabetes Association Interest Group on Foot Care have recommended that neurologic assessment in patients with diabetes should include routine screening with SWM in addition to 1 other clinical test (eg, 128-Hz tuning fork, pinprick sensation, ankle reflexes, or vibration perception threshold testing).²² Once the status of the patient’s protective sensation is identified, other factors, including peripheral arterial disease and deformity, are used to categorize them into a risk classification system that provides treatment recommendations and suggested follow-up protocols.²²

  Vibratory sensation is conventionally tested with a 128-Hz tuning fork at the interphalangeal joint of the hallux. An abnormal result occurs when the patient cannot perceive the vibratory stimulation from the tuning fork, while the clinician can simultaneously detect the vibration. Although this method does not differentiate the severity of sensory deficit, it may allow early detection of sensory neuropathy.²³ The faster-conducting, thickly myelinated fibers that mediate vibration and proprioception are usually damaged before the slower-conducting, thinly myelinated, and unmyelinated fibers that transmit pain, temperature, and light touch.²³ Absent vibratory sensation at the hallux has shown to correlate with the development of foot ulcerations.²⁴ In the authors’ observations, vibratory testing with a tuning fork is less frequently used compared to SWM testing; however, vibratory testing is subjective and relies on the patient’s perceptions, similar to SWM testing. In addition, the vibratory tone is not standardized, as it is a function of how hard the examiner manually strikes the tuning fork to stimulate the vibratory resonance.

  Vibration perception threshold testing (VPT) is performed using a handheld device (Bio-Thesiometer, Bio-medical Instrument Company, Newbury, OH). This instrument quantitatively tests vibratory sensation with a specialized probe set at 100-Hz and has an adjustable amplitude ranging from 0-50 volts.²³ The clinical technique employed is similar to the tuning fork, where the probe is placed at the distal hallux. The amplitude on the device is adjusted until the patient can distinctly sense a vibratory stimulus. Several prospective studies have determined that a vibratory threshold of > 25 volts is an abnormal finding which correlates with a strong predictor of ulceration.^25-27 This instrument has the unique advantage of providing quantitative data, so sensory loss can be objectively documented during serial examinations; however, the equipment costs more than $730, which may potentially limit its availability and render it impractical for routine office use.

  Reflex testing to screen for sensory neuropathy primarily focuses on examining the ankle reflex. The clinician aligns the ankle into a neutral position and strikes the Achilles tendon with a neurological hammer. An abnormal result is demonstrated by complete absence of ankle plantarflexion. Compared to other lower extremity reflexes, the ankle reflex is the most sensitive for detecting early signs of neuropathy. In a multicenter study, ankle reflex testing had sufficient reproducibility and moderate agreement among examiners;²⁰ however, the test has limited clinical application as it has been shown to be a poor predictor of ulceration.²⁴

  Pinprick testing involves the superficial application of a sterile safety pin at the forefoot with enough manual pressure to slightly deform the skin. Abnormal findings are concluded if a patient cannot detect the sharp stimulus. One of the primary indications for this testing modality is to locate a focal area of sensory loss. However, this method is highly subjective, thus the results are poorly reproducible.²⁰ In addition, the concern for sterility and inadvertent skin puncture may limit its clinical application.

  Aside from being a rapid exam, the Ipswich Touch Test (IpTT) is the most simplified technique for sensory screening. Essentially, it involves lightly touching the distal tufts of the first, third, and fifth toes with the examiner’s index finger for 1-2 seconds. The practitioner must avoid pushing, prodding, poking, or tapping during the light touch exam to avoid a potential misinterpretation of those sensations by the patient.²⁸ Identification of 2 or more insensate areas is considered a positive result. In a prospective study of patients with diabetes who had VPT ≥ 25 volts, the IpTT had a similar sensitivity, specificity, and operating characteristics when compared with the SWM test.²⁸ If this technique actually parallels the outcomes of the monofilament exam, the authors question why a clinician would choose to screen with a method that is more time consuming and requires instrumentation.

  Two-point discrimination testing functions to measure nerve fiber density in a localized area,²⁹ and has been routinely used to evaluate nerve injuries in the hand and, more recently, expanded to encompass lower extremity sensory testing.³⁰ With this method, a device is applied to the skin to measure the minimum distance a patient can detect between 2 discrete points; however, it should be noted that the amount of pressure applied to the skin during the examination creates a source of subjectivity.

