Anatomic variation of palmar digital nerve pathways were reported in several cases. Selective exploration of palmar digital nerves with a nerve conduction study has been challenging, because of technical issues. We report a patient who received bilateral carpal tunnel release operation, complaining of a tingling sensation, and hypoesthesia on the middle and ring fingers. An electrodiagnostic study revealed a sensory neuropathy of palmar digital nerve of the left median nerve, supplying the ulnar side of the middle finger, and radial side of the ring finger. She underwent re-operation of open left carpal tunnel release, and a branching site of common digital nerves of the median nerve was identified not at the palm, but at a far proximal site around the distal wrist crease. 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.
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 [
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 [
A 33-year-old healthy woman with a 1-year history of progressive sensory change on the left hand, visited the Department of Physical Medicine and Rehabilitation August 30, 2017. She had received bilateral open carpal tunnel release operation May 24, 2017, at a private orthopedic clinic. Tingling sensation and hypoesthesia of the thumb and index finger were relieved after surgery, but symptoms of the middle and ring fingers persisted and were aggravated. On physical examination, all deep tendon reflexes were normoactive in bilateral upper extremities, and no muscle wasting was noted including hand intrinsic muscles. Muscle strength was normal in bilateral upper extremities, including thumb abduction and opposition. Hypoesthesia was noted on the ulnar side of middle finger, and the radial side of ring finger, on the palmar side of the left hand. Tinel’s sign was positive, with proximal wrist crease tapping.
An electrodiagnostic study was performed September 5, 2017. On NCS, left median compound muscle action potential was within normal range. Left antidromic median sensory response with the middle finger recording was of low amplitude, compared to the sound side. To eliminate the possibility of volume conducted response, the orthodromic sensory conduction study was performed (
Ultrasonography was performed on the same day as the electrodiagnostic study. No abnormal findings such as nerve swelling and impingement were found at the palm and carpal tunnel. However, compression and swelling of the median nerve with fluid collection were noted at the distal wrist crease. Considering the above findings, compression of common digital branch of the median nerve at distal wrist crease was suspected (
The patient underwent re-operation of open left carpal tunnel release October 26, 2017. Interestingly, a branching site of common digital nerves of the median nerve was identified not at the palm, but at a far proximal site around the distal wrist crease (
Six months after surgery, pain on the left middle and ring fingers reduced by 70%. A follow-up electrodiagnostic study was performed April 10, 2018 (
Median nerve variations are not uncommon [
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 [
Complications after carpal tunnel release surgery are rare (0.19%–0.49%) [
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.
No potential conflict of interest relevant to this article was reported.
Conceptualization: Kwon HK. Methodology: Kwon HK, Park JW. Formal analysis: None. Funding acquisition: none. Project administration: none. Visualization: none. Writing – original draft: Noh JS. Writing – review and editing: Kwon HK. Approval of final manuscript: all authors.
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Orthodromic sensory conduction study of palmar digital nerve, supplying the ulnar side of the middle finger (A) and radial side of the ring finger (B).
Orthodromic median sensory response was not obtainable with the left ulnar side of III digit and the radial side of IV digit stimulations, in preoperative study (A and C), but obtainable with low amplitude postoperatively (B and D).
Divergence of common palmar digital nerves (arrowhead) was found, around a distal wrist crease level. A traumatic neuroma (0.5×0.5 cm, arrow) was identified, proximal to a diverging site (A). Neuroma excision and microscope-assisted to direct the end-to-end neurorrhaphy were performed (B).
Motor and sensory nerve conduction study
Stimulation site | Recording site | Latency (ms) | Amplitude | Distance (cm) | NCV (m/s) | F wave (ms) | |
---|---|---|---|---|---|---|---|
Motor | |||||||
Rt. median | Wrist | APB | 3.5 | 11.3 | 19.5 | 59 | 25.5 |
Rt. ulnar | Wrist | ADM | 3.0 | 16.6 | 15.5 | 53 | 25.5 |
Below elbow | ADM | 5.9 | 16.3 | 10.0 | 56 | ||
Above elbow | ADM | 7.7 | 16.2 | ||||
Sensory | |||||||
Lt. median | Wrist | Thumb | 2.0/2.5 | 56 | 10 | - | - |
Wrist | II digit | 2.5/3.3 | 41 | 14 | - | - | |
Wrist | III digit | 2.5/3.1 | 19* | 14 | - | - | |
III digit (radial side) | Wrist | 3.2/3.5 | 19 | 14 | - | - | |
III digit (ulnar side) | Wrist | - | NR* | - | - | - | |
IV digit (radial side) | Wrist | - | NR* | - | - | - | |
Lt. ulnar | Wrist | IV digit (ulnar side) | 2.3/3.2 | 37 | 14 | - | - |
Wrist | V digit | 2.3/3.1 | 54 | 14 | - | - | |
Rt. median | Wrist | III digit | 2.8/3.4 | 40 | 14 | - | - |
III digit (radial side) | Wrist | 3.1/3.6 | 18 | 14 | - | - | |
III digit (ulnar side) | Wrist | 3.1/3.7 | 18 | 14 | - | - | |
IV digit (radial side) | Wrist | 3.2/3.6 | 17 | 14 | - | - | |
Rt. ulnar | Wrist | IV digit (ulnar side) | 2.2/2.9 | 29 | 14 | - | - |
Latency of sensory nerve is divided into onset/peak. Amplitudes are measured in millivolt (mV, motor nerve) and in microvolt (μV, sensory nerve). Abnormal values are represented with asterisk (*).
NCV, nerve conduction velocity; APB, abductor pollicis brevis; ADM, abductor digiti minimi; NR, no response.
Sensory nerve conduction study (follow-up)
Stimulation site | Recording site | Latency (ms) |
Amplitude (uV) | Distance (cm) | ||
---|---|---|---|---|---|---|
Onset | Peak | |||||
Lt. median | Wrist | III digit | 2.9 | 3.4 | 11* | 14 |
III digit (radial side) | Wrist | 2.2 | 3.0 | 6* | 14 | |
III digit (ulnar side) | Wrist | 3.1 | 3.7 | 1* | 14 | |
IV digit (radial side) | Wrist | 2.9 | 3.4 | 2* | 14 |
Abnormal values are represented with asterisk (*).