Muscular Dystrophy

Introduction

Muscular dystrophy (MD) refers to a group of genetic diseases associated with progressive weakness and loss of muscle mass. Mutations in genes responsible for the production of proteins key to healthy muscle development (e.g. dystrophin) result in progressive muscle degeneration.¹

The two most common forms of MD are:

• Duchenne muscular dystrophy (DMD)
• Becker muscular dystrophy (BMD)

Other less common types of MD include:

• Emery-Dreifuss muscular dystrophy (humeroperoneal MD – affecting the arms and distal legs)
• Limb-girdle muscular dystrophy
• Facioscapulohumeral muscular dystrophy (affecting the face, scapula and arms)

Aetiology

DMD and BMD are classic examples of X-linked recessive conditions, meaning they mostly affect males, with females acting as carriers. However, one-third of DMD cases occur as a result of spontaneous mutations.²

Other types of MD have different modes of inheritance:

• Autosomal dominant (AD): facioscapulohumeral, distal, ocular, oculopharyngeal MD
• Autosomal recessive (AR): limb-girdle MD
• Multiple forms of inheritance: Emery-Dreifuss MD (can be X-linked recessive, AD or AR)

Clinical features

History

Typical presenting features of muscular dystrophy include:

• Progressive weakness: starting proximally and moving distally with the lower limbs being affected before the upper limbs.
• Delayed motor milestones: typically the ability to walk independently.
• Waddling gait

Other presenting features of muscular dystrophy may include:

• Faltering growth
• Fatigue
• Intellectual impairment (e.g. delayed speech milestones)
• Behavioural issues: attention deficit hyperactivity disorder, autism and obsessive-compulsive disorder.
• Leg pain

In children with DMD, although histological and laboratory evidence of myopathy may be present from birth, the typical clinical features of the condition (e.g. muscle weakness) do not usually become apparent until after the age of 2.

Children may first present with delayed motor milestones, such as the inability to walk. As a result, it is important to check the creatine kinase level in children who cannot walk by 18 months of age to screen for MD. Once children with MD start to walk, parents often describe them as ‘clumsy’ or ‘falling all the time’. A slow and ungainly run is a common presentation.

DMD patients usually become wheelchair users by the age of twelve. These children in particular tend to have evidence of scoliosis and poor pulmonary function.

Death usually occurs in the second or third decade of life due to cardiac or pulmonary complications.³

BMD is similar to DMD, but the manifestations of BMD typically develop later and are milder. Although muscle involvement is less severe than in DMD, cardiac involvement is often a predominant feature of the presentation in BMD. Patients with BMD tend to live past the fourth or fifth decade of life.¹

Clinical examination

Typical clinical findings of muscular dystrophy include:

• Weakness: typically the proximal and distal leg muscles in the earlier phases of the disease.
• Calf pseudohypertrophy: due to the accumulation of connective tissue and fat replacing muscle tissue.
• Waddling gait: typically exacerbated when attempting to run.
• Tip-toe walking: occurs due to shortening of the Achilles tendon.
• Gower’s sign: the child climbs up their legs when rising from the floor.
• Hyporeflexia or areflexia
• Loss of the arches of the feet (i.e. flat feet)
• Difficulty or inability to squat

Clinical findings associated with rarer forms of MD ³

Clinical findings associated with facioscapulohumeral MD include:

• Winging of the scapula
• Upper extremity weakness
• Facial weakness

Clinical findings associated with oculopharyngeal MD include:

• Dysarthria
• Dysphagia

Clinical findings associated with Emery-Dreifuss MD include:

• Contractures affecting the ankles, elbows and neck extension
• Increased tone in the lumbar and paravertebral muscles.
• Cardiac abnormalities such as first-degree heart block.

Investigations and diagnosis

The initial investigation used to screen for muscular dystrophy is serum creatine kinase (CK), an enzyme which leaks out of damaged muscle cells. The level of CK is typically elevated from birth and peaks around the age of 2, after which it steadily declines due to progressive muscle wasting. As a result, CK is not as reliable for screening in those who are already wheelchair users.

Definitive diagnosis is achieved via a combination of:

• Genetic analysis: to identify known muscular dystrophy mutations.
• Muscle biopsy: to enable analysis of the dystrophin protein.
• Findings on clinical examination

Other investigations which may be performed include:

• Electromyography: myopathic changes are apparent including small polyphasic potentials.
• ECG and echocardiography: to screen for cardiomyopathy.
• Lung function testing: to identify restrictive lung disease secondary to muscular weakness.

