* I gratefully acknowledge Bill Buford, bioengineer, University of Texas at Galveston, formerly at the Paul W. Brand Research Laboratory, Gillis W. Long Hansen’s Disease Center, Carville, Louisiana, for his help in reviewing the monofilament calculations, in developing instrument measurements, and collaborating on sensibility test design.
Accurate sensibility tests are useful for early recognition of peripheral nerve problems and allow early intervention and monitoring.
Screening of touch threshold perception according to the zones and areas of peripheral nerve innervations and illustrating the results using a color-coded hand map is possible using Semmes–Weinstein-style monofilaments.
Other sensibility tests, such as those for two-point discrimination, may add to the overall assessment of patient status.
Assessments of sensory and motor nerve conduction velocity along with sensibility tests form a crucial basis for treatment decisions in patients with peripheral nerve problems.
Neurophysiologists are interested in “normal” sensory function. Clinicians are interested in accurately assessing abnormal sensibility and detection thresholds. The examiner of sensibility must determine how a client’s performance on the spectrum of sensibility tests compares with a normal baseline and, if abnormal, be able to quantify the degree of change in measurable increments. In this chapter, abnormal results for sensibility measured by clinicians via various tests are referenced against normal values so that degrees of loss can be accurately assessed and monitored.
Test Instrument Considerations
Sensibility measurement instruments must have instrument integrity before test results can be considered valid in clinical studies . Instrument accuracy can only be as good as the quality of stimulus input the instrument provides. For example, a sensibility test instrument that is not repeatable in force of application lacks the needed accuracy, does little to clarify, and can actually be misleading in results.
In any given skin area of 10 mm there are more than 3000 sensory end organs. Sensibility tests are intended to measure the physiologic function of the peripheral nerves by assessing response of their respective end-organ mechanoreceptors in the skin. All force and pressure tests used to excite sensory mechanoreceptor nerve endings in the skin (which respond to stretch or deformation) should be in repeatable force or pressure units defined by the National Institute of Standards and Technology (NIST).
Pathomechanics and Degrees of Injury
The examiner of sensibility should be aware of the normal patterns of sensory nerve innervation in the hand and upper extremity as well as the typical sensory signs and symptoms that result from different levels of lesions along nerve pathways ( Figs. 11-1 and 11-2 ). The prognosis for recovery of function of the sensory nerves depends on which nerve structures are involved, and their degree of damage (axons, endoneurium, perineurium, or epineurium). Modes of nerve injury are mechanical, thermal, chemical, or ischemic. Swelling and inflammation can exacerbate internal and external nerve compression and compromise vital nutrition to nerve axons, particularly where the nerves must pass through tight structures. Direct injury can result in a nerve lesion in continuity (axonal conduction disruption) or in complete transection. ,
Observation and interview before testing may help to identify apprehensive patients, those with exaggerated symptoms, and those whose intention is secondary gain from an injury. Often, patients provide clues as to their condition by how they hold their arms, sign papers, manipulate objects, and present themselves. If patients have a history of symptoms suggestive of peripheral nerve involvement, but test within normal limits, the examiner may use certain provocative positions, or otherwise stress the nerve in question in order to provoke symptoms that can then be measured. Stress testing can be static (e.g., sustained wrist flexion or extension at end range for 1 minute), or dynamic (e.g., putty squeezed for 5 minutes, or provocative work and activity), with sensibility measurement before and after stress.
Nerve Conduction Velocity: A Companion Assessment
The objective of nerve conduction velocity (NCV) assessment is to determine the speed of neural conduction and, if slowed or abnormal, to determine whether more than one nerve or site is involved. NCV does not determine if and how much a patient can and cannot actually “feel.” In order to determine a subject’s touch threshold detection and discrimination, tests of sensibility need to be done along with NCV. , It is important for clinicians to understand how NCV test results fit in with sensibility test results to enable an overall interpretation of peripheral nerve status.
