Ulnar Nerve

Introduction

Ulnar neuropathy is relatively common and occurs either following an acute trauma or in the setting of a chronic compression of the nerve. In fact, ulnar nerve pathology is the most common reason for hospitalization compared with all other upper extremity neuropathies. Due to the rare occurrence of refractory ulnar neuropathic pain, there are limited treatment options once conservative management and surgical treatment have been exhausted. Peripheral nerve stimulation (PNS) is a potential intervention for patients who have refractory ulnar nerve pathology symptoms and also represents an opportunity to improve quality of life for these patients. Beginning as a terminal cord of the brachial plexus and traveling down the entire length of the arm, the ulnar nerve has a number of potential sites for injury or compression that can cause pain. In this chapter, we will examine the clinical presentation of ulnar nerve neuropathic pain, relevant anatomy, history and physical exam, imaging, and diagnostic nerve blocks, in addition to the technique for using PNS as a therapeutic modality.

Anatomy

The ulnar nerve is a mixed nerve, containing both motor and sensory axons. It arises from the ventral rami of the C8-T1 nerve roots and exits the brachial plexus as the terminal branch of the medial cord. Distally, at the wrist, it bifurcates into superficial and deep terminal branches.

The nerve travels down the upper arm parallel to the brachial artery and, as it reaches halfway down the upper arm, it traverses the medial intermuscular septum (the arcade of Struthers) ( Fig. 13.1 ) and descends posteriorly, where it lies close to the humerus and the medial head of the triceps brachii muscle. Down at the elbow, the ulnar nerve passes between the groove of the medial epicondyle of the humerus and the ulnar olecranon, where it lies superficially and is often palpable. As the nerve exits the groove, it passes within the cubital tunnel formed by the humeroulnar arcade, an aponeurotic arch connecting the humeral and ulnar heads of the flexor carpi ulnaris muscle ( Fig. 13.1 ). While in the cubital tunnel, the ulnar nerve gives off its motor branches to the flexor carpi ulnaris muscle and the ulnar side of the flexor digitorum profundus.

Once at the distal third of the forearm, the ulnar nerve gives off the dorsal cutaneous branch, which passes medial to the flexor carpi ulnaris muscle. The dorsal cutaneous branch of the ulnar nerve enters the dorsal ulnar aspect of the forearm by penetrating through the flexor carpi ulnaris muscle’s fascia. The dorsal cutaneous branch then divides into a radial and ulnar branch, providing sensation to the dorso-ulnar aspect of the hand as well as the fourth and fifth digits.

The main trunk of ulnar nerve enters the hand in tandem with the ulnar artery via Guyon’s canal, which is formed by the pisiform bone and the hook of the hamate ( Fig. 13.2 ). In the hand, the ulnar nerve bifurcates into superficial and deep terminal branches. The superficial terminal branch supplies the cutaneous ulnar border of the palm and the palmar surfaces of the fourth and fifth digits. In contrast, the deep branch innervates the opponens digiti muscle, the hypothenar muscles, all the interossei muscles, the third and fourth lumbricals, the adductor pollicis, and variable portions of the flexor pollicis brevis muscles.

Etiology

Ulnar nerve pathology most commonly arises at the elbow but can also occur at the wrist. In fact, ulnar neuropathy at the elbow is the second most common compression neuropathy affecting the upper extremities, after carpel tunnel syndrome. At the elbow, ulnar nerve compression most often occurs in the ulnar groove or the cubital tunnel. In the ulnar groove, the nerve is quite superficial and unprotected, making it susceptible to external compression. Prolonged elbow pressure, blunt trauma, or fracture can also cause ulnar nerve palsies either acutely or in a delayed fashion. In the cubital tunnel, the ulnar nerve is prone to compression and stretching, particularly with elbow flexion. Although less frequent, another site of ulnar compression is at the medial intermuscular septum in the upper arm. This may be seen in patients following an anterior transposition of the ulnar nerve or with congenitally unstable ulnar nerves that slide over the medial epicondyle of the humerus.

Less commonly, injury to the ulnar nerve can also occur at the wrist, typically in or slightly distal to Guyon’s canal. Ulnar neuropathies of the wrist are classified as one of three types. A type I ulnar nerve lesion is just proximal to or within the Guyon canal, involving the deep and superficial branches, causing mixed motor and sensory deficits. A type II lesion involves the deep branch, which causes a pure motor deficit. A type III lesion is limited to the superficial branch, causing purely sensory deficits to the palmar aspect of the medial half of the fourth digit and the fifth digit.

Clinical Presentation

Ulnar nerve injuries are broadly categorized as high or low pathologies. For high injuries, which occur at the level of the elbow, numbness or paresthesia over the palmar aspect of the fourth and fifth digits may be the earliest manifestation. Symptoms usually begin intermittently and flare at night if the patient sleeps with their elbow flexed. Medial elbow pain may be present, along with referred pain along the medial forearm. In an elbow ulnar neuropathy, motor symptoms are less common than sensory symptoms. However, these motor symptoms can include weakness of intrinsic hand muscles, claw hand deformity, and decreased grip strength.

In contrast, for low injuries, which occur at the level of the wrist, motor symptoms are more common, because the injury is distal to the motor branch takeoffs of the ulnar nerve. Patients may present with hand weakness and muscle atrophy, loss of dexterity, and variable sensory involvement. The extent of clawing of digits four and five can be worse with lesions at the wrist than at the elbow due to sparing of the flexor digitorum profundus and weakness of the third and fourth lumbricals, resulting in greater muscle imbalance.

Diagnosis