Pearls & Oy-sters: False positives in short-segment nerve conduction studies due to ulnar nerve dislocation
B. J. Kim, MD, PhD,
S. B. Koh, MD, PhD,
K. W. Park, MD, PhD,
S. J. Kim, MD, PhD and
J. S. Yoon, MD, PhD
From the Departments of Neurology (B.J.K., S.B.K., K.W.P.) and Rehabilitation Medicine (S.J.K., J.S.Y.), College of Medicine, Korea University, Seoul.
Address correspondence and reprint requests to Dr. Joon Shik Yoon, Department of Rehabilitation Medicine, Korea University Guro Hospital, Korea University College of Medicine, 80, Guro-dong, Guro-gu, Seoul, 152-703, Korea rehab46{at}korea.ac.kr.
The possibility that a technical error may occur during nerveconduction studies due to ulnar nerve dislocation when the elbowis flexed has recently been suggested. We investigated normalvolunteers using ultrasonography to observe the effects of ulnarnerve dislocation during elbow flexion on short-segment nerveconduction studies. We found significant conduction block inall of the subjects with ulnar nerve dislocation, and the findingwas defined as a technical error caused by volume conduction.The results of the present study suggest that caution shouldbe exercised when interpreting the results of short-segmentnerve conduction studies at the across-elbow segment due tothe possibility of technical error induced by ulnar nerve dislocation.
Abbreviations: NCS = nerve conduction study; S-NCS = short-segment NCS; UNE = ulnar neuropathy at the elbow.
A nerve conduction study (NCS) is the most valuable diagnostictool in patients with ulnar neuropathy at the elbow (UNE). However,the possibility that a technical error induced by ulnar nervedislocation may occur when the elbow is flexed has recentlybeen suggested.1 Indeed, the position of the elbow may havea significant influence on the calculated ulnar nerve conductionvelocity across the elbow,2 and when the elbow is extended,it is thought that the ulnar nerve may become loose or redoubledin the ulnar groove and that surface measurements do not accuratelyreflect the true distance of the underlying nerve.2,3 Elbowflexion may stretch the nerve; therefore, the measurement ofthe ulnar groove distance is a more accurate reflection of thedistance along the nerve in the ulnar groove. The recommendedposition of the elbow during ulnar NCS is flexion at 90 or 135degrees in order to straighten the ulnar nerve.4 However, significanttechnical error may be caused by ulnar nerve displacement whenthe elbow is flexed.1
NCS with short-segment interval stimulations has been commonlyused to increase the sensitivity for very focal lesions, becausethe effect of the abnormally slow segment is progressively dilutedas more and more normal segments are included in the examination.5A short-segment NCS (S-NCS) can be performed by stimulatingthe nerve at 2-cm intervals across the elbow in the flexed position.6A significant conduction delay or focal change in amplitudeor waveform morphology across a segment is generally consideredabnormal; however, ulnar nerve displacement by elbow flexionmay cause a more significant technical error in S-NCS than inroutine NCS at the across-elbow segment.
In the present study, we investigated the effects of ulnar nervedisplacement on S-NCS at the across-elbow segment. We also measuredthe angle of the elbow when the ulnar nerve was dislocated byelbow flexion.
Participants and evaluation.
We studied 234 elbows from 117 healthy volunteers (52 men, 65women), ages 20 to 50 years (mean age, 30.4 y), who had normalNCS results in their upper extremities. Elbow flexion at approximately135° was used for ulnar NCS in all subjects. None of theparticipants had any history or symptoms of elbow pain or trauma,entrapment neuropathy, or systemic disease. Approval from thehospital institutional review board was obtained prior to conductingthe study. Written informed consent, in which the potentialrisks of ultrasonography were outlined, was obtained from theparticipants before the procedure.
Real-time ultrasonographic study.