Management

Initial management

After diagnosis, the family should be provided with appropriate information and support, including the offer of screening for carrier status.

Other things to arrange after diagnosis include:

• Referral to any relevant specialists and/or multi-disciplinary teams (e.g. paediatric neurologist, cardiologist).
• Immunisations: including influenza and pneumococcal vaccines.

Early management

Early management encompasses the period in which the child is still walking.

Corticosteroids

Corticosteroids have been shown to prolong the child’s ability to walk by 6-24 months. They also may help to slow the progression of impaired respiratory function and cardiomyopathy.⁵

Side effects of steroids can include sleep disturbance, weight gain and osteoporosis.

Vitamin D and calcium supplements

Vitamin D and calcium supplements are often prescribed to enhance bone health.

Physiotherapy

Regular physiotherapy is essential to prevent the development of contractures.

Orthoses

Orthoses may be required to stabilise the knee, ankle and foot to prolong the child’s ability to walk.

Serial casting of the ankles

Serial casting of the ankles is performed to prevent shortening of the Achilles tendon and the associated gait abnormalities developing (e.g. tip-toe walking).

Later stage management

In the later stages of the disease, when the child is unable to walk, different management strategies are required.

Wheelchair

An electric wheelchair will be required to allow the child to continue to mobilise.

Counselling

Counselling may be required, particularly for adolescents.

Orthopaedic input

Orthopaedic input including orthotics and surgery for contractures and scoliosis.

Cardiac and respiratory surveillance

Cardiac and respiratory function will progressively deteriorate over the course of the disease and therefore effective surveillance is required to identify and treat problems early.

Respiratory surveillance and treatments options include:

• Monitoring of forced vital capacity, peak flow and pulse oximetry
• Regular review by a respiratory physician
• Low threshold for treating respiratory infections
• Chest physiotherapy
• Non-invasive ventilation (NIV)
• Tracheostomy

Cardiovascular surveillance and treatments options include:

• Regular cardiology reviews to identify signs of cardiac failure (e.g. 6-monthly)
• ECG and echocardiography to identify arrhythmias and impaired function
• ACE inhibitors, diuretics and beta-blockers for heart failure

Advanced planning and palliative care

Given the progressive nature of the disease, it is important that discussions surrounding appropriate ceilings of care are carried out in advance. Palliative care input should also be sought early in the terminal phase of the disease.

Key points

• Muscular dystrophy (MD) refers to a group of genetic diseases associated with progressive weakness and loss of muscle mass.
• Mutations in genes responsible for the production of proteins key to healthy muscle development (e.g. dystrophin) result in progressive muscle degeneration.
• The two most common forms of MD are Duchenne muscular dystrophy and Becker muscular dystrophy.
• Typical presenting features of muscular dystrophy include progressive weakness, delayed motor milestones, waddling gait and an inability to run or jump.
• Typical clinical findings of muscular dystrophy include muscle weakness (typically the lower limbs in early disease), calf pseudohypertrophy, and waddling gait.
• Definitive diagnosis is achieved via a combination of genetic analysis, muscle biopsy and findings on clinical examination.
• Early management involves corticosteroids, physiotherapy, orthoses, vitamin D and calcium supplements and serial casting of the ankles.
• Later stage management involves mobility aids (e.g. electric wheelchair), orthopaedic input and cardiorespiratory surveillance.
• End-stage disease requires advanced planning and palliative care input.

Further reading

If you’re interested in learning more about neuromuscular disorders, take a look at the RCPCH e-learning module developed by Dr Vanruiten, Consultant Paediatric Neurologist in the Great North Children’s Hospital.

Reviewer

Paediatric Consultant

References

1. Up to Date. Duchenne and Becker muscular dystrophy. Clinical features and diagnosis. Available from: [LINK].
2. Medscape. Population frequencies of inherited neuromuscular diseases – a world survey. Available from: [LINK].
3. Emedicine Medscape. Muscular Dystrophy Treatment and Management. Available from: [LINK].
4. Up to Date. Muscular Dystrophy, Duchenne and Becker muscular dystrophy: Management and Prognosis. Available from: [LINK].
5. Manzur A, Kuntzer T, Pike M, et al. Glucocorticoid corticosteroids for Duchenne muscular dystrophy. Cochrane Database Syst Rev. Available from: [LINK].