Ulnar Nerve Entrapment

Ulnar nerve entrapment generally occurs in the cubital tunnel at the level of the elbow or in the ulnar tunnel at the level of the wrist. The exact location of the compression will affect the presentation. To understand the symptoms seen in patients with ulnar nerve entrapment, it is necessary to understand the anatomy of the ulnar nerve.The ulnar nerve is the continuation of the medial cord of the brachial plexus and lies posteromedial to the brachial artery in the anterior compartment of the upper arm. It pierces through the medial intermuscular septum at the arcade of Struthers, which is located approximately 8 cm proximal to the medial epicondyle. Traveling behind the medial epicondyle with the superior ulnar collateral artery, the ulnar nerve enters the forearm through the cubital tunnel, which is defined as the space between the posterior and transverse bands of the medial collateral ligament (deep) and Obsbourne's ligament (superficial), which refers to the extension of the fascia spanning the two heads of the flexor carpi ulnaris (FCU). Compression at the level of the cubital tunnel is most frequently due to compression by Osborne's ligament.In the forearm, the ulnar nerve travels between the flexor carpi ulnaris (FCU) and the flexor digitorum profundus (FDP). The ulnar nerve provides motor innervation to both the FCU and the FDP controlling flexion of the distal interphalangeal joint (DIP) of the ring and small fingers.The ulnar nerve and ulnar artery enter Guyon's canal, or the ulnar tunnel at the wrist, which is bordered by the transverse carpal ligament (deep), volar carpal ligament (superficial), pisiform (ulnar), and hook of the hamate (radial). The ulnar nerve lies ulnarly and posteriorly to the ulnar artery within Guyon's canal. Guyon's canal is further subdivided into 3 zones: zone 1 is proximal to the bifurcation of the ulnar nerve into the deep motor branch and the superficial sensory branch. Zone 2 is distal to zone 1, surrounding only the motor branch. Zone 3 is distal to zone 2, and surrounding only the superficial sensory branch of the ulnar nerve.Therefore, compression or pathology at the cubital tunnel or zone 1 of Guyon's canal results in a motor and sensory deficit of the ulnar nerve. Pathology in zone 2 of Guyon's canal will result in an isolated motor deficit, while dysfunction in zone 3 of Guyon's canal will result in an isolated sensory deficit.The ulnar nerve is crucial for innervating the intrinsic musculature of the hand, which includes the adductor pollicis and the deep head of flexor pollicis brevis in the thenar eminence. Additionally, the ulnar nerve innervates the interossei, the third and fourth lumbricals, the abductor digiti minimi, opponens digiti minimi, and flexor digiti minimi in the hypothenar eminence.The ulnar nerve relays sensory information back to the central nervous system in the hand from the ulnar border of the ring finger, the small finger, and spanning the dorsal and volar aspect of the hypothenar region of the hand.