While helping a trainee with placement of an epidural catheter, you are able to aspirate clear fluid consistent with CSF. Which space is the catheter in?
a) Epidural space
b) Subdural space
c) Subarachnoid space
d) Subpial space
Answer c. The subarachnoid space is filled with CSF and may be inadvertently entered during epidural catheter placement. The epidural space is a potential space filled with fat, veins, and spinal nerve roots and should not have any fluid in it. The subdural space is also a potential space. Neuraxial blocks which enter the subdural space can have a less reliable distribution of analgesia. Additionally, the small volume between the closely approximated dura mater and arachnoid mater can result in a larger spread of local anesthetic than would be expected during an epidural injection. There is no anatomic location named the subpial space; the pia mater is adherent to the underlying neural tissue.