Skeletal Muscle Relaxants and Analgesic Balms

Ausim Chaghtai

Charles E. Argoff

Skeletal Muscle Relaxants

The class of medications termed skeletal muscle relaxants is ill defined leading to much confusion for both clinicians and scientists. This confusion results from the assumption that these medications all act in a similar fashion and produce reliable skeletal muscle relaxation. Neither is true. These medications produce a range of effects that remain poorly defined (Table 80.1).¹^,²^,³^,⁴ The class includes carisoprodol, chlorzoxazone, cyclobenzaprine, metaxalone, methocarbamol, and orphenadrine.⁵ These medications are approved by the U.S. Food and Drug Administration (FDA) for indications including spasticity, spasms, and/or musculoskeletal conditions (Table 80.2). Medications are often utilized for off-label uses and skeletal muscle relaxants are no exception. Benzodiazepines, principally diazepam, also are commonly used for adjunctive relief of skeletal muscle spasm. Chief among the challenges to clinicians using these drugs is discerning the role of the agents across the continuum of painful disorders. It should be noted that the order of medications within the subsections do not indicate management priority or expert preference.

TABLE 80.1 Pharmacotherapies Commonly Used for Muscle Spasm

Drug	Onset	Duration	Common Dosing	Side Effects	Important Drug Interactions
*Sedative*
Carisoprodol (Soma)	30 min	4-6 h	350 PO QID	Ataxia, dizziness, drowsiness, N/V, withdrawal potential	Additive effects with alcohol and other CNS depressants
Chlorzoxazone (Parafon Forte)	˜1 h	3-4 h	250-750 mg PO TID-QID	Dizziness, drowsiness, headache, N/V
Metaxalone (Skelaxin)	1 h	4-6 h	400-800 mg PO TID	Dizziness, drowsiness, headache, N/V, rash
Methocarbamol (Robaxin)	30 min (PO)	N/A	750-1,000 mg PO QID	Blurred vision, dizziness, drowsiness
*TCA-like*
Cyclobenzaprine (Flexeril)	˜1 h	12-24 h	5-10 mg PO TID	Drowsiness, dizziness, dry mouth	Additive effects with alcohol and other CNS depressants; seizures with tramadol and MAOIs; additive effects with TCAs
*Antihistamine*
Orphenadrine (Norflex)	1 h (PO)	4-6 h	100 mg PO BID	Tachycardia, light-headedness, N/V, dry mouth	Additive effects with alcohol and other CNS depressants; coadministration with propoxyphene can lead to confusion, anxiety, and/or tremors
*GABA Type*
Diazepam (Valium)	30 min (PO)	Variable, depending on elimination	2-10 mg PO TID	Sedation, fatigue, hypotension, ataxia, respiratory depression	Potentiation of effects when taken with phenothiazines, opioids, barbiturates, MAOIs
Baclofen (Lioresal)	3-4 d (PO) 30 min (IT)	Variable (PO) 4-6 h (IT)	5 mg PO TID titrated up to 40-80 mg/d	Drowsiness, slurred speech, hypotension, constipation, urinary retention	Antidepressants (short-term memory loss); additive effects with imipramine
*Central α₂ Agonists*
Tizanidine (Tizanidine)	2 wk	Variable	2-8 mg PO TID-QID	Drowsiness, dry mouth, dizziness, hypotension, increased spasm/tone	Additive effects with alcohol and other CNS depressants; reduced clearance with oral contraceptives
CNS, central nervous system; GABA, γ-aminobutyric acid; IT, intrathecal; MAOIs, monoamine oxidase inhibitors; N/A, not applicable; N/V, nausea/vomiting; PO, orally; QID, four times a day; TCA, tricyclic antidepressant; TID, three times a day.

