[Position of the spinal cord and the depth of the peridural space as a function of spinal column kyphosis and age].

A dry tap may occur when performing a spinal anaesthetic, especially when there is an extreme kyphosis. The puncture can be at normal level or higher. The reason for this is in our experience, excluding other known possibilities, is that spinal cord and the cauda equina come to lie dorsally against the dura. The cerebrospinal fluid is thereby displaced ventrally. This fact is especially important in anatomical length variations of the medulla spinalis. If all the causes for a dry tap are borne in mind then this can easily be prevented by a simple manoeuvre, which we describe in the text.