Duchenne Muscular Dystrophy


Article Author:
Vijay Venugopal


Article Editor:
Steven Pavlakis


Editors In Chief:
Temekis Hampton


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Trevor Nezwek
Radia Jamil
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
6/4/2019 12:35:44 PM

Introduction

Duchenne muscular dystrophy (DMD) one of the most severe forms of inherited muscular dystrophies. It is the most common hereditary neuromuscular disease and does not exhibit a predilection for any race or ethnic group. Mutations in the dystrophin gene lead to progressive muscle fiber degeneration and weakness. This weakness may present initially with difficulty in ambulation but progressively advances to such an extent that affected patients are unable to carry out activities of daily living and become wheelchair bound. Cardiac and orthopedic complications are common, and death usually occurs in the twenties due to respiratory muscle weakness or cardiomyopathy. Current therapy is centered on treatment with glucocorticoids and physiotherapy to prevent orthopedic complications.[1][2][3]

Etiology

DMD is a genetic disease due to the mutation of the Dystrophin gene, located on chromosome Xp21. It is inherited as an X- linked recessive trait however approximately 30% of cases are due to new mutations.[4][5]

Mutations in the dystrophin gene result in diseases known as dystrophinopathies which encompass Duchenne muscular dystrophy, Becker muscular dystrophy, and an intermediate form. Mutations result in a limited production of the dystrophin protein, which results in loss of the myofiber membrane integrity with repeated cycles of necrosis and regeneration. Fibrous connective tissue and fat progressively replace muscle leading to clinical features.

Carrier females show no evidence of muscular weakness; however, symptomatic female carriers have been described. About 2.5% to 20% of female carriers may be affected. This can be explained by the Lyon hypothesis in which the normal X chromosome become inactivated, and the X chromosome with the mutation is expressed.

Female carriers can become symptomatic if they are associated with Turners syndrome (45X) or mosaic Turner karyotype, balanced X autosome translocations with breakpoints within the dystrophin gene and preferential inactivation of the normal X, and females with a normal karyotype but with nonrandom X chromosome inactivation with diminished expression of the normal dystrophin allele.

The dystrophin gene is one of the largest genes in the human genome. Containing 79 exons of a coding sequence and 2.5 Mb of DNA, this gene codes for the protein dystrophin measuring 427 kDa.

Most mutations are deletions and duplications, and this accounts for 70% to 80% of the mutations. Point mutations are seen in 20% to 30% of patients.

Dystrophin is expressed in the striated and cardiac muscle as well as the brain and the retina. The distribution in the brain is less than that in the muscle; however, this does explain some of the central nervous system manifestations of this disease.[6]

Epidemiology

As DMD is inherited as an X linked recessive manner, boys are more frequently affected than girls. The estimated incidence is 1 in 3600 male live-born infants. Some studies have estimated the prevalence of DMD as 2 per 10,000 in the United States. It is one of the most common and most severe congenital myopathies.

Pathophysiology

Dystrophin is a large cytoskeletal protein that facilitates interactions between the cytoskeleton, cell membrane, and extracellular matrix. It is located at the plasma membrane in both muscle and non-muscle tissues. Dystrophin is a critical part of the dystrophin-glycoprotein complex (DGC), which plays an important role as being a structural unit of muscle. In DMD, both dystrophin and DGC proteins are missing, leading to excessive membrane fragility and permeability, dysregulation of calcium homeostasis, oxidative damage. These factors play a crucial role in muscle cell necrosis. As patients with DMD age, the regenerative capacity of the muscles appears to be exhausted, and connective and adipose tissue gradually replaces muscle fibers.

Histopathology

A muscle biopsy will demonstrate endomysial connective tissue proliferation, scattered degeneration, and regeneration of myofibers, muscle fiber necrosis with a mononuclear cell infiltrate and replacement of muscle with adipose tissue and fat.

