Ulnar Nerve Entrapment

Etiology

Ulnar nerve entrapment occurs most commonly in the cubital tunnel or the ulnar tunnel, i.e., Guyon's canal. At the cubital tunnel, compression between Osborne's ligament, defined as the fascia connecting the two heads of the FCU superficially and the MCL of the elbow deep, can lead to symptoms of compressive neuropathy in the forearm and hand.[3]

In the ulnar tunnel or Guyon's canal, the zone of compression determines the physical presentation. Compression in zone 1 at the proximal end of Guyon's canal will result in a mixed motor and sensory deficit and is most likely caused by ganglia or hook of the hamate fractures. Compression in zone 2 after the bifurcation and surrounding the deep motor branch will cause an isolated motor deficit of the ulnar nerve, and as in the case of zone 1 pathology, is also most likely caused by ganglia or hook of the hamate fractures. Compression in zone 3 surrounding the superficial sensory branch of the ulnar nerve will result in an isolated sensory deficit in the distribution of the ulnar nerve in the hand and is most likely caused by an ulnar artery thrombosis or aneurysm.[4]

Epidemiology

A general population study in Italy revealed that men are affected at twice the rate of women for compressive neuropathies at the elbow.[5]

History and Physical

Symptoms of ulnar nerve compression have an insidious onset that typically begins with numbness and paresthesias over the small finger, ulnar half of the ring finger, and hypothenar eminence. Weakness and atrophy of the hand intrinsics may follow if compression occurs proximal to zone 3 of Guyon's canal. Wasting and atrophy of the hand often follows, and leads to clumsiness and decreased pinch strength exhibited by Froment's test positivity. Froment's test is a component of the physical examination where a patient is asked to hold a piece of paper between the thumb and radial side of the index finger to isolate adductor pollicis, which as noted above is the only muscle of the thenar eminence that is solely innervated by the ulnar nerve. Flexion of the thumb at the IP joint to utilize flexor pollicis longus (FPL) rather than the adductor pollicis indicates dysfunction of the ulnar nerve and constitutes a positive Froment's sign.[6]

Similarly, Tinel's sign at the cubital and ulnar tunnels may reproduce symptoms of paresthesias and numbness, indicating a likely compressive neuropathy at that location.

Wartenberg's sign is a test for ulnar nerve motor weakness where a patient is asked to hold his or her fingers fully adducted with the metacarpophalangeal joint, proximal interphalangeal, and distal interphalangeal joints (i.e., MCP, PIP, and DIPs) in full extension. If in so doing, it is found that the small finger drifts away from the others into abduction, this is known as a positive Wartenberg sign. This finding represents a deficiency of the palmar interossei, which are innervated by the ulnar nerve and function to adduct the fingers. An abducted drift of the little finger is caused by unopposed action from the extensor digit minimi (EDM) and the extensor digitorum communis (EDC) muscles to the small finger, which are innervated by the posterior interosseus nerve (PIN).[7]

As noted above, it is critical to understand the different areas and presentations of ulnar nerve dysfunction, and a good physical exam must differentiate between the various sites of compression. For example, a patient who has an isolated motor deficit with no sensory deficit raises suspicion of compression within zone 2 of Guyon's canal, while an isolated sensory deficit may be indicative of a zone 3 compression, or alternatively an early compressive neuropathy more proximally. Reproduction of symptoms with flexion of the elbow raises a strong suspicion of compression at the level of the elbow in the cubital tunnel. It is important to inquire if the patient notices a "snapping" or "popping" in the elbow, which indicates nerve mobility within the cubital tunnel and indicates ulnar nerve transposition may be appropriate management.

Lastly, the examiner must consider alternative diagnoses, including C8 radiculopathy. The sensory deficits associated with C8 radiculopathy and peripheral nerve compressive neuropathy are similar and must be distinguished. By understanding the anatomy, differentiation of these two mimicking clinical pictures becomes much simpler. The medial forearm receives its sensory innervation from the medial antebrachial cutaneous nerve and rises from the medial cord of the brachial plexus. Therefore, neurological changes in the medial forearm rather than isolation to the hand are consistent with C8 radiculopathy rather than an ulnar nerve compression more distally at the elbow. Muscles of the thenar eminence, including abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis (OP), as well as the radial two lumbricals, are innervated by the C8 nerve root through the median nerve, and would be weak in C8 radiculopathy but not in a peripheral ulnar nerve compressive neuropathy.[8]

Evaluation

X-rays of the affected extremity at the elbow and wrist should be obtained to rule out any osseous deformity that may cause nerve entrapment, as well as cervical spine radiographs that may reveal sources of radiculopathy or first rib involvement. Finally, a chest x-ray should be obtained to rule out compression of the medial chord by an apical lung or Pancoast tumor, particularly in a patient with a positive history for smoking.

X-rays of the elbow may reveal congenital abnormalities, post-traumatic changes, or valgus alignment of the elbow that can result in neuropraxia at the elbow. X-rays of the wrist should include carpal tunnel and oblique views to assess for hamate fractures, which is a common cause of ulnar nerve compression symptoms in the hand. If there is high clinical suspicion for a hook of the hamate fracture, a CT scan can better assess for pathology, as x-ray has a low sensitivity for detecting such fractures.

