Prevalence of Martin-Gruber Anastomosis in Healthy Subjects: An Electrophysiological Study from Raigarh, Chhattisgarh
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.325369
Source of Support: None, Conflict of Interest: None
Keywords: Anomalous innervation, electrophysiology, Martin-Gruber anastomosis, median nerve, ulnar nerveKey Message: Analysis of Martin –Gruber Anastomosis is important to prevent misdiagnosis in ulnar and median nerve conduction studies.
In the electrophysiological assessment of muscle and nerve anomalous innervation of various peripheral nerves may be mistaken as technical pitfall or actual pathology. One of the most common anomaly is Martin Gruber anastomosis (MGA) which is the crossing of motor nerve fibers from median nerve or one of its branches mainly AIN (Anterior Interosseous nerve) and innervate intrinsic hand muscles such as Abductor digiti minimi (ADM), First dorsal interossei (FDI), Adductor Pollicis brevis and deep head of Flexor Pollicis brevis which are normally supplied by Ulnar nerve. This was first described by Martin in 1763 and Gruber in 1870.,
The anastomosis in the nerve fibres may be due to the altered signalling between the neuronal growth cones and mesenchymal cells antenatally which persist postnatally, or failure of differentiation of brachial plexus cords may lead to aberrant course. These anomalies have shown a phylogenetic significance as many mammals including primates have shown similar connections between median nerve and ulnar nerve.,
In various studies Martin Gruber Anastomosis has been reported from 6% to 43.7%.,,, In Cadaveric studies prevalence of Martin Gruber Anastomosis was reported between 10% to 30.6%,,, while in electrophysiological studies the anomaly was reported between 5% to 40%.,,, This wide range of variation in prevalence can be attributed to various diagnostic criteria of MGA. It is found to be present bilateral as well as unilateral. Although this anomaly is asymptomatic and usually identified during assessment of peripheral nerves of forearm.,, But it may lead to misinterpretation of various entrapment lesions or traumatic median and ulnar nerves so it is important to identify this anastomosis.
Though the anomaly is frequent in occurrence no study determining the prevalence of MGA in residents of Raigarh, Chhattisgarh has been done. The aim of this study was to determine the prevalence of MGA in Raigarh so that it may help the neurologist and orthopedics about the prevalence in the general population and prevent misdiagnosis in various nerve conduction studies and Electromyography.
This is a cross-sectional study which was carried out after obtaining approval from the institutional ethical committee. The sample size was calculated with confidence level of 95% and 5% margin of error. 140 healthy subject from different families with age group 17–25 years and exclusion of subjects with any peripheral neuropathy, ulnar neuropathy and carpal tunnel syndrome (CTS) and normal clinical neurological examination of both upper limbs were recruited for this study.
After obtaining informed consent from the subjects, height and weight of subjects were measured. Electrophysiological evaluation for presence of Martin Gruber anastomosis was performed by Allenger Scorpio 2 Channel EMG-NCV machine on 280 forearms of 140 subjects. The filter was kept between 2Hz to 5 kHz with sweep speed 5 ms per division, sensitivity as 5 mV/division and duration of electric pulse was set at 100 μs. A bipolar stimulator having a current production ability of up to 100 mA with pulse duration 0.2 ms was used. Temperature of the forearm was maintained at 30°C. Amplitude of Compound muscle action potential (CMAP) was measured from peak to peak.,
The median and ulnar motor nerve conduction studies by stimulating with supramaximal strength at wrist (distal) and elbow (proximal) was performed for hypothenar muscle Abductor Digiti Minimi muscle (ADM), First Dorsal Interosseous Muscle (FDI), and thenar muscle Abductor Pollicis Brevis Muscle (APB) muscles. Precaution was taken to prevent alteration of results due to technical factors such as submaximal stimulation and costimulation of nearby nerves. Supramaximal stimulation was obtained by slowly increasing the intensity of current stimulus to a stimulus of 20% greater voltage/current than required until there was no further increase in amplitude and CMAP was recorded.