History and Physical

Development in the first few years of life is typically normal with milestones achieved at a slightly delayed if not normal rate. Growth velocity, however, is slower leading to short stature. Mild hypotonia in an infant may be present, and poor head control in an infant may be an initial sign. Patients do not have atypical facies, but with the onset of facial muscle weakness, a transverse or horizontal sign may be seen in later childhood. Weakness and difficulty in ambulation in typically first noted between 2 and three years of life. This manifests as toe walking, difficulty running, climbing up stairs, and frequently falling. Weakness is more pronounced in proximal than distal muscles and the lower limb more than the upper limb.

In patients who are ambulatory, increased incidence of fractures is noted as a consequence of the frequent falls.

Lumbar lordosis and scoliosis with muscle contractures occur. As a result of scoliosis, pulmonary function may be impaired which can lead to pulmonary compromise.

Contractures of the ankles, knees, hips, and elbows may be seen.

Enlargement of the calves with wasting of the thigh muscles result in pseudohypertrophy of the calves, which is a classical feature. Aside from the calves, hypertrophy of the tongue and muscles of the forearm may be seen but are less classical.

A waddling or Trendelenburg gait is common, and patients become wheelchair bound by the age of 12.

Pharyngeal weakness can result in episodes of aspiration, nasal regurgitation of liquids, and a nasal quality of voice.

Incontinence of urine and stools due to urethral and anal sphincter weakness is uncommon and if present, is a late manifestation.

Rarely, malignant hyperthermia after anesthesia may be a presenting sign.

Symptomatic female carriers may have an early onset and progressive muscular dystrophy.

Intellectual Disability

Intellectual impairment is seen in all patients however only 20% to 30% of patients have an Intelligence quotient (IQ) less than 70. The degree of impairment does not correlate with disease severity. Most patients have only a mild form of learning impairment and can function in a regular classroom. Epilepsy is more common than in the general population, and uncommonly, autism-like behavior has been described.

DMD Associated Cardiomyopathy

Symptoms of cardiomyopathy can develop in the early teens and are present in almost all patients in their twenties. Persistent tachycardia and heart failure maybe presenting signs. In affected patients, dilated cardiomyopathy is characterized by extensive fibrosis of the posterobasal left the ventricular wall. As the disease progresses, fibrosis can spread to the lateral free wall of the left ventricle. With the involvement of the posterior papillary muscle, significant mitral regurgitation can occur.  Inter and intraatrial conduction abnormalities, possibly involving the AV node can be seen. Arrhythmias, particularly supraventricular arrhythmias are also associated with the developing cardiomyopathy.

Physical exam shows pseudohypertrophy of the calf muscle and occasionally the quadriceps muscle. Shortening of the Achilles tendon may be noted, and the patient may have hyporeflexia or areflexia. Ankle reflexes are preserved till late in the disease unless contractures develop. Knee deep tendon reflexes (DTR’s) are less brisk than the ankle and can be lost by age 6. The brachioradialis reflex is brisker than the biceps or triceps DTR’S. Typically, children are noted to use their arms to lift themselves up from a seated position on the ground. This is known as Gowers sign.

Evaluation

A dystrophinopathy should be suspected in patients with symptoms of weakness, characteristic physical exam and a possible family history of the disease. Laboratory testing involves creatinine kinase measurements, muscle biopsies, gene testing, and ECG findings for cardiomyopathy.[7][8][9]

Serum Creatinine Kinase (CK)

Serum CK measurements are elevated before the development of clinical symptoms and signs and may also be elevated in newborns. Levels peak by age 2 and can be more than 10 to 20 times above the upper limit of normal. As age and disease progress, serum CK levels decrease as fibrosis and fat progressively replace muscle. Other muscle enzymes such as aldolase levels and AST levels may also elevate.

Asymptomatic carriers may also have elevated CK levels. This is seen in about 80% of cases, and the highest levels are noted between ages 8 and 12.

