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SIGNIFICANCE OF NCS/EMG IN BILATERAL MARTIN-GRUBER ANASTOMOSIS WITH CARPAL TUNNEL SYNDROME.

Byline: Zaheer Ahmed Gill, Farooq Azam Rathore, Akhtar Waheed and Zahid Rustam

Article

INTRODUCTION

Martin-Gruber Anastomosis (MGA) is a well known neuronal anomalous connection occurring at various levels between median and ulnar nerves in the forearm. Motor fibers from median nerve cross the forearm to join ulnar nerve. Swedish anatomist Martin in 1763- and later Gruber in 1870 first described this anomaly in detail and hence referred to as Martin-Gruber anastomosis1.

The anastomosis almost exclusively involves motor axons. Sensory involvement though unusual but has been reported in a case by Santotro et al2. Physiological studies report the incidence between 15-39 % but anatomists report a narrow range of 11-24 % in their papers3 MGA can be diagnosed by detecting the difference in the compound muscle action potential (CMAP) recorded from the hand muscles when median and ulnar nerves are stimulated electrically, respectively, at wrist and elbow. Presence of carpal tunnel syndrome with MGA can give rise to false impression of ulnar nerve compression across elbow leading to unnecessary surgical explorations. We report here the first such case diagnosed on NCS/EMG from Pakistan.

CASE REPORT

A 45 years old house wife was referred to Electro Diagnostic Department at Armed Forces Institute of Rehabilitation Medicine (AFIRM) Rawalpindi for carpal tunnel syndrome (CTS). She had 1-year history of mild to moderate pain in both forearms with paresthesias localized to 3 radial fingers of both hands. Symptoms were pronounced at night and exacerbated after household activities like cooking and washing. She reported waking up at night and shaking her hands to relieve the tingling in her hands (Flick sign). On examination there was no obvious wasting of intrinsic hand or forearm muscles but pin prick sensations were impaired in median nerve (MN) distribution more so on right side. Deep tendon reflexes were intact on both sides. Electro diagnosis testing was done using Medtronic Keypoint Version4 EMG machine (Medtronic Corporation, Minnepolis, MN, USA). Disposable concentric needle and surface electrodes were used for EMG and NCS studies respectively.

Median and ulnar nerve motor conduction studies were performed by recording the CMAP from the abductor pollicis brevis (APB) and abductor digiti mini (ADM) muscles respectively with disposable surface electrodes placed in a belly-tendon arrangement. Stimulation of median nerve at the wrist (MW) on right side evoked a biphasic CMAP of (2.9mV; normal: greater than 4.0mV) with increased distal latency (4.8ms; normal: greater than 3.7ms) at APB (Fig 1). Stimulation of median nerve at elbow (ME) evoked an irregular CMAP at APB of 5.5mV with positive deflection onset, which was 2.6mV (89%) larger than that evoked at wrist. This response had a latency only 1.7ms prolonged than that obtained at wrist thus indicating the activation of hand muscles by axons not compressed at carpal tunnel (Fig 1).

MW stimulation on left side, evoked similar biphasic motor response (Fig 1). Median motor conduction velocities were relatively faster 135m/s and 115m/s (normal greater than 55m/s) on right and left respectively (Table 1).

Table-1. Motornerve conduction studies

MNCV###Latency Amplitude CV###Amp

Data###(mc)###(mv)###(m/sec %

R Median

Wrist-APB###4.8###2.9

BE- wrist###6.5###5.5###135.3###89

###N: less###greater

###than- 4ms N:###than- 4mv

L Median

Wrist-APB###4.5###3.8

BE- wrist###6.5###6.7###115.0 74

R Ulnar###2.3###10.2

Wrist-AD\f 5.4###7.5###71 -26

BE- wrist###N:less

###than-2ms###N: greater

###than-6mv

LUlnar###2.2###9.9

Wrist ADM###5.7###8.4###65.7###-15

BE- wrist

MNCV= motornerve conduction velocity.

Amp=amplitude, CV=conduction velocity

Table 2. Sensorynerve conduction studies

SNCV Data###CV

R Median

Palm- wrist###4.1###11.9###34

L Median

Palm- wrist###3.3###16.7

R Ulnar

Digit V-wrist

L Ulnar

Dig V-wrist###2.0###38.7

SNCV =Sensorynerve conduction velocity,

Amp=amplitude, CV= conduction velocity

Median sensory studies showed prolonged sensory latencies of 4.1ms and 3.5 ms R and L respectively (normal 3.0ms) with marked reduction in sensory nerve action potential (SNAP) amplitudes indicating bilateral carpal tunnel syndrome (Fig. 2).

Stimulation of ulnar nerves at wrist (UW) and elbow (UE) evoked biphasic negative-onset CMAPs over ADM with normal amplitudes, latencies and conduction velocities. The CMAP amplitudes however were larger at wrists (UW) than that obtained at elbow (UE) as shown in fig 1.

