The influence of muscle relaxers on physiologic processes and exercise

Introduction 

It is important to understand the effects muscle relaxers have on the physiological processes of the body because as therapists it is likely some patients will be using these drugs. Muscle relaxers are commonly used to decrease muscle spasticity or spasms and lessen low back pain from musculoskeletal diseases or injuries . Muscle relaxers are a class of drug that effectively decreases skeletal muscle function which in turn produces a tranquilizing effect. Common types of muscle relaxers include:

• Baclofen (Lioresal, Baclosan)
• Cyclobenzaprine (Flexeril)
• Metaxalone (Skelaxin)
• Carisoprodol (Soma, Carisoma, Sodol)
• Methocarbamol (Marbaxin, Robaxin)
• Chlorzoxazone (Parafon Forte, Remular)
• Orphenadrine (Flexon, Norgesic, Norflex)
• Quinine
• Dantrolene (Dantrium)
• Tizanidine (Zanaflex)
• Botulinum toxin type-A

Botulinum Toxin Type-A (BoNT-A) & Botulinum Toxin

Botulinum toxin type-A (BoNT-A) is one type of muscle relaxer that could be commonly seen within a rehabilitation type setting. Research has shown that BoNT-A injections can produce positive outcomes when it comes to controlling spasticity and muscle tone. When considering the effects of exercise and BoNT-A, children who have cerebral palsy (CP) are a good target population to consider given the spasticity that often accompanies the disease. Research has shown that children who receive BoNT-A injections and strength training programs show both an increase in muscle volume as well as a rise in strength . The timing of the injections in relation to strength training did not seem to be a factor in strength improvement, and overall children with CP who received this injection had positive outcomes in their functional ability . This is clinically significant because therapists will be able to increase strength in areas that may be contracted with the assistance of BoNT-A injections.
Another Study looked at the effects of BoNT-A injections for treating spasticity in individuals who had suffered a stroke. As one could imagine, spasticity in the lower extremities can have detrimental effects on an individual’s ability to walk, and this can directly influence their ability to ambulate and gain back independence in life. Many individuals who have been affected by a stroke will have BoNT-A injections to reduce spasticity, but do not include therapeutic activities to either increase strength or function . The results of this study show that those who receive BoNT-A injections and a self-rehabilitation program improve their maximal gait speed, distance covered and max speed during 6MWT-modifeid (6 minute walk test), and time taken to go up or down a flight of stairs in comparison to those who only receive the injections . This research shows that a home stretching and strengthening program can help prevent muscle wasting, and can also improve gait patterns which will then increase an individual's ability to move around independently. 

Botulism Toxin, after being purified from botulism, also works as a muscle relaxer. It attaches to presynaptic terminal membranes in the skeletomuscular junction. After binding, it destroys proteins that help to make acetylcholine. When it is injected into a specific muscle, the muscle is less excitable, due to the smaller amount of acetylcholine. This allows the muscle to relax. It can also be used to help someone dress better, wash their hands, and other activities of daily living that use extension in the wrist, elbows, and fingers.

Like many other muscle relaxers, using botulinum can have adverse effects. Long term use inhibits the release of acetylcholine. A decrease in acetylcholine levels causes the muscle to relax, but a lack of acetylcholine paralyzes the muscle fibers. The paralysis in those muscle fibers can spread, and cause loss of function in the muscle.

Diazepam

Diazepam is another type of muscle relaxer. Diazepam works by inhibiting the excitability of the central nervous system, or CNS. This happens when Diazepam binds to receptors at GABA synapses and boost the GABA inhibiting effects. This mechanism allows Diazepam to act as a muscle relaxer, and increases the GABA effect on alpha motor neuron activity in the spinal cord. One side effect of this drug is relaxation of the skeletal muscle, which will lead to sedation and less general motor activity. This can help someone who is recovering from a musculoskeletal injury. The sedation leads the patient to rest, which will lead to better healing during the initial time after the injury. Another side effect of this drug is not good. If a patient continues to use this drug, they can become addicted. Withdrawal symptoms can include seizures, anxiety, agitation, tachycardia, and sometimes death. This drug is best used for short-term relief for an injury.

Baclofen

In patients without complications like stroke or a traumatic brain injury, drugs such as Diazepam and Baclofen can interfere with normal neuron changes that help improve motor activities. Lundbye-Jensen, J., Neilsen, J. B., Peterson, T. H., & Willerslev-Olsen, M. (2011) studied healthy patients learning a visuomotor skill. They taught the skill to 16 healthy subjects and used the primary motor cortex leg area in the brain to create responses in the anterior tibialis muscle. The authors found that Diazepam and Baclofen interrupt some of the neuroplastic changes that help patients improve their motor performance. Physiotherapists should use these medications with much caution, especially in healthy subjects. 

Other complications to consider when taking intrathecal baclofen are instances of infection, seizures, and withdrawal due to the use of an intrathecal baclofen pump. According to a study by Zdolsex, H. A., Olesch, C., Antolovich, G., & Reddihough, D. (2011), eighteen children with cerebral palsy were observed retrospecitvely to determine whether intrathecal baclofen was an effective treatment for them. In this study, researchers group complications associated with intrathecal baclofen into three categories. The categories were defined as follows:

"Problems were defined as difficulties that did not require surgical intervention and fully resolved before the end of treatment. Obstacles were defined as difficulties that required surgical intervention but that were fully resolved before the end of treatment. True complications were intra-and postoperative problems not fully resolved before the end of treatment."

At the end of the study, researchers identified eight problems, two obstacles, and seven true complications associated with intrathecal baclofen. These complications ranged from infection, depression, catheter malfunctions, and withdrawal symptoms.

In another study, researchers observed ninety nine patients with multiple sclerosis who were being treated with intrathecal baclofen. These subjects were being observed for the prevalence of seizures while taking intrathecal baclofen. Seven patients reported having seizures during the study. Each seizure was related to an overdose of intrathecal baclofen due to a malfunction in the pump. In a case report of a forty one year old woman with multiple sclerosis, epileptic seizures were observed after the woman started taking intrathecal baclofen. Researchers found that the baclofen was having a negative reaction with the woman's medication for a psychotic disorder. Seizures are another complication to be considered related to intrathecal baclofen. In each instance, the seizures seemed to occur as a reaction to other factors.

The final complication related to intrathecal baclofen pumps is the threat of withdrawal. This complication is similar to the possibility of overdose because it is related to a malfunction in the pump itself. In a study of eighty eight patients who were using SynchroMed pumps to administer their medication, twenty one patients experienced withdrawal from their medication. The patients who were taking intrathecal baclofen experienced withdrawal symptoms such as "pruritus without rash, generalized paresthesias, and sudden increased spasticity." While this complication can be very dangerous, it can be avoided. It is important to ensure that the pump and catheter are working properly, and that the patients are not taking any other medications that may react poorly with the muscle relaxant.

Risk of Injury

Skeletal muscle relaxants can produce effects in the central nervous system and are often associated with the symptoms of drowsiness and dizziness. Patients in the age group of 60 and over taking skeletal muscle relaxants are more likely to experience injuries than the same age group of people not taking them. Common injuries suffered were fractures, contusions, falls, and lacerations. Patients 60 years and older accounted for 16% of skeletal muscle relaxant consumers. While skeletal muscle relaxants can be beneficial for some conditions, they can produce negative side effects especially in the elderly population. Before working with a patient, the therapist should determine whether the patient is using muscle relaxants. If the patient is using them, the therapist should be prepared to monitor the patient accordingly and be prepared to assist the patient in order to keep them safe.