Pharmacology of spinal cord injury: Basic mechanism of action and side effects of commonly used drugs

Journal of Neurologic Physical Therapy, Sep 2003 by Scelza, William, Shatzer, Matthew

Over time, the pain from acute trauma subsides and the SCI patient is slowly weaned from the stronger narcotics to milder medications. Tramadol (Ultram) is a common 'stepdown' medication. Tramadol has been shown to have a dual action. It acts partially as an opioid agonist, however to a much lesser degree and also as a monoaminc uptake inhibitor.15 Side effects are milder than for the stronger opioids, however this medication does lower seizure threshold.

Muscle spasms are another source of pain after SCI. When these spasms cause pain, muscle relaxants are used. A positive aspect of these medications is that they may decrease the need for narcotics.16 However, these medications commonly cause sedation and may slow progress in the rehabilitation setting and should be used only when physical modalities are either contraindicated or ineffective. A list of some muscle relaxants derived from the benzodiazepine family are as follows: Carisprodol (Soma), Chlorzoxazone (Paraflex), Cyclobenzaprine (Flexeril), Diazepam (Valium), Metaxalone (Skelaxin), Methocarbamol (Robaxin), Orphenadrine (Norflex).

Nonsteroidal anti-inflammatory drugs (NSAIDs) are another class of medications frequently used to manage musculoskeletal pain. The NSAIDs act to inhibit the enzyme cyclooxygenase, which is responsible for the production of pain mediating substances, prostaglandins, and thromboxanes. These compounds are involved in many functions within the body including the mediation of pain, inflammation, fever, anti-platelet effects, gastric mucosal health, and renal homeostasis. All of these medications share a similar side effect profile. Gastric complications (upset stomach, GI bleed) as well as kidney and liver problems may result from use of these medications. Also, there is some concern that the healing potential of bone may be reduced with the use of NSAIDS.17 Nonsteroidal anti-inflammatory drugs commonly used in the rehabilitation medicine setting are aspirin, ibuprofen (Motrin), naproxen (Naprosyn), indomethacin (Indocin), and ketorolac (Toradol). Recently, the development of a new class of NSAIDs has been developed. These medications are similar to other NSAIDs but act specifically at the cyclooxygenase-2 (COX-2) subtype. This enzyme has been shown to selectively inhibit the pathway that is directed primarily at the prostaglandins that gencrate pain and inflammation and thus have a much better gastrointestinal tolerability.18 Examples of these medications are Celecoxib (Celebrex), Refecoxib (Vioxx), and Meloxicam (Mobic).

The treatment of pain is as much an art as it is a science. Therefore, there is no one 'cocktail' that can help treat every patient's pain. Rather, the clinician and patient work in conjunction to find the appropriate regimen of medications to help treat pain.

Neuropathic Pain

The first line pharmacological agent used to treat neuropathic pain is the anti-epileptic drug (AED), Gabapentin (Neurontin). Gabapentin structurally looks very similar to the GARA neurotransmitter however it does not interact with the GABA receptor. Actually the precise mechanism of Gabapentin is not known.19 Carbamazepine (Tegretol), another AED, has also been efficacious in the treatment of neuropathic pain. Carbamazepine acts to block the conductance of sodium channels.20 Its most significant side effects include bone marrow suppression and an elevation of liver enzymes so these parameters must be monitored periodically. Anti-epileptic drugs, like narcotics and muscle relaxants, also produce sedation that can interfere with rehabilitation.