A common peripheral nerve condition in the upper extremity is median nerve compression at the level of the wrist, thus the median nerve is frequently initially targeted for NCV testing when numbness or tingling occur in a patient’s fingers. But the astute clinician is aware that other peripheral nerves can be involved and more than one level of involvement, referred to as a “double-crush” syndrome. Furthermore, there can be, and frequently is, bilateral involvement. Patients can have nerve lesions in continuity at all segments of upper extremity nerves, including brachial plexus and thoracic outlet. Sensory involvement usually precedes motor. Hunter and others maintain that high-level traction neuropathies commonly result from high-speed vehicular trauma, falls on the outstretched arm, repetitive assembly work (lateral abduction or overhead lifting), overuse (when heavy work exceeds physical capacity), and poor posture. NCV testing from the neck to the fingers bilaterally is recommended when the initial history and physical do not readily suggest the level, type, and degree of involvement. Although valuable, NCV testing is known to vary according to the time of day, temperature of the extremity, size of the electrodes, placement of the electrodes, and instrument. In skilled hands using correctly calibrated machines, NCV testing can be accurate and repeatable. , Results of NCV and sensibility tests do loosely correlate but do not always directly correlate as evidence of peripheral nerve involvement. , “Slowing” of sensory NCV, or abnormal amplitude, along with abnormal sensibility testing, help confirm abnormal nerve function.
When NCV examination results show a “slowed” nerve conduction response, and light touch threshold tests such as the Semmes–Weinstein monofilament test demonstrate results that are “within normal limits,” the clinician can assume that there is not yet a detectable change in sensory threshold detection. NCV can be reported as “absent,” while the heaviest monofilaments, or a pinprick, can still be detected in some instances, signaling the nerve does have residual viable function that could potentially improve if treated. These differences in the test results do not mean that one is more sensitive or objective, but that they are different tests and measurements of neural physiologic function. See Chapter 15 for a detailed discussion of nerve conduction studies.
Hierarchy and Categories of Sensibility Tests
Five hierarchal levels of sensibility testing have been described by Fess and LaMotte. They include the following: autonomic/sympathetic response, detection of touch, touch discrimination, quantification, and identification.
Callahan divides sensibility tests into four categories: threshold tests, functional tests, objective tests and provocative or stress tests. Selected tests from these categories are discussed in the following sections. For another perspective and more details, the reader is referred to Callahan’s archived chapter on sensibility assessment on the companion Web site of this text .
Touch–Pressure Threshold Testing (Semmes–Weinstein-Style Monofilaments)
Touch–pressure threshold testing involves the use of nylon monofilaments of standard length and increasing diameters that provide controlled gradients of force to the mechanoreceptors in the skin to determine light-touch to deep-pressure detection thresholds ( Fig. 11-3 ). The advantage of monofilament testing is that it provides clear, quantified, and repeatable information about the patient’s detection of touch. The pattern of sensibility loss reflected by the monofilament testing helps to identify pathology.
It is important to note any changes in touch–pressure detection thresholds with repeated testing, but this requires the clinician’s interpretation to guide the course of further treatment. Improvement in light-touch to deep-pressure thresholds may be seen even in chronic conditions, episodes of worsening may occur, and the course of recovery following nerve release or suture can be predictable or complicated by scar and fibrosis. All of these occurrences can be reflected with repeat testing of touch–pressure thresholds.
The monofilament form of testing is used in patients with neuropathies, entrapment or compression syndromes, lacerations, and other abnormalities, including patients with diabetes. The test can reveal sensibility losses in patients with Hansen’s disease (HD), in which early changes are often superficial, and with worsening of the disease process, changes can mimic other peripheral nerve lesions or compression. Early testing in this situation can lead to prevention, reversal, or improvement of neural damage with timely use of steroids and other anti-inflammatory medications.
Monofilament testing is easy to perform yet profound in the information it provides. Results are understood by the examiner, physician, insurance provider, health care gatekeeper, the patient, and others involved in treatment and employment. The versatile monofilament form of testing can be used in a 10-minute screen or for a full map showing clearly and completely in detail the area and degree of abnormality. Screening examinations are used to evaluate the autonomous areas of cutaneous innervation of the median, ulnar, or radial nerves or other sites of potential or suggested involvement.
