Muscle Relaxants for Acute and Chronic Pain

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Key points

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    Types of muscle relaxants and adverse effects are examined.

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    Pharmacologic indications for each type of muscle relaxant are highlighted.

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    Guidance concerning appropriate utilization of muscle relaxants when treating acute and chronic low back pain is provided.

Background

Skeletal muscle relaxants are poorly defined and understood. Conventionally defined as pharmacologic agents that are thought to produce relaxation of skeletal muscles, the various types of skeletal muscle relaxants each function in a different fashion1, 2, 3, 4 (Table 1). Little is known about the mechanism of action of several of these agents. These studies are limited in number, and most of them are based on animal models alone.5 Furthermore, there is a lack of a consolidated source for

Pathophysiology

Basic muscle physiology will be described to better understand the regulatory systems and pathways associated with muscle activity. Muscle regulation is generally considered to act centrally or peripherally. The central nervous system (CNS) involves a complex network of excitatory and inhibitory pathways that synapse to an interneuron, which in turn, makes postsynaptic connection to alpha motor neurons in the ventral horn of the spinal cord. Efferent pathway eventually synapses with the

Antispasmodic Versus Antispasticity Versus Myofascial Pain

Spasticity is defined as a velocity-dependent resistance to passive movement of the affected muscles at rest. It presents as incessant increased tone, and often involves irregularity in posturing during ambulation or with noxious stimuli. Spasticity results from a centralized etiology such as stroke, cerebral palsy, or spinal cord injury among other causes. Symptoms include constant stiffness, hypertonicity, and hyperreflexia. Antispasticity agents include baclofen, diazepam, tizanidine, and

Metaxalone

FDA approved in 1964,15 metaxalone is a centrally acting muscle relaxant with an onset of 1 hour. Duration is 4 to 6 hours. The exact mechanism of action is unclear. Metaxalone is generally considered to be well tolerated with the exception of it being known cause of hemolytic anemia in rare cases.16 Caution is advised with patients with hepatic or renal impairment.

Methocarbamol

FDA approved in 1964,17 methocarbamol is available for oral, intramuscular, or intravenous use. The mechanism of action in humans

Cyclobenzaprine

With a chemical structure similar to tricyclic antidepressants (TCAs), cyclobenzaprine was FDA approved in 1977.6 It is thought to relieve muscle spasm of local origin without disrupting motor function. The mechanism of action is similar to the likes of amitriptyline, with serotonin receptors at the spinal cord or brain stem to block alpha motor neuron excitation.23 Animal studies suggest reduced hyperactivity of skeletal muscles. Duration is 8 to 37 hours. Therapeutic doses are achieved in 3

Diazepam

Diazepam centrally acts as a GABA agonist at the presynaptic GABA a receptors.29 It serves to inhibit monosynaptic and polysynaptic spinal reflex pathways. The overall effect is membrane hyperpolarization and decreased neuronal firing. Diazepam has well known diverse clinical utility with a plethora of clinical effects. Depending on the prescriber’s subspecialty, diazepam is used as an anxiolytic, antiepileptic, or an antispasmodic agent. What is also known about the class of benzodiazepines is

Tizanidine

Tizanidine is FDA approved for treatment of muscle spasticity resulting from CNS disorders such as spinal cord injury and stroke. Although it has a similar chemical structure to clonidine, tizanidine does not share the antihypertensive effects or exert any benefit for treatment of dysautonomia.30 Tizanidine has a short half-life, which results in frequent dosing. Studies show equivocal data for effective monotherapy treatment for muscle spasm, or myofascial pain.31,32 There might be more

Orphenadrine

The structure of orphenadrine is nearly identical to that of diphenhydramine with the addition of a methane group. It shares both antihistaminic and anticholinergic properties. It may be considered in patients with a history of bronchospasms/common allergies or Parkinson disease to reduce tremors and stiffness, respectively. Severe adverse effects include blurred vision, tachycardia, and urinary retention.35

Dantrolene

Dantrolene is the only known skeletal muscle relaxant that acts peripherally at the level of the striated muscle itself. It blocks the calcium release of the sarcoplasmic reticulum, which in turn, reduces extrafusal muscle fiber contractility and sensitivity.23 It is known to exert the most influence on fast-twitch motor units.29 It also exerts minor influence on smooth and cardiac muscle.

Skeletal Muscle Relaxants for Treatment of Acute and Chronic Low Back Pain

Skeletal muscle relaxants are frequently used in the treatment of acute and chronic back pain. According to

Disclosure

The author has nothing to disclose.

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