making muscle wasting history

The muscular dystrophies are an inherited group of progressive myopathic disorders resulting from defects in a number of genes required for normal muscle function ¹ . The Duchenne and Becker muscular dystrophies (as well as a third intermediate form) are caused by mutations of the dystrophin gene and are therefore named dystrophinopathies. Weakness is the principal symptom as muscle fiber degeneration is the primary pathologic process.

The dystrophinopathies are inherited as X-linked recessive traits and have varying clinical characteristics:

Duchenne muscular dystrophy (DMD) is associated with the most severe clinical symptoms
Becker muscular dystrophy (BMD) has a similar presentation to DMD, but a relatively milder clinical course
An intermediate group of patients, known as “outliers,” may be classified clinically as having either mild DMD or severe BMD.
The management and treatment of Duchenne and Becker muscular dystrophy will be discussed in this review. The genetics, pathogenesis, and clinical characteristics of the Duchenne and Becker muscular dystrophy are reviewed separately.

SYMPTOM MANAGEMENT
In addition to muscle weakness, cardiac, pulmonary, and orthopedic complications are frequently associated with Duchenne (DMD) muscular dystrophy and Becker (BMD). The anticipation and early detection of organ involvement is important for optimal therapy. Furthermore, patients should be evaluated by pulmonary and cardiac specialists prior to any surgery ² .

The American Thoracic Society guidelines recommend that all patients with DMD should receive the pneumococcal vaccine and an annual influenza vaccination ² . The pneumococcal vaccine can provide immunity for 5 to 10 years.

In our clinic, we recommend influenza and pneumococcal immunizations for wheelchair-bound patients with DMD or BMD if they have cardiomyopathy or declining pulmonary function tests (eg, a timed forced expiratory volume [FEV1] or forced vital capacity [FVC] less than 80 percent of the predicted value for age). Clinical experience suggests that 10-year old wheelchair-bound patients with DMD most probably do not need annual influenza vaccinations if they have normal pulmonary and cardiac function (ie, normal PFTs and no cardiomyopathy).

While ambulatory patients with DMD or BMD usually have normal or minimally diminished pulmonary testing by PFTs, we do recommend pneumococcal vaccination and annual influenza vaccination for those who have low PFTs (unusual) or cardiomyopathy (not unusual for an ambulatory patient with BMD).

Cardiac disease — Slowing progression of the cardiomyopathy associated with dystrophinopathies such as DMD and BMD is an area of active research. Initial clinical studies have evaluated the effect of angiotensin converting enzyme (ACE) inhibitors and beta blockers, both of which are used to treat asymptomatic left ventricular dysfunction and overt heart failure. A further rationale for such therapy is the suggestion that the magnitude of force generation is an important determinant of cell injury in the dystrophinopathies ³ .

The possible preventive efficacy of ACE inhibitors, which are beneficial in asymptomatic left ventricular dysfunction in adults, was evaluated in a randomized trial of 57 children with BMD (mean age 10.7 years) who had a mean left ventricular ejection fraction (LVEF) of 65 percent ⁴ .

The children were randomly assigned to perindopril (2 to 4 mg/day) or placebo. There was no difference in LVEF at three years. All of the children were then treated with perindopril for two more years. Although the mean LVEF was still not different (58.6 versus 56.0 percent with placebo), an LVEF less than 45 percent was noted in only one of the 27 patients originally treated with perindopril compared to 8 of the 29 treated with placebo. After an additional year, there were three deaths in the placebo group, all among the eight patients with an LVEF <45 percent.

More data are needed to prove that ACE inhibitors slow progression of heart disease in children with DMD who have a normal LVEF.

Results from a retrospective observational study suggest that early diagnosis and treatment of dilated cardiomyopathy (DCM) in patients with DMD and BMD may lead to ventricular remodeling ⁵ . Among 69 affected boys, an ACE inhibitor was started in 27 with DMD and four with BMD after the first abnormal echocardiogram indicative of DCM (eg, LVEF <55 percent or left ventricular dilation); the mean age was 15 years. A beta blocker (carvedilol or metoprolol) was added after three months if echocardiography showed no improvement.

At a mean follow-up of 3.3 years among 29 of the 31 patients who had repeat echocardiography, left ventricular size and function showed normalization, improvement, or stabilization in 19, 8, and 2 patients, respectively (66, 26, and 8 percent). In addition, the mean LVEF increased from 36 to 53 percent and there was evidence of improved ventricular geometry as measured by reduced sphericity.

At our institution, early treatment of DCM with an ACE inhibitor and/or a beta blocker is common practice in children with DMD or BMD. Overt heart failure is also treated with other heart failure therapies such as diuretics and digoxin as needed. Cardiac transplantation is an option in BMD patients with severe DCM and limited or no clinical evidence of skeletal muscle disease.

Surveillance — Echocardiography or cardiac MRI should be obtained around age 10 years in boys with DMD and BMD and then repeated annually or biannually ⁶ . Cardiac evaluation of female carriers should begin after teenage years ⁷ .

