Carpal Tunnel Syndrome

The carpal tunnel is defined from the pisiform bone proximally until the level of the hamulus of the hamate bone distally. Squeezing of the median nerve during this pathway may give rise to a carpal tunnel syndrome. The diameter of the median nerve will increase at the P level and the nerve will be flattened at the H level. In addition, on MRI, the signal intensity increases on T2-WI and fasciculation appears in case of progressive CTS. The median nerve is swollen at the level of the pisiform bone and flattened at the level of the hamulus of the hamate bone. The flexor retinaculum forms a slight concave arch over the tunnel. The width of the median nerve is compared distally to the reduced height and an index of more than 3 is considered abnormal. Bowing of the fairly horizontal flexor retinaculum exceeding 3–4 mm is also considered abnormal. Variants of the median nerve are frequent. They should be noticed as well as causal abnormalities to inform the surgeon in case of a planned intervention. Persisting symptoms after CTS surgery may be due to hematoma, seroma, insufficient resection of the flexor retinaculum (proximally and/or distally) or fibrous “repair” of the flexor retinaculum, which can all be demonstrated by MRI.