Volume 87, Issue 3 , Pages 408-410, March 2006
Mixed Latency Difference for Diagnosis of Ulnar Neuropathy at the Elbow
Article Outline
Abstract
Heise CO, Toledo SM. Mixed latency difference for diagnosis of ulnar neuropathy at the elbow.
Objective
To provide reference values and to compare this technique with the standard motor conduction velocity (MCV) of the ulnar nerve.
Design
Retrospective unmasked study.
Setting
Private and institutional practice.
Participants
The reference group included 57 healthy volunteers. Patients included 100 subjects with suspected ulnar neuropathy at the elbow (UNE) referred for neurophysiologic evaluation. This group was subdivided into 2 groups: group A was composed of 45 patients with UNE confirmed by MCV of the ulnar nerve, and group B included 55 patients with suspected UNE in whom the diagnosis could not be established by MCV of the ulnar nerve.
Interventions
Not applicable.
Main Outcome Measures
Differences between peak latencies of ulnar and median mixed nerve action potentials at the arm, after stimulating these nerves at the wrist. This was called mixed latency difference.
Results
The upper normative limit of the mixed latency difference was 1.1ms, and there was a significant correlation with height. In group A, the mixed latency difference was abnormal in 80% of the cases and could not be calculated in 18%. In group B, the mixed latency difference was abnormal in 8 (15%) patients. All of these had abnormal “inching” of the ulnar nerve across the elbow.
Conclusions
The mixed latency difference was particularly useful in cases of mild UNE.
Key Words: Diagnosis , Elbow , Nerve conduction , Rehabilitation , Ulnar neuropathies
ULNAR NEUROPATHY AT the elbow (UNE) is a common challenge for the electromyographer.1 Motor conduction velocity (MCV) of the ulnar nerve is recommended as the standard neurophysiologic diagnostic technique for this condition by the American Association of Electrodiagnostic Medicine, the American Academy of Neurology, and the American Academy of Physical Medicine and Rehabilitation.2 Nevertheless, this technique relies on an accurate determination of the nerve length across the elbow, which is not easy because of its nonlinearity.3
Merlevede et al4 described a method for diagnosis of UNE that compares the latency of ulnar and median mixed nerve action potentials at the medial aspect of the arm after stimulating these nerves at the wrist. This is a very simple and fast technique that avoids the troublesome measurement of the segment across the elbow. However, it is not clear whether this method has any advantage over the classic MCV of the ulnar nerve.1
Methods
The local ethics commission approved this study. Subjects were examined lying in a supine position with the shoulder slightly abducted, the elbow extended, and the forearm supinated. Skin temperature was kept above 32°C. We placed the recording electrodes 3cm apart on the medial aspect of the arm, 10cm above the medial epicondyle. A Portable Keypointa was used for the recordings. First, we stimulated the median nerve at the wrist and recorded a mixed nerve action potential (NAP). Then, we stimulated the ulnar nerve at the wrist, parallel to the median nerve–stimulating site. Without moving the recording electrodes, we recorded another mixed NAP with a somewhat longer latency. We measured the difference between the peak latencies of these potentials and called it mixed latency difference.
The mixed latency difference was measured in 2 groups. The first group was composed of 57 healthy volunteers to establish our reference values. The subjects’ age ranged from 20 to 77 years (median, 39y). Only 1 arm was tested in each subject. The exclusion criteria included any complaint in the tested arm (ie, pain, weakness, numbness, paresthesia), diabetes mellitus, or any other known cause of neuropathy. We used the 98th percentile as the upper normative limit. We also looked for any relation of the mixed latency difference to age, height, or body mass index. Statistical analysis was done with the analysis of variance test by using the Epi Info 2002 program.b
The second group was composed of 100 patients retrospectively evaluated with persistent sensory complaints at the 4th and 5th fingers, with or without Tinel sign at the elbow. Patients with other findings in the electrodiagnostic evaluation, such as median nerve neuropathy at the wrist (n=27), polyneuropathy (n=6), cervical radiculopathy (n=5), ulnar nerve neuropathy at the wrist (n=2), or lower brachial plexus lesion (n=2), were excluded. Seven patients were excluded because of a history of trauma involving the ulnar nerve. We also excluded another 7 showing evidence of Martin-Gruber anastomosis. Nine had bilateral symptoms, and only the left side was considered for the purpose of analysis.
The group of symptomatic subjects was subdivided into 2 groups. In group A, 45 patients had ulnar neuropathy at the elbow confirmed by standard MCV of the ulnar nerve. For this test, the shoulder was abducted, the arm was externally rotated, and the elbow was flexed at 90°. The ulnar nerve was stimulated at the wrist, below and above the elbow. The segment across the elbow was 10cm long. The compound muscle action potential (CMAP) was recorded on the abductor digiti minimi muscle. A positive test required 2 of 3 criteria: (1) MCV of the ulnar nerve across the elbow below 50m/s, (2) difference between MCV of the ulnar nerve in the forearm and across the elbow above 10m/s, and (3) drop of the amplitude of the CMAP across the elbow of at least 20%.
