*2.5.6.2 Guyon's duct syndrome (Ulnar tunnel syndrome)*

**Anatomy:** After the ulnar nerve passes through the anterior part of the forearm, it comes to the wrist through the Guyon canal, a fibro-osseous tunnel located between the os pisiforme and the anchor of the os hamatum (**Figure 17**) [7, 8, 54]. The roof of the Guyon canal consists of the palmar fascia and the palmaris brevis muscle. There are pisiform and hamate bones around this canal. As the ulnar nerve passes through the Guyon canal, it divides into superficial and deep branches. After the superficial part of the nerve branches into the palmaris brevis muscle, it gives the sensory branches innervating ulnar side of the palm of the hand and all surface of the 4th finger and 5th finger. Deep part of the nerve gives branches to the hypotenar muscles. Subsequently, the deep part gives all the interosseous and branches innervating the 3rd and 4th lumbrical muscles. It ends by giving the terminal branches to the adductor pollicis and flexor pollicis brevis [16, 55].

*Upper Extremity Entrapment Neuropathy DOI: http://dx.doi.org/10.5772/intechopen.98279*

**Figure 17.** *The schematic drawing of the Guyon canal and ulnar nerve course inside the Guyon canal.*

**Description:** It is characterized by compression of the ulnar nerve in the Guyon canal [1].

**Causes:** Using of tools, bicycle, handlebars (cyclist's palsy), crutches, using wheelchairs and work machines, osteophyte, arthritis, synovitis, ganglion, fibrous bands, subluxation of the ulnar nerve over the medial epicondyle, the presence of the Struthers arch and anconeus internus muscle, presence of os hamuli proprium, presence of an accessory abductor digiti minimi muscle and accessory or reversed palmaris longus muscle, hypertrophic flexor carpi ulnaris muscle are factors that can cause compression [1, 7, 8, 35, 55].

**Clinical features:** Sensory loss occurs on the palmar-ulnar side of the little finger and ring finger. Weakness and atrophy can be seen in the intrinsic muscles of the hand innervated by the ulnar nerve. Disruption of the balance between the intrinsic and extrinsic muscles of the hand causes the physiological arcs of the hand to collapse and classic claw hand deformity occurs. Froment and Wartenberg findings are positive [1, 8, 35, 55].

*Demyelination Disorders*