Muscle Biopsy

A muscle biopsy will demonstrate endomysial connective tissue proliferation, scattered degeneration, and regeneration of myofibers, muscle fiber necrosis with a mononuclear cell infiltrate and replacement of muscle with adipose tissue and fat.

The common muscles biopsies are the quadriceps femoris and the gastrocnemius.

Electromyography

Characteristic myopathic features can be seen; however, this is nonspecific. Motor and sensory nerve conduction velocities are normal, and denervation is not present.

Gene Analysis

Patients with DMD demonstrate the complete or near complete absence of dystrophin gene. Dystrophin immunoblotting can be used to predict the severity of the disease. In DMD, patients are found to have less than 5% of the normal quantity of dystrophin.

Polymerase chain reactions (PCR) can also be used and detect up to 98% of mutations. Multiplex ligation-dependent probe amplification (MPLA) is also used to identify duplications and deletions.  Duplications can lead to in-frame or out of frame transcription products. Fluorescence in situ hybridization (FISH) is used less frequently but is useful to identify small point mutations.

Dystrophin immunocytochemistry can also be sued to detect cases not identifies by PCR.

Electrocardiogram (ECG)

Characteristic ECG changes are tall R waves in V1-V6 with an increased R/S ratio and deep Q waves in leads I,aVL and V5-6. Conduction abnormalities with arrhythmias may be identified with telemetry. As mentioned previously, supraventricular arrhythmias are more common. Intra-atrial conduction abnormalities are more common than AV or infra-nodal defects in DMD.

Echocardiogram

Evidence of dilated cardiomyopathy is present in almost all patients by the end of their teens or in their 20s.

Treatment / Management

No medical cure exists for this congenital dystrophy, and the disease has a poor prognosis. Treatment is centered on glucocorticoid therapy, prevention of contractures and medical care of cardiomyopathy and respiratory compromise.[10][11]

Glucocorticoid Therapy

Glucocorticoid therapy decreases the rate of apoptosis of myotubes and can decelerate myofiber necrosis. Prednisone is used in patients four years and older in whom muscle function is declining or plateauing.

Prednisone is recommended at a dosage of (0.75 mg/kg per day or 10 mg/kg per week is given over 2 weekend days).

Deflazacort, an oxazoline derivative of prednisone is sometimes preferred over prednisone as it has a better side effect profile and has an estimated dosage equivalency of 1:1.3 compared with prednisone. The recommended dosage is 0.9 mg/kg/day.

Studies have shown that glucocorticoid treatment is associated with the improved pulmonary function, delayed development of scoliosis, reduces incidence and progression of cardiomyopathy and overall improved mortality.

Cardiomyopathy

Treatment with angiotensin-converting enzyme (ACE) inhibitors and/or beta blockers is recommended. Early studies suggest that early treatment with ACE inhibitors may slow progression of the disease and prevent the onset of heart failure.

Overt heart failure is treated with digoxins and diuretics as in other patients with cardiomyopathy.

Surveillance consists of a cardiology assessment with ECG and ECHO. This should be performed at the time of diagnosis or by the age of 6 years. Routine surveillance should be performed once every 2 years until the age of 10 and then yearly after that. If evidence of cardiomyopathy is present, surveillance every 6 months is indicated.

Pulmonary Interventions

Pulmonary function must be tested prior to wheelchair confinement. This should be repeated twice a year once the patient reaches 12 years of age, is wheelchair confined or vital capacity is found to be less than 80% of predicted.

Orthopedic Interventions

Physiotherapy to prevent contractures is the mainstay of the orthopedic interventions. Based upon patient requirements, passive stretching exercises, plastic ankle-foot orthosis during sleep, long leg braces to assist in ambulation may be used. Surgery to release contractures may be required for advanced disease. Surgery to correct scoliosis may improve pulmonary function.

Nutrition

Patients are at risk for malnutrition including obesity. Calcium and Vitamin D should be supplemented to prevent osteoporosis secondary to chronic steroid use. DEXA scanning should be obtained at age 3 and then repeated yearly.

