The interscalenic block prolonged by a catheter allows an optimal analgesia for important surgery of the shoulder. Its realization is easy but exposes to potentially serious complications. We report a case of spinal anaesthesia due to the accidental catheterization of the medullar canal at the time of an interscalenic block. An examination by tomodensitometry showed images of myelography explained by the injection of non ionic contrast media agent in the catheter at the time of X-ray control. The regression of neurological signs was fast and without after-effect. This observation recalls that in spite of all the safety measures, the realization of locoregional anaesthesia can be burdened with accidents. The proscription of a major sedation and the use of short needles are elementary rules to realize the interscalenic block. The improvement of puncture techniques, ultra-sound location or use of stimulative catheters can decrease the risk of this kind of accident.