First Report.

The first paper describing the NDE was presented by Rosier and Blair[<12] at the 21st annual Rocky Mountain Bioengineering Symposium and 21st ISA Biomedical Sciences Instrumentation Symposium in 1984 in Boulder, Colorado. The report met two of the six criteria (1 and 3). Twenty patients with symptoms of median or ulnar nerve compression at the wrist were studied at an orthopedic clinic. Distal motor latencies obtained by the NDE in 38 median and 20 ulnar nerves were compared to latencies obtained by standard nerve conduction tests. The conclusion of this paper was that the correlation between the NDE's data and standard NCS data was excellent.

No details are given about the clinical evaluations or the final clinical diagnoses. The stimulating and recording methods are ambiguously described: placement sites of recording electrodes are not precisely defined, and stimulation intensity was adjusted so as to obtain a vigorous muscle twitch. The report states that the neurometer is not powerful enough to reliably allow stimulation of the palm segment. If this is the case, however, the present authors wonder how supramaximal stimulation can be ensured reliably even in the wrist segment.

Limb temperature was not monitored. Control values were based on results of standard NCSs, not on values obtained from the NDE in a control population. Even though all patients were symptomatic, 20 NDE values (15 median and 5 ulnar nerves) were called normal and the rest abnormal, based on the results of standard NCSs; 2 nerves with abnormal standard NCSs were designated as normal for reasons that are unclear. Although comparable numbers of nerves with these arbitrarily defined normal and abnormal values were tested by the NDE and standard NCSs, the range of these values varied greatly. For example, the mean abnormal value for the ulnar nerve was 4.10 ms by the standard NCS method, but 8.12 ms by the NDE; one measurement taken by the NDE was 17 ms, but none obtained by corresponding NCSs exceeded 5.5 ms.

Subsequent Reports.

Feierstein, 1988.

A paper presented by Feierstein[8>] in 1988 met two of the six criteria (1 and 3). The study evaluated 15 patients whose CTS had failed to respond to conservative management and who were undergoing surgical release of transverse carpal ligament, and were studied by the NDE preoperatively and postoperatively. The author concluded that the latency values of 11 patients improved postoperatively.

The time between preoperative and postoperative studies varied from 2 weeks to almost 1 year. The value assigned as the upper limit of normal for the latency value was taken from a previous study from a different laboratory that averaged 10 responses, whereas this study averaged only 5 such responses. Since the test-retest reliability of the NDE has not yet been determined, these changes raise questions about the choice of the normative data. The bases for clinical diagnoses are not given, limb temperature was not monitored, and comparisons to standard techniques are not documented.

Osterman and Colleagues, 1989.

A scientific exhibit by Osterman and colleagues[10] met two of the six criteria (1 and 2). Thirty-five patients were evaluated by NCSs and by the NDE to determine the concordance rate of diagnoses of CTS between these two methods of study. The patients were divided into two groups: those with and those without clinical symptoms of CTS. There was no control population, and the abnormal value used was arbitrarily based on the suggestion of Nervepace Electroneurometer manufacturers. The patients were divided into groups with mild, moderate, and severe CTS based on the arbitrarily chosen NDE values. Interestingly, one patient with clinically severe CTS had normal NDE values, whereas two who did not have CTS had NDE values in the moderately severe range. Eighteen more patients with CTS were studied postoperatively And reductions in NDE values reported, but standard median NCSs were not performed postoperatively. Limb temperature was not monitored.

This study was not designed to provide sufficient information on the patient population, control values, and techniques used. In addition, the study was based in part on the incorrect assumption that a prolonged distal motor latency is the most frequently used electrodiagnostic measure for confirming the diagnosis of CTS.

Steinberg and Colleagues, 1992.

