Abstract

The Riche-Cannieu anastomosis is a neural connection between the deep branch of the ulnar nerve and the recurrent motor branch of the median nerve to the thenar. Rarely, it provides complete or nearly complete ulnar innervation of the thenar muscles (all-ulnar hand). This may lead to confusing clinical and electrodiagnostic findings in patients with median or ulnar nerve pathology, or during workup for suspected nerve lesions.

Two patients referred for electrodiagnostic assessment of carpal tunnel syndrome and one patient with suspected peripheral neuropathy presented with absent or very low compound motor action potential from the thenar after median nerve stimulation. This occurred in spite of normal thenar muscles’ bulk and function, in one hand (one subject) or both hands (two subjects). Ulnar nerve stimulation at wrist and elbow while recording over abductor pollicis brevis, first and second lumbrical muscles and needle electromyography allowed us to recognize Riche-Cannieu anastomosis and avoid diagnostic errors.

The rare innervation variant of all-ulnar hand should be considered in less typical presentations of hand disorders, especially when there is a discrepancy between the clinical findings and the expected electrodiagnostic changes.

carpal tunnel syndrome, hand, median nerve, nerve conduction study, Riche-Cannieu anastomosis, ulnar nerve

The textbook innervation of the thenar suggests the median nerve as predominantly supplying the abductor pollicis brevis, opponens pollicis, and the first and second lumbrical muscles. The flexor pollicis brevis muscle most often has dual median and ulnar supply.^[1] However, significant variability of innervation exists in the hand, partially due to crossovers between the median and ulnar nerves.^[2] The Riche-Cannieu anastomosis (RCA), named so after Riché and Cannieu who described it first, is a loop-shaped communication between the deep ulnar nerve branch and the recurrent thenar branch of the median nerve.^[2,3] Recently, RCA is defined in the broader sense of ulnar-to-median motor connections at the hand, including patterns different from the classic thenar ansa. With microdissection, such branches are seen in almost all hands^[3], but most of these seem to be of dubious significance because of their minute size, unclear direction, and fiber content.

Rarely, the ulnar nerve innervates entirely or almost entirely the intrinsic hand muscles. This extreme form of RCA is referred to as all-ulnar hand.^[4,5] It results in confusing clinical presentations and clinical-electrodiagnostic discrepancies and may cause diagnostic difficulties. The clinical manifestations of this innervation variant are mostly reported as single case reports or brief case series.^[4,5]

This prospective case series was approved by the Hospital Ethics Committee and written consent was obtained from each patient. After recognition of the first case in January 2015, we were vigilant for this condition. Until April 2020, upper limb nerve conduction studies (NCS) were performed on 5428 patients at the Clinical Neurophysiology Unit which serves a population of about 560 000. During this period, we identified three cases of clinically significant RCA.

Case 3

A 30-year-old lady with an unrevealing medical history and exam was referred to us to “exclude peripheral neuropathy” because of transient glove-stoking paresthesias. Her NCS revealed low CMAP with normal latency from APB, LM1 and LM2 on both sides after median nerve stimulation, but normal size and shape after ulnar stimulation at the wrist and elbow. Sensory NCS were normal. The needle EMG of both APB muscles was also normal. She was referred back to her general practitioner to rule out electrolyte disturbances or hyperventilation syndrome. We have no follow-up for this patient.

Selected graphs of patients’ NCS are presented in Figs 1, 2, 3 .

Figure 1.

Patient 1. The absence of CMAP from APB and first lumbrical muscles after median nerve stimulation at the wrist is evident. Responses appear after ulnar stimulation. On the left, conduction to the first lumbrical muscle is normal (presented for comparison).

Figure 2.

Patient 2. Very low response from the APB, first and second lumbrical muscles after median stimulation, high CMAPs after ulnar stimulation.

Figure 3.

Patient 3. Findings similar to those in patient 2, results for both upper limbs are presented.

While the incidence of frequency of Riche-Cannieu anastomosis or anomaly varies widely according to its definition, the all-ulnar hand is a rarity.^[4,5] In the neurologically healthy person, it is asymptomatic and may be discovered incidentally in subjects referred for NCS for symptoms unrelated to median/ulnar pathology.^[6]

In median neuropathy below the elbow (most frequently CTS), the RCA may prove misleading in two ways. In extreme cases of CTS or complete median transection, the clinically preserved thenar function via the anastomosis may mask the real severity of the lesion; out of 101 CTS patients with absent CMAP, over 10% had mild or no muscle atrophy and intact thumb opposition and could be considered mild on purely clinical grounds.^[7] Indeed, many hand surgeons tend to neglect the NCS and emphasise on thenar atrophy as a main decision-making factor in CTS workup.^[8] The reverse is also possible: over-relying on the electrodiagnostic absence of thenar CMAP may lead to an erroneous diagnosis of “extreme” CTS while the neuropathy is mild or even absent.^[9]

With ulnar nerve lesions, RCA is reported as casuistic in single patients but is even more deceptive. Deep ulnar branch lesion in the hand (posttraumatic, piso-hamate hiatus syndrome) leads to painless atrophy of both the thenar and the interossei muscles, imitating the “split hand atrophy” of motor neuron disease.^[10,11] With the ominous prognosis it carries, such misdiagnosis becomes very burdensome. In proximal ulnar lesions, the manifest thenar involvement may falsely suggest plexus or root lesion and cause unnecessary investigations and delay in treatment.^[12]

In such conflicting clinical-electrodiagnostic situations, one should compare the median and ulnar conduction to the thenar after stimulation at the wrist and elbow, strictly avoiding the possibility of co-stimulation of both nerves or co-registration of CMAP from different muscles.^[4,5] This may require needle recording and near-nerve needle stimulation. Recording from the lumbrical muscles and needle EMG study of thenar muscles are also needed. Recently, MRI or ultrasound imaging of the deep ulnar branch in the hand has been applied in diagnosis.^[11]

In case of apparent clinical and electrodiagnostic discrepancies during workup of suspected median and/or ulnar nerve pathology, the rare all-ulnar hand innervation variant (an extreme form of RCA) should be considered to avoid possible diagnostic errors and delay.

All authors fulfil the authorship criteria by their contributions to data acquisition, designing, drafting, editing, and approving the paper. The authors declare no conflict of interest. No funding was received for the production of this manuscript.