[Report of a rare variation in ulnar nerve and its importance in surgery of carpal tunnel syndrome]

To report a variation in ulnar nerve and its importance in carpal tunnel syndrome surgery. The case is a patient that due to pain in carpal region and parasthesia in the second and third digits of both hands had referred. In physical examination there were phalan test and tinel sign. Electromyography [EMG] and Nerve Conducting [NC] confirmed the carpal tunnel syndrome and the surgery was conducted on the left hand. During surgery an aberrant branch found on retinaculum flexore that separated from ulnar nerve in Gugon's canal. The branch crossed with retinaculum flexor and then tended toward thenar muscles Forming a neuroma. Regarding the crossing of the aberrant branch with retinaculum flexor, it should be distinguished during releasing of retinaculum flexor in carpal tunnel syndrome surgery. Carelessness about this aberrant branch and its cutting leads to impairment of thumb movement