Digital nerves are among the most frequently injured peripheral nerves in the upper extremity1. Digital nerve injuries are seen following penetrating injuries, lacerations, and blunt force trauma. Patients more commonly present with numbness along an anatomic nerve distribution, rather than pain. Pain can be associated with the injury, particularly when associated with a concomitant injury such as a fracture. A review of the literature reports 1 case of partial digital nerve injury that presented with intermittent “lancinating” pain, and 2 cases of partial digital nerve injuries with subsequent neuroma formation that presented with constant, severe pain2,3. This case series reports 3 acute cases of partial digital nerve lacerations where patients presented with a similar electric shock–like sensation, the “electric shock sign” radiating distally along the digital nerve distribution. The lancinating pain commonly occurs with active or passive finger extension.
Complete digital nerve transection is characterized by a loss of sensation in a discrete anatomic distribution. Partial nerve injuries, however, can have a variable clinical presentation, including pain in combination with numbness. This case series reports 3 cases of partial digital nerve injuries where all 3 patients presented with an electric, shock-like sensation radiating down the digit with active or passive extension, a feature unique to partial digital nerve injuries.
This case report proposal was submitted to the IRB who determined it was exempt from IRB committee review and waived the requirement for patient informed consent.
Case 1. A 30-year-old woman presented with a 4-day-old left index finger laceration sustained while using a kitchen knife to cut an avocado. A 1-cm laceration was present over the midproximal phalanx with an exit wound along the dorsal ulnar aspect of the index finger (Figs. 1-A and 1-B). She reported numbness along the ulnar aspect of the index finger and a shooting, electric-type pain with finger extension. She had full active and passive range of motion. Two-point discrimination testing was 4 mm radially versus >15 mm ulnarly. An electric shock–like, shooting pain was reproducible with passive extension of the left index finger. The physical examination was consistent with an ulnar digital nerve injury, although the nature and extent of the injury to the nerve was unknown. Surgery was performed 8 days postinjury. A 75% digital nerve laceration (Fig. 2-A) underwent a simple epineurial repair using 8-0 nylon sutures (Fig. 2-B). At 4.5 months postoperatively, she had full finger motion. She reported slight subjective numbness along the ulnar aspect of the digit, but the shooting pain with passive digital extension had resolved. Two-point discrimination was 4 mm on both the radial and ulnar aspects of the digit.
Case 2. A 37-year-old woman presented 4 days following a left long finger laceration sustained while using a fabric cutter. A 1-cm laceration was present over the radial aspect of the long finger just distal to the proximal interphalangeal joint flexion crease. She reported subjective numbness along the radial aspect of the left long finger. She noted an electric shock sensation with active or passive finger extension. Physical examination demonstrated left long finger full active and passive range of motion. Two-point discrimination was >15 mm radially vs. 5 mm ulnarly. Surgery was performed 2 weeks postinjury. A 50% laceration of the radial digital nerve underwent a simple epineurial repair using 8-0 nylon sutures. At 8 months postoperatively, the patient had full finger range of motion. Two-point discrimination was 9 mm along the radial aspect of the digit. The patient denied subjective pain, and passive digital extension of the digit did not demonstrate the electric-type shooting pain.
Case 3. A 50-year-old man sustained an accidental laceration to his left palm with a #15 blade scalpel during surgery. He noted immediate numbness in his third web space. Flexor tendon function was intact. He had a 1-cm laceration in central aspect of his palm, located between the third and fourth metacarpals in line with the proximal palmar flexion crease. He had full active and passive range of motion. Two-point sensation showed greater the 15-mm 2-point sensation on the ulnar aspect of the long finger and radial aspect of the ring finger. With active and passive finger extension, he experienced an electric shock sensation along the third web space common digital nerve distribution. Surgery was performed within 1 week of injury. A 50% laceration of the third common digital nerve underwent a simple epineurial repair using 9-0 nylon sutures. At 11-year postoperative follow-up, he has full digital range of motion and 2-point sensation of 5 mm in the third web space common digital nerve distribution. He no longer had the electric shock sensation with finger extension.
