Friedreich's Ataxia
Friedreich’s ataxia is a spinocerebellar degeneration syndrome with the onset of symptoms before age 20 years. This autosomal-recessive condition has been linked in one subtype to chromosome 9q13-21.1 (FRDA), with the protein implicated being termed “frataxin.” A second subtype referred to as FRDA2 is linked to chromosome 9p23-p11.
The incidence of Friedreich’s ataxia is 1 in 25,000 to 50,000. Carrier frequency is 1 in 60 to 110. Age of onset is usually concerns regarding overwork weakness in dystrophic myopathies. The dominant upper limb has been found to be weaker in persons with FSHD muscular dystrophy than the nondominant, providing circumstantial evidence for overwork weakness (67,121). A single subject with scapuloperoneal muscular dystrophy had a reversal of rapid strength decline after reducing daily physical activity. Other studies evaluating strengthening intervention in DMD subjects have shown maintenance of strength or even mild improvement in strength over the period of the investigation. However, these studies are limited by use of primarily nonquantitative measures (122), lack of a control group (123), and use of the opposite limb as a control without considering the effects of cross training (124). Animal work utilizing dystrophic dogs has shown significant increases in creatine kinase values immediately following exercise.
No systemic studies using the DMD population have shown any deleterious effects of resistance exercise. Based on the theoretic susceptibility of the dystrophin-deficient sarcolemmal membrane to mechanical injury and the relative paucity of investigations, it is prudent to recommend a submaximal strengthening program in DMD and other rapidly progressive dystrophic disorders. A great concern is how to incorporate these activities effectively into the daily routine of the child, avoiding use of mundane and tedious regimens that employ progressive resistive exercises.
Incorporation of the activity into recreational pursuits and aquatic-based therapy are probably the most reasonable approaches for the preadolescent child.Strengthening Exercise in Slowly Progressive Neuromuscular Diseases
Only supervised strengthening programs in this population have been advocated. Recently, a moderate resistance home exercise program (using a less supervised approach) was devised that demonstrated similar strength gains in both neuromuscular disease patients and normal control subjects without evidence of overwork weakness (125). Based on this encouraging result, the home program was advanced to high resistance training in similar subjects without apparent additive beneficial effects; in fact, eccentrically measured elbow flexor strength actually decreased significantly (126).
Based on these investigations, the author believes that there is adequate evidence to generally advocate a submaximal strengthening program for persons with slowly progressive NMD. There seems to be no additional benefit to high-resistance, low-repetition training sets, and the risk of actually increasing weakness becomes greater. Improvement in strength will hopefully translate to more functional issues such as improved endurance and mobility.
Aerobic Exercise in Neuromuscular Disease
Aerobic exercise refers to rhythmic, prolonged activity of the level sufficient to provide a beneficial training stimulus to the cardiopulmonary and muscular systems but below the threshold where anaerobic metabolism of fuels is the primary source of energy. The response of normal skeletal muscle to this type of training includes increased capillary density in the muscle to improve substrate transfer, increased skeletal muscle mitochondrial size and density, higher concentrations of skeletal muscle oxidative enzymes, and improvement in utilization of fat as an energy source for muscular activity. Patients with neuromuscular disease have a diminished capacity for exercise.
Children with Duchenne muscular dystrophy have been demonstrated to have low cardiovascular capacity and peripheral oxygen utilization with higher resting heart rate compared with controls (127). Physical ability and exercise capacity is more likely to be limited by muscle strength than by deterioration of cardiorespiratory function. In a recent study using a home-based aerobic walking program, slowly progressive neuromuscular disease subjects showed modest improvement in aerobic capacity without evidence of overwork weakness or excessive fatigue (128). It is likely that alternative exercise approaches, such as aquatic-based therapy, will need to be utilized in children with more severe neuromuscular diseases who are nonambulatory and have less-than-antigravity muscle strength.One group recently studied the effect of endurance training on conditioning and strength in adult Becker muscular dystrophy (BMD). Eleven patients with BMD and seven matched, healthy subjects cycled 50 30-minute sessions at 65% of their maximal oxygen uptake (VO2max) over 12 weeks, and six patients continued cycling for 1 year. Endurance training for 12 weeks significantly improved VO2max by 47 ± 11% and maximal workload by 80 ± 19% in patients. This was significantly higher than in healthy subjects (16 ± 2% and 17 ± 2%). CK levels did not increase with training. Strength in muscles involved in the cycle exercise (knee extension, and dorsi- and plantarflexion) increased significantly by 13% to 40%. Cardiac pump function, measured by echocardiography, did not change with training. All improvements and safety markers were maintained after one year of training. Endurance training was demonstrated to be a safe method to increase exercise performance and daily function in patients with BMD, and the findings support an active approach to rehabilitation of patients with BMD (129).