Handbook on the Late Effects of Poliomyelitis for Physicians and Survivors©
New muscle weakness is the hallmark of post-polio syndrome and can significantly impact activities of daily living. Some amount of new muscle weakness is likely to occur in about half of post-polio individuals (Jubelt & Drucker, 1999). Muscle weakness is most likely to occur in muscles previously affected during the acute poliomyelitis followed by a partial or full recovery (Cashman et al., 1987; Dalakas & Illa, 1991). Less frequently, "previously unaffected" muscles may also develop this new progressive weakness. However, these "previously unaffected" muscles had subclinical involvement at the time of the acute poliomyelitis. About 50% of the motor nerve cells supplying a muscle must die before a muscle is noticeably (clinically) weak in usual daily activities.
The new progressive weakness has many of the characteristics of weakness seen during the acute polio, i.e., it is usually asymmetric, can be proximal or distal, and is often patchy (weakness may affect one or two muscles in one extremity and skip the other muscles). New muscle weakness can involve specific muscle groups and can result in respiratory insufficiency (Bach, 1995), bulbar muscle weakness (swallowing difficulties, aspiration, loss of voice) (Sonies & Dalakas, 1995), and sleep apnea. Other symptoms resulting from new neuromuscular dysfunction are increased muscle fatiguability, described as the rapid weakening of a muscle with exercise that then recovers quickly (minutes to hours) depending on the amount of overuse (Sharma et al., 1994), and muscle pain and tenderness.
The new muscle weakness of post-polio syndrome has been estimated to typically progress at 1-2% per year. In these studies, strength was assessed by either manual muscle strength testing (Dalakas et al., 1986) or by mechanical strength measurements (Agre et al., 1995; Stålberg & Grimby, 1995). The course of new weakness is variable. Some survivors experience slow, continuous progression while others report a stepwise course with plateaus between periods of progression. Because of the slow and variable course of the weakness, it may be difficult to demonstrate progression (Jubelt & Drucker, 1999).
The exact cause of new muscle weakness (see Theories) is not known, but it is clear that there is progressive deterioration of the motor unit (Wiechers & Hubbell, 1981) (see Pathology). It frequently has been hypothesized that the increased metabolic demand on the nerve cell to maintain more muscle fibers than normal results in the premature exhaustion or death of the nerve cell, beginning with its distal processes. The overuse of muscles resulting in excessive muscle fatigue may contribute to this premature exhaustion of the motor units (Grimby et al., 1996; Sharma et al., 1994; Sivakumar et al., 1995).
To manage new weakness, it is important to use energy conservation measures (see Conservation of Energy) and to stop overusing weak muscles (Agre et al., 1996). This can be accomplished by pacing activities (see Pacing ) and using orthotics (see Orthotics) to support weak muscles. Also, it now has been demonstrated that nonfatiguing exercise programs can improve the strength of mild to moderately weak muscles (Agre et al., 1996; Jubelt & Drucker, 1999; Spector et al., 1996). The important point is to avoid frequent or continuous muscle overuse, or muscle exhaustion, and to use a nonfatiguing exercise program (see Exercise).