Fifty-five patients who did not fit the MCV of the ulnar nerve criteria made up group B. Many of them were also tested by using motor short-segment incremental studies (“inching”) across the elbow, including all patients with an abnormal mixed latency difference. An increase of latency of at least 0.7ms in a 2-cm step was considered abnormal. All those with abnormal short-segment incremental studies also had a clear increased gap or abrupt change of amplitude between successive curves in the superimposed mode by visual inspection.
Results
The mixed latency difference from the control group ranged from 0 to 1.1ms (median, 0.5ms). The 98th percentile was also 1.1ms, which was set as our upper normative value. The histogram of the mixed latency difference values in the control group is shown in figure 1. There was no correlation between the mixed latency difference and age or body mass index (P>0.5), but we noted a significant correlation with height (P<.01), with a linear Pearson correlation coefficient of .47. We developed the following equation for height correction: MLD – .015 × H + 2.67, where MLD is the mixed latency difference and H is height (in centimeters).
Fig 1.
Uncorrected mixed latency difference values in the control group, in the group of patients with UNE confirmed by MCV of the ulnar nerve (group A), and in the group of patients with suspected UNE not confirmed by MCV of the ulnar nerve (group B).
In group A, 18 (40%) patients fulfilled all 3 MCV of the ulnar nerve criteria, 26 (58%) fulfilled criteria 1 and 2, and 1 (2%) fulfilled criteria 1 and 3. Fifteen (33%) patients showed absence or amplitude reduction of the antidromic sensory nerve action potential recorded at the 5th finger (<9μV). Twenty (44%) showed signs of denervation at the first dorsal interosseous muscle. Thirty-five (78%) had an abnormal uncorrected mixed latency difference, and 36 (80%) had an abnormal height-corrected mixed latency difference. The histogram of uncorrected mixed latency difference values in group A is shown in figure 1. The mixed latency difference could not be determined in 8 patients because the ulnar mixed NAP was absent (18%). These patients had either a major conduction block at the elbow or marked axonal degeneration of the ulnar nerve. One patient had a normative borderline value, even after height correction.
In group B, 4 patients fulfilled only MCV of the ulnar nerve criterion 2 and 2 fulfilled only MCV of the ulnar nerve criterion 1 (all in the 45–49m/s range). No patient showed reduction of the amplitude of the sensory nerve action potential at the 5th finger or signs of denervation at the first dorsal interosseous muscle. However, 7 (13%) had an abnormal uncorrected mixed latency difference, and 8 (15%) had an abnormal height-corrected mixed latency difference. The histogram of uncorrected mixed latency difference values in group B is also shown in figure 1. Four patients with abnormal mixed latency differences met only 1 criterion of abnormal MCV of the ulnar nerve (2 met criterion 1, 2 met criterion 2). Meanwhile, short-segment incremental studies disclosed focal demyelination in all patients with abnormal mixed latency differences.
Discussion
UNE is the second most common entrapment neuropathy, and paresthesia at the 4th and 5th fingers is its most common symptom.5 The differential diagnosis includes C8 radiculopathy, lower brachial plexus lesion, and ulnar neuropathy at the wrist.6, 7 None of these are expected to produce an abnormal mixed latency difference, although patients with severe axonal degeneration of the ulnar nerve could have an unobtainable ulnar mixed NAP. An ulnar nerve lesion at the forearm or lower arm could produce an abnormal mixed latency difference, but it is rare.
Modern technology has greatly reduced the error of latency placement, yet this is not the case for distance measures.3 The ulnar nerve segment across the elbow is particularly prone to errors because of its nonlinearity. Landau et al3 showed that the experimental error for MCV of the ulnar nerve determination across the elbow could be as much as 10m/s. Our MCV of the ulnar nerve criteria for UNE diagnosis was based on the American Association of Electrodiagnostic Medicine’s guideline.4 Recording the CMAP over the first dorsal interosseous muscle1, 6, 7 could have enhanced sensitivity, whereas some studies have failed to disclose any advantage of this method over the classic technique.8, 9
Short-segment incremental studies (or inching) is probably the most sensitive neurophysiologic method for the diagnosis of UNE.10 Nevertheless, it has the following drawbacks: (1) it also critically depends on an accurate measure of the nerve length across the elbow, (2) it is time consuming, (3) it is not available in all machines, (4) the exact point of nerve excitation may be uncertain when the nerve is deeper in relation to the cathode,11 and (5) overstimulation at a given point would depolarize the nerve distal to the point underneath the cathode.12
We modified the original technique developed by Merlevede et al.4 This is probably why our normative values differed. We used peak latencies instead of the takeoff. Automatic markers can easily place the peak latencies in position, whereas defining the exact point of the onset latency can be difficult when dealing with low-amplitude potentials or noisy baselines. We also changed the limb position. In the original technique, the elbow was flexed at 15° and the forearm was pronated. Contrary to the belief of some, the elbow position does influence the mixed latency difference. During the control phase of this study, we saw that the mixed latency difference increases when the elbow is flexed. Because 15° can be difficult to judge subjectively, we placed the elbow in full extension. It is also easier to stimulate the median and ulnar nerves with the forearm supinated. The spread of current from the ulnar nerve to the median nerve is a pitfall of this technique, and care must be taken to not overstimulate the ulnar nerve.
