Cubital tunnel syndrome is a type of ulnar nerve compression neuropathy due to pathological compression of the ulnar nerve along its course within the cubital tunnel.
The compression of the ulnar nerve at the elbow is the second most common peripheral neuropathy of the upper extremity 1,3.
Ulnar nerve compression can result in altered sensation in the little and ring fingers. In many patients, the sensory loss is often the first symptom to be reported. As the condition progresses, there may be hand clumsiness, as the ulnar nerve is the principal motor supply to the intrinsic muscles of the hand. In well‐established cases, there may be marked wasting of the small muscles of the hand and the ulnar‐sided muscles of the forearm 4.
The ulnar nerve is vulnerable to stretching and compression injury as it crosses the elbow joint, which undergoes a large arc of flexion during normal range of motion. The cubital tunnel represents one of several small passages through which the ulnar nerve passes near the elbow, and is considered the most common specific site of injury 1.
During normal elbow flexion, the ulnar nerve experiences tension and axial compression due to increased pressure within the cubital tunnel, up to twenty-fold 1. Any local structural abnormality may exacerbate the mechanical forces on the nerve, which may result in neuropathy.
Other potential points of compression near the elbow include the arcade of Struthers, against the medial humeral epicondyle just proximal to the cubital tunnel, and between the humeral and ulnar heads of the flexor carpi ulnaris distal to the cubital tunnel.
Specific causes of nerve compression at the cubital tunnel include:
- subluxation/dislocation of the ulnar nerve from congenital laxity in fibrous tissue
- extrinsic compression
- humeral fracture with loose bodies or callus formation
- osteophytic bony spur arising from the epicondyle or olecranon
- compression from an accessory muscle, namely anconeus epitrochlearis muscle
- soft-tissue mass/tumor
- synovitis secondary to rheumatoid arthritis
- infective process (e.g. tuberculosis)
- thickened cubital tunnel retinaculum (or arcuate ligament) of the flexor carpi ulnaris muscle
Exact imaging features may vary dependent on the underlying cause. The thickening of the ulnar nerve can be a commonly observed feature.
Osseous spurring within the ulnar groove can be seen in patients with chronic nerve irritation due to overuse or posteromedial elbow impingement.
Ulnar nerve thickening and edematous changes are suggestive features.
Some authors have suggested a cut-off value for the cross-sectional area of the asymptomatic ulnar nerve of 9 mm2, derived as the upper limit of the 95% confidence interval 9.
Other suggestive features according to additional studies include
- a ratio of 1.5:1, comparing the ulnar nerve area at the level of the cubital tunnel with that proximal to the cubital tunnel 9
- 8.3 mm2 cross-sectional area of the ulnar nerve at the epicondyle level 9
The ulnar nerve in patients with cubital tunnel syndrome is usually hypoechoic on ultrasound due to neural edema
The following ulnar nerve changes within the cubital tunnel are suggestive of ulnar neuropathy:
- ulnar nerve thickening
- keep in mind that cross-sectional area of ulnar nerve varies according to the degree of elbow flexion; thus, comparison (e.g. to contralateral elbow) must be done carefully
- ulnar nerve T2 hyperintensity
- independently, this is non-specific and seen in up to 60% of asymptomatic elbows
- the degree of hyperintensity may be relatively higher in clinically-significant ulnar neuropathy 7
- edema-like signal changes or atrophy of the flexor carpi ulnaris and flexor digitorum profundus muscles may be seen secondary to ulnar neuropathy
Accurate assessment for ulnar neuropathy on MRI can be challenging, as the primary imaging features (nerve thickening, increased T2 intensity) may be present in asymptomatic cases.
Ulnar neuropathy near the elbow may result from compression at three other sites, which may require a different type of treatment:
- arcade of Struthers - a variable fascial thickening along the medial distal arm overlying the ulnar nerve
- posterior to medial humeral epicondyle (just proximal to the cubital tunnel)
- between the ulnar and humeral heads of flexor carpi ulnaris (just distal to the cubital tunnel)
Alternatively, the ulnar nerve may be injured at the elbow from repetitive ulnar nerve subluxation/dislocation during resisted elbow extension, e.g. bench press or pitching 1. This may present with a medial elbow "snapping" sensation (although the snapping may actually represent associated myotendinous subluxation rather than nerve motion). The resulting friction neuropathy may cause similar presentation as cubital tunnel syndrome.
- dynamic ultrasound imaging may demonstrate ulnar nerve subluxation/dislocation across the medial humeral epicondyle in real-time
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