Pathophysiology

The amazing capacity of a peripheral nerve to regenerate after injury enables functional recovery in many cases. Unfortunately, this regenerative capability, when it goes awry, can produce a painful scar at the site of injury. Neuromas can affect any nerve: large, small, or microscopic. The key process that contributes to neuroma formation is the regrowth of injured axons into impenetrable scar. A traumatic neuroma is a dense, minimally vascular fibrous mass containing many small nerve fibers connected to a nerve.¹ The fibrous connective tissue that makes up the outer layer of the neuroma is continuous with the perineurium of the involved nerve.² The injured nerve fibers in a traumatic neuroma cause pain by multiple mechanisms. Spontaneous firing of nociceptors sends impulses centrally, leading to neuropathic pain.³ Additionally, nerve fibers in close proximity to each other may stimulate impulse transmission even in the absence of a synapse between them.⁴ Further stimulation may occur through the abnormal release of chemical pain mediators.⁵

Neuromas may form along nerves after injury, although it is a mystery exactly why they become painful in some cases and not in others. Typically, the traumatic event lacerates the nerve either partially or completely, although essentially any type of injury, such as compression, gunshot wound, or traction, can result in neuroma formation. When the nerve is completely severed (neurotmesis),⁶ the ends often retract apart. The proximal end sometimes forms a bulbous neuroma, whereas the distal end withers into scar (Fig. 170-1). Because the cut ends are physically separate, the probability of meaningful recovery of axons across the injured segment is vanishingly low.⁷ Sharp and blunt lacerating injuries often cause this type of neuroma.

FIGURE 170-1 Intraoperative photograph after exposure of a median neuroma at a mid-humeral level. This patient sustained a median nerve transection by glass from a broken window 5 months previously. The proximal nerve stump (P) has formed a bulbous neuroma, whereas the distal end (D) has thinned and scarred down to adjacent tissues. Note the significant retraction that has occurred, producing a substantial gap between the proximal and distal ends.

When the axons are severed but the epineurium remains intact (axonotmesis),⁶ this yields a neuroma-in-continuity. Compression, gunshot wound, and stretch injuries generally cause this type of neuroma. A fusiform neuroma may form along the injured segment (Fig. 170-2). Because the cut ends are together, meaningful recovery of axons across the injured segment often occurs (regenerating neuroma-in-continuity).⁸ The presence of scar, however, prevents recovery in some cases (nonconducting neuroma-in-continuity). These forms are distinguished most reliably by using intraoperative nerve action potential recordings. Recovering nerve action potentials are seen with a conducting neuroma-in-continuity, but not with a nonconducting neuroma-in-continuity (Fig. 170-3).

FIGURE 170-2 Intraoperative photograph after excision of a sural neuroma at a mid-calf level. This patient underwent a partial-thickness sural nerve biopsy 1 year previously. A fusiform neuroma has formed at the biopsy site, leaving the nerve in continuity.

FIGURE 170-3 Intraoperative photograph after external neurolysis of a sciatic nerve that sustained a blunt injury several months previously. The nerve has been subjected to an internal neurolysis as well, dividing it into its tibial and peroneal divisions. Hook electrodes are applied proximally and distally to the injured area, a stimulus (S) is applied, and intraoperative nerve action potential recordings (NAP) are obtained. The tibial division (top) shows a recovering nerve action potential, indicative of a conducting neuroma-in-continuity; whereas the peroneal division (bottom) shows a flat tracing, indicative of a nonconducting neuroma-in-continuity.

Clinical Presentation

Neuromas form after nerve trauma. Often, the patient is able to give a clear history of the traumatic event. In many cases, the nerve injury is obvious. Sometimes, neuromas form unexpectedly after surgery following an unrecognized nerve injury. In other cases, neuromas form following a prolonged period of mild, repetitive injury. At times, a large, named nerve is injured, and in other cases, a small, unnamed sensory nerve forms the neuroma. When a nerve is acutely injured, it may be sensitive to touch or pressure immediately after injury. Sometimes this sensitivity persists until a neuroma forms. Sometimes nerves are not particularly painful in the early period after injury but become sensitive later on once the neuroma forms. It is unclear why some nerve injuries are painful immediately and others become so in a delayed fashion.

Traumatic neuromas are firm, slow-growing, and sometimes palpable nodules that can be associated with pain and paresthesias in the affected area. The hallmark finding for a painful neuroma is an exquisitely sensitive, focal area along the course of a previously injured nerve. Pain is evoked by palpation of the region and may remain local or radiate along the course of the peripheral nerve sensory distribution (Tinel’s sign).⁴ Patients can also have tenderness and hypersensitivity to normal tactile stimuli in the surrounding regions.² When the injured nerve is nonfunctional, there is often loss of sensation in the distribution of that nerve.

Diagnosis

The diagnosis of a neuroma can be made clinically: pain in the region of a scar, altered sensation in the distribution of the affected nerve, and Tinel’s sign. Confirmatory testing may include injection of local anesthetic into the vicinity of the neuroma⁹ or more proximally along the course of the affected nerve.¹⁰ The presence of temporary pain relief helps make the diagnosis of a painful neuroma and additionally may help to identify the exact nerve involved. This may be helpful if multiple candidate nerves are in the vicinity, such as in the inguinal region. Some practitioners use control injections of saline or local anesthetics of differing durations, one long acting and one short acting, in an effort to increase reliability of this diagnostic modality. Such a strategy may help reduce unwanted placebo effects, which can complicate the treatment of any pain disorder.

Imaging may be used to identify larger neuromas. Magnetic resonance imaging and magnetic resonance neurography may delineate the neuroma clearly in some cases.¹¹ Ultrasonography has been additionally used routinely to image Morton’s neuromas, affecting the plantar digital nerves that form near the metatarsal heads after repetitive compression trauma.¹²

Perhaps the most reliable way to identify a neuroma is with surgical exploration in the awake patient. Following external neurolysis of the neuroma, when palpation of the neuroma reproduces the patient’s exact pain, little diagnostic uncertainty remains. This strategy is essentially identical, in theory, to the conscious pain mapping procedures used by gynecologists during laparoscopic procedures to identify pelvic pain generators.¹³

Treatment

When a painful neuroma is diagnosed, a comprehensive, pain management–oriented approach may be helpful. Initial efforts may involve over-the-counter pain medications and protecting the painful area from additional trauma. Anticonvulsant¹⁴ and antidepressant¹⁵ medications may provide relief for neuropathic pain. Opiates¹⁶ may also reduce pain levels. Complementary and alternative therapies such as acupuncture may be useful.¹⁷

When these noninvasive efforts are insufficient, a more aggressive strategy may be required. Steroid injections, sometimes coupled with a local anesthetic, may provide temporary and, in some cases, durable pain relief.¹⁸ Ablative techniques such as radiofrequency ablation, pulsed radiofrequency ablation, or chemical neurectomy¹⁹ may destroy the associated nerve or neuroma, providing the necessary pain relief. There is a paucity of high-quality clinical data supporting the use of these techniques, however.

Open surgery is another option in the treatment of painful neuromas. A major problem in neuroma excision is the recurrence of the neuroma, and prevention of growth of a secondary neuroma is a key part of surgical treatment. In the past, treatment of neuromas involved neuroma excision followed by capping the nerve with many substances, including silicone and surplus epineurium, to prevent new neuroma formation.²⁰ Additionally, chemical treatment of the cut proximal end of the nerve has been used.²¹ One study reported that oblique transection of peripheral nerves, as opposed to perpendicular transection, greatly reduces the rate of classic neuroma development.²²