Monofilament test results are mapped to identify the area and degree of abnormality, and repeat testing reflects changes over time. For normative studies , it is critical to include a lighter, above-threshold monofilament from the set of 20 so the study includes above-threshold sensitivities. Szabo and others use all of the lighter monofilaments in the within-normal-limits functional level, along with the hand screen monofilament sizes when testing patients with possible compression or entrapment, considering it important to use the most sensitive possible. This is an accepted variation of the test when time allows, which adds additional data to the hand screen. See studies by Szabo, Gelberman, and Lundborg for their protocols.
Examiners sometime include additional monofilaments in the heavy range with the hand screen monofilaments. The 5.07, 10-g level monofilament is most frequently added, specifically for testing of the diabetic foot for measuring gross protective sensation of the plantar surface. , Some prefer 4.56, 4-g level monofilament (already included in the hand screen kit) for screening protective sensation of the foot. ,
It is important to consider that the patient’s activities immediately prior to testing can influence the testing results. For example, if the patient has had a relatively stress-free morning, after sleeping late, having a good breakfast, and a more quietly paced and lighter-duty work schedule than is normal routine, the results of testing may be better than after heavy-duty work of a few hours’ duration. Hunter terms this condition transient stress neuropathy and recommends monofilament testing after activities and positions that reproduce their symptoms.
Monofilament testing can detect abnormal sensory threshold responses all over the body, even on the face, where sensitivities exceed that of the hand, and on the plantar contact area of the foot, where slightly heavier detection thresholds allow for callus. , , The force range of the monofilaments in available diameter sizes is 4.5 mg, for the lightest, to over 300 g for the heaviest. Semmes and Weinstein and Weinstein , found that within normal subjects, differences can be found between men and women, the left- and right-handed, and among age groups. For most clinical testing, however, it is not as important to use the very lightest above-threshold monofilaments as it is to determine if the patient is normal or not. The 2.83 marking number (50-mg level monofilament) is then the most important of the monofilaments available. It is the last of the lightest monofilaments falling within normal limits for screening of men and women and right and left hands.
Nylon monofilament force of application was examined by Bell-Krotoski and Buford at the Paul W. Brand Research Laboratory. Nylon material was obtained, tested, and used for sets researched and produced at the former Gillis W. Long Hansen’s Disease Center (GWLHDC), Carville, Louisiana, in 1989. All sets were made to Weinstein’s original specifications for diameter size and length of 38 mm . Bell-Krotoski provided these original specifications for sets first produced by North Coast Medical (NCM). Both the GWLHDC sets and NCM sets were then used in a normative study by six examiners who tested 131 subjects (262 hands, 520 tests; 182 feet, total 364 tests). In this study, which used the standard protocol detailed in this chapter and included all of the lighter monofilaments , the 2.83 (marking number) 50-mg level monofilament was confirmed as the optimal size for within-normal-limits screening for males and females, right and left hands, and all over the body, except the plantar contact area of the foot where the 3.61 (marking number) 200-mg level was found to be a better predictor of normal ( Fig. 11-4 , online).
A normal person is not expected to detect the 2.83, 50-mg level monofilament 100% of the time, but a normal person can detect this level of monofilament force of application most of the times it is applied. The correlation of monofilament sizes with functional levels of detection was developed by von Prince and Butler, Werner and Omer, and as used today by Bell (Bell Krotoski). Those who detect a 3.61-size monofilament (but not lighter) also have difficulty in discerning textures and symbols drawn on the fingertips ( “diminished light touch” ) ( Table 11-1 ).
|Filament Markings *||Calculated Force (g)|
|Blue||Diminished light touch||3.22– 3.61||0.166–0.408|
|Purple||Diminished protective sensation||3.84– 4.31||0.697–2.06|
|Red||Loss of protective sensation||4.56–6.65||3.63–447|
* Minikit monofilaments are in bold . Descriptive levels based on other scales of interpretation and collapse of data from 200 patient tests.