Exercise

Guidelines recommend all patients participate in a gentle exercise to avoid disuse atrophy. A combination of swimming pool and recreation based exercises is recommended. Activity should be reduced if myoglobinuria is noted or significant muscle pain develops.

Novel Therapies

Gene therapies include medications that bind RNA and skip over the defective codon. This produces a shorter but potentially functional protein. Eteplirsen and Dispersion are exon 51 skipping antisense oligonucleotides medications used for this purpose. Eteplirsen has been approved by the FDA for this purpose.

Differential Diagnosis

Beckers Muscular Dystrophy (BMD)

BMD has a later onset, and the length of survival is longer. Patients typically have higher concentrations of dystrophin protein.

Intermediate form of Muscular Dystrophy

Patients with this form of dystrophy have dystrophin levels between DMD and BMD.

Myotonic Muscular Dystrophy

Inherited as an autosomal dominant disorder, distal muscles are more commonly affected the ability to walk is often preserved.

Limb-Girdle Muscular Dystrophy

This inherited dystrophy primarily affects muscles of the hip and shoulder girdles

Congenital Myotonic Dystrophies

This encompasses a group of inherited disorders associated with muscular dystrophy. The dystrophy is characterized by an increased severity at birth but has a benign course through life. There is a higher association with brain malformations. This includes diseases such as Ullrich type of muscular dystrophy, Fukuyama type of congenital muscular dystrophy and muscular dystrophy associated with Walker-Warburg syndrome to name a few.

Prognosis

The prognosis is typically poor for affected patients. Patients are typically wheelchair bound by the age of 12 years. Death occurs as a result of respiratory or cardiac complications in the teens or 20s. Other causes of death are pneumonia, aspiration, or airway obstruction.

Enhancing Healthcare Team Outcomes

The management of patients with DMD is best done with a multidisciplinary team that includes palliative care nurses and therapists. There is no cure for the disorder and all the treatments are palliative. The most important thing is to ensure that the patients lead a decent quality of life. Aggressive treatments may often do more harm than good.[12]


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Duchenne Muscular Dystrophy - Questions

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Which condition is seen in patients with Gower sign?



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A 3-year-old male is diagnosed with Duchenne muscular dystrophy. What commonly is the presenting symptom(s) of this disorder?



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A 3-year-old male is diagnosed with Duchenne muscular dystrophy. Which of the following is often the first symptom?



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"Pseudohypertrophy" of the calf is seen in which of the following conditions?



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A 14-year-old teen is affected by a genetic disease that does not affect his father who is also not a carrier. The father has siblings. His brother and a sister are not carriers and are not affected. He has a brother who has the disease. He also has a sister who is a carrier. Which of the following diseases could be present?



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Which disorder tends to present with pseudo-hypertrophy of muscles?



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A 14-year-old male presents to a healthcare clinic. He has been wheelchair-bound since 12 years of age. He has severe scoliosis and follows up with the cardiology clinic. Examination reveals enlargement of the muscles of the tongue and calf. Gene analysis will reveal an abnormality in coding for which protein?



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A 33-year-old female has been identified genetically to be a carrier of the gene causing Duchenne muscular dystrophy. Which of the following laboratory values will be elevated?



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What is the prevalence of Duchenne muscular dystophy?



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A 22-month-old female child presents with progressive muscular weakness. In order for her to have Duchenne muscular dystrophy, which of the following must also be TRUE?



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The development of the dystrophin gene in Duchenne muscular dystrophy is felt to be the result of spontaneous mutation in a high number of cases due to which of the following?



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Which is true of the mental and cognitive ability in patients with Duchenne muscular dystrophy?



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A 12-year-old with Duchenne muscular dystrophy has pseudohypertrophy of his muscles. This feature is most commonly seen in which location?



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Patients with Duchenne muscular dystrophy do NOT have which symptom?