Costimulation of nearby nerves was avoided by preventing increase in intensity of current stimulus on observation of sudden changes in the morphology of the recorded CMAP.
Placement of electrodes
For Ulnar nerve studies. - Wrist stimulation was given by bipolar stimulator electrode at the wrist just lateral to the flexor carpi ulnaris tendon 7 cm proximal to the active recording electrode. Below elbow stimulation was given by bipolar stimulator electrode at 3 cm distal to the medial epicondyle.
For Median nerve studies. -Wrist stimulation was given by bipolar stimulator electrode between the palmaris longus tendon and flexor carpi radialis tendon at a distance 7 cm from the active recording electrode. Antecubital fossa stimulation was given by bipolar stimulator electrode over the brachial artery pulse medial to the tendon of biceps brachii. The CMAP amplitudes of the ulnar and median nerve studies were taken for analysis.
MGA is classified into three types. In Type I anastomosis the cross over fibers terminate in the Hypothenar muscles (ADM), in the type II anastomosis the cross over fibers terminate in the First Dorsal Interossei (FDI), in the type III anastomosis the cross over fibers terminate in the thenar muscles., The diagnosis of various type of MGA was done using following criteria as in [Table 1].
Recording of Ulnar nerve and Median nerve at wrist and proximal site in various type of MGA is shown in [Figure 1].
Statistical analysis of data was done by using the SPSS (version 17) software. Descriptive measures (count, frequency, minimum, maximum, mean and standard deviation (SD), as well as analytic measures (Pearson's Chi-square test and Fisher's exact test), were used. Statistical significance was considered to any P value at less than 0.05.
In this study, 280 forearms of 140 subjects were studied out of which 74 female subject (52.8%) and 66 male subjects (47.14%) participated. The mean age of the subjects was 20.68 ± 1.87 with mean age of female participants was 21.31 ± 1.87 while that of male participants was 20.35 ± 1.39.
On electrophysiological Examination MGA was found in 44 (15.71%) forearms of 30 (21.4%) subjects, out of which 21 (15.9%) forearms of 14 (21.1%) male subjects and 23 (15.5%) forearms of 16 (21.6%) female subjects. Statistically no significant difference between the prevalence of MGA in male and female was seen. Any other type of anomaly fsuch as Marinacci Communications were not found in our study.
Out of the 44 forearms 22 (50%) were of right side and 22 (50%) were of left side. In this study MGA to FDI muscle (Type II) was most common with 33 forearms (11.7%) of 22 subjects (15.7%) having this anastomosis. 2nd most common type of anomaly was MGA to thenar muscle (Type III) and the least common was MGA to ADM (Type I). The gender-based distribution of different type of MGA is given in [Table 2].
Out of 30 subjects having MGA 14 (46%), subjects had MGA bilaterally while rest 16 (54%) had MGA in only one forearm. Among the subjects with bilateral MGA, 13 had same type of MGA in both the forearms. MGA to FDI muscle (Type II) was present in both forearms of 11 subjects while MGA to thenar muscle (Type III) was present in both forearms of 3 subjects. 1 subject had different types of MGA in both forearm.
Among the 16 subjects having unilateral MGA, 8 subjects had MGA in right forearm and 8 subjects had MGA in left forearm. MGA with combination of more than one type in a forearm was seen in 7 forearms of 7 different subjects. Out of those 5 forearms had MGA Type II and MGA type III simultaneously. One forearm had MGA Type II and MGA type III and one subject had all three type of MGA. Frequency of various combination and isolated MGA in subjects is given in [Figure 2].
