Bifid median nerve entrapment by forearm musculature – a case report and systematic literature review

A 46-year-old man presented with neuropathic pain in his left hand following a laceration to his anterior forearm caused by a glass cut. He underwent surgery to repair his radial artery and flexor carpi radialis tendon. During the procedure, it was noted that he had an intact palmar cutaneous branch of the median nerve (PCBMN). Postoperatively he experienced altered light-touch sensation in his palm, while his fingers and the back of his hand remained unaffected. He experienced marked scar sensitivity. Initial treatment included desensitization exercises and gabapentin. Six months later, his symptoms had not changed with dysesthesia, particularly in the proximal palm. He also had ongoing scar sensitivity and a positive Tinel’s sign at the level of the flexor carpi radialis tendon repair. There was no motor weakness in his median intrinsics, and he reported an undisturbed subjective light-touch sensation in his median-innervated digits of 10/10. We concluded that this combination of symptoms was most likely caused by compression or entrapment of the PCBMN by scar tissue. Nine months after his original injury and repair, he underwent operative exploration of his median nerve. An anatomical variant of the median nerve was found: an accessory muscle belly of the flexor digitorum superficialis (FDS) to the middle finger was passing through the median nerve (Figure 1). This was proximal to the take-off point of the PCBMN, which was intact and free from scar tissue. We performed tenolysis of the median nerve and released the tendinous edge of the anomalous muscle. Two months after surgery, his symptoms had largely resolved. His dysesthesia had completely disappeared.

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Figure 1.

Entrapment of a bifid median nerve at the level of the mid-distal forearm by an accessory belly of flexor digitorum superficialis muscle, as indicated by the arrow at the distal extent of the muscle belly traversing between both portions of the nerve which rejoins distally.

We conducted a systematic literature review according to the PRISMA protocol to identify all reported cases of bifid median nerve compression caused by a forearm muscle and to establish whether this case was novel. The results are summarized in Table 1. Sixteen papers, dating from 1969 to 2023, were identified. These represented 19 cases, six of which were cadavers and 13 of which were living patients. In the cadavers, the sites at which the bifid nerves were associated with muscle included the wrist and both the proximal and distal forearm. In living patients, compression sites were limited to the wrist and distal forearm. The muscles and tendons causing compression were a reversed palmaris longus, a palmaris profundus, an accessory palmaris longus, an accessory lumbrical, a FDS muscle, a FDS tendon, a flexor pollicis longus (FPL), and anomalous connections between the FDS and FPL muscles and tendons. Nerve compression was caused by the association between the bifid median nerve and muscle/tendon in 10 out of 13 cases. Ten out of 13 patients had abnormal nerve conduction studies/electromyography results. Twelve patients underwent surgery, with 10 achieving complete resolution of symptoms and two achieving partial resolution.

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Table 1.

Published cases of bifid median nerve compression by forearm/wrist musculature.

Authors	Year	Cases	Living/cadaveric	Symptomatic	Site of bifid nerve compression	Site of bifurcation	Muscular abnormality	Symptoms relieved by surgery	Abnormal NCS/EMG	Aetiology
El-Hossiny et al.	2023	1	Cadaveric	N/A	Wrist	Distal forearm	Reversed palmaris longus	N/A	N/A	N/A
Kostoris et al.	2019	1	Living	Yes	Wrist	Not traced	Palmaris profundus	Yes – just tunnel release	Yes	Idiopathic
Bale et al.	2018	1	Cadaveric	N/A	Distal foreram	Distal forearm	Ulnar branch of nerve pierces FDS to middle finger	N/A	N/A	N/A
Atoni et al.	2017	1	Cadaveric	N/A	Proximal forearm	Proximal forearm	Anomalous FDS to FDP muscle	N/A	N/A	N/A
Park et al.	2017	1	Living	Yes	Distal forearm	Distal forearm	FDS tendon separated branches	Yes	Yes	Idiopathic
Natsis et al.	2016	1	Cadaveric	N/A	Distal foreram	Distal forearm	FDS muscle at wrist	N/A	N/A	N/A
De Franco et al.	2013	2	Living	Yes	Wrist	Not traced	Reversed palmaris longus – compression	No surgery	Unclear	Idiopathic
			Living	Yes	Distal forearm	Distal forearm	Reversed palmaris longus – compression	Yes	Yes	Idiopathic
McClelland et al.	2012	1	Living	Yes	Wrist	Not traced	Palmaris profundus overlying nerve	Yes – just tunnel release	Yes	Idiopathic
Won et al.	2010	1	Cadaveric	N/A	Proximal forearm	Proximal forearm	Tendon from FDS to FDP	N/A	N/A	N/A
Skie et al.	2010	1	Living	Yes	Distal forearm	Distal forearm	FDS to index separated branches	Yes – partially	Yes	Previous surgery
Jones	2006	2	Living	Yes	Wrist	Wrist	Palmaris profundus	Yes	Yes	Idiopathic
			Living	Yes	Wrist	Wrist	Palmaris profundus	Yes	Yes	Idiopathic
Chou et al.	2001	1	Cadaveric	N/A	Distal foreram	Mid forearm	Palmaris profundus	N/A	N/A	N/A
Fernandez-Garcia et al.	1994	2	Living	Yes	Distal forearm	Distal forearm	Passed through hole in FDS tendon to middle finger at musculocutendinous junction	Yes	Not done	Idiopathic
			Living	Yes	Distal forearm	Distal forearm	Passed through hole in FDS tendon to middle finger at musculocutendinous junction	Yes	Yes	Idiopathic
Crandall et al.	1979	1	Living	Yes	Wrist	Not traced	Belly of FDS to middle proximally, accessory palmaris longus distally	Yes –unclear if just tunnel release	Yes	Idiopathic
Schultz et al.	1973	1	Living	Yes	Wrist	Not traced	Unknown muscle – probably accessory lumbrical	Yes	Yes	Idiopathic
Kessler	1969	1	Living	Yes	Wrist	Distal forearm	Lateral branch compressed by FPL	Yes – partially	Not done	Previous trauma

Abbreviations: NCS: nerve conduction studies; EMG, electromyography; FDS, flexor digitorum superficialis; FDP, flexor digitorum profundus; FPL, flexor pollicis longus (FPL).

Abnormal anatomy of the median nerve can cause neuropathy. Abnormalities of median nerve anatomy within the carpal tunnel are classified according to Lanz (1977). Divisions of the median nerve proximal to the carpal tunnel are classified as type III. Although a recent review documented various cases of type III Lanz neuropathy (Corte et al., 2021), these were limited to bifid median nerves at the level of the carpal tunnel.

We present a case of median neuropathy caused by a bifid nerve in the mid-forearm. We encourage surgeons to bear such anomalies in mind when assessing patients with symptoms of median nerve compression.