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

It is not always necessary to treat Spasticity as sometimes it may actually be of benefit to the patient. Spasticity should only be treated if it interferes with function, has the potential to cause contractures, negatively affect seating in a wheelchair, or cause significant pain. Pharmacological treatment of Spasticity may also unmask underlying weakness in muscles and thus make some functional tasks more difficult.

Spasticity may be exacerbated by other noxious stimuli. Examples of these include urinary tract infections (UTI), bladder stones, bowel impaction, skin breakdown, heterotopic ossification, fractures, DVT, and abdominal pathology. Treatment of these conditions will often allow the spasticity to abate or return to the baseline and thus they are important to recognize. If spasticity abruptly increases and no obvious cause is identified, further investigation by a physician is warranted.

Use of medications in the treatment of spasticity requires careful titration of drugs to achieve the correct balance of tone and side effects. Baclofen is a first line agent for spasticity of spinal origin. It works as a GAHA (gamma-amino-butyric acid, an inhibitory neurotransmitter) agonist and binds prcsynaptically at the GABA^sub b^ receptor.30 Baclofen has been found to decrease flexor spasms, increase range of motion, and decrease tone. Potential side effects of Baclofen that may be problematic during physical therapy are sedation, fatigue, weakness, nausea, dizziness, paresthesias, and hallucinations. One must be careful when discontinuing baclofen as it needs be weaned slowly. If this medication is stopped abruptly, patients have been reported to have seizures, visual disturbances, and hallucinations.31

Benzodiazepines are also used to treat spasticity. Like Baclofen, these medications enhance the affect of the inhibitory neurotransmitter GABA by binding presynaptically at the GABAa receptor.30 The primary medication used in this class is diazepam (Valium). Side effects with this medication that may negatively affect therapy include sedation, hypotension, and depression. The sedative side effects may interfere with the ability of the patient to stay awake and participate in therapies and therefore doses should be started low and advanced slowly. In addition, this class of medication should not be used in a spinal cord injured patient with a concomitant brain injury because they have been Ibund to inhibit new learning and memory recovery.32

The class of medications known as alpha 2-adrenergic agonists has also been used to treat spasticity. Examples of these medications include clonidine (Catapress) and tizanidine (Zanaflex). These agents bind to presynaptic alpha-2 receptors in the dorsal horn of the spinal cord and result in a decrease of polysynaptic reflexes by decreasing the release of excitatory neurotransmitters and facilitation of inhibitory neurotransmitters.33 As with the benzodiazepines, these medications can cause fatigue and hypotension (clonidine is also used as an anti-hypertensive agent). Bradycardia is also a side effect seen with clonidine, and to a lesser extent, tizanidine. Therefore, attention must be paid to vital signs during physical therapy for patients on these medications.