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Which of the following is not true about Duchenne muscular dystrophy?



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Which of the following muscular disorders are associated with pseudohypertrophy?



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Which of the following best describes Duchenne muscular dystrophy?



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Which protein is abnormal in Duchenne muscular dystrophy?



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A 23-year-old wheelchair-bound male from New York was diagnosed with congestive heart failure in an emergency department and presents for follow up. He has been in the wheelchair for most of his life. He has severe scoliosis but no history of chest pain or hypercholesterolemia. The exam shows S1S2 regular with S3 but no murmurs. There are bilateral crackles in the lungs but no cyanosis. An ECG shows deep Q waves in V5 and V6 with tall R waves in V1 and V2. Global, severe left ventricular dysfunction is seen on echocardiogram. Select the most likely diagnosis.



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Select the incorrect statement about Duchenne muscular dystrophy.



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What is the gene location for Duchenne muscular dystrophy?



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A 3-year-old boy is diagnosed with muscular dystrophy. Which gene is most often responsible for this condition?



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A 30-year-old female is a known carrier of the gene for Duchenne muscular dystrophy. Which of the following may be markedly elevated on blood testing?



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What is the incidence of Duchenne muscular dystrophy?



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Which of the following could result in a female having Duchenne muscular dystrophy?



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Why is it presumed that there is a high spontaneous mutation rate for the dystrophin gene?



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How can the average intellectual function of children with Duchenne muscular dystrophy be characterized?



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Select the correct statement about pseudohypertrophy in patients with Duchenne muscular dystrophy.



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A 7 year old male child presented with progressive muscle weakness. He was also noticed to have hypertrophied calf muscles. Muscle biopsy would show which of the following?



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Which of the following is the most common childhood inherited myopathy?



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Which of the following is a reasonable diagnostic test in patient thought to potentially have Duchenne muscular dystrophy?



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Duchenne muscular dystrophy is transmitted through:



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What is the most common cause of death in children with Duchenne muscular dystrophy?



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An 8 year old child has Duchenne muscular dystrophy. Which developmental issue is most important to address?



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A child with Duchenne muscular dystrophy uses a manual chair at school but tires when traveling between classes. Which of the following would be the best solution?



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What kind of inheritance does Duchenne muscular dystrophy show?



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Which of the following accurately depicts Duchenne muscular dystrophy?



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A 15-year-old male is dependent on mechanical ventilation. He was healthy until he was 4, but then developed progressive symmetrical muscle weakness. Initially, it involved the pelvic girdle but currently affects all muscles. Early in the disease, there was bilateral calf enlargement. Biopsy showed random alternating muscle hypertrophy and atrophy and fatty pseudohypertrophy. Select the most probable diagnosis.



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You are in the clinic seeing a 7-year-old boy. The mother provides you with a long history. The child has been intellectually impaired since birth and has had great difficulty walking. Even when he started to walk at the age of 4, his gait was abnormal. He fell frequently and had difficulty going upstairs. While she is providing you with more history, you quickly glance at the back of the child's legs and notice that the calves are enlarged. The child also appears to have an enlarged tongue. What is the most likely diagnosis?



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A four-year-old boy presents with muscle weakness and muscle loss. Intellectual ability is diminished. A plain film of the back suggests the presence of scoliosis. Which of the following findings would not be expected?



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Presently, what is the only pharmacologic agent with documented benefits in both ambulation and cardiac function for patients with Duchenne muscular dystrophy?



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Which of the following maneuvers is associated with Duchenne muscular dystrophy?



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A 17-year-old wheelchair bound male presents to the emergency department with complaints of shortness of breath. On exam, he is tachycardic and tachypneic. Physical exam shows severe scoliosis, diffuse rales on auscultation of the lung, and hypertrophy of the calf muscles. What is the most common mode of inheritance of this disorder?