Ulnar sensory studies on both sides with D4 recording were also normal (Table 2).

The drop of CMAP amplitudes in ulnar motor studies Rt -26% and left -15% gives a false impression of ulnar nerve compression at cubital tunnel but normal sensory studies, with normal DML and normal EMG of ulnar innervated muscle i.e. flexor carpi ulnaris (FCU), ADM and FDI ruled out this possibility. Median nerve electrodiagnostic studies also painted an interesting picture. Larger CMAP amplitudes at elbow (RT +89% and Lt +74%) as compared to wrist gave a false impression of bilateral median nerve compression somewhere across forearm being supported by abnormal median sensory studies. However normal median sensory velocities and normal EMG findings of median innervated forearm muscles i.e. prontor teres (PT), flexor digitorum superficalis (FDS), pronator quadratus (PQ) and flexor pollicic longis (FPL) ruled out this possibility. Prolongation of distal median motor and sensory latencies with abnormal EMG of median innervated thenar muscles only proved it to be a case of bilateral CTS.

Other unusual findings were explained by possibility of an anomalous median to ulnar motor connections (Martin-Gruber anastomosis).

DISCUSSION

Martin-Gruber anastomosis is a usual finding with incidence ranging from 5% to 34%4. It is the most common form of anomalous innervations of the extremities, being found in about 15% of cadavers and is mainly a motor variant5. Six different types of MGA have been reported in literature6.

In our patient clinical assessment was suggestive of bilateral median neuropathy at wrist (carpal tunnel syndrome). However, the conduction block identified on routine ulnar motor studies was not matching with clinical findings and other electrodiagnostic findings. In fact, this so called conduction block between below- and above-elbow ulnar nerve stimulation sites was explained by a possible Martin-Gruber anastomosis (MGA). More over there was hardly any conduction slowing across elbow in bilateral ulnar motor studies further solidifying the coexistence of anomalous MGA with bilateral carpal tunnel syndrome. The misinterpretation of Martin-Gruber anastomosis for ulnar motor conduction block is remarkably important considering that conduction block is taken by some of the investigators as the most reliable indicator of a focal ulnar nerve lesion across elbow7.

Routine EDX screening test for ulnar neuropathy at elbow consists of recording an ulnar innervated hand muscle (ADM), with stimulation at elbow and 5cm above the elbow. If a waveform change is identified, it is standard practice in many laboratories to stimulate 5 cm below the elbow and determine whether the waveform change occurs in the wrist-to-BE segment of the nerve, or in the BE-to-AE ulnar nerve segment. If the waveform change occurs at the wrist to-BE segment of the nerve, the explanation is usually MGA8.

A case of bilateral carpal tunnel syndrome with a coexisting MGA like this, the failure to recognize a proximal MGA may lead to over estimation of ulnar neuropathy at elbow may end up aggressive surgical exploration. Furthermore presence of MGA may lead to an over estimation of conduction block in coexisting ulnar neuropathy at elbow.

CONCLUSION

Martin-Gruber anastomosis should be kept in mind in all patients undergoing electrophysiological studies of upper limbs as it can give rise to faulty interpretation especially in case of suspected compression neuropathies of median and ulnar nerves across wrist and elbow.

Reference

1. Erdem H R, Ergun S, Erturk C, Ozel S. Electrophysiological evaluation of the incidence of Martin-Gruber Anastmosis in healthy subjects. Yonsei Med J Vol 43, 3:2002)

2. Santor L, Rosato R, Caruso G. Median ulnar nerve communication : electrophysilogical demonstration of motor and sensory fibre cross-over. J neurol 1983; 229:227-35

3. Lee K S, Oh C S, Chung I H, Sunwoo I N. An anatomic study of martin gruber anastomosis: Electrodiagnostic implications. Muscle Nerve 2005; 31:95-7

4. Leibovic SJ, Hastings H. Martin-Gruber revisited. J hand Surg 1992; 17A:47-53

5. Srinivasan R, Rhodes J. The median-ulnar anastomosis (Martin-Gruber) in normal and congenitally abnormal fetuses. Arch Neurol 38:418-419

6. Lee K, Oh C, Chung I, Sunwoo I. An anatomic study of Martin-Gruber Anastomosis: Electrodiagnostic implications. Muscle Nerve 2005; 31:95-7

7. Marras C, Midroni G. Proximal martin-gruber anastomosis mimicking ulnar neuropathy at the elbow. Muscle Nerve 1999; 22: 1132-35

8. Preston DC, Shapiro BE. Electromyography and neuromuscular disorders: clinical-electrophysiological correlations. Boston: Butterworth-Heinemann; 1998.
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Author:Gill, Zaheer Ahmed; Rathore, Farooq Azam; Waheed, Akhtar; Rustam, Zahid
Publication:Pakistan Armed Forces Medical Journal
Article Type:Report
Geographic Code:9PAKI
Date:Mar 31, 2012
Words:1437
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