In repeated measurements over time using standard specifications for this size monofilament (7 mil diameter, and 38-mm length) the 3.61 monofilament size was found to measure a more sensitive 200-mg level, not actually reaching the 400-mg level shown in Weinstein’s original calculated forces ( Table 11-2 ). Both 200- and 400-mg forces fall in the diminished-light-touch functional level of detection. It is important that manufacturers of the monofilaments appreciate the fact that if the length or diameter of a monofilament is changed from that used in the normative and clinical studies according to Weinstein’s original specifications, these studies no longer apply for interpretation of test results with the changed monofilaments for accuracy and reliability. It is recommended that the force of application for each monofilament should remain that of the original design with the standard 38-mm length and specified diameters.
|MN||CF (g)||S-WF (g)||LMF (g)||B-TMMAF (g)||ASD (g)||Diameters (inches)||Diameters (mm)|
|Filament Index||Application Force Mean, Minikit Filaments from Standard Kit (g)||Application Force Mean, Previous Minikit Test Results (g)|
|Minikit Screen filaments||−2.83||0.080||0.072|
Touch–pressure detection thresholds increase to gram levels with more severe degrees of nerve loss. Hand screening determines magnitude of response in established functional sensibility levels, beginning with within-normal limits and progressing to diminished light touch, “diminished protective sensation,” “loss of protective sensation,” “deep pressure sensation,” or unresponsive.
The monofilaments clearly have been shown to be accurate and their results repeatable if the instrument is calibrated correctly. The monofilaments bend when the predetermined threshold for that size is applied to the patient and cannot go beyond if applied correctly . The elastic properties of the nylon monofilament material provides the instrument with the unique ability to dampen the vibration of the examiner’s hand that occurs with hand-applied devices that do not control for this vibration. In extensive materials testing, characteristics of the nylon used to manufacture the monofilaments was found to be important. Additives during manufacture can change nylon’s physical properties, that is, the force of application. Nylon fishing line is beginning to be used by some manufacturers of the filaments, but it is extruded on rolls instead of in straight lengths. Any rolled nylon when extruded does not hold repeatable calibration, even if artificially heat-straightened and, thus, cannot be recommended. Clinicians should recognize and replace any nylon monofilament that is bent and stays bent.
Straight-length, extruded nylon holds calibration, even if curved slightly, until damaged, because nylon has an indefinite shelf life. The elasticity of straight-length nylon monofilament and its bending and recovery at a specific force means the force it can apply is limited and controlled, thus the monofilament form of testing with pure straight nylon is force-controlled.
Some of the monofilaments in the full set of 20 have been found to be so close in force of application that they occasionally overlap and represent the same force (see Table 11-2 ). Those examiners concerned with losing test sensitivity because they are not using all 20 monofilaments need to consider that sensitivity is better using the hand screen set where the forces never overlap, rather than the full set, where some overlap is possible.
Sidney Weinstein and his physicist son Curt Weinstein developed the Weinstein Enhanced Sensory Test ( WEST) following review and discussion with Bell Krotoski and Fess regarding abnormal functional levels, and the Hand Screen set. , The Weinsteins improved on Bell Krotoski’s “pocket filament” prototype of the Hand Screen monofilaments in one handle, by rounding monofilament tips, and designing a less fragile handle ( Fig. 11-5 ). The Weinsteins certify the WEST monofilament force of application, and do slightly adjust the lengths toward Weinstein’s original calculated forces. The WEST monofilament set is recommended for research (available through Connecticut Bioinstruments, Connecticut) but the difference in stimulus needs to be considered when attempting to compare the WEST with threshold and functional scales of interpretation developed from Weinstein’s standard style monofilaments. The tip geometry has changed, and force of application differs slightly from standard sets used. Although the instruments are very close in stimuli, additional clinical testing is needed to determine how the WEST compares with the original standard monofilament design in test stimuli and if the interpretation scales are applicable for the WEST.
