Last updated on December 19, 2019
Lying immediately deep (dorsal) to the superficial palmar arch are the common digital nerves. They pass distally to the webs, and divide into proper digital nerves. Here the nerves lie superficial (volar) to the arteries There is a dorsal branch of each proper digital nerve arising just distal to the MP joint. They give branches to the joints, and trifurcate just distal to the DIP joints. Terminal branches supply the fingertip pulp and the nail beds.
The superficial (sensory) branch of the ulnar nerve divides into two branches; the ulnar one supplies the ulnar side of the little finger, the radial one supplies the 4th web space and adjacent sides of little and ring fingers.
The median nerve enters the palm beneath the flexor retinaculum and divides into three branches. The ulnar branch divides into two and supplies the 3rd web and adjacent sides of ring and middle fingers and the 2nd web and adjacent sides of middle and index fingers. The latter branch supplies the second lumbrical muscle. The radial branch supplies the radial side of the index, the whole of the thumb and its web on the palmar surface, and distal part of the dorsal surface. The branch to the index supplies the first lumbrical.
Mechanism of Injury
A nerve can be injured by sharp or blunt mechanisms, and the severity of the injury is proportional to the energy involved. Nerve injury can be classified using the Seddon or Sunderland classification schemes.
From Seddon’s original publication in 1942, the severity of the injury is inversely proportional to the number of the classification:
“1. Neurotmesis (a ‘cutting’ which implies a separation of related parts), which describes the state of a nerve that has been completely divided. The injury produces a lesion which is in every sense complete.
2. Axonotmesis: Here the essential lesion is damage to the nerve fibres of such severity that complete peripheral degeneration has followed; and yet the sheath and the more intimate supporting structures of the nerve have not been completely divided. which means that the nerve as a mass of tissue is still in continuity.
3. Neurapraxia (non-action) is used to describe those cases in which there is a short-lived paralysis so short that recovery could not possibly be explained in terms of true regeneration.”
Common usage reverses the numbers so that the severity is proportional to the number.
From Sunderland’s original publication in 1951:
“Five degrees of nerve injury may now be defined on the basis of changes induced in the normal structure of the nerve. The injuries are arranged in ascending order of severity from the first to the fifth degree and affect successively (i) conduction in the axon, (ii) the continuity of the axon. (iii) the endoneurial tube and its contents, (iv) the funiculus and its contents, and finally (v) the entire nerve trunk.”
The two classifications correspond in the following manner:
Sunderland 1 corresponds to neurapraxia
Sunderland 2, 3, and 4 correspond to axonotmesis, with increasing levels of disruption of the architecture of the nerve involving the axon, endoneurium, and perineurium.
Sunderland 5 corresponds to neurotmesis – complete disruption of the nerve.
Mackinnon added a 6th level of injury describing a mixed picture with different levels of injury in the cross section of the nerve.
Patients may complain of numbness or paresthesias. These two symptoms should be distinguished, as they may represent different stages of injury or recovery. Numbness probably represents complete inactivity of the nerve – such as loss of polarization of the axon membrane in neurapraxia, or transection of the axon – both resulting in inability to conduct an action potential to the cell body. Paresthesias may be seen in a partial nerve injury or a recovering nerve – they represent transmission of abnormal action potentials, such as seen in demyelination, recovering nerve fibers, or neuromas.
There is little benefit to looking in a wound in the emergency room to look for a nerve injury. Bleeding, patient pain, poor lighting, and other factors limit one’s assessment of the anatomic structures, even with loupe magnification. More information can be obtained by careful history and sensory examination. All sensory testing is subjective (stimulus is interpreted by the brain, and response is given verbally), and thus susceptible to other factors, such as pain, anxiety, and other emotional influences. Light touch examination is not very predictive of the severity of nerve injury. Patients can have altered light touch sensibility with an intact nerve, and can claim normal sensation with a divided nerve. Consequently, it may be difficult to classify the degree of nerve injury at the initial presentation. All 6 levels of nerve injury may have similar initial physical findings, and the decision to explore should be based on evaluation of the history and the density of the sensory loss. Two-point discrimination is easy to perform (a paper clip can be bent to an appropriate shape to perform the test), and even though it is still a subjective test, it is more likely to aid in the diagnosis of nerve injury (citation needed). Normal 2-pt discrimination (3-4mm) is incompatible with a divided nerve. Absent 2-pt discrimination (greater than 15mm) means a divided nerve is likely. Abnormal, but not absent, 2-pt discrimination may indicate a partial nerve injury. Interestingly, even after allowing time for full evolution of the nerve injury, it may still be difficult to classify nerve injuries exactly. For example, stages IV, V, and VI may all end up as neuroma-in-continuity.
A digital nerve injury does not need to be repaired emergently, but excessive delay should be avoided to reduce the need for a nerve graft, which has a lower success of sensory recovery. If treatment is to be performed electively, the skin should be closed, and a dressing applied. Splinting is not necessary, and range of motion should be encouraged pending repair of the nerve. Repair should be performed in the operating room. Loupe magnification has been shown to be adequate, but use of the operating microscope may confer some benefit in terms of fascicular alignment (citation needed). An epineurial repair with 9-0 nylon sutures is standard, and fibrin glue or nerve wrap has been shown to increase the number of axons crossing the repair (citation needed). A nerve graft should be performed to prevent tension on the repair if there has been loss of substance (citation needed). Autologous nerve graft is the gold standard (citation needed). The terminal branch of the posterior interosseous nerve is a useful donor nerve – it provides sufficient fascicles, with the the benefit of no sensory loss (citation needed).
A detailed history and careful sensory physical exam are essential. The degree of nerve injury can’t always be determined by sensory testing, but 2-pt discrimination can help determine if exploration of the nerve is indicated. Repair should be undertaken within 3 weeks, and epineurial repair should be performed with magnification.