Revision Aid: Entry #14
Topic #14: Needle Insertion: Lumbar punctures, nerve blocks, epidurals and pnemo-/haemothoraxes:
- In a LUMBAR PUNCTURE, a needle is inserted into the space between the arachnoid mater and pia mater. This is the SUBARACHNOID SPACE, which is filled with cerebrospinal fluid - samples of this need to be taken for a number of reasons, such as testing for infections of the CNS (meningitis, syphilis, polio). Alternatively, fluid may be drained to relieve intracranial pressure (e.g. from hydrocephalus) or medications may be delivered in this manner.
- The needle is inserted into the L3/L4 or L4/L5 space, to avoid spinal cord damage (the cord ends at the lower border of L1.) It must be inserted at an upward angle such that it is parallel to and between the two spinous processes of the lumbar vertebrae on either side of the intervertebral space. An initial 'give' indicates the dura has been pierced; a second will indicate the arachnoid has been pierced, and the needle is deep enough.
- An EPIDURAL may be administered in much the same way, and to the same region, except, as the name suggests, it needs only be injected into the epidural space, outside all three meningeal layers.
- To perform an INTERCOSTAL NERVE BLOCK, the needle should be inserted high in the intercostal space to produce effective anaesthesia. This may be performed to alleviate the pain of broken ribs.
- When inserting a chest drain, for example to drain a haemothorax, the trochar needs to be placed low in the intercostal space, to avoid damage to the neurovascular bundle which lies in the costal groove immediately below the rib.