The term “intrathecal” is derived from the Latin prefix intra- (“within”) and the Greek word theca (“sheath” or “covering”). This medical term describes a method of directly administering substances into the space holding the fluid surrounding the brain and spinal cord. It is a specialized delivery route used to ensure medications reach the central nervous system quickly and effectively, especially when traditional oral or intravenous methods are ineffective.
Defining the Intrathecal Space
The “theca” refers to the meninges, the three protective layers that encase the central nervous system. These layers are the dura mater (the tough outer layer), the arachnoid mater (the middle layer), and the pia mater (the delicate inner layer covering the spinal cord). These membranes function as a physical cushion and protective barrier for the delicate neural tissue.
The specific target for intrathecal delivery is the subarachnoid space, located between the arachnoid and pia mater. This space is filled with Cerebrospinal Fluid (CSF), a clear, colorless fluid that bathes and mechanically cushions the brain and spinal cord. Injecting into the CSF allows therapeutic agents to be introduced directly into the fluid circulating around the neural structures.
Why Intrathecal Delivery is Necessary
The primary reason for using this delivery method is to overcome the Blood-Brain Barrier (BBB). The BBB is a highly selective border of endothelial cells that shields the central nervous system from circulating toxins, pathogens, and chemicals in the bloodstream.
While protective, the BBB severely limits the passage of many therapeutic drugs, especially large-molecule compounds like certain antibodies or chemotherapy agents. Systemic administration results in very little medication reaching the intended target tissue in the brain or spinal cord. Intrathecal delivery bypasses the BBB entirely by placing the drug directly into the CSF.
This direct route allows the medication to achieve high concentrations precisely where needed with minimal systemic exposure. Delivering the drug straight to the CNS receptors significantly reduces the overall dosage required compared to oral or intravenous routes, thereby diminishing the risk of systemic side effects, such as widespread organ toxicity.
Common Medical Applications
Intrathecal drug delivery is employed for a variety of conditions requiring targeted action within the central nervous system.
Regional Anesthesia
One common use is in regional anesthesia, where a single injection, often called a spinal block, provides potent, localized numbness for surgical procedures. This allows for rapid and complete pain relief in a targeted area of the body.
Chronic Pain and Spasticity Management
The method is frequently used for chronic pain management that is severe or refractory to oral treatments. Medications like potent opioids (e.g., morphine) or non-opioid options, including ziconotide, are delivered directly to spinal cord pain receptors for long-term relief. Patients with severe muscle spasticity, often related to cerebral palsy or spinal cord injury, also benefit from intrathecal delivery of muscle relaxants like baclofen.
Oncology
A significant application is in oncology, specifically for treating cancers affecting the central nervous system, such as certain leukemias and lymphomas. Intrathecal chemotherapy ensures that cancer-fighting drugs reach malignant cells within the CSF, a sanctuary site often inaccessible to systemic chemotherapy alone. This localized treatment helps prevent or manage neurological complications of the disease.
The Intrathecal Procedure
Intrathecal procedures utilize two primary methods, depending on the therapeutic goal.
Single Injection (Lumbar Puncture)
The first method is a single, acute injection, commonly known as a lumbar puncture or spinal tap. A thin needle is carefully inserted between the vertebrae in the lower back, typically below the L2 level (at L3/L4 or L4/L5), where the spinal cord has ended and only the CSF-filled thecal sac remains.
This bolus injection is routinely used for administering spinal anesthesia for surgery, collecting CSF for diagnostic testing, or delivering a single dose of chemotherapy. The procedure is quick and performed under strict sterile conditions.
Implanted Pump System
The second method involves continuous, long-term delivery via an implanted intrathecal pump system. This system is typically reserved for chronic conditions like severe, persistent pain or spasticity.
The device, which is usually a round reservoir, is surgically implanted under the skin of the abdomen or flank. A small, flexible catheter is then tunneled under the skin and threaded into the subarachnoid space to continuously infuse the medication directly into the CSF.