Historically, skeletal muscle relaxants have been prescribed for acute and chronic conditions associated with muscle related pain. The majority of these agents are indicated for use during the initial presentation of acute low back pain, which often results from soft tissue mechanical injury and normally occurs in the muscles, ligaments, and/or tendons, structures around the lumbar spine. Acute pain may include local pain and tenderness, muscle spasm, and limited range of motion, but what actually constitutes painful muscle spasm remains controversial.⁶ Muscle spasm may be a variant of the myofascial pain presentation, and as such not really a spasm.⁷

To better understand the potential pharmacologic benefit of these agents, consider the regulation of muscle activity in both the peripheral tissues and central nervous system (CNS). At the level of the peripheral muscle, tissues are composed of intrafusal fibers that signal changes in muscle length. These lie
in parallel with extrafusal muscle fibers that normally serve to contract or stabilize joints. When muscle tissue is stretched, the intrafusal fibers stretch resulting in an increase in neural discharges carried by afferent nerve fibers. This signal is transmitted to the dorsal horn and synapses with α-motoneurons in the ventral horn, producing excitatory postsynaptic potentials. The result is a type of negative feedback, with muscle contraction of the intrafusal muscle fibers where the original stretch signal originated. These muscle fibers also maintain an efferent component, facilitated by small γ-motoneurons that originate in the ventral horn of the spinal cord and travel together with the α-motoneurons that innervate extrafusal muscle fibers. The γ-motoneurons adjust the sensitivity of the muscle fibers and regulate muscle tension over a wide range of muscle lengths. This complex system of afferent and efferent signaling through the motoneurons when at homeostasis leads to stabilization of muscle structures (Fig. 80.1).

TABLE 80.2 U.S. Food and Drug Administration-Approved Therapies and Indications

Medication	Indication
Baclofen	Spasticity
Dantrolene	Chronic spasticity Malignant hyperthermia
Tizanidine	Muscle spasticity
Carisoprodol	Musculoskeletal conditions
Metaxalone	Musculoskeletal conditions
Chlorzoxazone	Musculoskeletal conditions
Cyclobenzaprine	Muscle spasm
Methocarbamol	Muscle spasm Tetanus
Orphenadrine	Muscle spasms

FIGURE 80.1 Neural regulation of muscle tone: possible sites of action of skeletal muscle relaxants. See text for details. +, excitatory effect; -, inhibitory effect; α, α-motoneuron; Ca²⁺, calcium ion; E, excitatory interneuron; γ, γ-motoneuron; I, inhibitory interneuron.

In the dorsal horn, a complex network of excitatory and inhibitory interneurons mediates motor reflexes in response to deep and cutaneous stimulation. Such reflexes mediate ipsilateral flexion and contralateral extension in response to noxious stimuli to coordinate a protective or escape response. Impulses from cutaneous afferents travel through the dorsal horn of the spinal cord and terminate on excitatory interneurons, which in turn terminate on presynaptic terminals of the intrafusal fibers further promoting excitation at the ventral horn α-motoneuron. Inhibitory centers in the bulbar reticular formation and facilitatory centers from several brain regions further regulate both corticospinal and reflex muscle activity.⁸^,⁹

Excitatory neurotransmitters in the CNS play a major role in the modulation of movement in the spinal cord and include substances like glutamate, aspartate, and substance P. These neurotransmitters are released from the terminals of primary afferent fibers to mediate reflexes that enhance motor tone at the spinal level.¹⁰ γ-Aminobutyric acid (GABA) is a major inhibitory CNS neurotransmitter that emanates from supraspinal and interneuronal inputs. GABA is believed to play a major role in presynaptic inhibition of motor neurons in the dorsal horn.¹⁰

Any change in the homeostasis of the peripheral or CNS components related to maintaining proper muscle tone can lead to production of an acute reflex muscle spasm. When this occurs, there are two main potential issues. Either a reflex increase in muscle tone activates polysynaptic reflexes and produces hyperexcitability of α- and/or γ-motoneurons, or there is supraspinal activation of descending facilitatory systems.¹¹ In settings of chronic muscle spasticity, the processes appear to be
more involved, with pathology from supraspinal CNS descending pathways that produce excessive excitation or diminished inhibition of α-motoneurons in the dorsal horn.¹²

TABLE 80.3 Agents by Proposed Mechanism of Action

CNS Depressants

Antihistamine

Orphenadrine
Sedatives

Carisoprodol, chlorzoxazone, metaxalone, methocarbamol
TCA-like

Cyclobenzaprine

Central α₂ Agonists

Tizanidine

GABA Agonists

Baclofen

Benzodiazepines

CNS, central nervous system; GABA, γ-aminobutyric acid; TCA, tricyclic antidepressant.