Pulmonary complications — Overnight mouth intermittent positive pressure can be used to treat symptomatic nocturnal hypoventilation, and respiratory assistance may be used during periods of respiratory infection.

In addition, patients with neuromuscular weakness and atelectasis may benefit from mechanical insufflation-exsufflation.

Baseline pulmonary function tests should be obtained prior to wheelchair confinement, usually around 9 or 10 years of age. Pediatric pulmonology evaluations should be obtained two times a year after any one of the following occurs: wheelchair confinement, vital capacity less than 80 percent predicted, or age 12 years ² .

Management of the respiratory complications of DMD is discussed in a consensus statement published by the American Thoracic Society in 2004 ² and available online at http://ajrccm.atsjournals.org/cgi/content/full/170/4/456.

Complications related to anesthesia and sedation — Patients with DMD have a high risk of complications when they undergo procedures requiring anesthesia or sedation ⁸ . These potentially life-threatening risks include the following:

Reactions to inhaled anesthetics and certain muscle relaxants
Upper airway obstruction
Hypoventilation
Atelectasis
Respiratory failure
Difficulty weaning from mechanical ventilation
Cardiac arrhythmias
Heart failure
As an example, DMD may be associated with susceptibility to malignant hyperthermia or episodes that mimic malignant hyperthermia, with clinical manifestations that include rhabdomyolysis, hyperkalemia, and sudden cardiac arrest [8-16] . Avoidance of potential triggering agents, particularly succinylcholine and inhalational anesthetics such as halothane, isoflurane, and sevoflurane, may reduce the risk of these events in patients with DMD or BMD.

The American College of Chest Physicians (ACCP) published a consensus statement in 2007 regarding the management of patients with DMD undergoing anesthesia or sedation ⁸ .

Orthopedic interventions — Therapeutic interventions in DMD/BMD are specifically aimed at maintaining function, preventing contractures, and providing psychologic support. Patients should receive physical therapy to encourage mobility and to prevent or reduce the risk of contractures. The mainstays of physical therapy are passive stretching exercises to prevent contractures of the iliotibial band, the Achilles tendons, and flexors of the hip.

Multiple additional interventions and/or modalities may be used based upon the patient’s requirements and severity of disease:

Lightweight plastic ankle-foot orthoses should be applied if the foot remains in plantar flexion during sleep.
Standing and/or walking may be maintained by using long-leg braces.
Surgery may be performed to release contractures of the hip flexors, iliotibial bands, and Achilles tendons.
Standing and ambulation may prevent scoliosis.
Spine surgery to stabilize or correct scoliosis may improve patient comfort, particularly for those confined to a wheelchair, and may benefit pulmonary function [2,17] ,
Orthopedic evaluations should monitor for scoliosis and other complications and surgical interventions should be utilized as needed. Chest and spine radiography should be ordered on an as-needed basis.

Nutrition — Exposure to sunshine and a balanced diet that is rich in vitamin D and calcium is important to improve bone density and reduce the risk of fractures. We recommend vitamin D supplementation if the serum concentration of vitamin D is less than 20 ng/mL [18-20] . Weight should be monitored and controlled to avoid obesity. It is recommended that patients receive routine evaluation by a nutritionist.

TREATMENT
Glucocorticoids are the mainstay of treatment for DMD ²¹ , and are offered as treatment for boys who are over the age of five years.

Prednisone — Prednisone is beneficial in the treatment of DMD and is associated with a significant increase in strength, muscle function, and pulmonary function ²² . A practice parameter developed by the American Academy of Neurology (AAN) and the Child Neurology Society (CNS) in 2005 ²³ identified six high quality randomized controlled trials of prednisone therapy [22,24-28] , and one of prednisolone therapy ²⁹ for DMD. The following observations were made ²³ :

Average muscle strength increased by 11 percent with prednisone (0.75 mg/kg per day) treatment compared with placebo. Strength increased significantly by 10 days, reached a maximum at three months, and was maintained at six and 18 months.
Urinary creatinine excretion, a surrogate for muscle mass, increased with prednisone treatment after six months and 18 months by 30.5 and 36 percent compared with placebo. This finding supports an anabolic action of prednisone in DMD in contrast to its catabolic action on normal skeletal muscle in unaffected people.
The results of standardized timed function testing (eg, time to climb stairs, walk nine meters, or arise from supine to standing) improved significantly with prednisone treatment compared with placebo. As an example, the average time to climb four stairs was approximately 43 percent faster with prednisone treatment compared with placebo (4 versus 7 seconds respectively).
Forced vital capacity (FVC) improved significantly (10.5 percent higher) after six months of daily prednisone (0.75 mg/kg per day) treatment compared with placebo.
Average muscle strength, improvements in timed function tests, and increases in FVC were significantly greater for prednisone 0.75 mg/kg per day than for 0.3 mg/kg per day in studies that examined the prednisone dose-response.
Alternate day therapy with prednisone (1.25 and 2.5 mg/kg every other day) was not sufficient to achieve the sustained benefit associated with daily prednisone ranging from 0.3 to 1.5 mg/kg per day. One small trial of 14 boys found that prednisolone 5.0 mg/kg every other day was effective in preventing loss of ambulation after 36 months of treatment ²⁹ .
Insufficient data exist regarding the optimal age to begin treatment with glucocorticoids, or the optimal duration of such treatment.
The most common side effects after 6 to 18 months of treatment with daily prednisone were weight gain and cushingoid facial appearance. There was no significant increase in hypertension, diabetes, gastrointestinal bleeding, psychosis, compression fractures, or cataracts. However, retrospective data suggest that long-term use of glucocorticoids does increase the risk of vertebral compression fractures and long bone fractures.
After 18 months, significantly more patients treated with prednisone developed weight gain (>20 percent of baseline weight) compared with placebo (75 to 80 versus 20 to 24 percent respectively).
Weight gain in patients with DMD is not solely an undesirable side effect because it is associated with an increase in muscle mass as noted above. In addition, one study found that ambulatory patients treated with prednisone did not have significantly greater weight gain than placebo treated patients ²⁷ . In contrast, nonambulatory patients treated with prednisone did have a significantly greater weight gain.

