Analisi della letteratura sulle varianti anatomiche del ramo motorio del nervo mediano diretto all'eminenza tenar e studio elettrofisiologico della mononeuropatia di questo ramo

Introduction. The aims of this study are: 1. to review the literature on anatomical variations of thenar motor branch of the median nerve to hypothesize the pathogenesis of mononeuropathy of this branch; 2. to report a retrospective electrophysiological study of 28 consecutive cases affected by thenar motor neuropathy (TMN) of the median nerve. Analysis of literature. All scientific publications, searched in "PubMed", reporting studies on anatomical variations of course, source and number of motor branch of the median nerve, were reviewed. Regarding the nerve course in respect to the transverse ligament, there is an unanimous agreement since the most frequent course is extralegamentosous. As regards the intraneural topographic distribution of motor fascicles there is no agreement. For some authors, the most frequent origin and orientation of thenar motor fascicles is the center-volar site, for others they are placed at the radial site of the nerve. Regarding the number of motor branches the presence of a single branch that then divides into 3 successive terminal branches, one for each thenar muscle, is the most common. However, many anatomical variants and motor accessory branch are described. Patients. 28 consecutive cases with TMN were diagnosed in an outpatient EMG lab from 1995 to 2008. They were 11 women, 17 men, mean age 48.8 years (range 26-77); 14 agricultural/industrial workers, 3 women employed in a clothes factory, 4 walked with a stick, 2 wheelchair bound, 3 office workers, 1 hand-milker, 1 pizza-maker. They had weakness of thenar muscles of the dominant hand, without sensory symptoms associated in 15 cases to hypotrophy or atrophy of thenar muscles. They underwent neurography of median, ulnar, sensory radial and palmar nerves and EMG of hand and forearm muscles. In 21 cases, distal motor latency (DML) recording from II interosseus and II lumbrical muscles were also obtained and the Boston Questionnaire (BQ) was administered. Electrophysiological results. Motor neurography showed complete denervation of abductor pollicis brevis (APB) muscle in one case and delayed median DML in all, associated in 14 with reduction of compound muscle action potential amplitude. APB EMG showed neurogenic pattern in 18 cases with denervation activity at rest in 10. The sensory conduction velocity of the median nerve (I, III and IV finger-wrist), radial (I finger-wrist) and ulnar (IV and V finger-wrist), the conduction velocity along the palmar branches, DMLs of the ulnar nerve recording from abductor digiti mini, and those of the contralateral median nerve were normal. DMLs for recording from II lumbricalis and II interosseous muscles were also normal in affected and healthy sides. BQ showed a high score only for the "function of the hand" of the affected side. Conclusions. TMN may recognize different pathogenesis. It could be considered a variant of the common CTS with only involvement of thenar motor branch, favoured by anatomical variations of the branch that might arise from the anterior surface of the common trunk in the carpal tunnel rather than from the radial site. However, because the neuropathy preferred males, dominant hand, and especially the manual work, the neuropathy may be due to chronic direct compression of the nerve, in some cases favoured by anatomical abnormalities. Indeed, the branch motor could run above or across the transverse ligament predisposing the nerve to direct trauma.