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Dystrophinopathies
Introduction
Dystropinopathies are a group of X-linked muscle disorders, with their most recognized pathology being Duchenne muscular dystrophy (DMD), followed by Becker muscular dystrophy (BMD) and the relatively new phenotype DMD associated dilated cardiomyopathy (DCM). The most severe phenotype, DMD, usually presents in childhood with a series of developmental motor problems, while BMD, it is a late-onset entity. All pathologies are inherited in an X-linked manner, affecting mainly males, but females can also present with a related condition (DCM).
Etiology
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Etiology
DMD is 2.2 MB gene with 79 exons, located on the X chromosome. Deletions of exons, particularly in the exonic regions 2 to 20 and 44 to 53, comprise 60% to 70% of pathogenic variants. Dystrophin is membrane-associated protein in muscle and neurons, part of a protein complex linking cytoskeleton and membrane proteins, who later on bind to extracellular matrix proteins.[1]
Epidemiology
Prevalence for all age groups in the United States is 1.12 for DMD and 0.36 for BMD. There is a higher predominance in Hispanic individuals.[2]
Pathophysiology
Dystrophin defect leads to the general disorganization of the dystrophin-associated protein complex, resulting in higher muscular susceptibility for membrane damage caused by myocyte contractions. [3]
Histopathology
Histopathology shows variation in fiber size with foci of necrosis, leading to atrophy, inflammatory cell activation, and fibro-fatty infiltration.[4]
History and Physical
Three main phenotypes have been described[5]: DMD, BMD, and DCM, with the first two, sharing symptoms and history, but with an important difference, DMD patients tend to require wheelchair use by age 13, while BMD after age 16 or later.
DMD
Presents in early childhood, delayed motor milestones, most noticeably independent walking and standing from the floor. Consistent with parents' complaints, as well as gait disturbances (toe walking and flat feet). The main affected area is proximal, leading to difficulty climbing stairs, jumping, running.[6] The Gower maneuver is both a major clinical finding and a technique for affected children to stand from seating position on the floor. Calf muscles progressively turned firm due to fat infiltration.
Cardiomyopathy presents in one-third of individuals by adolescence, turning 100% prevalent in all children at age 18.[7]
Some children may present a deficit in executive functioning [8], leading to decreased visuospatial skills.
The most common cause of death is respiratory failure or cardiomyopathy.[9]
BMD
BMD shows late-onset muscle weakness, sometimes developing symptoms after age 30.[10]
Heart failure due to cardiomyopathy is the most common cause of lethality.[11] No cognitive impairment evidenced.
DMD-Associated DCM
Rapidly progressive course, related to ventricular arrhythmias, with no skeletal muscle involvement. Affected females exhibit a mild form around age 40.
Evaluation
Creatine phosphokinase (CK) is the best initial test to perform. Values to consider DMD are above 10-times normal limits, while for BMD are above 5-times the normal limit. DCM CK values could present in a wide range above normal reference. Other tests that may present with high values are aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH).
Transaminases could also be from hepatic origin. In order to differentiate, gamma-glutamyl transferase needs to be ordered. If values are normal, then it is likely to be secondary to a myopathy like DMD. [12]
The most accurate test is DMD gene deletion-duplication analysis, as 60% to 70% of patients show this abnormality. Should it not be positive on the basis of a strong clinical picture, sequencing would be recommended to assess for the rest (20% to 30%).
Treatment / Management
Specialties to Involved after Diagnosis
Physical therapy, developmental specialist, cardiology (age 6), and clinical genetics/counselor are consulted.
Cardiomyopathy
When left ventricle section fraction is below 55%, some institutions start angiotensin-converting enzyme (ACE) inhibitors or beta-blockers [13] to improve left ventricular function. Should there be intolerance to ACE inhibitors, angiotensin II-receptor blockers are similarly effective[14]. When cardiac failure is present, digoxin and diuretics should be added. Cardiac transplant had been recommended for patients with severe cardiomyopathy and mild BMD.(A1)
Patients who develop scoliosis may require bracing and surgery (spinal fusion).