MRI is an imaging modality that can be further used to confirm clinical suspicions of cervical radiculopathy in the neck, or of a ganglion or other space-occupying lesions in the wrist.

Ultrasound of the nerve at the elbow and wrist can be used to measure the size of the ulnar nerve compared to controls, as well as to identify a thrombosis of the ulnar artery that can lead to ulnar nerve symptoms originating in Guyon's canal.[9]

Electromyography is also commonly used in the diagnosis of compression neuropathy with muscle denervation. Compressive neuropathies result in increased distal latency and decreased conduction velocity. Thus in patients with cubital tunnel syndrome, one is likely to identify a slowing of conduction in the ulnar nerve segment crossing the elbow.[9][10]

Treatment / Management

Initial treatment for patients with ulnar neuropathies that lack atrophy should include nonsurgical modalities such as anti-inflammatory medications, external padding of the nerve, and night splinting with the elbow in 45 degrees of flexion, or of the wrist in neutral position depending on the etiology of the compression. Additionally, occupational therapy that educates on appropriate positioning of the extremity may help to alleviate symptoms.[9][11]

Cubital Tunnel Syndrome Surgical Management

For patients with significant atrophy and weakness, or those who have failed, conservative nonsurgical treatment should consider surgical management. Surgical release of Osborne's ligament, which spans the heads of FCU, can release pressure on the nerve at the elbow. The release can be extended proximally through the medial intermuscular septum and distally through the flexor-pronator aponeurosis to allow for free movement of the nerve. Ulnar nerve transposition may also serve to relieve the traction and compression of the ulnar nerve when placing the nerve superficial to the flexor-pronator origin, which positions the nerve anterior to the axis of the elbow motion.[12][13]

Alternative treatments of cubital tunnel syndrome may include a medial epicondylectomy, which has a lower incidence of ulnar nerve injury compared to transposition but has the disadvantage of an increased risk of elbow instability and flexor-pronator weakness.[14][15][16]

Ulnar Tunnel Syndrome Surgical Management

Surgical management of compression at Guyon's canal should address the specific pathology causing the nerve compression. Ganglia and other local masses should be excised when possible and results in favorable patient outcomes. Typical release of Guyon's canal should include all four compartments spanning from proximal to the wrist flexion crease and extending to the mid-palmar line. It is important, however, to note that an isolated loss of motor function indicates a zone 2 compression, and therefore should only be managed with release of the pisohamate ligament and exploration for a deep ganglion cyst.[9][17]

Differential Diagnosis

C8-T1 radiculopathies are relatively rare in comparison to peripheral ulnar nerve compression syndromes, and given that it has a similar presentation to peripheral ulnar nerve compression, the condition is frequently overlooked by diagnosticians.[9][18] Ultimately, a thorough understanding of the anatomy of the ulnar nerve is paramount to arriving at an accurate diagnosis. Therefore, a thorough physical examination that includes both a motor and sensory examination, along with appropriate diagnostic studies such as electromyography and MRI imaging, is critical to achieving the proper diagnosis.

The ulnar nerve provides innervation to much of the hand, including the muscles of the hypothenar eminence, the palmar and dorsal interossei, the third and fourth lumbricals, and a portion of the thenar eminence including adductor pollicis and the deep head of the flexor pollicis brevis (FPB). The median nerve alternatively enters the hand through the carpal tunnel. It innervates the remainder of the thenar eminence, including the abductor pollicis, opponens pollicis, and the remainder of the flexor pollicis brevis (FPB), as well as the first and second lumbricals. In a purely isolated ulnar nerve compressive neuropathy, these five muscles would theoretically remain unaffected, while C8-T1 radiculopathy often does manifest in weakness of these muscles via the median nerve.[8]

In most patients, the ulnar nerve provides sensation to the ulnar half of the fourth finger, the small finger, and the hypothenar region of the hand on both the palmar and dorsal side. However, the ulnar nerve does not provide sensation to the medial forearm. Instead, sensation to the medial forearm is provided by the medial antebrachial cutaneous nerve, which arises from the medial cord of the brachial plexus. Therefore sensory deficits in the medial forearm should prompt evaluation for radiculopathy or Pancoast tumors that may compress the brachial plexus. Intact sensation to the medial forearm should alternatively prompt a search for a more peripheral ulnar nerve compressive syndrome.[8]

Prognosis

A review of 50 published reports of ulnar nerve compression suggests that patients with a minimal degree of compression can have an excellent outcome 50% of the time with non-operative techniques such as anti-inflammatories, external padding, and positional modification. Surgical intervention results in nearly 100% relief of symptoms, provided the location of nerve compression is properly identified and released with anterior transposition of the nerve where appropriate.[19][9]

Complications

Symptom recurrence after surgery is typically the result of either incomplete decompression of the nerve or perineural scarring.[20] The surgical approach to the cubital tunnel also places several structures at risk. Damage to the medial antebrachial cutaneous nerve can result in hyperesthesia and hyperalgesia in the medial forearm. Reports of damage to the medial collateral ligament of the elbow is also a known complication of transposition, which can lead to chronic elbow instability.[9] Surgeons should similarly take great care during surgical release of the ulnar tunnel. It is possible to damage the ulnar artery, which lies anterior and radial to the ulnar nerve, or the carpal ligaments, leading to instability in the wrist.