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Intrathecal (intra- + theca, "within a sheath") is an adjective that refers to something occurring in or introduced into the anatomic space or potential space inside a sheath, most commonly the arachnoid membrane of the brain or spinal cord (under which is the subarachnoid space). For example, intrathecal immunoglobulin production is production of antibodies in the spinal cord, and an intrathecal injection (often simply called "an intrathecal") is a route of administration for drugs via an injection into the spinal canal, more specifically into the subarachnoid space so that it reaches the CSF and is useful in spinal anaesthesia, chemotherapy, or pain management applications. This route is also used to introduce drugs that fight certain infections, particularly post-neurosurgical. The drug needs to be given this way to avoid the blood brain barrier. The same drug given orally must enter the blood stream and may not be able to pass out and into the brain. Drugs given intrathecally often have to be made up specially by a pharmacist or technician because they cannot contain any preservative or other potentially harmful inactive ingredients that are sometimes found in standard injectable drug preparations.
Ziconotide itself has been limited to a specific subset of pumps, due to its affinity to metals. However, the compounds created are not introduced to the intrathecal space.
Often reserved for spastic cerebral palsy, intrathecally-administered baclofen is done through an intrathecal pump implanted just below the skin of the stomach, (or behind the chest wall, depending on the surgeon implanting the device, and patient preferences), with a tube (called the 'catheter') connected directly to the base of the spine, where it bathes the spinal cord using a dose about one thousand times smaller than that required by orally-administered baclofen. Intrathecal baclofen also carries none of the side effects, such as sleepiness, that typically occur with oral baclofen. However, intrathecal baclofen pumps carry serious clinical risks, such as infection or a possibly fatal sudden malfunction, that oral baclofen does not.
A tremendous amount of care is taken to ensure the optimal location of the pump and catheter, based upon medical considerations and patient requirements.