  In 1989, Dellon³¹ created a novel technique to evaluate 2-point discrimination with the introduction of the PSSD,³ which uses a computer-assisted program to measure pressure threshold on the skin for both 1-point and 2-point static touch. This unique instrument contains 1-2 metallic prongs positioned at specific intervals. The distance, in millimeters, between the prongs is inversely related to the innervation density for that anatomic site; therefore, an increase in 2-point discrimination distance outside of the standard deviation correlates with the degree of axonal damage.³¹

  Like most other screening modalities, the PSSD is subjective because results are based entirely upon the patient’s sensory perception; however, according to Dellon, the PSSD is the most accurate computerized system to date capable of documenting sensation in the lower extremity.³¹ For practical purposes, this technique is often restricted to an academic setting due to the expense of the equipment and data analysis. Moreover, the PSSD does not directly influence treatment decisions. Its primary role in sensory testing is to document the progress following operative treatment including nerve decompression and microneurolysis.

  A patient with a nonpainful gait in the presence of foot deformity or mal perforans ulceration invariably has some level of sensory impairment; therefore, there is no justification to perform SWM testing within this cohort of patients. Consequently, the authors propose a tool called the Quick & Easy assessment system to document sensory alteration prior to the development of a wound. Once a preulcerative lesion has progressed to an ulceration, wound healing measures are instituted. This simplified technique can be used as a clinical guideline for anesthesia requirements for local wound care and wound bed preparation with debridement. Therefore, the clinician can quickly determine the patient’s anesthetic needs during dressing changes, in-office visits, bedside debridement, and surgical procedures.

  For patients with wounds and wound precursors, sensory perception is classified using a 0-2 grade scale, with 0 indicating total absence of sensation, 1 indicating impaired perception of pain, and 2 indicating normal sensation (Table 2). For a patient with normal sensation, full anesthesia with or without monitored anesthesia care and local field blocks is required (grade 2). Clinically, these patients often present without gross deformities and their tactile (ie, light touch) sensation in the feet equals that in their hands. For a patient with diminished sensation (grade 1), palpation of deformities and manipulation of the wound elicits mild to moderate discomfort. Most debridement procedures can be adequately performed once topical and/or local anesthesia has been administered. Although patients can detect pressure and may notice some discomfort during the procedure, this minimal use of anesthesia usually suffices to allow the procedure to be performed without difficulty. For patients with total loss of sensation (grade 0), debridements can be carried down to and including the level of bone without need for any anesthetic prior to the procedure. This cohort tolerates digital amputations, tenotomies, extensive debridement, and suturing without the need for anesthesia.

  When the level of sensation is intermediate between 2 grades, half points can be designated to fine-tune the anesthesia required for the procedure. If the patient does not have a wound, palpation of a deformity will reveal if there is underlying DPN. Likewise, in a patient with a wound, palpation of the periwound area can be used to gauge the level of pain perception. Finally, a comparison of the difference between light touch sensation of the hands and the feet may be used to confirm the grade of sensory neuropathy. If foot sensation is intact and equal in the hands and feet, this is graded as 2 points. However, if foot sensation is decreased by approximately 50%, the grade is reduced to 1 point. A complete absence of light touch sensation of the foot is graded as 0 points.

  This assessment system can obviate the need for time-consuming and highly subjective SWM testing. More importantly, it rapidly provides essential information for determining the level of anesthesia required for in-office and bedside debridement or other surgical procedures. This method quickly distinguishes procedures that can be successfully performed in the outpatient clinic versus those that require anesthesia in the operating room. A secondary benefit is that this grading system can bypass the patient’s exposure to anesthetic and sedative medications, if the procedure can be easily tolerated in the clinic without them. A tertiary benefit is the grading system integrates cost effectiveness by providing guidelines for debridement and other surgical procedures in a setting outside the operating room.

  Application of the assessment tool. Clinical observations were conducted on 50 patients using the Quick & Easy sensory assessment system.^32,33 The wounds were evaluated using the Wound Score, a 10 point objective scoring system and component of Strauss and colleagues’ Master Algorithm, to quantify the severity and guide management of the wound.^34,35 All of the wounds were designated as a “problem” type, where healing is observed in approximately 90% of the wounds after the 5 components of strategic management, as outlined by Strauss and coauthors,³⁶ are utilized. Strategic wound management includes wound base debridement, protection and stabilization of the wound, optimal medical management of the patient, selection of appropriate wound dressing agents, and mitigation of wound ischemia/hypoxia.

