Duchenne Muscular Dystrophy

Original Editor - Rachael Lowe

Top Contributors - Alyssa Aquino, Rachael Lowe, Rucha Gadgil, Laura Ritchie and Faye Underwood  

Introduction

Duchenne muscular dystrophy (DMD) is a genetic condition which affects the muscles, causing muscle weakness. It is a serious condition which starts in early childhood. The muscle weakness is not noticeable at birth, even though the child is born with the gene which causes it. The weakness develops gradually, usually noticeable by the age of three. Symptoms are mild at first, but become more severe as the child gets older.


Duchenne muscular dystrophy, the most common type, is one of more than 20 muscular dystrophies. The incidence of DMD globally is every 1/3500 male births[1] That means that there is approximately 2400 individuals living with DMD in the UK alone[2].


All types of muscular dystrophies are caused by faults in genes (the units of inheritance that parents pass on to their children) which result in progressive muscle weakness due to muscle cells breaking down and gradually becoming lost. The Duchenne type dystrophy is an X-linked genetic disorder affecting primarily boys (with extremely rare exceptions) and a problem in this gene is known to result in a defect in a single important protein in muscle fibres called dystrophin. It is named after Dr Duchenne de Boulogne who worked in Paris in the mid-19th century who was one of the first people to study the muscular dystrophies.

Clinically Relevant Anatomy

Dystrophin is responsible for connecting the cytoskeleton of each muscle fiber to the underlying basal lamina. The absence of dystrophin stops calcium entering the cell membrane affecting the signalling of the cell, water enters the mitochondira causing the cell the burst. In a complex cascading process that involves several pathways, increased oxidative stress within the cell damages the sarcolemma resulting in the death of the cell and muscle fibers undergo necrosis and are replaced with connective tissue.

Small amounts of dystrophin are also made in nerve cells (neurons) in specific parts of the brain, including the hippocampus. The hippocampus is the part of the brain involved in learning and memory, as well as emotions.The non-progressive memory and learning problems, as well as social behavioral problems, in some boys with DMD are most likely linked to loss of dystrophin in the neurons of the hippocampus and other parts of the brain where dystrophin is normally produced in small amounts, howvwer studies are being carried out to find out why only a small no. of DMD individuals are affected by this.[3]

Mechanism of Injury / Pathological Process

Histopathology of gastrocnemius muscle from patient who died of pseudohypertrophic muscular dystrophy, Duchenne type. Cross section of muscle shows extensive replacement of muscle fibers by adipose cells

The cause is a mutation in the  gene that encodes the 427-kDa cytoskeletal protein dystrophin which affects the muscles. People with DMD have a shortage of dystrophin in their muscles. The lack of dystrophin leads to muscle fibre damage and a gradual weakening of the muscles.

Clinical Presentation

The muscle weakness is mainly in the 'proximal' muscles, which are those near the trunk of the body, around the hips and the shoulders. Weakness typically starts proximally in the lower extremities, then moves distally. Weakness in the upper extremities tends to appear later[1]. This means that fine movements, such as those using the hands and fingers, are less affected than movements like walking.

The symptoms usually start around age 1-3 years, and may include:

  • Difficulty with walking, running, jumping and climbing stairs. Walking may look different with a 'waddling' type of walk. The boy may be late in starting to walk (although many children without DMD also walk late).
  • When you pick the child up, you may feel as if he 'slips through your hands', due to looseness of the muscles around the shoulder.
  • Toe-walking
  • Frequent falls
  • The calf muscles may look bulky, although they are not strong.
  • As he gets older, the child may use his hands to help him get up, looking as if he is 'climbing up his legs'. This is called 'Gower's sign'.
  • Some boys with DMD also have a learning difficulty. Usually this is not severe.
  • Sometimes, a delay in development may be the first sign of DMD. The child's speech development may also be delayed. Therefore, a boy whose development is delayed, may be offered a screening test for DMD. However, DMD is only one of the possible causes of developmental delay - there are many other causes not related to DMD.
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Diagnostic Procedures

The diagnosis may be suspected because of the child's symptoms (above). When looking for signs of DMD, it is important to watch the child running and getting up from the floor - the muscle weakness is more noticeable during these activities.

Tests are needed before DMD can be diagnosed. The first step in making the diagnosis is a blood test. This tests for creatine kinase. Children with DMD always have a very high level of creatine kinase (about 10-100 times normal). Therefore if a child's creatine kinase level is normal, then DMD is ruled out. If the creatine kinase level is high, further tests are needed to see whether this is due to DMD or to some other condition.

The next step in diagnosing DMD involves either a muscle biopsy and/or genetic tests:

  1. A muscle biopsy involves taking a small sample of a muscle, under local anaesthetic. The sample is examined under a microscope using special techniques to look at the muscle fibres and the dystrophin protein.
  2. Genetic tests are done using a blood sample. The DNA in the blood is tested to look at the dystrophin gene. This test can diagnose most cases of DMD.

Outcome Measures

Outcome measures for a DMD individual vary according to the progression of the disease.

