, Ali T. Ghouse2 and Raghav Govindarajan3
(1)
Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada
(2)
McMaster University Department of Medicine, Hamilton, ON, Canada
(3)
Department of Neurology, University of Missouri, Columbia, MO, USA
Ulnar Nerve Anatomy
The ulnar nerve is derived from the C8 and T1 roots with a probable minor component from C7. The ulnar fibers traverse through the lower trunk and the medial cord of the brachial plexus. The terminal extension of the medial cord is the destined ulnar nerve see Fig. 14.1.
In the upper arm, the ulnar nerve descends medially without giving off any sensory or motor branches. It pierces the intramuscular septum in the mid arm and passes through the arcade of Struthers (a band of dense fibrous tissue that extends from the medial head of the triceps) and the internal brachial ligament. The ulnar nerve travels medially and distally toward the elbow. At the elbow, it enters the ulnar groove between the medial epicondyle (ME) and the olecranon process. It travels under the arch formed by the two heads of the flexor carpi ulnaris (FCU) muscle. This is the cubital tunnel, also known as the humeral-ulnar aponeurosis.
Motor branches are given off to the FCU and the medial division of the flexor digitorum profundus for the fourth and the fifth digits. In the forearm, the nerve descends through the medial forearm up to the wrist. At about 5–8 cm proximal to the wrist, the dorsal ulnar cutaneous sensory branch is given off, and this supplies the dorsal medial hand and the dorsal surfaces of the fourth and the fifth digits. The palmar cutaneous sensory branch is given off at the level of the ulnar styloid, and it supplies sensation to the proximal medial palm.
At the medial wrist, the nerve enters Guyon’s canal and supplies sensation to the palmar aspect of the fourth and fifth digits, and muscular and motor branches are supplied to the hypothenar muscles. Also, the nerve supplies the palmar and the dorsal interossei, the third and the fourth lumbricals, and two muscles in the thenar eminence, that is, the adductor pollicis and the deep head of the flexor pollicis brevis.
Figure 14.1
Ulnar nerve
Ulnar Neuropathy at the Elbow
Anatomy
SEGMENT 1. The ulnar nerve courses between the posterior compartment and the entrance to the condylar groove. It passes beneath the medial head of the triceps and the intermuscular septum (arcade of Struthers).
SEGMENT 2. Courses in the condylar groove; posterior to the ME; subluxable; compressible.
SEGMENT 3. Courses from the level of the ME and olecranon (position 0) through the fibrous arch of the FCU (fibro-osseous cubital tunnel); related to the medial ligament of the elbow. Flexion narrows the tunnel.
SEGMENT 4. Courses from between the two heads of the FCU, and passes through the aponeurosis separating the FCU from the flexor digitorum profundus and the flexor digitorum superficialis.
Clinical Presentation
The ulnar nerve can be entrapped in the upper arm (arcade of Struthers), at the elbow (cubital tunnel), in the forearm, at the wrist (Guyon’s canal), or in the hand.
Factors Predisposing to Ulnar Nerve Injury
Stretching and tension with elbow at 90° flexion.
Fibrous tissue in the cubital tunnel or condylar groove.
Positional pressure changes: increased intraneural pressure with arm elevated, elbow flexed, and wrist extended.
Subluxation of the ulnar nerve out of the condylar groove to posterior and anterior position relative to the ME; this is related to shallow groove, short ME, blunt ME, bulging of the medial head of the triceps, or trauma to the post-condylar groove.
Occupational factors of repetitive flexion-extension, persistent pronation, or sustained pressure, such as when driving.
Risk in surgical patients, particularly in patients undergoing cardiac surgery.
Predisposing factors in debilitated patients.
Wheelchair users resting their arms on the armrests.
Fractures and dislocations at the elbow.
Space-occupying lesions such as ganglionic cysts, lipomas, or other tumors.
Inflammatory disorders (rheumatoid arthritis [RA], synovitis).
Osteoarthritis and hypertrophic bony changes.
Double crush syndrome.
Vascular compromise caused by surgery or vasculitis.
Electrodiagnostic Testing
The ulnar motor component is steadied by recording from the abductor digiti minimi and stimulating the nerve at the wrist, below the elbow, at the elbow, above the elbow, at the axilla, or at Erb’s point. The ulnar sensory responses are recorded from the fourth and fifth digits.
Abnormalities noted could be reduced sensory nerve action potentials (SNAPs), conduction slowing in the ulnar-innervated fourth and the fifth digital nerves, low amplitude of the ulnar compound muscle action potential (CMAP), prolonged distal latency of the ulnar nerve at the wrist, conduction block, or conduction slowing of the ulnar nerve segment at the elbow or in the forearm.
Other techniques for evaluation include inching studies across the elbow and recording the dorsal ulnar cutaneous and medial antebrachial cutaneous SNAPs, as well as performing needle electromyography (EMG) of the ulnar-innervated muscles in the hand and forearm. For the differential diagnosis, muscles supplied by the C7, C8, and the T1 roots would be studied.
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