Lipoma of Guyon’s canal causing ulnar neuropathy

Lipoma is a benign soft tissue tumor which rarely causes neuropathy. In closed compartments such as Guyon’s canal, even small volume loss can lead to compression of nerve. Hence in such areas, even innocuous tumors such as lipomas can cause neuropathy and warrant surgery. We present one such case of ulnar neuropathy caused by lipoma of Guyon’s canal.


Introduction
Lipoma is a benign pliable tumor composed of mature adipocytes. It is one of the commonest benign tumors, but rarely causes neuropathy. Very few cases of ulnar neuropathy due to lipoma in Guyon's canal have been reported in literature.

Case report
A 36-year-old female presented with burning pain and diminished sensations along the little finger and ulnar aspect of the ring finger of right hand for 6 months. On examination, there was a swelling on the hypothenar eminence and fullness of medial aspect of wrist. The swelling was soft but tense. There was atrophy of muscles on hypothenar aspect of hand. The power of the little and ring fingers was diminished.
Conduction velocity studies of the right ulnar nerve showed prolonged distal motor latency from the wrist to the abductor digiti minimi of 8 ms (normal \4.0 ms), diminished motor conduction velocity of the ulnar nerve in the elbow-to-wrist segment of 42 m/s (normal 45-75 m/s), and prolonged distal sensory latency of 4.2 ms (normal \3.5 ms). Conduction velocity studies of the left ulnar nerve were normal.
Ultrasound of the swelling showed a well-defined echogenic mass in the hypothenar eminence. Subsequent magnetic resonance imaging (MRI) showed a well-defined lobulated mass hyperintense on both T1W and T2W images and suppression of signal on STIR images s/o fat (lipoma) (Fig. 1a-c). The mass was lying within the lateral aspect of abductor digiti minimi muscle. Part of it was insinuating between flexor tendons of the hand and metacarpals and another part was insinuating into zones 1 and 2 of Guyon's canal (ulnar tunnel). The mass also extended distally for a considerable distance along the deep branch of the ulnar nerve. The ulnar nerve was stretched over the mass in zone 2. The carpal tunnel was displaced radially (Fig. 2a, b).
The patient was subsequently operated and the mass was removed. Histopathology showed mature adipose tissue consistent with lipoma. Postoperative course was uneventful with progressive resolution of symptoms and good functional recovery.
The patient provided consent for the publication of this case report.

Discussion
Guyon's canal (ulnar tunnel) is a fibroosseous tunnel along the anteromedial portion of the wrist that contains the ulnar nerve and artery. It extends from the palmar carpal ligament at the proximal edge of the pisiform bone to the origin of the hypothenar muscles at the level of hamulus [1]. The importance of the tunnel lies in the fact that it is a closed compartment and any compromise in its volume can lead to ulnar nerve compression causing Guyon's canal syndrome.
Guyon's canal consists of three zones. Zone 1 is the portion of the tunnel proximal to the bifurcation of the  Lipoma extending distally into the wrist along the deep branch of ulnar nerve ulnar nerve. Zone 2 encompasses the deep motor branch of the nerve which is bounded superficially by the tendinous arcade and deeply by the pisohamate ligament. Zone 3 surrounds the superficial branch consisting mainly of sensory fibres [2]. Clinical symptoms correlate with the zone in which ulnar nerve compression occurs: combined motor and sensory deficits occur in zone 1 lesion, pure motor deficits in zone 2 lesions, and isolated sensory deficits in zone 3 lesions [3]. Our patient had both sensory and motor ulnar neuropathies as there was compression in zone 1 and also zone 2.
Lipoma is a soft tumor and does not normally compress surrounding structures, but even it can cause compression when occurring in a closed compartment such as ulnar tunnel. Timely surgical removal can be curative in such cases.