neurologic disorders
How To Do Lumbar Puncture
Relevant Anatomy
The desired insertion point of the needle is the L3-L4 or L4-L5 interspace; thus, the needle is inserted below the level of the spinal cord. The spinous process of L4 lies along an imaginary line between the top of the posterior superior iliac crests. In amidline insertion(most common approach), the spinal needle first traverses the supraspinous and interspinous ligaments between the vertebral spinous processes, before reaching the ligamentum flavum. A lateral insertion (eg, 1 cm both lateral and caudad to the midline skin insertion point) bypasses the midline ligaments and may facilitate lumbar puncture in older patients whose ligaments are calcified and sclerotic. The ligamentum flavum is thick connective tissue, and a distinct pop may (or may not) be palpable when the needle traverses it. The needle then passes through epidural space (fat tissue and internal vertebral venous plexus) and then may palpably pop—again—through the dura mater (and simultaneously through the adjacent subarachnoid membrane) to enter the subarachnoid space (CSF space). Note that in many cases, no "pop" can be appreciated. In children or infants, whose fibrous tissue offers little resistance to penetration, a pop may not be palpable as the needle passes the ligamentum flavum or dura. 