Let us review a little anatomy so that you can understand where to do your lumbar puncture. At birth, the inferior end of the spinal cord is opposite the body of the third lumbar vertebrae (L3). Distal to this point is the cauda equina and its nerve roots. As the child grows, the vertebral column grows much faster than the spinal cord itself, and by adulthood, the spinal cord only reaches the inferior border of the L1 vertebra, or the superior aspect of L2. Distal to this point is the cauda equina. In order to avoid transfixing the spinal cord during LP, the needle is placed distal to L2. This means the needle enters the subarachnoid space at the level of the mobile cauda equina.

Landmarking the interspace is quite easy, as an imaginary line that crosses the lumbar region of the back joining the posterior superior iliac crests will cross the L3-L4 interspace. Thus, one can easily identify the L2-L3(above the line), L3-L4(at the line), or L4-L5(below the line) interspaces, all of which are suitable for LP.

The CSF itself resides in the subarachnoid space between the pia mater and the arachnoid mater. In order to place the needle into the subarachnoid space, the needle passes between two vertebral processes and continues through the interspinal tissues and into the subarachnoid space. The tissues pierced are (in order): skin, subcutaneous tissue, supraspinal ligament, interspinal ligament, ligamentum flavum, dura mater, the arachnoid mater and into the subarachnoid space.

Figure 2. Local spinal anatomy