Instrument handle variations may not affect the specified monofilament force of application if the nylon is maintained at a 90-degree angle to the handle, and force of the monofilament applied to the skin is correct and at a distance from the examiner’s hand. These two criticisms of Semmes–Weinstein-style monofilaments have been addressed in a new handle design: the examiner’s difficulty in seeing the tip of the lightest monofilaments on application, and the monofilaments’ breaking at the point it leaves the handle. The new handle design extends over the monofilament where it exits from the handle and includes a light to illuminate the skin being tested, while keeping the 90-degree orientation of the monofilament to the rod handle. This new handle design prevents the monofilament from being sheared off by a neighboring monofilament, from being laid down upon itself, and from damage when inadvertently dropped (available at timelyneuropathytesting.com) ( Fig. 11-6 ).
If made of pure nylon, and the diameter size and length are correct, the standard Semmes–Weinstein-style monofilament instrument stimulus has been found to be repeatable within a small specified standard deviation. , The monofilament length can be checked to be 38 mm with a millimeter ruler. Diameters can be checked with a micrometer. Because the monofilaments are not always perfectly round, diameters are taken three times and averaged. (See diameters in Table 11-2 ).
Clinicians either need to request actual monofilament calibration measurements on test sets they use or measure their sets to confirm calibration. Monofilaments are applied 10 times and averaged for application force measurement. Nevertheless, it is most accurate to use the same set for retesting. It is not enough just to state in studies that a calibrated instrument was used, or say sets used are made to specifications based on a published calculated table of application force.
A known problem in measurement of monofilament application force is the use of top-loading balance scales in an attempt to measure monofilament force of application. For most who attempt monofilament calibration, these scales are inaccurate in measuring the monofilament “dynamic” force of application because the instruments are intended for measuring static weight, and depend on an internal spring mechanism that works against the elasticity of the nylon.
Bell and Buford specifically engineered an instrument measurement system to be sensitive enough to measure dynamic force application and range of the monofilaments, in addition to any other hand-held sensory testing instruments ( Fig. 11-7 ). The signature of a monofilament repeatedly applied was accurately displayed in real time on an oscilloscope, measured, and examined for spikes in force, vibration of the tester’s hand, or subthreshold application. , The lightest monofilaments were applied to a calibrated strain gauge accurate to less than 1 g. If applied too quickly (less than 1.5 seconds) and “bounced” against the skin, the monofilament force of application will spike, overshoot, and exceed intended force of application, thus technique of application is important. A spectrum analyzer (not shown) was used in the measurement system to detect the force frequency of application. Frequency signal outputs from the lightest to the heaviest monofilament were detected throughout the available frequency spectrum, negating claims that the monofilaments only test low- or high-frequency (slowly or quickly adapting) end-organ response. ,
In recent years, mechanical engineer researchers James Foto and Dave Giurintano have reproduced the earlier strain-gauge transducer design and engineered the force output to be read directly on a computer. This system, like the original, can measure any hand-held sensibility testing device, but allows former analog measurements to be digital in real time. Computer calculations of force of application and standard deviation help eliminate potential examiner error in calculations of average force (developed in Labview scientific program, National Instruments, Austin, Texas). A still more recent development adds a motorized attachment to apply the monofilament to the measurement system ( Fig. 11-8 ). All variations in hand-held application are thus eliminated for measurement. Foto and Giurintano initiated this automation in a project between Louisiana State University and the Paul W. Brand Research Laboratory, National Hansen’s Disease, Programs (NHDP) now in Baton Rouge, Louisiana.
Three applications of the lightest monofilaments are used in clinical testing even though the patient usually responds to the first application. It was found in instrument testing that one touch of these extremely light monofilaments may not reach the required threshold, but one out of three always reaches intended threshold. Clinical studies and papers that require two out of three, three out of five, or one out of five, and so forth for a correct response are incorrect , as this is not the test protocol used in normative studies and clinical studies for functional levels (which requires one correct response out of three trials).
When calibrated and applied correctly, the monofilaments are a valid test for determining sensibility detection thresholds. Studies have clearly demonstrated their ability to accurately detect intended clinical conditions. Used in standard consistent protocols, the monofilament test is able to compare patient data in individual and multicenter studies and is providing information regarding peripheral nerve changes with treatment not previously available with less sensitive and uncontrolled instruments. When calibrated correctly, it is one of the few, if not the only, sensibility measurement instrument that approaches requirements for an objective test.