Steinberg and colleagues [13] studied the utility of the NDE for patients with CTS. The article met three of the six criteria (1, 2, and 5). NDE studies of 51 hands of 28 patients with symptomatic CTS were compared to standard NCSs of 18 hands of 10 control subjects. A close relationship was observed between the two modes of testing (correlation coefficient of 0.93); however, nine patients with a clinical diagnosis of CTS tested normal on the NDE (sensitivity of 69%). Interestingly, a number of patients were excluded from the sensitivity calculations because no readings could be obtained from them by the NDE. In three patients (one with peripheral neuropathy, one with gout, and one with radius fracture), no recordings could be obtained by NDE, although the response was clearly present as shown by standard NCSs. Control measurements were presented; however, the reasons for identifying a measurement as abnormal are not made clear. Room temperature was defined, but skin temperature was not monitored.

Grant and Colleagues, 1992.<

A paper authored by Grant and colleagues [9] met four of the six criteria (1, 2, 3, and 6). Two hundred fifty-two hands of 132 female individuals were assessed, and divided into several groups. Twenty-two patients (32 hands) with physician-diagnosed CTS (confirmed by conventional NCSs), 63 plant workers (126 hands) with and without symptoms of CTS, and 47 healthy asymptomatic controls (94 hands) were studied. This study was prospective And included the generation of reference values from a control population expressed in terms of means and standard deviations. The authors concluded that while the mean values for the four groups differed significantly, A H igh false-negative rate limited the usefulness of NDE as a screening procedure for CTS.

Room temperature was recorded but limb temperatures were not monitored in this study. The conventional NCS procedures employed were not described in detail. It is unclear whether studies were performed by multiple operators. Although Attempts were made to establish a clinical diagnosis independent of electrodiagnostic studies, it is unclear if this assessment was limited to the use of A H and diagram, or included other findings on physical examination. Although mean Values between the groups differed significantly (3.6 ms for controls, 3.9 ms for asymptomatic plant workers, 4.2 ms for symptomatic plant workers, 4.4 for physician-diagnosed CTS patients), the standard deviations were sufficiently large to result in a substantial overlap of the values for each group, making it difficult to assess the significance of any single value in an individual patient. Among CTS patients (diagnosed clinically by physicians with confirmatory NCSs), 47% fell within two standard deviations of the control mean, reflecting A High false-negative rate. This high false-negative rate suggests that conventional NCSs represent a more sensitive method for diagnosing CTS than NDE.

Beckenbaugh and Simonian, 1995.

A paper by Beckenbaugh and Simonian[4] met two of six criteria (1 and 3). Seventy-two median nerves in 45 patients with suspected CTS were studied by the NDE, and compared with standard EMG performed on 64 of these nerves. The NDE data were also compared to actual nerve compression observed at surgery in 23 cases, and to 4 physical tests (Phalen's, Tinel's, two-point discrimination, and a median nerve compression test). The article concluded that NDE assessment gave a more accurate prediction of CTS than physical examination, with A H igh degree of sensitivity (85.7%) and specificity (87.5%). NDE results were found to correlate positively with results from routine electromyography.

Although this study attempted to correlate NDE and NCS data, the article provided no description of the conventional nerve conduction study techniques employed (EMG values were obtained in the standard fashion). Additionally, NDE and NCS data were not obtained on the same day (average 12.2 days between assessments). Temperature was not controlled. Normative data for the NDE technique were not generated on a control population. Criteria for abnormality were not defined in statistical terms (mean, standard deviation, range); an NDE latency determination of >3.9 ms was arbitrarily chosen as abnormal in order to maximize both sensitivity and specificity. The diagnosis of CTS was not established by clinical criteria, independent of electrophysiologic techniques. Rather, an EMG value >4.5 ms (presumably representing the median distal motor latency) was considered a positive screening Value for CTS. Although the article noted that the complete EMG examination included sensory latency values and comparison to ulnar latency values, no NCS data were reported apart from distal motor latencies. In the 23 surgical cases, all with evidence of nerve compression at surgery, the article noted a sensitivity of 87% for NDE and 63.6% for EMG. However, the NCS sensitivity was again based on an absolute median distal motor latency value of >4.5ms, rather than other NCS parameters that have been shown to be more sensitive (e.g., median to ulnar palmar mixed nerve study comparisons).[2]

