Another way the word is used is to refer to an intrathecal injection (often simply called "an intrathecal"). This is an injection into the spinal canal, more specifically into the sub-arachnoid 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.
Often reserved for spasticcerebral palsy, intrathecally-administered baclofen is done through an intrathecal pump implanted just below the skin of the stomach with a tube connected directly to the base of the spine, where it bathes the appropriate nerves 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.
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^Qweider M, Gilsbach JM, Rohde V (March 2007). "Inadvertent intrathecal vincristine administration: a neurosurgical emergency. Case report". Journal of Neurosurgery. Spine6 (3): 280–3. doi:10.3171/spi.2007.6.3.280. PMID17355029.