A 7.5 to 12 MHz linear transducer (5000 System HDI; PhilipsMedical Systems NA, WA) was applied on the line between themedial epicondyle and olecranon of subjects lying supine onan examining table. We used the B mode to view the ulnar nervein the postcondylar groove between the medial epicondyle andolecranon and used the color Doppler mode to differentiate arteries.A baseline nerve location was determined at zero degrees ofelbow flexion, and we observed whether the ulnar nerve was displacedat any angle during elbow flexion using real-time ultrasonography.We classified the position of the ulnar nerve (during elbowflexion) into one of three types according to the classificationsystem of a previous report.7 Type N (no dislocation) classificationwas assigned when the nerve moved anteriorly, but not to thetip of the epicondyle, during elbow flexion. The ulnar nerveposition was classified as type S (subluxation) when the nervemoved onto the tip of the epicondyle. The position of the ulnarnerve was classified as type D (dislocation) when the nervemoved anteriorly beyond the tip of the epicondyle. In casesof type D, we measured the angle of the elbow using a goniometerwhen the ulnar nerve was dislocated (figure 1). To measure theangle, an axis of the goniometer was located on the skin ofthe medial epicondyle tip, and a proximal fixed arm of the goniometerwas aligned parallel to the long axis of the upper arm. A distalmovable arm was then attached to the medial surface of the forearmlong axis. The elbow angle was defined as the angle made bythe movement of the distal movable arm with forearm flexionfrom the extended position.
Figure 1 Angle of the elbow when the ulnar nerve was dislocated
The ulnar nerve (*) moved onto the tip of the medial epicondyle when the elbow was flexed and finally dislocated at 125 degrees of flexion.
Short-segment nerve conduction study.
S-NCSs for the ulnar nerve were performed in subjects with typeS and D classifications by an electromyographer who was blindto the classification type. The S-NCSs were performed at anelbow position of approximately 135°. Prior to the S-NCS,we confirmed that none of the subjects among those with typeS and D classifications had weakness in the ulnar innervatedmuscles or sensory changes in the fourth and fifth fingers.To achieve approximately 135° of elbow flexion in the mostcomfortable position possible, subjects were positioned supineon an examining table and asked to abduct their upper arm to90° and to rest it on a pillow such that the hand approachedthe ipsilateral ear, resulting in elbow flexion. The midpointof the ulnar groove between the tip of the medial epicondyleand the olecranon was marked (point P; figure 2. From this point,marks were placed along the course of the ulnar nerve at 2-cmintervals from 6 cm proximal to 4 cm distal.
Figure 2 Stimulation points for the short-segment ulnar nerve conduction study
The black dots represent the stimulation points for the ulnar nerve located in the ulnar groove (marked without prime). The ragged dots indicate the stimulation points for the dislocated ulnar nerve tracked by ultrasonography (marked with a prime). P = midpoint in the ulnar groove between the tip of the medial epicondyle and the olecranon. P2 = 2 cm proximal to point P. D2 = 2 cm distal to point P. P' = the closest ulnar nerve point from the tip of the medial epicondyle tracked by ultrasonography. MET = medial epicondyle tip.
The ulnar nerve was stimulated with abductor digiti minimi recordingat each mark. The recording electrode was placed over the bellyof the abductor digiti minimi, and the reference electrode wasplaced over the distal tendon. A constant current stimulator(2-cm intervals between cathode and anode) and a disposable,flat-surface electrode 1 cm in diameter were used for recording.Equipment used for the study was manufactured by Nicolet Biomedical(Viking IV, Madison, WI). A sensitivity setting of 5 mV/division,a sweep speed of 2 msec/division, and a filter setting of 5Hz to 5 KHz was maintained throughout all measurements. Theskin temperature of the subjects during the study was also maintainedat or above 32 °C. Before the measurements were performed,we ensured that supramaximal stimulation was achieved and thatadequate pressure was applied to the stimulating electrodesin order to enable focal stimulation without spread.
Latencies were measured from the stimulus to CMAP onset, andthe amplitude measurements were calculated from baseline tonegative peak. A conduction delay of greater than 0.5 msec orconduction block with an amplitude reduction of 50% or moreacross 2-cm segments were defined as abnormal.8
After the S-NCS, the nerves that were displaced by elbow flexionwere tracked using ultrasonography and marked as ragged dotswhen elbow flexion was at 135°. The closest point from thetip of the medial epicondyle was marked as point P'. Marks werethen placed along the course of the tracked ulnar nerve at 2-cmintervals from 6 cm proximal to 4 cm distal to point P' (raggeddots in figure 2). The S-NCSs were performed again by stimulatingthe ragged dots.
