Intrathecal

The dura mater is the outermost meninge and the arachnoid mater is the intermost meninge. Intrathecal injections place formulations in the subarachnoid space, e.g., underneath the arachnoid mater. This space is filled with the cerebral spinal fluid that circulates around the spinal cord and the brain. Intrathecal injections allow dosages that may be about one-tenth those given by epidural administration. However, intrathecal administration carries a greater risk and consequence of bacterial contamination than epidural administration because the cerebral spinal fluid is a good medium for bacteria growth. These injections may also cause "spinal headaches." Spinal headaches occur when the needle puncture does not seal off and cerebral spinal fluid continually leaks out into the epidural space.

Intrathecal administration is often used to give single injections of narcotics for postoperative pain management. Implantable infusion pumps are used to chronically administer medications to the intrathecal space. These administration techniques allow for very low doses of medications to be given in a controlled manner and reduces the incidence of side effects. The reservoirs in these pumps typically hold between 18 - 50 ml of solution; therefore, the solution concentrations will be high. Formulating high concentration solutions may entail problems with solubility, pH, buffering, etc. Also, these solutions must be preservative free to avoid nerve damage.