Laboratory Studies
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. [2] In traumatic hip dislocations, immediate assessment of nerve function is important because potentially reversible nerve injury from compression-induced ischemia can rapidly progress to permanent loss of function. [26] Routine electrodiagnostic testing can be used to support a clinical suspicion of nerve injury or to evaluate the nerve function in patients in whom a reliable neurologic examination is impossible. [26]
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 [1] :
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Do the symptoms and findings localize to a lesion in the central or peripheral nervous system?
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Are the symptoms and findings consistent with a focal or a diffuse type of peripheral nerve problem?
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Is the nerve injury complete or incomplete?
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What is the grade of the peripheral nerve injury?
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Does clinical evidence indicate recovery or further neurological deterioration?
As part of the physical examination, the strength of individual muscles or of muscle groups is graded. Additionally, a sensory examination is performed, which includes testing for light touch, pinprick, 2-point discrimination, vibration, and proprioception. [1] In anterior dislocation of the shoulder, the sensory distribution of the axillary and musculocutaneous nerves are tested to detect nerve injury in the early stages. [23]
Nerve injuries that result in decreased blood supply only result in delayed nerve conduction velocity. Significant external compression producing partial nerve ischemia is indicated by a 10% decrease in nerve conduction across a localized region (eg, the carpal tunnel). This is the most sensitive laboratory test to document a nerve compression syndrome. [5]
Imaging Studies
Imaging techniques, such as radiography, computed tomography (CT) scan, and magnetic resonance imaging (MRI) are valuable diagnostic tools for evaluating a peripheral nerve lesion. [1] Numerous clinical studies have used magnetic resonance neurography (MRN) to examine patients with peripheral nerve pathology. Assessment of peripheral nerve injury in patients using this technique has the potential to confirm acute nerve injury and monitor the recovery process. Despite this clinical work, little is understood about the sequence of imaging changes following nerve injuries and how they correlate with functional deficit. [36, 37]
Many peripheral nerve injuries can be associated with other soft tissue or bone injuries that can be revealed through radiographic findings. Radiographs of the injury site help identify fractures or foreign bodies. [2] For example, cervical spine fractures frequently are associated with brachial plexus injuries. In the presence of phrenic nerve paralysis, chest radiographs demonstrate unilateral elevation of the diaphragm. Midhumeral fractures are associated with radial nerve injuries; midforearm fractures of the ulna or radius are associated with median or ulnar nerve injuries, respectively. Hip and proximal femur fractures are associated with sciatic nerve injuries, and femur fractures that are more distal are associated with peroneal or tibial nerve injuries. [1]
From 2 to 3 months after median nerve entrapment following an elbow trauma, lucency in the supracondylar region appears on radiographs. This lucency helps diagnose median nerve entrapment radiographically. [3]
To rule out bony and ligamentous injuries, all patients with axillary nerve injury should have radiographs taken of the shoulder and cervical spine. [10]
For resolving the fine anatomic detail of soft tissue, MRI has proven to be much more effective than CT scan. Conventional MRI has been used to visualize both normal and abnormal peripheral nerve structures. In addition, MRI can help reveal signal changes in denervated muscle as early as 4 days after injury. In a prospective study, MRI was sensitive and specific for evaluating ulnar nerve entrapment at the elbow, with 97% diagnosis of ulnar neuropathy. [38] With short tau inversion recovery (STIR), signals of the thenar muscles on MRI images were found in 100% of patients with clinically advanced carpal tunnel syndrome. [1] With neurapraxic nerve injuries, STIR or T2-weighted signals in the innervated muscles remained normal. Therefore, following a peripheral nerve injury, obtaining an MRI of the muscle can be useful early in distinguishing a neurapraxic grade of injury from more severe axonotmetic and neurotmesis grades of injury.
Because CT scan and traditional MRI techniques have inherent limitations in their capacity to resolve and distinguish peripheral nerves from the surrounding structures, magnetic resonance neurography (MRN) has been developed. [1] An MRN can help visualize both normal and abnormal peripheral nerves in various regions of the body. [11] The injured peripheral nerve can be assessed by orienting the images along the damaged nerve course. For example, the loss of T2-weighted signals indicates damage to the myelin sheath. [5] In addition, loss of water content in denervated nerves of deep muscles can be assessed by MRN when needle electrode tests are difficult to perform. [5] Regarding the assessment of peripheral nerve trauma, the predictive value of MRN as a diagnostic tool has not been established.
At present, operative exploration with intraoperative electrophysiologic monitoring remains the criterion standard to treat traumatic peripheral nerve lesions that are not improving. [1]
Other Tests
Clinicians can use other tests to evaluate peripheral nerve injuries. For example, clinical threshold testing can be used to evaluate sensory function in peripheral nerves. These tests determine the level of stimulus necessary to elicit a response. Semmes-Weinstein monofilaments are fine filaments that exert a discrete amount of pressure on the fingertips. They are used to perform threshold testing. Vibratory sense can be assessed through clinical threshold testing using a range from low frequency (30 Hz) to high frequency (256 Hz). [5]
Injuries disrupting part or all of the nerve produce changes on electromyogram (EMG) findings within 3 weeks of injury. [5] In nerve dysfunction from elbow trauma, Ristic et al recommended that nerve conduction studies (NCS) be delayed 3-4 weeks. [3] After an anterior shoulder dislocation with paralysis or severe paresis, an EMG should be performed within 3 weeks. [23] Severe nerve compression syndromes also can produce axonal disruption resulting in EMG changes. [5] In axillary nerve injury, results of EMG studies may show signs of deltoid muscle denervation, including fibrillation potentials. [10] Improved results from EMG studies without voluntary muscle contraction warrant further conservative therapy. [4] Repeat EMG studies are warranted if no clinical improvement occurs. [10]
An NCS is effective for evaluating peripheral nerve injury. [12] Somatosensory evoked potential (SSEP) monitoring is accomplished through stimulation of a peripheral nerve. The SSEP is recorded from a scalp electrode over the contralateral sensory cortex. If signal conduction is disrupted along any segment of the circuit, an evoked potentiation is not produced. [1, 5]
An NCS is useful for distinguishing neurapraxic from more severe grades of injury. The hallmark of a neurapraxic injury is slowing or block of conduction across a section of nerve. The absence of nerve conduction distal to the injury indicates axonal loss related to a more severely graded injury. [1]
Nerve function and suspected injury can be tested with simple office techniques that do not require elaborate instruments. Impending sciatic nerve dysfunction from traumatic hip dislocation can be predicted by a positive result on a sciatic stretch test. In this test, the examiner passively extends the affected knee with the hip flexed, which produces pain in the sciatic nerve distribution. [26] An advancing Tinel sign is another office technique that can be used to establish the progression of axonal regeneration. [1]
Light microscopy of nerves injured by epineurectomy and nerve crush injury reveals widespread fiber degeneration and myelin debris in the subperineurial region. [35] The centrofascicular areas have relative preservation, as compared to the subperineurial regions. The central vessels are preserved within the centrofascicular area of the injured nerve. [35] In injured animals, the thickness of the myelinated axon area and myelin is less than in uninjured animals. [35]
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Peripheral nerve, cross-section. Image courtesy of the Department of Histology, Jagiellonian University Medical College.