Outcome measures to quantify disease progression, including: the 6‐minute walking test, the North Star ambulatory assessment scale, the time taken to climb four steps, the time taken to rise from the floor, and the performance of upper limb. One of the limitations of these measures is the fact they target either ambulant or non‐ambulant patients[7]. However as the disease progresses, the outcome measures change making it difficult to use a single outcome measure to analyse the patient. Studies are being carried out to create a uniform measure for the muscular dystrophies.

(see Outcome Measures Database)

Management / Interventions

There is no cure for DMD at present. The proactive symptom-based multidisciplinary team (MDT) management and access to non-invasive ventilation have enabled improved survival into adulthood[8]. Males with DMD, with intervention, can now be expected to live until their 30's and 40's[9]

Preschool age

Usually, at this stage, the child will be well and not need much treatment. What you will usually be offered is:

  • Provide information about DMD and patient support groups.
  • Referral to a specialist team (for example, a paediatrician or neurologist, physiotherapist and a specialist nurse) to monitor child health.
  • Advice about the right level of exercise.
  • Genetic advice for the family.

Age 5-8 years

At this age, some support may be needed for the legs and ankles. For example, using night time ankle splints, or with a knee-ankle-foot orthosis. There is no evidence of significant benefit from any intervention for increasing ankle range of motion[10].

Treatment with corticosteroids can help to maintain the child's muscle strength. This involves taking medication such as prednisolone or deflazacort as a long-term treatment, either continuously or in repeated courses.

8 years-late teenage years

At some time after the age of 8 years, the child's leg muscles become significantly weaker. Walking gradually gets more difficult, and a wheelchair is needed. The age at which this happens varies from person to person. Often it is around age 9-11 years, although with corticosteroid treatment, some boys can walk for longer.

After the child starts needing a wheelchair, this is also the time that complications tend to begin, so it is important to monitor the boy's health and to treat any complications early.

Practical support and equipment will be needed at this stage, for example, wheelchairs and adaptations to the child's home and school.

Counselling and emotional support for child and family may be helpful.

Late teenage years-twenties

At this stage, muscle weakness becomes more problematic. Increasing help and adaptations are needed. Complications such as chest infections are likely to increase, so more medical monitoring and treatment are required.

Physiotherapy

Due to the wasting of muscle caused by the lack of dystrophin, DMD patients will struggle with many everyday activities. Physiotherapists can help with management of the presenting neuromusculoskeletal problems. They can help slow the degression of range of motion, muscle strength, daily function, work to improve gait pattern and posture/alignment [9]. Physiotherapy can also address the pain that the patient may be experiencing.

Physiotherapists often implement stretching programs for DMD patients[11]. A stretching program can be a combination of passive range of motion, active range of motion, and active assisted range of motion [11]. It is suggested that stretching will help the patient cope with decreased muscle extensibility and muscle contractures. Regular stretching of the ankle, knee, and hip is necessary throughout the course of a patient's life[11]

As the patient's walking and standing abilities decline the physiotherapist may chose to impliment a standing program [11].

Pharmacology

In the ambulant stage, chronic cortical steroid treatment is an accepted practice. Glucocorticoid, a type of corticosteroid, is a standard of care among DMD patients.The use of steroids has been shown to help improve muscle strength and function, delay the loss of ambulation, and help maintain cardiac and respiratory function. Due to the fact that it helps with muscle strength, cortical steroids also reduces the risk of scoloisis.[12]

Treatment plans may vary, with some patients taking steroids everyday or every second day. Different dosages may also be taken.

As with all medications there are side effects which should be taken under consideration when treating the patient. Examples of know side effects are: Cushingoid features, weight game and growth inhibition, impaired fat and glucose metabolism, fluid retention and hypertension, osteroporosis with increased risk of vertebral fractures, and cataracts[12].

Respiratory Care

Respiratory function should be routinely checked in DMD patients. This allows physicians to monitor  who may need help with assisted coughing and ventilation in the future. Examples of what evaluation of respiratory function should include are: spirometric measurements of FVC, FEV, and maximal mid-expiartory flow rate, maximum inspiratory and expiratory pressure, and peak cough flow. Carbon dioxide levels should also be monitored[13]

Airway clearance is of importance to prevent atelectasis and pneumonia. A variety of manual techniques can be used in clinical practice to help clear the patient's airways. Manually assisted coughing techniques are useful in patients who have a low cough peak flow rate (below 160 L/min) because self-clearance of their airways are not adequate. Expiratory force can also be increased by applying pressure to the patients upper abdomen during their natural cough. Other manual techniques include air stacking, glossopharyngeal breathing, and positive pressure application [13].

Along with manual techniques mechanial techniques and mucus mobilization devices may also prove useful[13].

Complications

Anaesthetics

People with DMD need extra care if they have a general anaesthetic. Certain anaesthetic medicines can cause a harmful reaction for people who have DMD. Also, extra care for the chest and breathing is needed. It is important to have a pre-operative assessment and a senior anaesthetist providing the anaesthetic care.

