Martin Gruber

Martin-Gruber Anastomosis (MGA): Understanding Anomalous Nerve Communication

California Sports and Rehab | Dr. Hakimian, Nerve Specialist in Los Angeles and Orange County

Overview

The human nervous system is full of natural variations. One of the most fascinating—and clinically important—examples is the Martin-Gruber Anastomosis (MGA), a normal anatomical connection that can affect the way the median and ulnar nerves communicate in the forearm.

In simple terms, MGA occurs when small nerve fibers branch off from the median nerve (or its branch, the anterior interosseous nerve) and connect to the ulnar nerve.
This crossover changes how signals travel to the muscles in the hand and can alter the expected results of nerve tests or the presentation of certain nerve injuries.

At California Sports and Rehab, Dr. Hakimian and our team use electromyography (EMG) and nerve conduction studies (NCV) to identify variations such as Martin-Gruber Anastomosis, ensuring accurate diagnosis and tailored treatment for patients with hand or forearm nerve symptoms.

What Is the Martin-Gruber Anastomosis?

The Martin-Gruber Anastomosis is a naturally occurring communication between the median and ulnar nerves in the forearm.
Rather than being a problem, it’s a normal anatomical variant found in about 3 % – 40 % of the population.
The connection usually carries motor fibers (those that control muscle movement) from the median to the ulnar nerve.

Because of this crossover, muscles that are usually controlled by the ulnar nerve may instead receive some signals from the median nerve. This can change how muscle weakness or nerve damage appears on testing—and if not recognized, can be mistaken for an abnormal finding.

A Brief History

• First described by Martin (1763), who proposed that the two nerves might share fibers.
• Gruber (1870) confirmed the finding about a century later through anatomical dissections.
• The connection is now named in their honor.

Normal Hand Innervation (Without MGA)

However, studies show this textbook pattern appears in only one-third of individuals—meaning variations like MGA are quite common.

How Martin-Gruber Anastomosis Forms

In MGA, motor fibers from the median nerve cross over in the forearm and join the ulnar nerve before it reaches the hand.
These communicating fibers can innervate several intrinsic hand muscles usually supplied by the ulnar nerve, such as:

• Abductor Digiti Minimi (ADM)
• First Dorsal Interosseous (FDI)
• Deep head of Flexor Pollicis Brevis (FPB)
• Adductor Pollicis

The crossover can occur on one side (unilateral) or both sides (bilateral)—bilateral connections appear in roughly 10 – 40 % of cases.

Types of Martin-Gruber Anastomosis

Modern studies, including the Cavalheiro et al. (2015) classification, describe several forms of MGA based on where the crossover happens:

1. Type I – From the anterior interosseous nerve (AIN) to the ulnar nerve
2. Type II – Double connection between AIN and ulnar nerve
3. Type III – From the median nerve (before AIN branches) to ulnar nerve
4. Type IV – Loop between AIN and ulnar nerve supplying flexor digitorum profundus
5. Type V – Intramuscular connection within the flexor digitorum profundus (FDP)
6. Type VI – From a branch to the flexor digitorum superficialis joining the ulnar nerve

Each variation can influence which hand muscles are affected in nerve injuries or neuropathies.

Clinical Significance

Understanding MGA is crucial in neurology, orthopedics, and rehabilitation medicine because it can:

• Alter clinical findings: Some muscles may remain active even after an injury to the median or ulnar nerve.
• Affect EMG/NCV results: Without awareness of MGA, electrical tests can appear confusing or misleading.
• Mimic or mask pathology: MGA may make carpal tunnel or cubital tunnel syndrome appear more or less severe.
• Influence surgical decisions: Surgeons need to know about possible crossover fibers when operating near the forearm nerves.

Example

If the median nerve is injured above the level of communication, some hand muscles may still function normally because they receive extra input via the ulnar nerve. This could lead to an incorrect assumption that the nerve is intact.

Diagnosis

At California Sports and Rehab, we use advanced electrodiagnostic testing to detect variations like MGA:

• Electromyography (EMG): Evaluates muscle activity and identifies unusual nerve supply patterns.
• Nerve Conduction Studies (NCV): Measure how fast signals travel through the nerves and detect atypical connections.
• Ultrasound or MRI (in selected cases): Visualize nerve pathways if structural anomalies are suspected.

Correctly identifying MGA ensures your test results are interpreted accurately and helps avoid misdiagnosis of nerve damage.

Why Recognition Matters

Recognizing a Martin-Gruber Anastomosis can prevent:

• False-positive diagnoses of nerve injury
• Unnecessary surgery or treatment for presumed nerve entrapment
• Misinterpretation of carpal tunnel or ulnar neuropathy findings

For patients, it means more accurate testing, correct diagnosis, and the right treatment plan.

When to See a Nerve Specialist

Schedule an appointment with Dr. Hakimian if you experience:

• Numbness, tingling, or weakness in the hand or fingers
• Symptoms that don’t match typical carpal tunnel or ulnar neuropathy patterns
• Unusual EMG or nerve conduction results that need expert review

Early evaluation by a nerve specialist ensures proper diagnosis and management.

Frequently Asked Questions (FAQ)

Q: Is Martin-Gruber Anastomosis a disease?
A: No. MGA is a normal anatomical variation, not a disease. It simply changes how certain hand muscles are wired.

Q: Can MGA cause symptoms?
A: MGA itself doesn’t usually cause symptoms, but it can alter how other nerve problems—like carpal tunnel syndrome—appear or progress.

Q: How is MGA detected?
A: It’s usually identified through EMG and nerve conduction studies performed by a neurologist or electrodiagnostic specialist.

Q: Why is it important for EMG testing?
A: Recognizing MGA prevents misreading EMG/NCV results and ensures accurate diagnosis of nerve injuries.

Q: Can both arms have Martin-Gruber Anastomosis?
A: Yes. It can be unilateral or bilateral, with both forearms showing similar or different communication patterns.

Schedule an EMG or Nerve Evaluation

If you’ve been told you have unusual EMG findings or symptoms that don’t fit a classic nerve pattern, an evaluation with Dr. Hakimian at California Sports and Rehab can help.
Our EMG and nerve conduction studies accurately detect variations like Martin-Gruber Anastomosis, ensuring the most precise diagnosis and care.

📞 Call 310-652-6060 or book your nerve evaluation near you online today.