  Patient demographics included 29(58%) males and 21(42%) females, with ages ranging between 25 years and 81 years. The results showed that 50% of the patients were completely insensate at the periwound site (grade 0). The anesthetic requirements for wound debridement and surgical procedures based on this 0-2 grade scale were appropriate for all cases. In fact, none of the patients required a modification in the sensory grade after the Quick & Easy assessment. Also, there were no anesthetic modifications required during the surgical procedure once the appropriate level of anesthesia was administered based on the Quick & Easy sensory grading system.

Diabetic foot ulcers are predominantly attributed to repetitive cycles of mechanical pressure in the setting of sensory neuropathy. Soft tissue breakdown develops from an initial injury caused by compressive and/or shear stresses.³⁷ Patients with intact sensation experience a sense of discomfort, prompting them either to shift the weight-bearing stress to an adjacent area of the foot or to discontinue the activity. However, patients who have lost protective sensation unknowingly continue the activity and develop ulceration once the number of repetitions has exceeded the stress threshold of the soft tissue.

  Motor and autonomic components of peripheral neuropathy tend to be subjugated to the sensory assessment during the foot evaluation of patients with diabetes (Table 3). These factors may be the inciting cause or contribute to the advancement of the deformity component of the Treacherous Triad, defined as 3 primary factors that account for why chronic wounds fail to heal (Figure 4). The 3 components include resolving underlying infection, eliminating gross deformities, and improving ischemia/hypoxia at the wound site. Thus, the clinician must consider these factors when examining a patient who is at risk for developing foot wounds. Autonomic dysfunction manifests clinically with signs of dry skin, atrophy of soft tissue padding, and loss of sympathetic control of the microcirculation. These structural changes make the patient more susceptible to developing a wound, especially in the face of deformity and/or locations with disproportional amounts of pressure.

  Motor neuropathy can manifest itself as imbalances between lower extremity extrinsic and intrinsic muscles, resulting in contractures. If a joint contracture persists without treatment, then a fixed deformity results. This deformity often leads to concentrated areas of pressure, which, in turn, can evolve into a mal perforans ulceration (Figure 5). Therefore, the exam should always include detection of deforming forces, such as equinus, as well as bony prominences created from digital contractures and bunion deformities. Preulcerative lesions that show signs of erythema, dense callus formation, or superficial erosions should be identified and monitored closely during serial examinations.

  The history and physical examination remains the most fundamental tool to evaluate DPN. The SWM test can be time consuming and generate equivocal responses. In addition, there are concerns regarding its accuracy, reliability, and validity. Most clinicians in demanding practices do not have the time or resources to carefully document 10 testing sites per foot in patients with diabetes. In addition, the result from the SWM test rarely changes the course of treatment. Screening for sensory neuropathy with the SWM provides limited prognostic value once a wound has already developed. Any patient who walks into the clinic with a foot wound that is not adequately offloaded and lacks signs of discomfort has some stage of DPN.

  Results from this preliminary observation suggests the Quick & Easy assessment tool is accurate in predicting the level of anesthesia required for wound debridement and surgical procedures in the clinic as well as in the operating room.^32-33 It efficiently integrates sensory evaluation with wound management. Additional studies and utilization of this assessment tool will further clarify its clinical value.

  When sensory neuropathy is detected, the clinician should always initiate a wound prevention program. This plan should provide foot education for patients with diabetes with emphasis on daily inspection of the feet, implementation of the appropriate level of weight-bearing activity, selection of proper footwear, and proactive surgeries to prevent new wounds or address recalcitrant wounds. The Quick & Easy grading system incorporates well into the comprehensive management of the patient with diabetes.

Affiliations: The authors are from Long Beach Memorial Medical Center, Long Beach, CA.

Address correspondence to:
Alexander B. Craig, DPM
Long Beach Memorial Podiatric Surgery Residency
2801 Atlantic Avenue
Long Beach, CA 90806
acraig.dpm@gmail.com

Disclosure: Dr. Strauss and Dr. Miller disclose the receipt of royalties from Best Publishing Co.