How do I confirm a Martin‑Gruber anastomosis using nerve conduction studies?

Confirming Martin-Gruber Anastomosis Using Nerve Conduction Studies

To confirm Martin-Gruber anastomosis (MGA), stimulate the median nerve at the elbow and wrist while recording from ulnar-innervated hand muscles (first dorsal interosseous, abductor digiti minimi, or adductor pollicis); MGA is present when the compound muscle action potential (CMAP) amplitude is paradoxically larger with elbow stimulation compared to wrist stimulation, with a corresponding amplitude drop in ulnar nerve studies when comparing above-elbow to wrist stimulation. 1

Standard Testing Protocol

Recording Electrode Placement

• Place surface recording electrodes on the first dorsal interosseous (FDI) muscle as the primary target, as this provides the highest sensitivity for detecting MGA 2
• Include recordings from the abductor digiti minimi (ADM) and adductor pollicis (AP) muscles, though these are less sensitive 3, 2
• Note that MGA to FDI is found in all patients with this anastomosis, while MGA to ADM occurs in only 11% and to AP in 10% of cases 3

Stimulation Sites and Technique

• Deliver supramaximal stimulation to both median and ulnar nerves at the wrist and at/above the elbow 1
• For median nerve studies, stimulate at the wrist first, then at the elbow
• For ulnar nerve studies, stimulate at the wrist, then above the elbow 1, 3

Diagnostic Criteria for MGA

Median Nerve Findings (Recorded from Ulnar-Innervated Muscles)

• Increased CMAP amplitude after elbow stimulation compared to wrist stimulation when recording from FDI, ADM, or AP 1, 3
• Presence of a small initial positivity in the waveform with elbow stimulation 3
• This paradoxical amplitude increase occurs because median nerve fibers cross over to join the ulnar nerve in the forearm, contributing additional motor units when stimulated proximally 1

Ulnar Nerve Findings (Recorded from Same Muscles)

• Decreased CMAP amplitude with above-elbow stimulation compared to wrist stimulation 1, 3
• This creates a "conduction block-like" appearance in the forearm segment that is actually an artifact of the anastomosis 3, 4
• The amplitude drop corresponds exactly to the amplitude increase seen in median nerve studies 1

Classification of MGA Types

The anastomosis is classified based on which ulnar-innervated muscles receive median nerve fibers:

• Type I: Median fibers supply thenar muscles (rare, 3% of cases) 1
• Type II: Median fibers supply hypothenar muscles (most common, seen in 21 of 27 cases with MGA) 1
• Type III: Median fibers supply FDI muscle 1
• Combined patterns (Type I + Type II) can occur 1

Special Consideration: Proximal MGA at the Elbow

Recognition and Testing

• Proximal MGA occurs at or above the elbow rather than in the forearm and is frequently underrecognized 4, 2
• This variant can mimic ulnar neuropathy at the elbow by creating apparent conduction block that is disproportionate to clinical findings 4, 2
• Test for proximal MGA by stimulating the median nerve at the elbow while recording from FDI and ADM 2

Key Distinguishing Features

• Suspect proximal MGA when there is discrepancy between clinical presentation and electrodiagnostic findings—specifically, apparent severe conduction block without corresponding clinical weakness 4, 2
• The apparent conduction block is not associated with focal slowing of conduction velocity, unlike true ulnar neuropathy 2
• Detection of proximal MGA to FDI is more sensitive than to ADM 2

Clinical Prevalence and Implications

• MGA occurs in approximately 25-27% of the general population 1, 3
• The anastomosis is not pathological but represents a normal anatomical variant 1
• Failure to recognize MGA can lead to:
  • Misdiagnosis of median nerve lesions (falsely elevated CMAP amplitudes) 1
  • Misdiagnosis of ulnar neuropathy (false appearance of conduction block) 3, 4
  • Incorrect interpretation of needle EMG findings in median and ulnar nerve territories 1

Common Pitfalls to Avoid

• Do not interpret the amplitude drop in ulnar studies as true conduction block without considering MGA, especially if clinical findings are inconsistent 3, 4
• Do not overlook small initial positivity in the CMAP waveform with proximal median nerve stimulation, as this is a hallmark of MGA 3
• Always test for MGA when evaluating suspected ulnar neuropathy at the elbow if apparent conduction block seems disproportionate to clinical weakness 4, 2
• In cases of suspected median nerve lesions at the wrist or forearm, decreased amplitude with proximal stimulation may reflect MGA rather than nerve damage 5

Reversed Martin-Gruber Anastomosis (Ulnar-to-Median)

• A reversed anastomosis (ulnar-to-median) can also occur, though it is less common 5
• Suspect this variant when CMAP amplitude in opponens pollicis is larger with wrist stimulation than elbow stimulation of the median nerve, without clinical signs of pronator or anterior interosseous syndrome 5
• Use gradual, slow increase of stimulus voltage at the wrist to avoid simultaneously activating both median nerve and collateral ulnar branches, which can mask true median nerve latency 5