Martin Gruber Anastomosis is the anatomic variant of ulnar nerve innervation where there is an anomalous pattern of innervation occurring between the median and ulnar nerves in the forearm.
The crossover usually occurs between axons from the anterior interosseous nerve to the ulnar nerve in the forearm. Rarely, the crossover can occur between the main median nerve trunk and ulnar nerve.
In the former scenario, only motor fibers cross and no sensory fibers are involved in the crossover. But sensory fibers can cross when the median nerve trunk is involved.
The Martin-Gruber anastomosis occurs in 10-30% of individuals and is bilateral in about two-thirds of affected persons.
It can be familial though associated genes have not been found yet.
The fibers involved in crossover belong to C8-T1 nerve roots.
Types of Martin Gruber Anastomosis
Following three patterns are recognized
- Type I
- Crossover fibers innervate hypothenar muscles
- Type II
- Crossover fibers innervate the first dorsal interosseous muscle
- Type III
- Crossover fibers innervate thenar muscles- the adductor pollicis and the flexor pollicis brevis [not abductor pollicis brevis]
- Sometimes flexor digitorum superficialis and the flexor digitorum profundus too
Type II is most common and type III is the least common type.
Significance of Martin Gruber Anastomosis
This anomaly can alter the clinical findings in nerve entrapment. For example, in ulnar neuropathy at the elbow in type I, the hypothenar muscles can be spared as they would be supplied by crossed-over fibers.
This creates conflicting evidence in clinical findings as well as in findings of nerve conduction studies.
Therefore, in such unusual presentations, this anomaly should be suspected and confirmed.
Martin Gruber anastomosis can be confirmed by nerve conduction studies.
In a patient without Martin Gruber anastomosis, stimulating the median nerve at the wrist and elbow produces equal compound muscle action potential amplitude [CMAP] at the thenar eminence eg, abductor pollicis brevis.
But in a patient with the anomaly, the median nerve at the wrist carries lesser median nerve fibers due to crossover occurring in forearm and the stimulation of median nerve produces a smaller response. In contrast, elbow response because it would also stimulate ulnar nerve muscles activated by crossed median nerve fibers.
However, on stimulating ulnar nerve over the hypothenar eminence (abductor digiti quinti) or the first digital interosseous muscle, the wrist response would be higher due to median nerve fibers innervating ulnar muscles in the hand and the elbow response is smaller.
This could be mistaken for a conduction block.
Therefore, it becomes important to have Martin Gruber anastomosis should be excluded before an ulnar conduction block is diagnosed.
Confirmation of Martin Gruber Anastomosis
It is done by nerve conduction studies.
Type I- Crossover fibers innervate hypothenar muscles
In this type, the characteristic finding is ulnar stimulation at the wrist produces larger hypothenar CMAP than stimulation at the elbow.
To confirm, median nerve is stimulated at the elbow produces a response at hypothenar muscles [normally no response as median nerve does not supply hypothenar muscles] provides confirmation
As an additional confirmation, hypothenar CMAP from ulnar stimulation at wrist is equal to hypothenar CMAP from ulnar stimulation at elbow plus hypothenar CMAP from median stimulation at the elbow.
As discussed before, smaller response from proximal stimulation could be mistaken for conduction block.
Type II – Crossover fibers innervate first dorsal interossei [FDI] muscle.
In this type, ulnar stimulation at wrist produces larger CMAP than stimulation at the elbow when tested for first dorsal interossei.
Stimulation of the median nerve at the elbow which also produces a response at FDI, and is considered confirmatory.
Moreover, the FDI CMAP from ulnar stimulation at the wrist is equal to FDI CMAP from ulnar stimulation at elbow plus FDI CMAP from median stimulation at the elbow. This verifies the presence of type II.
III-Crossover fibers innervate thenar muscles
As we discussed earlier, typically adductor pollicis and flexor pollicis brevis muscles are innervated.
Typically, the elbow stimulation of median nerve produces greater thenar response than wrist stimulation. This occurs because the fibers have left the median nerve before the wrist for cross-over.
For confirmation, the following tests are employed
- Ulnar stimulation produces higher thenar CMAP with wrist stimulation than with elbow stimulation
- Median elbow stimulation amplitude is equal to median wrist stimulation amplitude plus ulnar wrist stimulation amplitude minus ulnar elbow stimulation amplitude
This can lead to confusing findings especially when carpal tunnel syndrome is involved
Riche-Cannieu anastomosis is communication between the recurrent branch of the median nerve and deep branch of the ulnar nerve in hand. It is present in about 80% of hands but often not enough to affect diagnostic findings at all.
There can be ulnar innervation to some muscles usually innervated by the median nerve, median innervation to muscles usually innervated by the ulnar nerve, or both.