Continuing Education Activity

Compressive radial mononeuropathy is a compressive neuropathy of the radial nerve from prolonged, direct pressure onto the upper medial arm or axilla by an object or surface. The radial nerve comprises the C5 to T1 nerve roots, which arise from the posterior segment of the brachial nerve plexus. It initially runs deep to the axillary artery before passing inferior to the teres minor and then wrapping down the medial aspect of the humerus, which lies in the spiral groove. Compression of the radial nerve leads to nerve palsy, causing motor and sensory deficits. This activity covers the etiology, anatomical definition, and clinical presentation of compressive radial mononeuropathy and highlights the role of the interprofessional team in diagnosing and treating patients with this condition.

Objectives:

• Identify common causative injuries that can lead to compressive radial mononeuropathy.

• Describe the clinical presentation and physical exam findings associated with compressive radial mononeuropathy.

• Implement the appropriate treatment for patients with compressive radial mononeuropathy.

• Collaborate with the team to improve outcomes for patients affected by compressive radial mononeuropathy.

Introduction

Compressive radial mononeuropathy is a compressive neuropathy of the radial nerve from prolonged, direct pressure onto the upper medial arm or axilla by an object or surface. The radial nerve comprises the C5 to T1 nerve roots, which arise from the posterior segment of the brachial nerve plexus. It initially runs deep to the axillary artery before passing inferior to the teres minor and then wrapping down the medial aspect of the humerus, where it lies in the spiral groove. A prolonged period of immobilization compresses the radial nerve, leading to nerve palsy and causing motor and sensory deficits.[1][2][3]

Etiology

Compressive radial mononeuropathy results after immobilization in an unnatural position, resulting in prolonged compressive damage to the radial nerve. This palsy classically involves an individual falling asleep with the arm hanging over a chair or other hard surface, leading to compression within the axilla. An example is immobilization, when an intoxicated individual may not retain the reflexive ability to correct body position while asleep. Another example is an individual falling asleep on the arm of another and consequently compressing that person's nerve, as when a couple sleeps in the same bed. While these are the classically described presentations, one must be aware that compressive radial mononeuropathy can be caused by any unnatural positioning or use of the limbs that can cause compression by a similar mechanism. This includes but is not limited to compressive clothing or accessories, improper use of crutches, prolonged blood pressure cuff usage, and more.[2][4]

Epidemiology

Compressive radial mononeuropathy is relatively common and has been reported to affect 2.97 per 100,000 men and 1.42 per 100,000 women. It is the fourth most common mononeuropathy in the United States and is similarly prevalent worldwide. Given the mechanism of injury, it is not restricted to any age group and has been seen in patients of all ages.[5]

History and Physical

Patients will often report symptom onset after consuming a large amount of alcohol and then sleeping in an unnatural position. Otherwise, patients report some other mechanism by which compression would have been unnaturally placed on the upper medial arm or axilla. It is possible that patients may not provide this info until prompted, as it can go unrecognized as an inciting event. Symptoms can also begin several days after the initial insult, leading to a delayed presentation. Patients may report numbness, weakness, tingling, pain, or any combination.  On physical exam, patients may demonstrate a characteristic wrist drop, which results from the loss of extensor muscle function controlled by the radial nerve branches and preservation of the flexor muscles controlled by other nerves in the hand and arm. This leads to an inability to extend the wrist and fingers at the level of the metacarpophalangeal joints. Patients also lose the ability to extend the thumb, resulting in difficulty opening the hand and grasping objects. Providers should be aware that patients can still extend their fingers at the level of the proximal and distal interphalangeal joints, as the ulnar nerve controls these. Additionally, patients may exhibit loss of the triceps reflex, which is controlled by radial nerve innervation. Sensory deficits often involve the posterior or lateral upper arm, with symptoms extending distally to affect the posterior forearm, posterior hand, and posterolateral aspect of the lateral three-and-a-half digits.[5][6]

Evaluation

The evaluation and diagnosis of compressive radial mononeuropathy are primarily clinical, and many patients who have a clear history and physical exam may not require further diagnostic measures. However, additional diagnostic tools can help evaluate alternative causes and complications and predict prognosis. Electromyography and nerve conduction studies can localize lesions anatomically, which can help differentiate between cervical radiculopathies, brachial plexopathies, and peripheral neuropathies. Ultrasound can be a low-cost, low-risk modality that can assist with visualizing the nerve and identifying areas of damage or disruption. It can also be highly beneficial in the early identification of obvious nerve disruption and hastening early surgical intervention for these cases. Magnetic resonance imaging (MRI) can provide fine detail that may not be visible on ultrasound and can also identify which muscles have been affected. It can also evaluate for additional disease processes, neurologic disorders, and soft-tissue masses. X-ray imaging can evaluate for fractures, dislocations, and bony tumors that may be the cause of nerve injury. Combining several imaging modalities may be appropriate when considering a specific case.[1][7]

