18.1 Riche-Cannieu Anastomosis and Accessory Fibular Nerve

18.1 Riche-Cannieu Anastomosis and Accessory Fibular Nerve

Quick Answer: Riche-Cannieu anastomosis (AAET handbook spelling Richie Cannieu) is an ulnar-to-median thenar crossover in the palm. Median motor recording from abductor pollicis brevis (APB) can look absent at the wrist while ulnar stimulation still activates thenar muscle. The accessory deep fibular (peroneal) nerve is extra EDB innervation that travels behind the lateral malleolus. Anterior-ankle stimulation then underestimates the EDB CMAP and fibular-head stimulation looks larger. That amplitude drop is not automatically conduction block. Stimulate behind the lateral malleolus to prove the variant. Do not mix either pattern with Martin-Gruber anastomosis (median-to-ulnar in the forearm).

Why anomalous innervation fools otherwise careful NCS

Routine nerve conduction studies assume a default wiring map. Median motor axons are expected to reach APB through the carpal tunnel. Deep fibular axons are expected to reach extensor digitorum brevis (EDB) across the anterior ankle. Ulnar motor axons are expected to remain ulnar until they terminate in hypothenar, interosseous, and ulnar thenar muscles. Extra communications do not mean the patient is diseased. They mean a stimulation site can miss axons that still reach the muscle by another route, so amplitude comparisons between sites stop being a pure measure of conduction block.

Independent OpenExamPrep teaching for AAET R.NCS.T. candidates treats these variants as recognition and demonstration skills. You record what each stimulation site does. The interpreting physician decides whether disease is also present. The high-yield failure is calling conduction block or complete median motor axon loss because a textbook nerve path was incomplete.

This section covers two outline items: palmar Riche-Cannieu (handbook Richie Cannieu) and accessory deep fibular / accessory peroneal innervation of EDB. Martin-Gruber anastomosis (MGA) is only contrasted here so the three names do not collapse into one “anomalous nerve” blur on exam items.

Riche-Cannieu anastomosis (Richie Cannieu)

Anatomic literature uses Riche-Cannieu. Some handbook items use Richie Cannieu. Treat them as the same palmar communication. Fibers connect the deep palmar motor branch of the ulnar nerve with the recurrent thenar motor branch of the median nerve. Direction is ulnar to median thenar. Target muscles may include APB, opponens pollicis, and flexor pollicis brevis in any combination. This is motor, in the palm, not a forearm sensory loop and not a Martin-Gruber fiber.

When the communication is small, median wrist stimulation still produces a healthy APB CMAP and you may never notice the variant. When it is generous — including the all-ulnar hand or all-ulnar thenar pattern — median stimulation at the wrist while recording APB is tiny or unobtainable, yet ulnar stimulation at the wrist activates the thenar eminence. The muscle can twitch. Axons are present. They simply are not traveling with the median nerve at your wrist cathode.

That recording is a famous impersonator of severe carpal tunnel syndrome with thenar wasting and of complete median nerve interruption. Surface clues that still belong to the technologist:

• A median sensory SNAP to digit 2 or digit 3 is often still recordable if the median nerve is otherwise intact. Complete median axon loss usually takes sensory and motor together.
• Ulnar motor to abductor digiti minimi (ADM) is typically preserved. The extra innervation is thenar, not a globally dead ulnar nerve.
• Leaving G1 on APB and stimulating ulnar at the wrist can produce a thenar CMAP that fills in the missing median response.
• A side-to-side median APB CMAP that is nearly absent on one side without a matching sensory crash should trigger the ulnar-to-thenar check before the worksheet reads as if thenar muscle were denervated.

How to demonstrate palmar ulnar-to-thenar innervation

Keep the same APB (thenar motor-point) montage you used for the median study. Deliver a truly supramaximal median wrist stimulus and confirm the CMAP stays tiny. Then stimulate the ulnar nerve at the wrist without moving G1/G2. Watch the thenar twitch under G1, not only the display. A stable thenar CMAP at reasonable ulnar intensity supports Riche-Cannieu.

Exclude current spread. A very high ulnar stimulus in a thin wrist can jump to median fibers and fake an anastomosis. Raise intensity stepwise. Spread usually appears as a sudden extra jump at painful, high milliamperes with a change in twitch location. A true thenar anastomosis appears as a thenar twitch that tracks ulnar stimulation at intensities you already use for ADM.

Exclude volume conduction from ADM or first dorsal interosseous (FDI). A hypothenar far-field can look like a small lump on an APB channel. Reposition G1 over the thenar motor point, compare an ADM channel, and believe the response that matches a visible APB/thenar belly twitch.

Median sensory recording remains your separator from a true high-grade median lesion: preserved digit SNAP plus all-ulnar thenar motor is a wiring story. Absent SNAP plus absent median motor is still a lesion story until proven otherwise. Anomalous innervation does not grant immunity from carpal tunnel disease; it only says do not use a flat APB CMAP alone as proof that thenar muscle has no axons.