The duration of six of the seven studies assessed by the practice parameter ²³ ranged from six to 18 months, which is insufficient to evaluate clinically important endpoints such as loss of ambulation or decline in FVC. However, one longer term study found that the benefits of daily prednisone therapy (improvements in arm and leg function, timed function tests, and FVC) were sustained for three years ³⁰ .

Similar conclusions regarding benefit and side effects of prednisone treatment for DMD were reached in a systematic review ³¹ . The mechanism of the beneficial effect of glucocorticoids in patients with DMD is not clear ²³ .

Little is known of the effect of prednisone in patients with BMD.

Deflazacort — Deflazacort appears to be effective for the treatment of DMD, with efficacy and side effect profiles similar to prednisone [23,32] . Deflazacort is an oxazoline derivative of prednisone, and has an estimated dosage equivalency of 1:1.3 compared with prednisone. Thus, 1.3 mg of deflazacort is approximately equivalent to 1.0 mg of prednisone.

Deflazacort is not approved by the US Food and Drug Administration (FDA) and is not available in the United States. It is available in Canada via the Special Access Programme (SAP), which provides access to nonmarketed drugs for practitioners treating patients with serious or life-threatening conditions when conventional therapies have failed, are unsuitable, or unavailable. Deflazacort is available as a licensed pharmaceutical in some European, Asian, and South American countries.

The 2005 practice parameter from the AAN and CNS examined two high quality studies of deflazacort for the treatment of DMD [33,34] and made the following observations ²³ :

Daily deflazacort (1.0 mg/kg per day) treatment for nine months was associated with increased muscle strength and function compared with placebo ³³ .
In contrast with prednisone, alternate day treatment with deflazacort (2.0 mg/kg every other day) for two years was beneficial in one study ³⁴ . The mean prolongation of ambulation was 13 months.
Side effects of deflazacort were similar to prednisone in the two high quality randomized controlled trials [33,34] . In two longer term studies of open treatment with daily deflazacort, asymptomatic cataracts were noted in 10 of 30 patients treated for a mean of 3.2 years ³⁵ and in 6 of 13 patients treated for a mean of 5.4 years ³⁶ .
Some experts outside the US routinely use deflazacort for DMD ³⁷ , and believe it offers a more favorable side effect profile than daily treatment with prednisone, particularly with regard to weight gain.

The available clinical data regarding the impact of deflazacort on cardiac function in patients with DMD are limited. In a retrospective cohort study of 33 patients with DMD, those receiving deflazacort for ≥3 years were significantly more likely to have preserved cardiac function (20/21 patients), defined as ejection fraction >45 percent, than those who had not received the medication (5/12) ³⁸ . Larger long term prospective studies are needed to determine the effect of deflazacort on cardiac function in patients with DMD.

Prednisone versus deflazacort — Daily prednisone and daily deflazacort produced similar benefit in boys with DMD when compared in three open label trials that were included in the 2005 AAN and CNS practice parameter ²³ . These trials ranged in duration from 12 to 24 months, and reported comparable improvements in muscle strength and function for prednisone and deflazacort treatment.

Side effect profiles of prednisone and deflazacort were also similar in these studies. One small study found that deflazacort treatment was associated with a lesser increase in body weight compared with prednisone after 12 months (9 versus 21.3 percent respectively) ³⁸ . The practice parameter concluded that the data are insufficient to determine whether deflazacort has fewer side effects than prednisone ²³ . Nonetheless, deflazacort is used in preference to prednisone in some centers (eg, in Canada) for patients with DMD who may be predisposed to obesity based on body habitus or family history.

Orthopedic outcomes of glucocorticoid therapy — Retrospective data suggest that long-term therapy with glucocorticoids for DMD reduces the risk of scoliosis and prolongs independent ambulation but increases the risk of osteoporosis and long bone and vertebral compression fractures ³⁹ .

There are also a number of other side effects of long-term glucocorticoid therapy.

