Pathology
The cubital tunnel is formed by the two heads of the flexor carpi ulnaris muscle. One head originates at the medial epicondyle of the humerus, while the other is derived from the olecranon process. The two heads join to form the belly of the flexor carpi ulnaris. The ulnar nerve courses between the two heads in a channel called the cubital tunnel. Cubital tunnel syndrome occurs when the nerve is compressed between the two heads of the muscle.

Cubital tunnel syndrome develops as either an acute or chronic injury. Though not common, acute cubital tunnel syndrome may occur from a blow directly to the posterior elbow. As a result, scar tissue could develop in the tunnel, creating compression. It is also possible that the blow could damage bones or ligaments in the region that press on the nerve. The blow might also sensitize the nerve to further pressure.

In chronic compression injuries, cubital tunnel syndrome usually develops from tightness in the flexor carpi ulnaris muscle or prolonged periods with the elbow in flexion. During elbow flexion the two heads of the flexor carpi ulnaris are pulled apart as the olecranon process moves slightly away from the humerus. The tunnel becomes narrower and consequently increases pressure on the ulnar nerve. It is common for people to experience the symptoms of ulnar nerve compression at night by holding the elbow in flexion for long periods during sleep.

The mechanism of damage in Guyon’s canal syndrome is somewhat different. Guyon's canal is located in the wrist, adjacent to the carpal tunnel. Like the carpal tunnel, Guyon's canal is bordered by the transverse carpal ligament (also called the flexor retinaculum). There is a division in the ligament near the ulnar side of the hand that creates Guyon's canal (see Figure 2). The ulnar nerve and artery pass through Guyon's canal. Unlike the carpal tunnel, there are no tendons that travel through Guyon's canal. Therefore pressure on the nerve from tenosynovitis does not occur in Guyon’s canal syndrome as it does in carpal tunnel syndrome.            

Because there are no tendons in the canal to press on the nerve, pathological compression in Guyon’s canal syndrome occurs from extrinsic factors. That means nerve compression occurs from forces outside the canal as opposed to pressure from within. A condition like carpal tunnel syndrome, on the other hand, involves intrinsic pressure because it comes from within the tunnel due to tendon swelling.