Brief contrast with Martin-Gruber anastomosis

MGA is a forearm communication, usually median to ulnar (often with anterior interosseous contributions), supplying ulnar-innervated hand muscles such as FDI, ADM, or ulnar thenar. Extra axons join the ulnar nerve between elbow and wrist. Ulnar stimulation at the wrist therefore includes them, while stimulation below or above the elbow misses them because those axons are still traveling in the median nerve proximally. The ulnar CMAP can be larger at the wrist than below the elbow, which mimics conduction block in the ulnar forearm. Median stimulation at the elbow may activate FDI or ADM and account for the apparent missing ulnar amplitude.

Riche-Cannieu does not produce that forearm block arithmetic. It produces a palmar thenar innervation surprise: median wrist to APB may be absent while ulnar wrist to thenar is present. Memory hooks:

• MGA: median → ulnar, forearm, fake ulnar block.
• Riche-Cannieu: ulnar → median thenar, palm, fake median motor loss at APB.
• Accessory fibular: extra EDB path behind the lateral malleolus, fake fibular block in the leg.

If a stem says APB is silent to median and alive to ulnar, think Riche-Cannieu. If the ulnar FDI or ADM CMAP is larger at the wrist than below the elbow, and median-elbow stimulation supplies the difference, think MGA after technical checks. If EDB is smaller at the anterior ankle than at the fibular head, think accessory deep fibular before anyone types conduction block.

Accessory deep fibular (accessory peroneal) nerve

Fibular and peroneal name the same nerve. The default EDB study stimulates the deep fibular nerve at the anterior ankle and the common fibular nerve at the fibular head. Most EDB axons travel with the deep fibular under the extensor retinaculum. In the accessory variant, a motor branch arises from the superficial fibular nerve, runs posterior to the lateral malleolus, and innervates EDB — often the lateral portion of the muscle.

Those accessory axons never pass under your anterior-ankle cathode. The anterior-ankle CMAP therefore underestimates the EDB motor axon pool. Fibular-head stimulation depolarizes the common fibular trunk, which still contains both deep and superficial pathways, so the proximal CMAP is larger. The tracing looks like partial conduction block between fibular head and ankle. It is not block if the missing axons are simply behind the malleolus.

Electrodiagnostic teaching commonly cites this variant in roughly 15–25% of limbs (anatomic series can run wider). Treat that range as a teaching frequency, not as a guarantee on the next patient. The variant may be unilateral. Side-to-side EDB comparisons are meaningless until each side’s accessory contribution is known.

How to stimulate behind the lateral malleolus

1. Record EDB with the same G1 (muscle belly on the dorsum of the foot) and G2 (distal, often fifth metatarsal or little-toe region) montage used for the standard fibular motor study.
2. Obtain the anterior-ankle deep fibular CMAP and the fibular-head CMAP. Confirm the anterior cathode is truly over the deep fibular nerve (EDB or toe-extensor twitch, not a pure peroneus longus story) and is supramaximal.
3. The trigger is a substantially larger fibular-head than anterior-ankle EDB CMAP — after technical causes of a small distal response are excluded (wrong site, submaximal distal stim, electrode off the muscle).
4. Move the stimulator posterior to the lateral malleolus, in the groove where the accessory nerve is taught to travel (behind the malleolus, not on the anterior ankle crease and not on the Achilles tendon itself).
5. Place the cathode over that course. Raise intensity only until the accessory CMAP plateaus. Watch for an EDB belly twitch under G1.
6. A reproducible EDB CMAP from this site demonstrates accessory deep fibular innervation. Document three sites: anterior ankle, posterior lateral malleolus, and fibular head.
7. Many labs teach that anterior-ankle CMAP plus accessory CMAP approximate the fibular-head CMAP. Phase cancellation and overlapping motor points mean the arithmetic is a consistency check, not a legal identity. Do not report fibular conduction block on a two-site amplitude drop until the retro-malleolar site has been stimulated.

If the retro-malleolar site is silent and both standard sites are truly supramaximal over the intended nerves, an amplitude drop may still be disease — but you earned that conclusion by excluding the variant, not by ignoring it.

Recognition table

Worked recognition

Thenar. Median motor to APB is unobtainable. Digit-2 SNAP is present. Ulnar ADM CMAP is normal. Ulnar wrist stimulation with G1 on APB yields a clear thenar CMAP and a thenar twitch. That is palmar ulnar-to-thenar innervation, not proof the median nerve was severed.

Foot. EDB is 2.0 mV at the anterior ankle and 6.5 mV at the fibular head. Distal latency is not dramatically long. Retro-malleolar stimulation yields 4.0 mV with an EDB twitch. Do not call conduction block.

Not this section’s variant. Ulnar FDI is 8 mV at the wrist and 3 mV below the elbow; median-elbow stimulation to FDI supplies the missing 5 mV. That is MGA territory (after technical checks), not Riche-Cannieu and not accessory fibular.

Exam traps

• Handbook Richie Cannieu equals anatomic Riche-Cannieu.
• Accessory deep fibular innervation of EDB arrives via a superficial fibular branch — the adjectives are easy to swap.
• Larger proximal CMAP is not automatically block.
• Do not use MGA, Riche-Cannieu, and accessory fibular as interchangeable labels.