In all 3 of our patients, operative repair of the digital nerve lacerations was performed. With follow-up ranging between 4.5 months and 11 years, there was complete with resolution of their pain. All patients had also gained full digital range of motion and had either protective or normal sensation.
The morbidity of digital nerve injuries includes a loss of sensation with diminished hand function and pain secondary to neuroma formation. One study reported that digital nerve injuries contributed to impaired activities of daily living and subjective complaints of cold sensitivity, clumsiness, and pain in 13% of patients despite surgical intervention4.
Nerve pain is elicited when an intense or damaging noxious stimulus activates high-threshold nociceptors or primary afferent neurons, which are sensitive to injuries or pain5. Stimulus-evoked pain is a common component of peripheral nerve injury and has 2 key features: hyperalgesia and allodynia. Hyperalgesia, an exaggerated sense of pain, is the result of abnormal processing of nociceptor input. Allodynia, or pain resulting from a noninjurious stimulus, can be produced in 2 ways: by the action of low-threshold myelinated Aβ fibers on an altered central nervous system or by a reduction in the threshold of nociceptor terminals in the periphery5.
Although digital nerve lacerations typically present with a loss of sensation, these 3 cases demonstrate a unique physical examination finding felt to be consistent with a partial digital nerve injury. With partial digital nerve injuries and an altered peripheral nerve, all patients experience an electric shock–type sensation and pain with digital extension. Only 1 previously reported case of a partial digital nerve laceration presenting with intermittent lancinating pain exists in the literature. Pain in this patient was not described with passive extension of the digit2. Inada et al. reported 2 cases of partial digital nerve lacerations with subsequent neuroma formation that presented with severe and unrelenting pain 12 and 19 months following the initial injury3. However, the time to treatment and neuroma formation complicate the comparison to our cases. All 3 patients in this series were treated for an acute nerve injury.
Partial nerve lacerations have been described in other locations of the body, although upper extremity nerve lacerations are far more common. One case report described a child who presented with stabbing pain in the right temple triggered by facial or jaw activity following orbital trauma. Surgical exploration demonstrated an incomplete laceration of the infraorbital nerve6. This case of sharp, stabbing pain associated with a partial nerve injury resembles our cases albeit in a different anatomic location.
It is therefore hypothesized that partial nerve lacerations frequently present with sharp, electric-type pain with passive or active stretching of the injured nerve, the so-called electric shock sign. This likely occurs as the lacerated fascicles are stretched with passive or active stretching of the affected body part. Intuitively, stretching of injured fascicles is far more likely if there is a partial transection with intact fascicles to maintain the overall length of the nerve. A total nerve transection leads to the retraction of the nerve endings and minimizes the continuity and length of the nerve. It is unknown, however, whether this electric shock sign is in fact due to stretching of the transected fascicles or of the remaining unlacerated fascicles. In 1915, Paul Hoffmann and Jules Tinel independently described the Hoffmann-Tinel sign. This sign was described to indicate an indication of distal nerve regeneration of a repaired injured nerve7. Stimulation of the nerve distal to the injury site produces sensation in the area of cutaneous sensory distribution. In contrast to this sign, the electric shock sign is an immediate noxious sensation produced by traction on a partially injured nerve.
In both complete and partial digital nerve transections, surgical exploration is warranted as partial transections in which greater than 50% of the nerve is involved do not fully recover without surgical intervention8. Thus, the diagnosis and repair of both complete and partial digital nerve lacerations is recommended. The electric shock sign is a physical examination finding that may aid in the diagnosis of partial digital nerve injuries. Patients who present with a sharp lancinating pain association with active or passive extension of the effected digit or extremity should be suspected as having a partial nerve injury. In our 3 cases, the pain associated with the injury resolves following repair of the injured nerve.
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