Corticosteroids have been proved to improve muscle strength and function. This is, therefore, the main therapy for children younger than 15 years of age, although not recommended for children below age 2.[15] Therapy can start with either prednisone (0.75 mg/kg per day, maximum 40 mg per day) or deflazacort (0.9 mg/kg per day, maximum 36 mg per day) when motor skills begin to decline. To assess efficacy, physicians can perform pulmonary function tests and timed muscle function tests, while also monitoring for side effects Cushing syndrome, short-stature, changes in behavior, gastrointestinal (GI) symptoms, and osteopenia (increased risk for vertebral or long bone fractures[16]). Should a severe side effect present, particularly excessive weight gain, doses can be decreased by 25% until reaching 50% of the original dose. There are conflicting views on the use of corticosteroids on BMD patients, as data supporting benefits are quite limited.(A1)
Prevention/Monitoring
- Yearly influenza vaccine
- Pneumococcal vaccine (PPS 23)[17]
- Assess for, in the presence of corticosteroid intake, weight gain, dysphagia, constipation, malnutrition or prior main surgeries
- Physical therapy to prevent muscle contractures. Promote daily or regular exercise, but if there is muscle pain, reduce activity intensity or frequency
- Monitor for serum calcium, phosphorus, alkaline phosphatase, 25-hydroxyvitamin D (per semester), magnesium, PTH, urine calcium, and creatinine; Dual-energy x-ray absorptiometry at age three and annually; spine x-rays; bone age, especially if under corticosteroid therapy[18]
- Consider biphosphonates if there is a history of symptomatic vertebral fractures, not as prophylaxis
- Cardiac evaluation every two years, from the time of diagnosis (electrocardiogram and echocardiogram or cardiac MRI)[19][18]; On heterozygous asymptomatic females, observation, and work up as considered by symptoms; routine cardiac surveillance every five years from age 25
- Baseline pulmonary function tests and biannually along with pediatric pulmonologist if the patient uses a wheelchair, age 12, or has a reduction of vital capacity of less than 80%
- Family members or caregivers should be educated regarding manual ventilation bag, mechanical insufflation-exsufflation device. (A1)
Contraindications
The use of botulinum toxin is contraindicated. Non-depolarizing anesthetics or succinylcholine are also contraindicated despite no increased risk for malignant hyperthermia; a small subset has been reported to show severe malignant hyperthermia-like reactions.[20](B2)
Differential Diagnosis
Emery-Dreifuss muscular dystrophy: There is a triad of childhood joint contractures, slowly progressive muscle weakness, and initial wasting in humeroperoneal distribution extending to scapular and pelvic girdle muscles. Cardiac involvement can occur after the second decade of life. Limb-girdle muscular dystrophy: This is an autosomal recessive or dominant with a defect on genes encoding sarcoglycans
Spinal muscular atrophy: Shows reduced muscle tone, weakness (sparing facial muscles), anterior horn cell involvement manifesting as tongue fasciculations, and absent deep tendon reflexes. Onset is from birth to adolescence
Barth syndrome: This condition is X-linked of the TAZ gene. Cardiomyopathy, neutropenia, skeletal myopathy, prepubertal growth delay occur.
Pertinent Studies and Ongoing Trials
To date, there are several therapies under investigation, from gene repair and therapy to drugs affecting the expression of dystrophin. Eteplirsen is one of the latter. Approved by the FDA for infusion in 2016, it works by skipping exon 51 during pre-mRNA splicing correcting dystrophin expression.[21] Ataluren is another drug on trials, with the goal of bypassing pathogenic variants through promoting ribosomal read-through, therefore continuing dystrophin expression.[22]
Pearls and Other Issues
Genotype Correlations
The difference between phenotypes (DMD and BMD), relies on a reading frame rule, stating that if the pathogenic variant does not alter the reading frame, then the expression will be milder, i.e., BMD; while, on the contrary, should express as DMD (severe phenotype).[23] This prediction is about 92% accurate.
DMD-associated DCM involve pathogenic variants affecting the muscle promoter and first exon.
Enhancing Healthcare Team Outcomes
There are several genetically inherited muscular dystrophy disorders, which are progressive and have no cure. Because of the high morbidity and mortality, these disorders are best managed by an interprofessional team that includes a neurologist, physical therapist, internist, pediatrician, and a geneticist. Many of these patients develop severe motor problems and either become bedridden or must use a wheelchair. It is important to involve social workers early in their treatment to help improve the home environment and the quality of life. Rehabilitation and neuroscience nurses participate in patient care and report changes to the team. Pharmacists review medication, educate families about proper administration and side effects and monitor compliance. [Level 5]
References
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