MECHANISM OF ACTION

In animal studies, skeletal muscle relaxants act at various CNS sites that are important in muscle activity regulation. The exact mechanism of action for these various agents is not clear. A variety of mechanisms appear to be associated with the activity of this diverse group of agents (Table 80.3). Animal models have historically shown that muscle relaxants exert their activity by blocking polysynaptic neurons in the spinal cord and inhibiting interneuronal activity within the descending reticular formation.¹ Mephenesin, an early predecessor to today’s muscle relaxants, in animal models affected monosynaptic and polysynaptic reflexes.¹³^,¹⁴ Subsequent animal data showed that mephenesin and methocarbamol prolonged the refractory period of skeletal muscle by a direct action on skeletal muscle fibers.¹⁵ Very little has been described about the effects of skeletal muscle relaxants such as cyclobenzaprine, methocarbamol, carisoprodol, and chlorzoxazone on neurotransmission. These medications are also known to have a significant sedative profile. This in fact was a quality that was exploited when older treatment paradigms included significant bed rest. Interestingly, other medications with sedative properties are also known to depress polysynaptic reflexes. This situation certainly sheds some degree of confusion as to the specific utility of skeletal muscle relaxants, especially in relation to the purported effects versus nonspecific sedation.

The pharmacologic capacity of other commonly used drugs is less well characterized in specific relation to muscle spasm. Diazepam, a benzodiazepine, suppressed polysynaptic reflexes in cats but required doses higher than would be used clinically.¹⁶ Benzodiazepines act by potentiating the postsynaptic effects of GABA within the CNS.¹² Baclofen (parachlorophenol GABA) is a lipophilic derivative of GABA that binds to GABA_B but not to GABA_A receptors and may exert its effect, in part, by inhibiting the evoked release of excitatory amino acids (e.g., glutamate) and substance P.¹⁰ Tizanidine, a newer antispasticity agent, is an α₂-adrenergic receptor agonist that may also act by decreasing spinal excitatory amino acid release.¹⁶

Types of Skeletal Muscle Relaxants

CENTRALLY ACTING SEDATIVE-HYPNOTIC MUSCLE RELAXANTS

Chlorzoxazone

Chlorzoxazone may be less effective than the other skeletal muscle relaxants.¹⁷ Chlorzoxazone does not have any significant drug interactions but does have a significant adverse effect profile that includes a rare idiosyncratic hepatocellular reaction.¹⁸ The use of this agent may be questionable considering the potential lack of efficacy and significant toxicity profile.¹⁹

Metaxalone

Metaxalone does not have any significant drug interactions and appears to have a fairly benign side effect profile. Hemolytic anemia and impaired liver function may occur but are uncommon. Fatalities attributed to the use of metaxalone have been reported.²⁰^,²¹ Metaxalone is contraindicated in patients with severe renal or hepatic impairment. Because this drug was FDA-approved over 30 years ago, there are few published placebo-controlled studies of metaxalone for musculoskeletal pain.²²

Methocarbamol

Methocarbamol is available in an oral form and a parenteral form for intravenous (IV) or intramuscular (IM) use. Complications with the injectable form include pain, skin sloughing, and thrombophlebitis. This drug also was FDA-approved over 30 years ago, and as a result, there are few published studies comparing it to placebo for the treatment of musculoskeletal pain.²³

Carisoprodol

Carisoprodol is a schedule IV controlled substance in the United States. It is hepatically metabolized to meprobamate. Meprobamate produces physical and psychological dependence.²⁴^,²⁵^,²⁶^,²⁷^,²⁸^,²⁹ Substance abuse appears to be problematic with carisoprodol, probably due to meprobamate formation. In recent years, several states have begun treating carisoprodol as a controlled substance within their state formularies. Due to the dependence potential, carisoprodol should be cautiously tapered as opposed to immediately discontinued following long-term use. At the end of November 2007, the European Medicines Agency recommended the suspension of marketing authorization for carisoprodol-containing products for its 12 member states. Its Committee for Medicinal Products for Human Use concluded that the risk of their use is greater than the benefits.³⁰ Given the unfavorable risk-benefit balance, which includes a high dependence potential, carisoprodol should generally be avoided.

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