Ultrasonography.
Ulnar nerve dislocation (Type D) occurred in 13 elbows (5.6%)of 13 subjects (8 men, 5 women). Ulnar nerve subluxation (TypeS) was found in 58 elbows (24.8%) of 49 subjects (22 men, 27women). The mean elbow angle associated with ulnar nerve dislocationwas 99.8° (range 75 to 135). The ulnar nerve was dislocatedat an elbow angle of less than 90° in 3 elbows; 2 elbowswere at 87°, and 1 elbow was at 75°.
Short-segment nerve conduction study.
All 13 cases of ulnar nerve dislocation at the elbow exhibiteddelayed latency (12 subjects; mean 0.62 msec, range 0.6 to 0.8msec) or a conduction block around the medial epicondyle duringthe S-NCS with black dot stimulation. Of the 13 subjects withconduction block, the amplitude drop points were P in 6 subjectsand P2 in 3 subjects; the mean amplitude drop rate in these9 subjects was 68.9% (range 58.8 to 92.2%). The other 4 subjectsshowed complete conduction block (no evoked compound muscleaction potential) at point P2. One of the 4 subjects with completeconduction block showed an amplitude drop of 34% at point P.However, no abnormal findings were detected in the elbows ofthe subjects with ulnar nerve subluxation during elbow flexion.
The abnormal findings in the 13 elbows with ulnar nerve dislocationwere transformed to normal waveforms and latencies using raggeddot stimulation (figure 3).
Figure 3 Example of short-segment ulnar nerve conduction study
Conduction block on the point P2 with traditional site (black dots in figure 2) stimulation was transformed to normal waves by stimulation with ultrasound-guided nerve localization (ragged dots in figure 2).
The prevalence of ulnar nerve displacement in the present studywas similar to the results of our previous study that showeddislocation in 3.8% and subluxation in 20.5% of subjects.1 Theresults of these two studies showed a high percentage of ulnarnerve displacement in comparison with a previous report,9 althoughthis is probably due to the increased precision of investigationwith high-resolution ultrasonography. On the other hand, thepercentage of ulnar nerve displacement in the present studywas low in comparison with that from another report that foundulnar nerve dislocation in 39 elbows (20%) and ulnar nerve subluxationin 53 elbows (27%) out of 200 healthy elbows.7 Considering thatthe report included older subjects (age 20 to 69 years) thanour study (age 20 to 50 years), the higher percentage reportedin that report was likely related to the loosening of soft tissuein the ulnar groove due to aging, which would result in a higherchance of ulnar nerve displacement.
UNE is the second most common upper extremity compression neuropathy10;however, the diagnosis of UNE is not straightforward. Most physicianssuspect UNE when a patient complains of occasional ulnar distributionnumbness and paresthesias; however, these symptoms could becaused by a variety of conditions, such as lower cervical radiculopathy,lower trunk involvement of the brachial plexus, Guyon canalpalsy, and even myofascial pain syndrome. The diagnostic yieldof electrodiagnosis of UNE is low, and the interpretation ofthe data is often quite difficult. In our previous study, wesuggested the possibility of false negatives in the detectionof UNE if a patient has ulnar nerve dislocation during elbowflexion.1 If a patient is seen who is suspected of having UNE,but nonetheless shows normal findings in routine NCS, S-NCScould be used as one of the next steps to increase sensitivityof the diagnosis. Indeed, the results of the present study showedthat the S-NCS performed with elbow flexion in patients withulnar nerve dislocation at the elbow could lead to a significantdiagnostic error. All elbows showed conduction block or significantlatency delays in the ulnar nerve at the across-elbow segment,which is important, because the identification of these signsare important criteria for the diagnosis of UNE.6 Therefore,significant caution is needed when using S-NCS. Although elbowflexion at 90 or 135° is recommended during NCS to straightenthe ulnar nerve in the ulnar groove, the technical error inducedby ulnar nerve dislocation during elbow flexion should not beoverlooked. In the present study, most elbow angles when theulnar nerves were dislocated were more than 90°. Only threeelbows were at an angle less than 90°, and two of the threeelbows were very close to 90°, both at an angle of 87°.This finding indicates that there is less chance of ulnar nervedislocation during NCS at the across-elbow segment when thedegree of elbow flexion is less than 90°.
