Imaging of Traumatic Peripheral Nerve Injuries

Abstract

Nerves are commonly injured in case of blunt or penetrating trauma to the extremities. Patients with nerve injuries have profound consequences and thus a timely decision for operative management is a very important. Conventionally, management decisions have been based on clinical findings, patient course and electrophysiological studies. However, imaging modalities have an enormous role not only in localizing and grading of the nerve injuries but also in the follow-up of the nerve recovery. High-resolution ultrasound (HUS) is the modality of choice for evaluation of peripheral nerves. Magnetic resonance neurography (MRN) plays a complementary role, enabling better assessment of muscle changes and deeper nerves. Corresponding to the injured layer of the cross-section of the nerve, imaging manifestations differ in different grades of injury. Since imaging cannot detect ultrastructural changes at the microscopic level, thus there may be overlap in the imaging findings. Herewith, we discuss the imaging findings in different grades of nerve injury and propose a simple 3-tier grading for imaging (HUS and MRN) assessment of peripheral nerve injuries.

Keywords

1. Peripheral nerve injuries

2. High-resolution ultrasound (HUS)

3. Magnetic resonance

4. Neurography (MRN)

1 Introduction

The global prevalence of traumatic peripheral nerve injuries is approximately 5%, including brachial and lumbar plexus injuries.1 These are an important cause of morbidity and disability in young individuals. The patients can present acutely (in case of trauma) or have a chronic course (due to chronic stretching or overuse microtrauma). The clinical presentation is in the form of either motor weakness/sensory impairment or tingling/numbness involving the nerve. Penetrating trauma is the most direct mechanism of nerve injury and may not carry a poor prognosis due to clean-cut ends. Sometimes, the nerves may get injured inadvertently during surgery or any other intervention e.g. secondary to interposition of the nerve during reduction of fractured bones, stretching injuries or due to impingement by screws or other implants. The radial, ulnar, and median nerves are reported to be the most commonly affected following trauma of the upper limbs; while the sciatic, peroneal, tibial, and femoral nerves are most often affected in the lower limbs.2 Upper limb nerves are more commonly affected because of higher chance of trauma to upper limbs.

2 Mechanism of injury

Traumatic peripheral nerve injuries can result from varied mechanisms. They can be due to blunt trauma, penetrating injuries, chronic traction/acute stretch injuries and less commonly due to local chemical injury, electric shock or freeze injury. Acute severe injury can result from direct compression of the nerve against rigid structures like bone or by penetrating trauma. Indirect compression of the nerve by hematoma, fracture fragment, aneurysm or scar usually results in subacute to chronic presentation. Chronic stretch injuries may be due to overuse microtrauma (profession and sports-related), dislocations or iatrogenic injuries and are usually mild. The injuries caused by explosions are associated with poor prognosis. It may be noted that the nerves are usually affected in combination with the adjacent tissues; and the management of the associated bony injuries, vascular injury or soft tissue loss usually takes priority.

3 Patient evaluation and role of imaging

Evaluation of peripheral nerve trauma and its postsurgical outcome has classically been based on clinical and electrophysiological methods.3,4 In addition to the initial clinical symptoms, the course of the neurological manifestations is also important. Sometimes the patients present late; because of missed initial diagnosis or lack of referral of the patients from peripheral centers. Electrophysiological studies (EPS) include measuring nerve conduction velocity (NCV) and electromyography (EMG). The function of the peripheral nerve is assessed by electrical stimulation of the nerve and recording the response at the muscle or the nerve.5,6 It can be used for sensory, motor as well as mixed nerves. The response elicited gives valuable information regarding demyelination/axonal loss of the testing nerve. The motor NCV is performed by giving a supramaximal stimulus to the testing nerve proximally and evaluating the response distally through an electrode placed inside the muscle supplied by testing nerve. The compound muscle action potential (CMAP) so generated is recorded by the electrode and NCV is calculated, using formula; NCV = distance/latency, where distance is the length between stimulating and response recording electrode and latency is the time taken from onset of stimulation to onset of response. EMG is performed to record the intrinsic electrical activity of the muscle, by placing an electrode into the muscle.5,6 Observations are made at rest, minimal voluntary contraction and after maximal contraction of the muscle. The MUP (Motor unit potential), so recorded is assessed for amplitude & duration, which are relatively constant for different muscle fibers. Abnormality in EMG is manifested by fibrillation or fasciculation. Similarly, sensory NCV is performed by electrical stimulation of the peripheral nerve and recording from a purely sensory portion of the nerve.

Limitations of EPS and Clinical features: The major decision in nerve injuries is whether the patient requires surgery or is likely to improve by conservative management. Up to half of all traumatic peripheral nerve injuries may require surgical intervention.1 Sometimes, the site and extent of injury however remains uncertain after physical examination and EMG due to unequivocal findings.7 EPS cannot reliably differentiate between axonotmesis and neurotmesis.7 EPS findings vary as per the timing since injury and EMG muscle changes become apparent after 3 weeks from injury. It has been found that a certain group of patients with high grade nerve injury may actually benefit from early surgery and thus this group cannot be detected based on EPS.8 The precise extent of nerve damage and whether to proceed conservatively or with surgical repair, thus may not be optimally decided with the combination of neurological examination and EMG.7

High-resolution ultrasound (HUS) and magnetic resonance neurography (MRN) are the main imaging modalities for evaluation of nerve injuries.9 HUS and MRN are considered adjunct to each other and both have their advantages and limitations. HUS is an easy, effective, and economical modality in evaluating peripheral nerve injuries. Obtaining preoperative, anatomical information of the injured nerve is crucial for surgical planning. Table 1 lists the role of imaging in evaluation of nerve injuries. Before imaging, it is better to know the nature of trauma, identify the exact anatomical region injured, look for scar corresponding to penetrating injuries, review the radiographs for fractures and implants and analyze the surgical details, if any. The imaging findings must be interpreted in context of the clinical course and EPS tests.

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