Treatment / Management

Treatment for compressive radial mononeuropathy is largely focused on physical rehabilitation. Physical therapy involves the use of a soft wrist splint that holds the wrist in extension. However, it is important to allow for full passive range of motion of the affected extremity during rehabilitation, which can be accomplished by using a dynamic splint. These measures can be supplemented with supportive care, including nonsteroidal anti-inflammatory drugs (NSAIDs), systemic corticosteroids, steroid injections, and rest from vigorous use. Some novel treatment strategies involve the use of ultrasound to administer localized injections to speed recovery. Surgical management is reserved for severe injuries of the radial nerve or for cases in which the compression results from an intrinsic process such as a mass, bone spur, or cyst.[1][8]

Differential Diagnosis

The differential diagnosis is extensive and includes many processes that can lead to radial nerve compromise. This includes traumatic causes, with humeral fractures being a prevalent cause of radial nerve injury. Additionally, severe blunt trauma, crush injuries, puncture wounds, and stab wounds are other common causes. Anterior glenohumeral shoulder dislocation can rarely lead to radial nerve injury and should be considered in any patient with consistent physical exam findings. Iatrogenic injury can be seen in any surgery or injection involving anatomy associated with the radial nerve path. Internal compression by growing cysts, masses, tumors, muscle hypertrophy, and fibrinous tissue can also cause nerve palsy. Rarely, repetitive overuse and neurologic diseases can cause isolated palsies, with some patients even being found to have acute ischemic strokes after presenting with isolated symptoms.[4][9][10]

Prognosis

The prognosis for compressive radial mononeuropathy depends on the extent of the injury, which is determined by the force and duration of compression. Mild damage results in neuropraxia, a transient conduction block without nerve degeneration. This type of injury will almost always result in complete recovery. Moderate damage results in axonotmesis, characterized by axonal damage and Wallerian degeneration that can have incomplete or late recovery. Severe damage results in neurotmesis, characterized by complete axon degradation and Schwann cell death with a low chance of full recovery. Patients with this degree of injury will often need surgical intervention. The degree of damage can be difficult to determine based on electromyography alone, and prognosis prediction can be difficult early on. Recovery is not rapid, with mild cases resolving at the earliest in 2-4 months and often much longer.[1][4][11]

Complications

Complications can arise from the failure to consider a broad differential diagnosis, which can lead to missing severe disease or illness. Determining the etiology of the radial nerve deficits is essential, as management can change drastically from case to case. In the case of true compressive radial mononeuropathy, the main complication arises from the failure of recovery, which can indicate surgical exploration. Surgical options include nerve grafting, nerve transfers, tendon or muscle transfers, and numerous other techniques. As with most surgical procedures, there can be a comprehensive set of additional complications related to intraoperative issues and post-surgical infections. Additionally, partial recovery is often achieved in these cases, and long-term disability can be challenging. Prolonged and persistent physical therapy can be burdensome but necessary for regaining some functionality.[1]

Deterrence and Patient Education

Patients should be counseled on preventing re-injury by not repeating the same mechanism that initially led to the insult. Physical rehabilitation should be emphasized to increase the chance of recovery. Patients should also be urged to follow up as scheduled to ensure that their treatment plan can be adjusted as necessary, especially in cases where early surgical intervention is warranted. It is also essential for patients to continue following up regularly in cases where the prolonged delay to recovery may necessitate late surgical exploration and intervention.

Enhancing Healthcare Team Outcomes

The management of compressive radial mononeuropathy is best accomplished with an interprofessional team approach. The initial provider to evaluate a patient with compressive radial mononeuropathy should thoroughly assess them to rule out alternative causes for a new-onset neurologic deficit. Furthermore, appropriate referral to a neurologist to plan for electromyography and additional diagnostic/therapeutic measures should be made. A physical therapy referral should also be made, and patients should be educated on supportive measures. An established timeline should be in place to ensure that surgical intervention can be pursued early if deemed necessary, and appropriate surgical follow-up should ensue in these cases. Patients in all cases should be given realistic expectations regarding the recovery process, which may not be as straightforward as expected or desired.