Patients with Martin-Gruber anastomosis were excluded from the analysis because its presence is a potential threat for the mixed latency difference method. It is not a problem in the control group because the sensory fibers seem to be responsible for the main part of the ulnar mixed NAP. However, in patients with UNE, ulnar mixed NAP amplitude is reduced because of sensory axonal degeneration and temporal dispersion of the potential at the elbow. In this situation, motor fibers traveling through the median nerve and Martin-Gruber anastomosis become more important and may produce a “double-peaked” ulnar mixed NAP (fig 2). In cases of severe UNE, only the first peak may be recorded, which would produce a normal mixed latency difference.
Fig 2.
Recordings from a patient with Martin-Gruber anastomosis and ulnar neuropathy at the elbow showing a double-peaked ulnar mixed NAP. (A) Median nerve mixed NAP: latency 1 equals 5.9ms. (B) Ulnar nerve mixed NAP: latency 2 equals 6.3ms (mixed latency difference = 0.4), and latency 3 equals 8.3ms (mixed latency difference = 2.4).
The mixed latency difference was not more sensitive than MCV of the ulnar nerve. However, the mixed latency difference detected 4 patients with short-segment incremental studies–confirmed UNE that MCV of the ulnar nerve completely failed to disclose. The mixed latency difference can also assure the diagnosis in cases of mild MCV of the ulnar nerve abnormalities by providing multiple internally consistent findings.2 The diagnosis of UNE should not be made based only in an abnormal mixed latency difference because it is not possible to localize the lesion by this method.4
The amplitudes of mixed NAP were too variable in the control group to have any diagnostic utility. Even the comparison of the amplitude of the potentials from the median nerve and the ulnar nerve showed a great variation, depending on the relative position of the recording electrodes in the sagittal plane (anterior or posterior).
The mixed latency difference showed significant correlation with height, and this has not been previously reported to our knowledge. It is not clear if the longer ulnar mixed NAP latency is related to a longer course of the ulnar nerve or to some small differences in the conduction velocity of the median and ulnar nerves. If we take into account the second explanation, it is easy to understand why the mixed latency difference is correlated with height. Height correction improved the sensitivity only in 4%. Two patients had borderline mixed latency difference values that became abnormal after height correction. Although we could not show any relation of the mixed latency difference to age, it is important to recognize that there is a bias in our controls because only 6 subjects were older than 60.
Conclusions
Mixed latency difference is, indeed, a simple and fast technique that can be helpful for the diagnosis of mild cases of ulnar neuropathy at the elbow. Height correction should be used when dealing with borderline values. On the other hand, the diagnostic contribution of mixed latency difference is reduced in cases of severe ulnar neuropathy, and the presence of Martin-Gruber anastomosis can produce equivocal results.
Suppliers
References
- . Ulnar neuropathy at the elbow . Muscle Nerve . 2000;23:450–452
- . Practice parameter for electrodiagnostic studies in ulnar neuropathy at the elbow (summary statement) . Arch Phys Med Rehabil . 1999;80:357–359
- . Changes in nerve conduction velocity across the elbow due to experimental error . Muscle Nerve . 2002;26:838–840
- . Diagnosis of ulnar neuropathy (a new approach) . Muscle Nerve . 2000;23:478–481
- Kincaid JC. Ulnar neuropathy at the elbow. In: AAEM Course H: Peripheral neuropathy. Joint Meeting of American Association of Electrodiagnostic Medicine and International Federation of Clinical Neurophysiology; 2003 Sept 16-20; San Francisco (CA). Rochester: AAEM; 2003. p H29-36.
- . Focal peripheral neuropathies . In: Dumitru D , Amato AA , Zwarts MJ editor. Electrodiagnostic medicine . 2nd ed.. Philadelphia: Hanley & Belfus; 2002;p. 1043–1126
- . Electrodiagnostic approach to the patient with suspected mononeuropathy of the upper extremity . Neurol Clin . 2002;20:451–478
- . Clinical, electrodiagnostic, and sonographic studies in ulnar neuropathy at the elbow . Muscle Nerve . 2004;30:202–208
- . Which motor nerve conduction study is best in ulnar neuropathy at the elbow? . Muscle Nerve . 2004;29:585–590
- . The utility of segmental nerve conduction studies in ulnar mononeuropathy at the elbow . Muscle Nerve . 2003;27:46–50
- . Ulnar nerve entrapment at the elbow localized by short segment stimulation . Arch Phys Med Rehabil . 1988;69:959–963
- . Short segment incremental studies in the evaluation of ulnar neuropathy at the elbow . Muscle Nerve . 1992;15:1050–1054
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated.
PII: S0003-9993(05)01384-5
doi:10.1016/j.apmr.2005.11.006
© 2006 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved.
Volume 87, Issue 3 , Pages 408-410, March 2006