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A 3-year-old male is brought in for frequent falls. His mother reports that he began to walk at the age of 15 months. She also reports that an uncle had a disorder in which he was in a wheelchair at 13 years of age and died of a "heart problem" in his twenties. Which of the following would be least likely during an evaluation of this child?



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A 10-year-old patient with pseudohypertrophy of his calves is observed when walking. He is noted to be waddling and appears unsteady. While bearing weight on the right leg, the right hip drops instead of rising. What is the term for this type of gait?



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Which of the following is not true about Duchenne muscular dystrophy?



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A 5-year-old male is brought to your clinic by his parents with concerns of abnormal development. They state that he has muscle weakness that is getting worse and is having trouble running and playing at recess. On physical exam, you note that the child is slumped over. When asked to stand, he uses his arms to support him first before standing upright. Which protein has a defect in the child?



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Duchenne Muscular Dystrophy - References

References

Bello L,Pegoraro E, The     [PubMed]
Tomar S,Moorthy V,Sethi R,Chai J,Low PS,Hong STK,Lai PS, Mutational spectrum of dystrophinopathies in Singapore: Insights for genetic diagnosis and precision therapy. American journal of medical genetics. Part C, Seminars in medical genetics. 2019 May 13;     [PubMed]
Paquin RS,Fischer R,Mansfield C,Mange B,Beaverson K,Ganot A,Martin AS,Morris C,Rensch C,Ricotti V,Russo LJ,Sadosky A,Smith EC,Peay HL, Priorities when deciding on participation in early-phase gene therapy trials for Duchenne muscular dystrophy: a best-worst scaling experiment in caregivers and adult patients. Orphanet journal of rare diseases. 2019 May 9;     [PubMed]
Cai A,Kong X, The development of CRISPR-mediated systems in the study of Duchenne muscular dystrophy. Human gene therapy methods. 2019 May 7;     [PubMed]
Landrum Peay H,Fischer R,Tzeng JP,Hesterlee SE,Morris C,Strong Martin A,Rensch C,Smith E,Ricotti V,Beaverson K,Wand H,Mansfield C, Gene therapy as a potential therapeutic option for Duchenne muscular dystrophy: A qualitative preference study of patients and parents. PloS one. 2019;     [PubMed]
Jones D, Duchenne muscular dystrophy awaits gene therapy. Nature biotechnology. 2019 Apr;     [PubMed]
Ke Q,Zhao ZY,Mendell JR,Baker M,Wiley V,Kwon JM,Alfano LN,Connolly AM,Jay C,Polari H,Ciafaloni E,Qi M,Griggs RC,Gatheridge MA, Progress in treatment and newborn screening for Duchenne muscular dystrophy and spinal muscular atrophy. World journal of pediatrics : WJP. 2019 Mar 23;     [PubMed]
Nakamura A, Mutation-Based Therapeutic Strategies for Duchenne Muscular Dystrophy: From Genetic Diagnosis to Therapy. Journal of personalized medicine. 2019 Mar 4;     [PubMed]
Zhang K,Yang X,Lin G,Han Y,Li J, Molecular genetic testing and diagnosis strategies for dystrophinopathies in the era of next generation sequencing. Clinica chimica acta; international journal of clinical chemistry. 2019 Apr;     [PubMed]
Shimizu-Motohashi Y,Komaki H,Motohashi N,Takeda S,Yokota T,Aoki Y, Restoring Dystrophin Expression in Duchenne Muscular Dystrophy: Current Status of Therapeutic Approaches. Journal of personalized medicine. 2019 Jan 7;     [PubMed]
McMillan HJ, Intermittent glucocorticoid regimes for younger boys with duchenne muscular dystrophy: Balancing efficacy with side effects. Muscle     [PubMed]
Andrews JG,Pandya S,Trout C,Jaff T,Matthews D,Cunniff C,Meaney FJ, Palliative care services in families of males with muscular dystrophy: Data from MD STARnet. SAGE open medicine. 2019;     [PubMed]

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