The article by Cherniack and colleagues[6] met two of six criteria (3 and 5). The study represented a prospective evaluation of 19 patients (98 hands) referred to A H ospital-based EMG laboratory. The Article did not specify whether all of the patients were referred for a CTS evaluation. A subset of 29 individuals (58 hands) were referred following an occupational medicine consultation with a clinical diagnosis of CTS and an Associated index of certainty. A control group consisted of 10 hospital workers (20 hands). All of these patients underwent median and ulnar motor and sensory nerve conduction studies, with an effort to control temperature, and utilizing Anatomical sites for stimulation and recording without distance measurements. The authors do not comment on exactly where the ulnar sensory responses were recorded, but stimulation was at the wrist. They compared these results to normal values from their laboratory but comment that no adjustments were made for age or sex. Special techniques such as palmar conduction studies were not performed. The authors used the manufacturer's recommendations regarding normal Values for the NDE which apparently were not adjusted for age or sex. (When looking at these three groups, there were significant differences in age). The results indicated that standard nerve conduction studies surpassed the NDE in diagnosing CTS. However, results of the NDE correlated well with formal NCS results in patients who were defined to have CTS both clinically and on standard NCSs. The authors comment on the controversy regarding normal values for the NDE. Apparently three different limits of normal have been proposed by the manufacturer and other authors. In some patients, they conclude that using the most stringent normal values would result in false-positive tests exceeding true positives. The authors comment that the normal cut-off value supplied by the manufacturer did not bear any relationship to their own normal control results. The value derived by the authors was considerably higher. In using their own normal values, the NDE became a relatively insensitive instrument.

All of these considerations were summarized by the authors in concluding that: (1) the NDE is clearly less discriminating than NCSs for CTS; (2) there is A H igh proportion of false-positive NDE tests, suggesting that patients should be screened clinically and then directly referred for NCSs without the intermediate screening step with the NDE; (3) that before screening studies such as the NDE can be applied for the evaluation of common entrapment disorders, more extensive observation will be needed; and (4) that despite the technical accomplishments of the NDE, these advances will not compensate for the many underlying uncertainties that surround the testing.

Dunne and Colleagues, 1996.

An article by Dunne and colleagues[7] met one of the six criteria (1). This study represented a prospective study of 25 patients referred consecutively to A H ospital-based EMG laboratory with a clinical diagnosis of CTS. The authors performed conventional NCSs, measuring motor and sensory latencies bilaterally as well as obtaining NDE measurements bilaterally. Concerning the methodology, the authors did not comment on controlling for temperature, nor did they describe precisely the routine NCS methods (i.e., specific identification of sites of stimulation and recording). The values obtained from the two techniques were compared statistically. The NCS distal motor latencies and the NDE results reportedly correlated well, particularly when the same criterion of abnormality (i.e., a motor latency of >4 ms) was used for both techniques. The authors did not identify a normal value for the NDE from their own experience or from citing normal values from the literature. They note that NCS-derived sensory latencies did not correlate well with the NDE findings.

The authors concluded that NCS-generated sensory latencies are more useful in diagnosing CTS, though the sensory latency data do not correlate well with the NDE results. They also note that the NDE does not produce a visual waveform to interpret, which compromises the validity of the interpretation. The authors also comment that NDE cannot be adapted to more sensitive methodology such as comparing median and ulnar values. Lastly, they note that patients who have underlying problems such as a polyneuropathy may not be suitable for NDE studies. They conclude that NDE provides A H ighly specific but relatively insensitive tool for studying CTS.

Atroshi and Johnsson, 1996.