Osteoporosis

People with DMD may develop osteoporosis. This is due to lack of mobility and also to steroid treatment. It is important to prevent osteoporosis as far as possible. A good intake of vitamin D and calcium can help.  Sometimes a blood test to check vitamin D levels is advised, and vitamin D supplements may be offered.

Joint and spinal complications

Muscle weakness can result in joint contractures. In DMD, it is often the ankle joint and Achilles' tendon which become tight. This can be treated either using orthotic devices or by surgical release of the tendon.

Scoliosis can occur due to muscle weakness. Usually, this happens at the begining of the second decade of life, after the patient has lost ambulation[13]. The scoliosis can cause discomfort and is not helpful for posture and breathing. Treatments which can help are a spinal brace, or surgery to the spine.  Surgery has been shown to improve function and quality of life[14]. Surgery should be done while the patient still has sufficient lung function and before cardiomyopathy becomes a major risk factor when putting the patient under anesthesia[13].

Nutrition and digestion

Some children with DMD are prone to being overweight, especially if taking steroid treatment. Teenagers and adults with DMD may be underweight, due to loss of muscle bulk. Dietary advice can be helpful in these situations.

Constipation can be a symptom for anyone who is not mobile. This can be treated with laxatives and a high fibre diet.

In the later stages of DMD (as a young adult and older), people with DMD may have difficulty with chewing and swallowing food. They may need a careful assessment and nutritional advice or supplements. If the problem is severe, then a gastrostomy may be needed.

Chest and breathing complications

During the teenage years, the breathing muscles weaken, causing shallow breathing and a less effective cough mechanism which can lead to chest infections.  All individuals with DMD will present with restrictive lung disease[1]. Clearance techniques and non-invasive ventilation may help.

As the breathing muscles get weaker, oxygen levels in the blood may be reduced, more so while sleeping. Because this develops gradually, the symptoms may not be obvious. Possible symptoms are tiredness, irritability, morning headaches, night time waking and vivid dreams.

Vital capacity will increase as in a normal individual until the age of ten. After the age of ten, an individal's vital capacity will decrease by approximately 8-12% each year[1].

It is helpful if breathing problems are detected and treated early; so patients with DMD will usually be offered regularlung function tests once they start to have significant muscle weakness. 

Cardiac (heart) complications

Teenagers and adults with DMD may develop cardiomyopathy.Cardiomyopathy generally develops around the age of 10 and by the age of 18 all individuals with DMD will present with cardiomyopathy [1]. In dilated cardiomyopathy it becomes difficult for the heart to pump blood to the body because the chambers have become enlaged and the heart wall has thinned [1].

With DMD, the cardiomyopathy does not usually cause much in the way of symptoms. Possible symptoms are tiredness, leg swelling, shortness of breath or an irregular heart beat. Cardiomyopathy can be helped by medication which seems to work best if started at an early stage, before symptoms are noticed. So people with DMD are usually offered regular heart check-ups, starting from early childhood. The check-ups usually involve an ECG.

Differential Diagnosis

Other types of muscular dystrophy - particularly Becker's Muscular dystroph, which is similar but progresses more slowly. The age of onset is usually later and clinical involvement milder. Muscle biopsy can be used as standard for diffentiating Becker's and Duchene's dystrophy.

Other myopathies- Creatinine Kinase levels are usually lower than those in DMD. Identification of deletion or mutation of relevant gene with DNA analysis confirms the diagnosis.

Polymyositis- Diagnosis is established by characteristic muscle biopsy findings of inflammation, including mononuclear invasion of non-necrotic muscle, CD8+ cytotoxic/suppressor T cells, macrophages, and absence of perifascicular atrophy of dermatomyositis. Usually affects proximal and limb girdle muscles.

Neurological causes of muscle weakness - eg, spinal cord lesions ,spinal muscular atrophies , motor neurone disease, multiple sclerosis. These conditions are likely to have additional features, such as sensory loss, upper motor neuron lesion signs or muscle fasciculation.

Increased transaminases (aspartate aminotransferase and alanine aminotransferase, which are produced by muscle as well as liver cells). The diagnosis of DMD should thus be considered before liver biopsy in any male child with increased transaminases[15].


Resources

References

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  2. Muscular Dystrophy UK. Muscular Dystrophy UK Fighting Muscle-wasting Conditions. http://www.musculardystrophyuk.org/about-muscle-wasting-conditions/?gclid=CjwKEAjwpYeqBRDOwq2DrLCB-UcSJAASIYLjj2w3LQ3hI43XFhdWOw7wXT-6OFurebTSskU9ckZCdhoCZwfw_wcB (Accessed 28 April 2009)
  3. Rae MG, O'Malley D. Cognitive dysfunction in Duchenne muscular dystrophy: a possible role for neuromodulatory immune molecules. J Neurophysiol. September 1 2016; 116(3):1304-15. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5023417/.
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  10. Rose KJ, Burns J, Wheeler DM, North KN. Interventions for increasing ankle range of motion in patients with neuromuscular disease. Cochrane Database Syst Rev. 2010 Feb 17;2:CD006973.fckLR.
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