Classification of nerve injury is based on the damage sustained by the nerve components, nerve functionality, and the ability for spontaneous recovery.
Classification systems for nerve injuries were established by Seddon in 1943 and Sunderland in 1951.
Seddon created a 3-grade classification, and Sunderland categorized nerve injuries into 5 grades. Neurapraxia is a reduction or complete block of conduction across a segment of a nerve with axonal continuity conserved.
Axonotmesis is a more severe grade of nerve injury than neurapraxia, and neurotmesis is the most severe grade of peripheral nerve injury.
Mechanisms of injury include mechanical, crush/percussion, blunt, penetrating, stretch, high velocity (motor vehicle, gunshot), and cold leading to necrosis. Trauma to peripheral nerves is relatively common. The most common injury is from blunt trauma or from penetrating missiles, such as bullets or other objects.
An important quality of the peripheral nervous system, as compared to the central nervous system, is its remarkable ability to recover after an injury through remyelination and regeneration of the axon.
Peripheral nerve dysfunction can be debilitating, because peripheral nerves generate the signals that govern both pain and peripheral motor function. An untreated acute injury to a nerve can progress to chronic nerve injury.
Acute nerve injuries are common in patients with multisystem trauma. Once a nerve injury occurs, effects on the nerve, neuromuscular junction, and muscle begin to occur. Such effects may be irreversible 18-24 months after denervation. Therefore, appropriate acute management without delay is important.
Differentiating between a peripheral nerve problem and an injury involving the spinal cord, brain, bone, or soft tissue is crucial. After establishing baseline physical examination findings, the physician must answer the following questions
Do the symptoms and findings localize to a lesion in the central or peripheral nervous system?
Are the symptoms and findings consistent with a focal or a diffuse type of peripheral nerve problem?
Is the nerve injury complete or incomplete?
What is the grade of the peripheral nerve injury?
Does clinical evidence indicate recovery or further neurological deterioration?
(See the image below.)
Peripheral nerve, cross-section. Image courtesy of the Department of Histology, Jagiellonian University Medical College.
The workup of every patient with acute nerve injury begins with a complete history and a physical examination. The site of injury can be accurately localized from a precise neurologic examination.
The strength of individual muscles or of muscle groups is graded, and a sensory examination is performed, which includes testing for light touch, pinprick, 2-point discrimination, vibration, and proprioception.
Imaging techniques, such as radiography, computed tomography (CT) scan, and magnetic resonance imaging (MRI) are valuable diagnostic tools for evaluating a peripheral nerve lesion.
To rule out bony and ligamentous injuries, all patients with axillary nerve injury should have radiographs taken of the shoulder and cervical spine.
CT scan and traditional MRI have certain limitations in distinguishing peripheral nerves from the surrounding structures, in which case magnetic resonance neurography (MRN) can help visualize both normal and abnormal peripheral nerves.
A nerve conduction study (NCS) can be effective in identifying peripheral nerve injury.
Peripheral nerves are stimulated by somatosensory evoked potentials (SSEPs), and if signal conduction is disrupted along any segment of the circuit, an evoked potentiation is not produced.
Technological advances in neurosurgical instrumentation and diagnostic imaging have led to great results in the repair of acute nerve injury.
Accurate grading of an acute traumatic injury is necessary because surgery is indicated for patients with neurotmesis (Sunderland grade III-V).
Surgical intervention is based on the extent of nerve damage and functional viability of the nerve. Surgeons need to consider injury location, extent of injury, age, and medical condition. It is important to determine whether function can be restored and whether benefit outweighs risk.
Primary repair is direct reconnection of the nerve immediately after injury. In an epineurial repair, the epineuriums of the separated nerve endings are sutured together
Best results occur when the nerves are either purely sensory or purely motor and when the intraneural connective tissue component is small and the fascicles have been clearly aligned.
Secondary repairs are delayed repairs. Many surgeons prefer delayed suture because it allows the wound to heal and decreases the risk of infection. Also,, during a delayed repair, scarred ends of the nerve can be defined more accurately and trimmed back to normal fasciculi.
In general, contraindications to surgery usually result when the risks of surgery outweigh the benefits. Surgery should not be performed when a poor outcome is expected. Some surgical repairs initially contraindicated may be performed later. If a swollen and discolored peroneal nerve is encountered during an acute knee repair, it should not be resected; rather, waiting several months is better.
In traumatic peripheral nerve injuries, the rule of 3 may be applied to surgical timing: immediate surgery within 3 days for clean and sharp injuries; early surgery within 3 weeks for blunt/contusion injuries; and delayed surgery 3 months after injury for closed injuries.