Martin Gruber anastomosis is one of the most common anomaly in the forearm between median and ulnar nerve. In this connection in forearm motor axons of median nerve innervates some of the muscles which are supplied by ulnar nerve. This pattern causes difficulty in interpretation of various median and ulnar nerve studies.,,
The diagnosis of MGA by electrophysiology method is noninvasive and the rationale behind the test is that due to the crossover of some motor fibers from median nerve to ulnar nerve it gains motor fibers distal to the connection. So due to more number of motor fibers fired at the distal site of ulnar nerve there is increase in amplitude of CMAP on wrist stimulation than that obtained on below-elbow site of stimulation of Ulnar nerve if supramaximal stimulation is applied. As some motor fibers from median nerve has crossed to ulnar nerve stimulation of median nerve proximal to the crossing over will evoke a potential for the muscle supplied by ulnar nerve but no or minimal potential will be observed on stimulation of Median nerve distal to crossing over.,
In our study, the prevalence of MGA in Raigarh population was found to be 21.4%. This is consistent with various other studies where a wide variation of prevalence in found from 3.3% to 40%.,,,,, the prevalence of MGA in various studies is given in [Table 3].,,,,,,,,
This wide range of variation may be attributed to no standardized diagnostic criteria of MGA. Some studies considered CMAP amplitude difference of more than 1mV between distal and proximal stimulation sites while some other studies considered change in amplitude more than 10% or 20% as diagnostic criteria for MGA.,, This variation in different population can also be due to familial predisposition as observed by Crutchfield and Guttmann. Among cadaveric study and Electrophysiological studies Amoiridis et al. hypothesized that small nerve fiber bundles which produce a potential of 0.6 -1.5mV upon activation are detected by electrophysiological techniques but might go undetected in cadaver studies leading to potential under-reporting of MGA incidence in anatomical cadaveric studies.
MGA is described as a motor anastomosis from median to ulnar nerve by Hasegawa et al. while some studies as Simonetti et al. described that sensory fibers are also involved in this anastomosis from ulnar to median nerves.
In our study, MGA was present bilaterally in 14 subjects (10%) which is similar to many study where it was present bilaterally in 10–40% of the subjects. seven subjects (5%) had presence of more than one type of MGA in same forearm., Statistically no significant difference between prevalence of MGA in right or left forearm in unilateral subjects was found. This is concurrent with many studies.,,, However, some studies showed more occurrence in right forearm, while some studies showed that it is more prevalent in left forearm.
In this study no statistically significant gender differences were found in prevalence of MGA or any subtype of MGA as in many studies.,, Saba et al. found no difference in prevalence of MGA in subjects but for Type II MGA it occurred more in females as compared to males. No significant gender differences can be attributed to autosomal dominant pattern of inheritance as observed by Crutchfield and Guttmann by pedigree analysis of five subjects. Srinivasan and Rhodes found a correlation between trisomy 21 and MGA in congenitally abnormal fetuses.
In this study most common type of MGA to FDI muscle (Type II) which was present in 33 (75%) forearm of the 44 forearm with MGA. In most study Type II MGA was most common.,,, In our study MGA to thenar muscles was second most common and MGA to ADM muscle was least prevalent. This pattern is seen in many studies,, while Hefty et al. observed that MGA to thenar muscles was least prevalent.
Due to MGA misinterpretation of several nerve conduction studies and electromyography occurs specially in peripheral nerve injuries carpel tunnel syndrome (CTS) cubital tunnel and leprosy.
Case studies have shown that although MGA might not have any clinical significance in ulnar neuropathy but it may present as conduction block in the forearm or elbow which might prevent lesion precise localization and cause misdiagnosis. The electrophysiological pattern of proximal MGA may be interpreted as conduction block in normal individual and may lead to inappropriate intervention for ulnar neuropathy
MGA to thenar muscle (Type III) causes difficulty in diagnosis of carpel tunnel syndrome. However, MGA with CTS have specific features which may help in diagnosis.
In leprosy neuropathic lesions of ulnar nerve at the elbow and median nerve at wrist are very common. In leprosy patients with MGA a number of intrinsic muscle may escape involvement and it may lead to erroneous conclusions about the innervation pattern.
The limitation of the study was that only motor fibers were studied and sensory fibers were not examined. A standardized criterion is required for diagnosis of MGA to prevent variation in prevalence reported in various studies. It is important to consider MGA while nerve conduction studies of median and ulnar nerves in forearms to prevent misinterpretation of various diagnosis.
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Conflicts of interest
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]