Besides local anesthetics, what other agents are administered in the subarachnoid space?

Besides local anesthetics, what other agents are administered in the subarachnoid space?

Opioids are another class of agents that may be admin-istered into the subarachnoid space. Intrathecal opioids work via opiate receptors within the second and third laminae of the substantia gelatinosa in the dorsal horn of the spinal cord to produce intense visceral analgesia without sympathetic nervous system denervation, skeletal muscle weakness, or loss of proprioception. Commonly used intrathecal opioids are the lipophilic agents such as fentanyl and sufentanil, and the hydrophilic agents such as morphine. The lipophilic agents have a much more local-ized effect, a rapid onset of action, and a duration of about 2–8 hours. The hydrophilic agent, morphine, has a greater spread of action and can last anywhere from 6 to 24 hours. Intrathecal opioids are also used as adjuncts to local anesthetics during spinal anesthesia. They may prolong the sensory blockade of the local anesthetic with-out increasing the postoperative duration of the motor blockade or time to voiding. Side-effects associated with the use of intrathecal opioids include nausea and vomiting, urinary retention, pruritus, and respiratory depression. Opioid antagonists or agonist/antagonist agents are useful in treating these side-effects.

Other types of agents that have been administered intrathecally are the α₂-agonist clonidine and the centrally acting muscle relaxant baclofen. Clonidine administered into the subarachnoid or epidural space produces intense analgesia, presumably by activating α₂-receptors in the substantia gelatinosa of the spinal cord. Unlike intrathecal opioids, clonidine does not produce nausea and vomiting, urinary retention, pruritus, or respiratory depression. The combination of local anesthetics and clonidine during a neuraxial blockade produces a longer duration of sensory and motor blockade compared with local anesthetics alone. This combination produces a greater drop in dias-tolic blood pressure, necessitating preloading these patients with intravenous fluids.

Baclofen is a gamma-aminobutyric acid (GABA) analog used to treat spasticity resulting from spinal cord disease such as multiple sclerosis or spinal cord injury. Patients who have not responded effectively to oral administration of baclofen may benefit from intrathecal administration. Baclofen has been shown to have spinal analgesic effects without increased muscle tone, both in animal models and in cancer patients who are resistant or tolerant to opioids. However, the use of baclofen is limited by its side-effects, which include skeletal muscle weakness, sedation, and confusion. Toxicity to baclofen results in coma, respiratory depression, and seizures.