There are no clinical studies in children to guide preventive measures for glucocorticoid-induced osteoporosis. In the absence of such data, we suggest the following measures to minimize bone loss in children with DMD who are receiving prolonged glucocorticoid therapy:

Dietary calcium and vitamin D supplementation is suggested, in the form of dairy products, other foods rich in calcium and vitamin D, and sunshine exposure. Referral to a dietitian for weight control and discussion of dietary calcium and vitamin D intake may also be beneficial.
Calcium supplementation (500 to 1000 mg/day) is suggested for children with diminished intake of calcium-containing foods (eg, for children who do not like milk and dairy products).
Standing and weight-bearing exercise, ideally for at least 30 minutes each day, are suggested when possible.
Dual energy x-ray absorptiometry (DXA) scanning to measure bone mineral density, and a 25-hydroxyvitamin D (25OHD) level, are suggested at baseline and yearly thereafter.
- Referral to a pediatric endocrinologist or bone specialist is suggested if the 25OHD level is less than 30 ng/mL (others use 20 ng/mL as the limit) or if DXA scan shows marked deviation from age-normal or baseline values.

We also refer children who develop vertebral or long bone fractures to a pediatric endocrinologist or bone specialist for evaluation and possible bisphosphonate treatment.

Preventive measures for adults are discussed in detail separately and include dietary calcium and vitamin D supplementation, bisphosphonates, and DXA scanning.

Other immunosuppressive agents — Other immunosuppressive therapies have been studied for the treatment of DMD. In a randomized clinical trial of 95 boys with DMD cited above, azathioprine (2.0 to 2.5 mg/kg per day) was added after six months to the patients who had been randomly assigned at enrollment to placebo or low dose prednisone ²⁶ . No improvement was observed with azathioprine.

Oxandrolone, an anabolic (androgenic) steroid, was found in a pilot study to have effects similar to prednisone, with fewer side effects ⁴⁰ . In addition, a randomized prospective trial reported that, although oxandrolone did not produce a significant change in the average manual muscle strength score as compared with placebo, there was a marked improvement in quantitative muscle strength ⁴¹ . The investigators felt that oxandrolone may be useful before initiating therapy with glucocorticoids because it is safe and accelerates linear growth. However, its beneficial effect in slowing the progression of weakness is not of sufficient magnitude to justify its routine use.

The administration of cyclosporine for eight weeks has also been reported to improve clinical function in children with DMD ⁴² . However, because of the rare reports of cyclosporine-induced myopathy in patients treated with certain other drugs (eg, statins), the use of cyclosporine in muscular dystrophy remains controversial.

Recommendations for immunosuppressive therapy — The following recommendations for immunosuppressive therapy are in accordance with the national practice parameter from the AAN and CNS ²³ on glucocorticoid therapy:

Prednisone (0.75 mg/kg per day) should be offered as treatment for boys with DMD who are over the age of five years. A balanced discussion of the potential benefits and risks of glucocorticoid treatment should be given to every patient prior to initiating therapy.
Benefits and side effects of glucocorticoid therapy must be monitored. Timed muscle function tests, pulmonary function tests, and age at loss of independent ambulation are useful parameters to assess benefits. Cognizance of potential glucocorticoid therapy side effects (eg, weight gain, cushingoid appearance, short stature, decrease in linear growth, long bone and vertebral fractures, acne, excessive hair growth, gastrointestinal symptoms, and behavioral changes) is important to assess risks.
Maintaining the prednisone dose at 0.75 mg/kg per day is optimal. This dose should be continued if the side effects are not severe. A gradual tapering of prednisone to as low as 0.3 mg/kg per day will give significant but less robust improvement.
The prednisone dose should be decreased to 0.5 mg/kg per day if excessive weight gain occurs (>20 percent over estimated normal weight for height over a 12 month period). The dose should be further decreased to 0.3 mg/kg per day after three to four months if excessive weight gain continues.
Deflazacort (0.9 mg/kg per day) where available can also be used to treat DMD. Patients should be monitored for asymptomatic cataracts and weight gain.
Immunosuppression with azathioprine is not beneficial.
In addition to these recommendations, we suggest preventive measures to minimize bone loss for patients with DMD who are receiving prolonged glucocorticoid therapy. Such measures include dietary calcium and vitamin D supplementation, and yearly DXA scanning and a 25-hydroxyvitamin D level.

NOVEL THERAPIES
Gene therapy — The identification of the gene responsible for the dystrophinopathies led to initial excitement that transfer of a functioning dystrophin protein (whether by transplanted myoblast or direct genetic manipulation) may provide substantial benefit to or even “cure” affected patients. Although myoblast transfer has been attempted in humans, the results have thus far not been encouraging ⁴³ . However, in one patient who received a bone marrow transplant for severe combined immunodeficiency, donor nuclei persisted in a small number (>1 percent) of myocytes for more than 10 years ⁴⁴ . Thus, cells derived from bone marrow may serve as vector for genetic therapies.

Early clinical studies are evaluating systemic gene transfer by intravascular administration of recombinant adeno-associated viral (rAAV) vectors that carry microdystrophin or minidystrophin genes [45,46] . Another investigational approach involves injection of antisense oligonucleotides that induce specific exon skipping during messenger RNA splicing, thereby correcting the open reading frame of the DMD gene and restoring dystrophin expression [47-49]

Aminoglycosides — The gene mutation in up to 15 percent of patients with DMD is a premature stop codon. Aminoglycoside treatment of cultured cells can suppress stop codons by creating misreading of RNA, thereby allowing the insertion of alternative amino acids at the site of the mutated stop codon. In vivo gentamicin therapy in the mdx mouse resulted in dystrophin expression of 10 to 20 percent of the levels detected in normal muscle ⁵⁰ . This amount provided a certain degree of functional protection against contraction-induced damage.

Aminoglycoside therapy has therefore been suggested as an alternative to gene therapy. This therapy could be aimed only at patients with premature stop codons. A preliminary study in four patients, using gentamicin (7.5 mg/kg per day) for two-weeks did not result in the appearance of full length dystrophin in the muscles of treated patients ⁵¹ . Some authors, unable to reproduce the results previously published for the mouse model of Duchenne muscular dystrophy, have called for more preclinical investigation of this potential therapy ⁵² .

PTC124 therapy — PTC124 is an investigational orally administered drug being developed for the treatment of genetic defects caused by nonsense (stop) mutations. PTC124 promotes ribosomal read-through of nonsense (stop) mutations, allowing bypass of the nonsense mutation and continuation of the translation process to production of a functioning protein. Encouraging results have been reported in preclinical efficacy studies, where PTC124 treatment of primary muscle cells from humans and mdx mice was associated with the production of dystrophin ⁵³ .

In January 2005, the US Food and Drug Administration (FDA) approved an indication for PTC124 in the treatment of DMD. The approval was based upon the results of phase 1 studies in healthy adult volunteers showing that the drug is orally bioavailable and well tolerated ⁵⁴ . In a phase 2 study of 26 boys with nonsense-mutation-mediated DMD, increased full-length dystrophin expression was observed in vitro and in vivo with PTC124, and serum muscle enzyme levels decreased within 28 days of treatment ⁵⁵ . However, there were only minimal changes in muscle strength and timed functions with PTC124 treatment. Further investigations are needed to determine whether PTC124 can improve muscle function and activity in patients with DMD and BMD.

Creatine — Creatine monohydrate has been studied for its potential to increase muscle strength in neuromuscular disorders and muscular dystrophies [56-58] . In a randomized, controlled, crossover trial of 30 boys with DMD, each participant received treatment with creatine (about 0.1 g/kg per day) for four months and placebo for four months ⁵⁹ . Creatine treatment was associated with improved grip strength of the dominant hand and increased fat free mass compared with placebo. The beneficial effects of creatine were independent of steroid use. However, creatine treatment was not associated with significant improvement on functional measures or activities of daily living. Creatine was well tolerated with no evidence of renal or liver dysfunction.

Another small controlled trial randomly assigned 50 boys with DMD to either creatine 5 g/day, glutamine 0.6 g/kg per day, or placebo ⁶⁰ . There was no statistically significant benefit for either treatment group compared with placebo as assessed by change in the modified manual muscle testing score, the primary outcome measure ⁶⁰ .

In light of the limited data and apparently modest benefit attributed to creatine in these studies, demonstration of clinically important improvement in larger trials is needed before recommending this treatment for patients with DMD.

Deacetylase inhibitors — Several structurally unrelated deacetylase inhibitors (trichostatin A, valproic acid, and phenylbutyrate) can enhance muscle differentiation ⁶¹ and increase muscle size by inducing the expression of follistatin ⁶² . In addition, trichostatin A treatment restored muscle function and morphology in dystrophin-deficient mdx mice and in alpha-sarcoglycan-deficient mice ⁶³ . Human trials are needed to determine if deacetylase inhibitors are beneficial for patients with Duchenne, Becker, or limb-girdle muscular dystrophy.

Myostatin inactivation — Myostatin is a protein that has an inhibitory effect on muscle growth. Mice that would otherwise express the DMD phenotype but lack myostatin have an increased muscle mass compared to those with a wild-type myostatin gene ⁶⁴ . Antibodies to myostatin also have a beneficial effect; treated animals have increased muscle mass, strength, lower serum creatine kinase, and less histologic evidence of muscle damage ⁶⁵ .

A myostatin mutation in a child with gross muscle hypertrophy has been identified ⁶⁶ , suggesting that myostatin inactivation could be a therapeutic target to increase muscle bulk and strength in muscle wasting diseases such as DMD ⁶⁷ . Clinical trials of myostatin inhibitors are underway ⁶⁸ .

Stem cell therapy — The use of stem cells in the treatment of DMD and BMD is under investigation but remains experimental [43,44,69,70] .

PROGNOSIS
Among those with DMD, there may be some improvement between three and six years of age. However, this is followed by gradual but relentless deterioration, leading to wheelchair confinement by the age of approximately 12 years ¹ . Most patients with DMD die in their late teens or twenties from respiratory insufficiency (most commonly) or arrhythmia secondary to cardiomyopathy. In some cases, the immediate cause of death is not apparent. Survival in DMD may be improving with advances in respiratory care and increased utilization of assisted ventilation [71,72] .

By comparison, patients with BMD typically remain ambulatory beyond the age of 16 years and into adult life; they usually survive beyond the age of 30 years and have a mean age of death in the mid 40s [1,73,74] . The most common cause of death is heart failure from dilated cardiomyopathy, which also causes considerable morbidity in these patients despite their milder skeletal muscle involvement ⁷⁵ .

REFERENCES
1. Emery, AE. The muscular dystrophies. Lancet 2002; 359:687.

2. Finder, JD, Birnkrant, D, Carl, J, et al. Respiratory care of the patient with Duchenne muscular dystrophy: ATS consensus statement. Am J Respir Crit Care Med 2004; 170:456.

3. Colan, SD. Evolving therapeutic strategies for dystrophinopathies: potential for conflict between cardiac and skeletal needs. Circulation 2005; 112:2756.

4. Duboc, D, Meune, C, Lerebours, G, et al. Effect of perindopril on the onset and progression of left ventricular dysfunction in Duchenne muscular dystrophy. J Am Coll Cardiol 2005; 45:855.

5. Jefferies, JL, Eidem, BW, Belmont, JW, et al. Genetic predictors and remodeling of dilated cardiomyopathy in muscular dystrophy. Circulation 2005; 112:2799.

6. Darras, B, Korf, B, Urion, D. Dystrophinopathies. GeneReviews 2005. Available at www.geneclinics.org/profiles/dbmd/index.html (Accessed 1/25/08).

7. Nolan, MA, Jones, OD, Pedersen, RL, Johnston, HM. Cardiac assessment in childhood carriers of Duchenne and Becker muscular dystrophies. Neuromuscul Disord 2003; 13:129.

8. Birnkrant, DJ, Panitch, HB, Benditt, JO, et al. American College of Chest Physicians consensus statement on the respiratory and related management of patients with Duchenne muscular dystrophy undergoing anesthesia or sedation. Chest 2007; 132:1977.

9. Oka, S, Igarashi, Y, Takagi, A, et al. Malignant hyperpyrexia and Duchenne muscular dystrophy: A case report. Can Anaesth Soc J 1982; 29:627.

10. Kelfer, HM, Singer, WD, Reynolds, RN. Malignant hyperthermia in a child with Duchenne muscular dystrophy. Pediatrics 1983; 71:118.

11. Wang, JM, Stanley, TH. Duchenne muscular dystrophy and malignant hyperthermia--two case reports. Can Anaesth Soc J 1986; 33:492.

12. Heiman-Patterson, TD, Natter, HM, Rosenberg, HR, et al. Malignant hyperthermia susceptibility in X-linked muscle dystrophies. Pediatr Neurol 1986; 2:356.

13. Wedel, DJ. Malignant hyperthermia and neuromuscular disease. Neuromuscul Disord 1992; 2:157.

14. Morris, P. Duchenne muscular dystrophy: a challenge for the anaesthetist. Paediatr Anaesth 1997; 7:1.

15. Breucking, E, Reimnitz, P, Schara, U, Mortier, W. [Anesthetic complications. The incidence of severe anesthetic complications in patients and families with progressive muscular dystrophy of the Duchenne and Becker types]. Anaesthesist 2000; 49:187.

16. Yemen, TA, McClain, C. Muscular dystrophy, anesthesia and the safety of inhalational agents revisited; again. Paediatr Anaesth 2006; 16:105.

17. Do, T. Orthopedic management of the muscular dystrophies. Curr Opin Pediatr 2002; 14:50.

18. Bachrach, LK. Taking steps towards reducing osteoporosis in Duchenne muscular dystrophy. Neuromuscul Disord 2005; 15:86.

19. Biggar, WD, Bachrach, LK, Henderson, RC, et al. Bone health in Duchenne muscular dystrophy: a workshop report from the meeting in Cincinnati, Ohio, July 8, 2004. Neuromuscul Disord 2005; 15:80.

20. Quinlivan, R, Roper, H, Davie, M, et al. Report of a Muscular Dystrophy Campaign funded workshop Birmingham, UK, January 16th 2004. Osteoporosis in Duchenne muscular dystrophy; its prevalence, treatment and prevention. Neuromuscul Disord 2005; 15:72.

21. Angelini, C. The role of corticosteroids in muscular dystrophy: a critical appraisal. Muscle Nerve 2007; 36:424.

22. Mendell, JR, Moxley, RT, Griggs, RC, et al. Randomized, double-blind six-month trial of prednisone in Duchenne’s muscular dystrophy. N Engl J Med 1989; 320:1592.

23. Moxley RT, 3rd, Ashwal, S, Pandya, S, et al. Practice parameter: corticosteroid treatment of Duchenne dystrophy: report of the Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Neurology 2005; 64:13.

24. Griggs, RC, Moxley, RT 3rd, Mendell, JR, et al. Prednisone in Duchenne dystrophy. A randomized, controlled trial defining the time course and dose response. Clinical Investigation of Duchenne Dystrophy Group. Arch Neurol 1991; 48:383.

25. Fenichel, GM, Mendell, JR, Moxley RT, 3rd, et al. A comparison of daily and alternate-day prednisone therapy in the treatment of Duchenne muscular dystrophy. Arch Neurol 1991; 48:575.

26. Griggs, RC, Moxley RT, 3rd, Mendell, JR, et al. Duchenne dystrophy: randomized, controlled trial of prednisone (18 months) and azathioprine (12 months). Neurology 1993; 43:520.

27. Backman, E, Henriksson, KG. Low-dose prednisolone treatment in Duchenne and Becker muscular dystrophy. Neuromuscul Disord 1995; 5:233.

28. Rahman, MM, Hannan, MA, Mondol, BA, et al. Prednisolone in Duchenne muscular dystrophy. Bangladesh Med Res Counc Bull 2001; 27:38.

29. Siegel, IM, Miller, JE, Ray, RD. Failure of corticosteroid in the treatment of Duchenne (pseudo-hypertrophic) muscular dystrophy. Report of a clinically matched three year double-blind study. IMJ Ill Med J 1974; 145:32.

30. Fenichel, GM., Florence, JM, Pestronk, A, et al. Long-term benefit from prednisone therapy in Duchenne muscular dystrophy. Neurology 1991; 41:1874.

31. Manzur, AY, Kuntzer, T, Pike, M, Swan, A. Glucocorticoid corticosteroids for Duchenne muscular dystrophy. Cochrane Database Syst Rev 2004; :CD003725.

32. Campbell, C, Jacob, P. Deflazacort for the treatment of Duchenne Dystrophy: a systematic review. BMC Neurol 2003; 3:7.

33. Mesa, LE, Dubrovsky, AL, Corderi, J, et al. Steroids in Duchenne muscular dystrophy--deflazacort trial. Neuromuscul Disord 1991; 1:261.

34. Angelini, C, Pegoraro, E, Turella, E, et al. Deflazacort in Duchenne dystrophy: study of long-term effect. Muscle Nerve 1994; 17:386.

35. Biggar, WD, Gingras, M, Fehlings, DL, et al. Deflazacort treatment of Duchenne muscular dystrophy. J Pediatr 2001; 138:45.

36. Schara, U, Mortier, J, Mortier, W. Long term steroid therapy in Duchenne dystrophy – positive results versus side effects. J Clin Neuromusc Dis 2001; 2:179.

37. Biggar, WD, Politano, L, Harris, VA, et al. Deflazacort in Duchenne muscular dystrophy: a comparison of two different protocols. Neuromuscul Disord 2004; 14:476.

38. Silversides, CK, Webb, GD, Harris, VA, Biggar, DW. Effects of deflazacort on left ventricular function in patients with Duchenne muscular dystrophy. Am J Cardiol 2003; 91:769.

39. King, WM, Ruttencutter, R, Nagaraja, HN, et al. Orthopedic outcomes of long-term daily corticosteroid treatment in Duchenne muscular dystrophy. Neurology 2007; 68:1607.

40. Fenichel, G, Pestronk, A, Florence, J, et al. A beneficial effect of oxandrolone in the treatment of Duchenne musculardystrophy: a pilot study. Neurology 1997; 48:1225.

41. Fenichel, GM, Griggs, RC, Kissel, J, et al. A randomized efficacy and safety trial of oxandrolone in the treatment of Duchenne dystrophy. Neurology 2001; 56:1075.

42. Sharma, KR, Mynhier, MA, Miller, RG. Cyclosporine increases muscular force generation in Duchenne muscular dystrophy. Neurology 1993; 43:527.

43. Gussoni, E, Blau, HM, Kunkel, LM. The fate of individual myoblasts after transplantation into muscles of DMD patients. Nat Med 1997; 3:970.

44. Gussoni, E, Bennett, RR, Muskiewicz, KR, et al. Long-term persistence of donor nuclei in a Duchenne muscular dystrophy patient receiving bone marrow transplantation. J Clin Invest 2002; 110:807.

45. Gregorevic, P, Allen, JM, Minami, E, et al. rAAV6-microdystrophin preserves muscle function and extends lifespan in severely dystrophic mice. Nat Med 2006; 12:787.

46. Rodino-Klapac, LR, Chicoine, LG, Kaspar, BK, Mendell, JR. Gene therapy for duchenne muscular dystrophy: expectations and challenges. Arch Neurol 2007; 64:1236.

47. van Deutekom, JC, Bremmer-Bout, M, Janson, AA, et al. Antisense-induced exon skipping restores dystrophin expression in DMD patient derived muscle cells. Hum Mol Genet 2001; 10:1547.

48. van Deutekom, JC, Janson, AA, Ginjaar, IB, et al. Local dystrophin restoration with antisense oligonucleotide PRO051. N Engl J Med 2007; 357:2677.