To our knowledge, although there have been no studies that havereported that dislocation of the ulnar nerve predisposes a patientto ulnar neuropathy, recurring ulnar nerve dislocation at theelbow may cause bothersome symptoms in some subjects via repeatedirritation or enhanced exposure to injury. Therefore, detectionof ulnar nerve dislocation using ultrasonography would be helpfulin evaluating patients with normal conduction velocities inroutine nerve conduction study at the across-elbow segment,despite clinical symptoms of highly suspected UNE. If ultrasonographyis not available in a clinic, an alternative study could includeS-NCS, which should be performed with two elbow positions, withone at less than 90 degrees and the other at full flexion. Theappearance of conduction delay or block with full flexion wouldsupport either dislocation or transient compressive neurapraxia,either of which might be important in a symptomatic patientafter other causes of symptoms have been excluded. Future studiesare needed to investigate the reliability of this potentialapproach and the importance of dislocation as a predisposingfactor for UNE.
S-NCS for assessment of the ulnar nerve at the across-elbowsegment requires special caution due to the possibility of technicalerror caused by elbow position. Many laboratories use a varietyof elbow angles in order to minimize measurement errors dueto an ulnar nerve redundancy; however, such practices may overlookanother source of artifact, especially since we observed thatulnar nerve dislocation induced by elbow overflexion could leadto significant technical error. The results of our study suggestthat S-NCS at the elbow should be performed with the elbow flexedat less than 90 degrees. Future studies are needed to investigatethe ideal range of the elbow angle during S-NCS in order toavoid technical error caused by the position of the elbow.
Disclosure: The authors report no conflicts of interest.
Kim BJ, Date ES, Lee SH, Yoon JS, Hur SY, Kim SJ. Distance measure error induced by displacement of the ulnar nerve when the elbow is flexed. Arch Phys Med Rehabil 2005;86:809–812.[Medline]
Checkles NS, Russakov AD, Piero DL. Ulnar nerve conduction velocity–effect of elbow position on measurement. Arch Phys Med Rehabil 1971;52:362–365.[Medline]
Nelson RM. Effects of elbow position on motor conduction velocity of the ulnar nerve. Phys Ther 1980;60:780–783.[Abstract/Free Full Text]
Kimura J. Assessment of individual nerves. In: Kimura J, ed. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice, 3rd ed. New York: Oxford University Press; 2001:146.
Campbell WW. The value of inching techniques in the diagnosis of focal nerve lesions. Inching is a useful technique. Muscle Nerve 1998;21:1554–1556; discussion 1561.[Medline]
Kanakamedala RV, Simons DG, Porter RW, Zucker RS. Ulnar nerve entrapment at the elbow localized by short segment stimulation. Arch Phys Med Rehabil 1988;69:959–963.[Medline]
Okamoto M, Abe M, Shirai H, Ueda N. Morphology and dynamics of the ulnar nerve in the cubital tunnel. Observation by ultrasonography. J Hand Surg 2000;25:85–89.[Medline]
Azrieli Y, Weimer L, Lovelace R, Gooch C. The utility of segmental nerve conduction studies in ulnar mononeuropathy at the elbow. Muscle Nerve 2003;27:46–50.[Medline]
Childress HM. Recurrent ulnar-nerve dislocation at the elbow. J Bone Joint Surg Am 1956;38-A:978–984.[Abstract/Free Full Text]
Campbell WW. Ulnar neuropathy at the elbow. Muscle Nerve 2000;23:450–452.[Medline]
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