Dystrophinopathies

Clinical characteristics: The dystrophinopathies cover a spectrum of X-linked muscle disease ranging from mild to severe that includes Duchenne muscular dystrophy, Becker muscular dystrophy, and DMD-associated dilated cardiomyopathy (DCM). The mild end of the spectrum includes the phenotypes of asymptomatic increase in serum concentration of creatine phosphokinase (CK) and muscle cramps with myoglobinuria. The severe end of the spectrum includes progressive muscle diseases that are classified as Duchenne/Becker muscular dystrophy when skeletal muscle is primarily affected and as DMD-associated DCM when the heart is primarily affected.

Duchenne muscular dystrophy (DMD) usually presents in early childhood with delayed motor milestones including delays in walking independently and standing up from a supine position. Proximal weakness causes a waddling gait and difficulty climbing stairs, running, jumping, and standing up from a squatting position. DMD is rapidly progressive, with affected children being wheelchair dependent by age 12 years. Cardiomyopathy occurs in almost all individuals with DMD after age 18 years. Few survive beyond the third decade, with respiratory complications and progressive cardiomyopathy being common causes of death.

Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness. With improved diagnostic techniques, it has been recognized that the mild end of the spectrum includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s. Despite the milder skeletal muscle involvement, heart failure from DCM is a common cause of morbidity and the most common cause of death in BMD. Mean age of death is in the mid-40s.

DMD-associated DCM is characterized by left ventricular dilatation and congestive heart failure. Females heterozygous for a DMD pathogenic variant are at increased risk for DCM.

Diagnosis/testing: The diagnosis of a dystrophinopathy is established in a proband with the characteristic clinical findings and elevated CK concentration and/or by identification of a hemizygous pathogenic variant in DMD on molecular genetic testing in a male and of a heterozygous pathogenic variant in DMD on molecular genetic testing in a female. Females may present with a classic dystrophinopathy or may be asymptomatic carriers.

Management: Treatment of manifestations: ACE inhibitors are used with or without beta blockers for cardiomyopathy in both DMD and BMD phenotypes. Congestive heart failure is treated with diuretics and oxygen as needed; cardiac transplantation is offered to persons with severe dilated cardiomyopathy and BMD with limited or no clinical evidence of skeletal muscle disease. Scoliosis is treated with bracing and surgery. Corticosteroid therapy improves muscle strength and function for individuals with DMD between ages five and 15 years; the same treatment is used in BMD, although the efficacy is less clear. Dystrophin restoration therapies have been developed by using synthetic antisense oligonucleotides to restore the reading frame by exon skipping for individuals with specific pathogenic variants in DMD.

Prevention of secondary complications: Evaluation by a pulmonologist and cardiologist before surgeries; pneumococcal and influenza immunizations annually; nutrition assessment; physical therapy to promote mobility and prevent contractures; sunshine and a balanced diet rich in vitamin D and calcium to improve bone density and reduce the risk of fractures; weight control to avoid obesity.

Surveillance: For males with DMD or BMD: annual or biannual evaluation by a cardiologist beginning at the time of diagnosis; monitoring for scoliosis; baseline pulmonary function testing before wheelchair dependence; frequent evaluations by a pediatric pulmonologist. For heterozygous females: cardiac evaluation at least once after the teenage years.

Agents/circumstances to avoid: Botulinum toxin injections; succinylcholine and inhalational anesthetics because of susceptibility to malignant hyperthermia or malignant hyperthermia-like reactions.

Evaluation of relatives at risk: Early identification of heterozygous females who are at increased risk for cardiomyopathy and, thus, need routine cardiac surveillance and prompt treatment.

Genetic counseling: The dystrophinopathies are inherited in an X-linked manner. The risk to the sibs of a proband depends on the genetic status of the mother. Heterozygous females have a 50% chance of transmitting the DMD pathogenic variant in each pregnancy. Sons who inherit the pathogenic variant will be affected; daughters who inherit the pathogenic variant are heterozygous and may have a range of clinical manifestations. Males with DMD usually do not reproduce. Males with BMD or DMD-associated DCM may reproduce: all of their daughters are heterozygotes; none of their sons inherit their father's DMD pathogenic variant. Carrier testing for at-risk females, prenatal testing, and preimplantation genetic testing are possible if the DMD pathogenic variant in the family is known.