An article by Atroshi and Johnsson[3] met four of the six criteria (1, 2, 3, and 5). This study prospectively addressed the sensitivity and specificity of the NDE in the diagnosis of CTS in patients undergoing surgical release. A new model of the NDE (presumably the NS), capable of measuring both distal motor and distal sensory latencies, was employed. The recorded sensory latency represented an absolute value obtained from an antidromic digital technique). Preoperative NDE distal motor and sensory latencies of the median nerve were obtained in 43 hands with CTS (assessed by history, signs, and postsurgical relief of symptoms), and in 60 hands of 30 asymptomatic volunteers. Sensitivity was reported at 58% for distal motor latencies and 65% for distal sensory latencies, with sensitivities of 87% and 92%, respectively.

Room temperature was recorded but limb temperatures were not monitored. Distances utilized for the motor and sensory latency determinations were not specified. The initial criteria of abnormality were not obtained using the NDE or from the reference population, but instead were drawn from prior studies employing conventional NCS techniques. The authors did, however, express the normative data from the 60 control hands in terms of means with standard deviations, And did suggest a change in criteria to optimize sensitivity and specificity. Sensory responses were unobtainable in 19 of 43 cases; an absent sensory response was considered confirmatory evidence for CTS, and these data were factored into the sensitivity determinations. An absent response, however, cannot be used to confirm the presence of A localized entrapment.

Finally, the patients preselected for this study were all refractory to non-operative treatments (not specified), and thus may have reflected a more severely affected group of patients. The absent sensory responses in 19 patients supports this suggestion. As such, the sensitivity and specificity of the NDE and NS in patients with milder cases of CTS could not be assessed. This study did not compare the NDE to conventional NCSs.

Pransky and Colleagues, 1997.

A paper by Pransky and colleagues[11] met three of six criteria (1, 3, and 4). Both the NDE and NS were compared with conventional NCSs in the workplace screening for CTS. Thirty-two working individuals without CTS were examined with NDE, NS, and NCSs, and were retested 1 week later. The testing procedures were precisely documented, and limb temperatures were controlled. Test-retest reliability was best with NCSs. Results with the NS correlated slightly better with NCSs than did the NDE results. The authors concluded that the NDE and NS devices were unlikely to be useful in confirming early CTS, when detailed NCSs may be necessary to detect nerve entrapment. The device was felt to have potential utility in assessing longitudinal changes in mean latency in a group, with the mean changes possibly reflecting real effects in a workplace.

The number of study subjects was small. Clinical examination was limited to Phalen's sign, Tinel's sign, and light touch sensation, without inclusion of pin-prick sensation, two-point discrimination, or motor examination. Though 5 of the 32 subjects gave A H istory of some symptoms suggestive of CTS, these individuals were not separated out in the analysis. Only antidromic digital sensory responses were recorded; no palmar studies were performed, and no median to ulnar comparisons made. The protocol described stimulus onset to peak response measurements for both motor and sensory responses; this would not represent A typical method for measuring a distal motor latency, which is usually defined by the interval between stimulus onset and the initial negative deflection of the compound muscle action potential. Not all digital sensory responses obtained with conventional NCSs were performed at the designated distance of 140 mm (used for the NS). To correct for this, the authors made an adjustment using a ratio of the test distance to 140 mm so that results from NCSs and the NS would be comparable. It is unclear how this adjustment would impact the results. The authors concluded that a single NDE or NS screening test result in an individual may not accurately predict an NCS's result or the presence of CTS. Both NDE and NS produce a single latency value, thus yielding less information than conventional NCSs. False-positive and false-negative rates were felt to be high. The authors commented that more complete NCS testing, including inching and median to ulnar comparisons, may be more useful than NDE or NS in the detection of early or mild CTS. However, this study did not evaluate patients with CTS.

CRITIQUE OF THE TECHNOLOGY

The advantages of the NDE and NS are that they are compact, lightweight (approximate weight of the NDE S-100 model is 12 oz; the NDE S-200 model is 5 lbs), portable, and relatively quick and easy to use on a large scale in an industrial facility. However, in the opinion of the AANEM, these machines cannot compare to standard electrodiagnostic test equipment, as they fail to provide several important features essential to an electrodiagnostic evaluation: