S.E. is a 54-year-old man with relapsing remitting multiple sclerosis (RR-MS) diagnosed 12 years ago. He lives in a single-level home with his wife and two teenage children. S.E. is able to perform stand-pivot transfers independently and can sometimes ambulate limited household distances with a four-wheeled walker. His major mode of mobility is an electric wheelchair; however, he states that he would like to stand and walk more if possible. He is self-employed as an architect and works approximately 30 hours per week. S.E. is slightly overweight (BMI 25 kg/m2) but has no other comorbidities. He is currently taking oral baclofen and tizanidine for bilateral lower extremity spasticity. S.E. was referred to physical therapy for evaluation of his ambulatory skills and assessment of assistive devices. Upon initial evaluation, the patient had limited movement and volitional activity of his lower extremities, but upper extremity strength was normal. He has significant functional strength, as he is able to lift himself into and out of his vehicle daily. The physical therapist initially fitted S.E. with custom molded ankle-foot orthoses and began a neuromuscular re-education program to improve his ambulatory skills. S.E. had significant spasticity in his lower extremities, which was exacerbated with transfers and lower extremity weightbearing activity. Because spasticity was limiting his ability to make functional improvements, S.E.’s baclofen was increased to a maximum tolerated oral dose along with a slight increase in his oral dose of tizanidine.
Drugs that affect skeletal muscles fall into two major therapeutic groups: neuromuscular blockers and skeletal muscle relaxants, or spasmolytics. Neuromuscular blockers are used primarily as adjuncts to general anesthesia to produce the muscle paralysis necessary for surgery, tracheal intubation, and control of ventilation. These compounds are nicotinic antagonists at the neuromuscular junction and lack central nervous system (CNS) activity. Neuromuscular blockers were discussed in Chapter 5.
Figure 33-1 broadly categorizes the diverse class of skeletal muscle relaxants into two groups: agents more commonly used to reduce spasticity in a variety of neurologic conditions and those agents used to reduce muscle spasm following muscle injury or inflammation. Spasmolytic drugs have traditionally been called centrally acting skeletal muscle relaxants because almost all act at multiple sites in the CNS rather than at the neuromuscular end plate—a fact that has significant clinical ramifications. Only two spasmolytic drugs—dantrolene and botulinum toxin—act in or near skeletal muscle with no significant central effects.
Skeletal muscle relaxants may be divided into antispastic agents that are used chronically to treat central nervous system (CNS) associated spasticity and agents used acutely to reduce muscle spasms following muscle injury or inflammation. Prototype drugs are in parentheses.
PATHOPHYSIOLOGY OF SPASTICITY AND MUSCLE SPASM
Spasticity is characterized by an increase in tonic stretch reflexes ...