Comparison with Other Tests of Sensibility
Weinstein found that the normal detection threshold for touch pressure does not vary widely over the entire body. The relative consistency is what makes light-touch/deep-pressure threshold mapping of the cutaneous innervation of the peripheral nervous system possible. This is in contrast to point localization and two-point discrimination thresholds.
Depending on the question, one may need more than one test of sensibility to obtain an adequate picture of neural abnormality. The monofilaments do not measure end-organ innervation density . Once monofilament threshold is screened or mapped to establish areas of abnormality, other tests such as two-point discrimination and point localization can be focused in abnormal areas to help further qualify and quantify abnormal sensibility as to innervation density and localization of touch.
It is known that monofilament testing can sometimes reveal peripheral nerve compression before conventional two-point discrimination tests and reveal return of innervation long before two-point discrimination is measurable at the fingertips. Authors and clinicians have traditionally championed one or more methods of sensibility testing, and clinicians should understand that to determine the relative control and validity of another instrument versus the monofilaments, a comparison study requires a valid protocol with direct comparison of instrument stimulus and results, not just opinion.
The first sign of nerve return after laceration and repair is not the heaviest monofilament but a positive Tinel’s test distal to the site of repair. A positive Tinel’s test—in which there is perception of shocking and shooting electrical sensations—is a valuable, albeit subjective, indicator of returning nerve physiologic response after laceration concurrent with or before the heaviest touch–pressure threshold can be measured. Since peripheral nerve return occurs from proximal to distal, Tinel’s test, like the monofilament test, shows improvement proximally to distally over time.
Background Needed for Understanding
Von Frey was the inventor of the monofilament form of testing using horsehairs only capable of producing light thresholds. Weinstein first invented the 20 nylon monofilaments, added heavier levels of detection, and did normative studies. The range of forces of the monofilaments occur simply from available diameter sizes of nylon. In studies, they cannot be treated as occurring at equal mathematical increments as some researchers have done. Today the monofilament log numbers are primarily used as marking numbers for ordering and specifying diameter size (see Table 11-1 ).
Von Prince was greatly influenced by Moberg’s emphasis on sensibility and hand function. She began investigating the residual function of patients who had sustained a variety of peripheral nerve injuries from war wounds. She observed that of two patients who could not tell a difference in testing between one and two points, one could feel a match that would burn his finger, and the other could not. She thus described the all important “ level of protective sensation” that was not being measured by the Weber two-point discrimination test frequently used in practice. She also noticed that of two patients who responded to a pinprick, one would have the ability to discriminate textures and one would not. Thus she first described a “level of light touch sensation” that could be equated with the patient’s ability to discriminate textures. As she searched for tests that would be able to discern these differences in patients, she found the answer in Weinstein’s monofilament test. Von Prince published her findings but was transferred overseas before fully completing her investigation. Omer realized the value of the monofilament test and insisted it be continued by Werner.
James M. Hunter, originator of the Hand Rehabilitation Center, Ltd., in Philadelphia, realized the value of monofilament testing in producing information on patient neural status that was not forthcoming from other examinations. He insisted on this form of testing for his patients, many of whom came to him with previously longstanding unresolved peripheral nerve problems. But the test originally took 2 hours when included with other sensory tests and was confounded by inconsistent coding and the inclusion of other tests for interpretation.
Working with Hunter in 1976, Bell (later Bell-Krotoski) made changes to the test to make consistent peripheral nerve mappings and eliminate variables. Changes included (1) a constant scale of interpretation for the entire upper extremity rather than the interpretation scale changed for thumb, fingers, and palm, (2) eliminating two-point discrimination as a requirement for light touch, (3) eliminating point localization as a requirement for a “yes” response,” and (4) adding consistent colors from cool to warm for quick recognition of increase in force required for detection of touch–pressure. Results of mappings serially compared for changes in neural status could then be easily recognized in seconds numerically and visually for extent and severity of peripheral nerve abnormality. The mappings were found to predict the rate of neural return or diminution, as well as of the quality of neural return or severity of diminution ( Figs. 11-9 to 11-11 ).