As the median nerve passes under the flexor retinaculum it enlarges, flattens, and usually divides into two portions: lateral and medial. The lateral portion then divides into 3 proper palmar digital nerves, in which 2 of them supply the sides of the thumb, and one supplies the radial side of the index finger. The medial portion of median nerve divides into 2 common palmar digital branches, after passing through the transverse carpal ligament. Each common palmar digital nerve runs towards the cleft between the index and middle fingers, and the middle and ring fingers. Then they split into 2 proper digital nerves, to the adjoining sides of the 2nd to 4th digits [1-3].

Selective exploration of palmar digital nerves with a nerve conduction study (NCS) has been challenging, because of technical issues such as innervation overlaps, volume conducted responses, and inadvertent activation of nearby nerves [2]. In our study, we report a rare variation of the course of the common palmar digital nerves of the median nerve, and its neuropathy. We also describe how we identified palmar digital neuropathy with an orthodromic sensory conduction study.

Median nerve variations are not uncommon [4,5]. These features are one of the reasons for atypical findings clinically and electrophysiologically, and can lead to diagnostic errors. Focal neuropathies and ischemic neuropathies can occur at any location. Therefore, clinicians should be aware of the anatomy of nerve structure, as well as its variations. Normally, median nerve lies between tendons of flexor carpi radialis and palmaris longus, proximal to flexor retinaculum [5]. Anatomic variations of palmar digital nerve pathways were reported in several cases. Usually, palmar digital nerves do not branch out, until the median nerve passes through the flexor retinaculum. Distal to the retinaculum, the nerve expands, and typically divides into 5 branches. Lanz [5] reported cases of median nerve divergence at far proximal parts, divided by the median artery. When the median nerve divides, the diameter of each part is reported variably: equal, radial dominant, or ulnar dominant. Schultz et al. [6] also found a far proximal division of the median nerve, with an accessory lumbrical muscle, between the 2 common palmar digital nerves. In our case, the diverging site was found at the distal wrist crease level, and it was neither divided by a vessel, nor muscle. The radial part supplying the 2nd to 3rd fingers, was thicker than the ulnar part supplying 3rd to 4th fingers. Also, the neuroma was found just proximal, to the divergence point. This neuroma was probably formed due to an incomplete decompression of previous carpal tunnel release surgery, and is most likely the culprit responsible for the patient’s hypoesthesia on the 3rd web space, and adjoining fingers. Recognition of possibilities of anatomical variations in common palmar digital nerves can enable us to recognize unexpected symptoms and signs which can occur in medical practice.

An orthodromic conduction study of the median nerve is widely performed in electrodiagnostic study, and has the following advantages: nerves that are stimulated in orthodromic technique are sensory, but those in antidromic technique are mixed. Thus, an occasional motor artifact possibly interfering with a sensory response occurs less [7]. Also, near-nerve recordings are better suited for the orthodromic, because inadvertent activation of a neighboring nerve is less common with orthodromic stimuli, than with antidromic stimuli. Therefore, eventual selective impairment of the palmar digital nerve in this case was better demonstrated with orthodromic sensory conduction studies.

Complications after carpal tunnel release surgery are rare (0.19%–0.49%) [8]. However, if the symptom persists, there are 2 possibilities: incomplete carpal tunnel release, or other associated musculoskeletal problems. Since it is difficult to quantify subjective symptoms and physical examination findings after carpal tunnel release surgery, objective measurements that can quantify symptom improvement are needed. Latency and amplitude of SNAP are possible sensitive parameters in that they started to show improvement 2 weeks after surgery, and persisted until 3 months postoperatively [9]. Additionally, ultrasonography may be beneficial in evaluating postoperative changes, and other possibly combined musculoskeletal problems. An ultrasonographic cross-sectional area (CSA) measured at the site of maximal median nerve enlargement, and 2 cm proximal from the distal wrist crease (DWC) in the forearm decreased 3 weeks after surgery [10]. Also, the ratio of CSA of maximum median nerve enlargement to that of the forearm showed correlation with electrophysiological findings, although the extent of decrement showed no correlation with symptom severity [10]. In short, physicians can choose appropriate NCS or ultrasonography in patients with persistent or residual symptoms, to objectively assess postoperative changes, and eventually practice proper management.

In this patient, while the previously unobtainable orthodromically conducted sensory responses were detectable in the ulnar side of the middle finger and radial side of the ring finger stimulation with clinical improvement after the surgery, left median SNAP amplitudes were diminished postoperatively in antidromic response with the middle finger recording, and orthodromically recorded sensory response in the radial side of the middle finger. These findings could occur after an operative process such as neuroma excision, and microscope-assisted direct end to end neurorrhaphy.

In conclusion, we report a patient with a palmar digital neuropathy, with a rare anatomical variation of the median nerve. Also, usefulness of an orthodromic sensory conduction study was clarified to eliminate volume conducted response or co-activation of nearby nerves in the patient with selective involvement of palmar digital nerve. Knowledge of anatomic variation of common palmar digital nerves is essential, as it may be damaged during surgery or in trauma.