49. Rando, TA. Get personal with gene therapy for muscular dystrophy. Lancet Neurol 2008; 7:196.

50. Barton-Davis, ER, Cordier, L, Shoturma, DI, et al. Aminoglycoside antibiotics restore dystrophin function to skeletal muscles of mdx mice. J Clin Invest 1999; 104:375.

51. Wagner, KR, Hamed, S, Hadley, DW, et al. Gentamicin treatment of Duchenne and Becker muscular dystrophy due to nonsense mutations. Ann Neurol 2001; 49:706.

52. Dunant, P, Walter, MC, Karpati, G, Lochmuller, H. Gentamicin fails to increase dystrophin expression in dystrophin-deficient muscle. Muscle Nerve 2003; 27:624.

53. Welch, EM, Barton, ER, Zhuo, J, et al. PTC124 targets genetic disorders caused by nonsense mutations. Nature 2007; 447:87.

54. Hirawat, S, Welch, EM, Elfring, GL, et al. Safety, tolerability, and pharmacokinetics of PTC124, a nonaminoglycoside nonsense mutation suppressor, following single- and multiple-dose administration to healthy male and female adult volunteers. J Clin Pharmacol 2007; 47:430.

55. Wong, B, Bonnemann, C, Finkel, R, et al. Phase 2 study of PTC124 for nonsense mutation suppression therapy of duchenne muscular dystrophy (DMD) [abstract]. Ann Neurol 2007; 62 Suppl 11:S96.

56. Tarnopolsky, M, Martin, J. Creatine monohydrate increases strength in patients with neuromuscular disease. Neurology 1999; 52:854.

57. Walter, MC, Lochmuller, H, Reilich, P, et al. Creatine monohydrate in muscular dystrophies: A double-blind, placebo-controlled clinical study. Neurology 2000; 54:1848.

58. Louis, M, Lebacq, J, Poortmans, JR, et al. Beneficial effects of creatine supplementation in dystrophic patients. Muscle Nerve 2003; 27:604.

59. Tarnopolsky, MA, Mahoney, DJ, Vajsar, J, et al. Creatine monohydrate enhances strength and body composition in Duchenne muscular dystrophy. Neurology 2004; 62:1771.

60. Escolar, DM, Buyse, G, Henricson, E, et al. CINRG randomized controlled trial of creatine and glutamine in Duchenne muscular dystrophy. Ann Neurol 2005; 58:151.

61. Iezzi, S, Cossu, G, Nervi, C, et al. Stage-specific modulation of skeletal myogenesis by inhibitors of nuclear deacetylases. Proc Natl Acad Sci U S A 2002; 99:7757.

62. Iezzi, S, Di Padova, M, Serra, C, et al. Deacetylase inhibitors increase muscle cell size by promoting myoblast recruitment and fusion through induction of follistatin. Dev Cell 2004; 6:673.

63. Minetti, GC, Colussi, C, Adami, R, et al. Functional and morphological recovery of dystrophic muscles in mice treated with deacetylase inhibitors. Nat Med 2006; 12:1147.

64. Wagner, KR, McPherron, AC, Winik, N, Lee, SJ. Loss of myostatin attenuates severity of muscular dystrophy in mdx mice. Ann Neurol 2002; 52:832.

65. Bogdanovich, S, Krag, TO, Barton, ER, et al. Functional improvement of dystrophic muscle by myostatin blockade. Nature 2002; 420:418.

66. Schuelke, M, Wagner, KR, Stolz, LE, et al. Myostatin mutation associated with gross muscle hypertrophy in a child. N Engl J Med 2004; 350:2682.

67. McNally, EM. Powerful genes--myostatin regulation of human muscle mass. N Engl J Med 2004; 350:2642.

68. Wagner, KR, Fleckenstein, JL, Amato, AA, et al. A phase I/IItrial of MYO-029 in adult subjects with muscular dystrophy. Ann Neurol 2008; 63:561.

69. Gussoni, E, Soneoka, Y, Strickland, CD, et al. Dystrophin expression in the mdx mouse restored by stem cell transplantation. Nature 1999; 401:390.

70. Skuk, D, Roy, B, Goulet, M, et al. Dystrophin expression in myofibers of Duchenne muscular dystrophy patients following intramuscular injections of normal myogenic cells. Mol Ther 2004; 9:475.

71. Simonds, AK, Muntoni, F, Heather, S, Fielding, S. Impact of nasal ventilation on survival in hypercapnic Duchenne muscular dystrophy. Thorax 1998; 53:949.

72. Eagle, M, Baudouin, SV, Chandler, C, et al. Survival in Duchenne muscular dystrophy: improvements in life expectancy since 1967 and the impact of home nocturnal ventilation. Neuromuscul Disord 2002; 12:926.

73. Bushby, KM, Gardner-Medwin, D. The clinical, genetic and dystrophin characteristics of Becker muscular dystrophy. I. Natural history. J Neurol 1993; 240:98.

74. Bushby, KM. The limb-girdle muscular dystrophies-multiple genes, multiple mechanisms. Hum Mol Genet 1999; 8:1875.

75. Cox, GF, Kunkel, LM. Dystrophies and heart disease. Curr Opin Cardiol 1997;