Muscular Dystrophy & Spinal Muscular Atrophy

Muscular Dystrophy

Introduction

There are several different types of muscular dystrophy, most of which are rare. All are progressive, hereditary disorders, in which muscle fibres degenerate and are replaced by fibrous tissue and fat, resulting in gradually increasing weakness. Only those muscular dystrophies most commonly encountered are described in detail. Their effects depend on the muscles involved and on the rate of progress of the disease.

Duchenne Muscular Dystrophy

This is the commonest and most severe form of muscular dystrophy. In about two-thirds of affected individuals, the condition is inherited through the mother, who is a carrier. In the remainder, the condition is a result of a change in genetic structure (mutation) in the egg (ovum). Only boys are affected by the disease - although a few female carriers show some of the symptoms, which rarely, is severe enough to cause wheelchair-dependency.

The condition usually becomes apparent between one and four years old, with delay in walking, a clumsy gait, inability to run properly, frequent falls and difficulty managing stairs. Learning difficulties and behavioural problems are relatively common, but usually mild to moderate.

Weakness of the leg and hip muscles gradually increases, and walking becomes more difficult, despite the use of aids until a wheelchair is necessary. All boys become completely wheelchair-dependent by the age of 12 years. There is progressive weakness of the upper limbs and by mid-adolescence the only useful residual function is finger movement, which allows use of a keyboard and control of an electric wheelchair. Progressive spinal weakness leads to increasing curvature of the spine (scoliosis) which compromises breathing and causes difficulty with posture in the wheelchair. Spinal surgery (insertion of steel rods) may be necessary. Breathing difficulties increase due to progressive chest muscle weakness and spinal curvature. Untreated, boys used to die in their late teens from ventilatory failure. The introduction of night-time, non-invasive ventilation over the last 20 years has led to marked prolongation of life (into the 30s) but with the added complication of increasing physical disability and heart involvement. Heart muscle involvement (cardiomyopathy) is inevitable and requires drug treatment to prevent and treat heart failure. Although survival has been prolonged with the use of ventilation, those living longer are more likely to develop symptomatic cardiomyopathy, which may be fatal. There is more information about cardiomyopathy in the A-Z of medical conditions guidance.

Becker Type Muscular Dystrophy

This is a variant of Duchenne dystrophy, again usually only affecting boys, although there are rare cases of symptomatic female carriers who may also develop cardiomyopathy. It is extremely variable in severity. At the mildest end of the spectrum, the only complaint may be of muscle pain on exercise or weakness may appear for the first time in middle age and progress only slowly. At the most severe end, it is essentially indistinguishable from Duchenne dystrophy – wheelchair dependency being a little later than in Duchenne i.e. in mid to late -teens. The most common form presents with weakness in childhood, affecting first the legs and later the arms. It progresses slowly and boys may remain ambulant (able to walk) with aids for many years. Wheelchair use becomes increasingly necessary from about the age of 40 years, but sometimes earlier. Heart muscle involvement (cardiomyopathy) is common and again variable in severity. In some, the cardiomyopathy is far more severe than limb involvement – rare cases require cardiac transplantation. More information on the effects and treatment of cardiomyopathy is provided in A-Z of medical conditions guidance.

Limb Girdle Dystrophy

This broad term covers more than twenty specific genetic entities and affects both sexes. In terms of severity, the range is from a condition as severe as Duchenne dystrophy, to first presentation in late-middle age. Most commonly it presents in adolescence and early adult life. Muscle weakness is usually noticed in the lower limbs before the upper. Progression is relatively slow. Significant numbers eventually become wheelchair-dependent. Some are associated with cardiac involvement, which may be severe. Many are associated with ventilatory insufficiency, which may require the use of non-invasive ventilation. In some, ventilatory failure may occur when the patient is still able to walk, but usually it develops when they have been wheelchair dependent for many years.

Facioscapulohumeral Dystrophy

This is generally a milder disorder than those already mentioned, and affects both sexes. Onset may be at any age from childhood to adult life, but comes on most commonly during adolescence. The muscles of the face and shoulders are most affected, causing increasing difficulty in lifting the arms, weakness of eye closure and lack of facial expression.

Despite the name, lower limb involvement is common. Initially this is most commonly in the form of foot drop, but in the later stages of the disease there may be marked weakness around the hips causing major walking difficulties. It is commonly associated with spinal muscle weakness and abnormal curvature of the lumbar spine – low back pain is a common feature. About 10% of patients become wheelchair-dependent, and some of those will develop ventilatory insufficiency requiring non-invasive ventilation.

Myotonic Dystrophy (dystrophia myotonica)

This condition is extremely variable in severity and differs from other forms of muscular dystrophy in being a multi-system disorder. The muscular problems may be accompanied by others involving the eyes, heart, lungs, hormone and immune systems and the brain. As well as weakness, there is difficulty in relaxing muscles after voluntary effort (myotonia).

It affects both sexes and is commonly found in adults, developing between the ages of twenty and fifty, though it may occasionally arise in childhood or even at birth (in children of affected women) when it tends to be very severe and associated with major learning disabilities. The first symptoms in adults are usually weakness of the hands and difficulty with walking. The major non-muscle symptoms relate to brain function. Many have excessive daytime sleepiness very similar to narcolepsy. This may greatly interfere with employment and domestic activities. In those with adult onset, IQ tends to be lower than average, but in addition, specific cognitive dysfunction is very common. This may be seen as “apathy”, and look superficially like depression, but there are characteristic features of frontal lobe dysfunction (e.g. difficulties planning and completing simple tasks – dysexecutive problems) which may significantly affect work performance as well as social functioning. Unlike other physical effects of the condition, cognitive function does not tend to worsen significantly during life.

The distribution of muscle weakness is unusual, affecting the extremities more severely than the shoulder and hip girdle muscles. Although the rate of progression of weakness is relatively slow, the specific involvement of the hands and ankles leads to profound functional disability, despite strong proximal muscles. In late stages, there may be wheelchair-dependency, but by then there will have been years of substantial disability due to distal weakness.

Breathing muscles are affected early and causes hypoventilation which increases the risk of chest infection, the risk of which is also exacerbated by the involvement of the swallowing muscles which leads to aspiration of food and fluids. Pneumonia is the commonest cause of death.

The heart conduction system is involved and may lead to blackouts requiring pacemaker insertion. In addition, malignant cardiac arrhythmias are a relatively common cause of premature death. The effects and treatment of cardiac arrhythmias are covered in A-Z of medical conditions guidance.

Spinal Muscular Atrophy

Although not a dystrophy, this condition may at first resemble one, and it causes similar problems. It is a form of motor neurone disease, but clinically very different to classical adult motor neurone disease. It is inherited in recessive fashion so typically the parents, and any children of those with condition, are unaffected. The sexes are affected equally. Three main clinical forms are recognised.

The infantile form (Werdnig-Hoffman disease) presents at or soon after birth. There is severe weakness of all muscles, except those of the face. Most die within the first year of life. Some may survive longer but they are ventilator-dependent.

"Intermediate" spinal muscular atrophy is a severe, chronic, generalised form of the disease which normally presents in the first year of life with severe muscle weakness and skeletal deformities. Despite the fact that they have severe weakness and physical disability, these children are intellectually normal.

The juvenile form (Kugelberg-Welander Syndrome) is milder, and occurs between early childhood and adolescence. It usually begins with difficulty in walking, climbing stairs and rising after a fall. Progress is spasmodic, periods of deterioration being interspersed with periods when it seems to be arrested, and the ability to walk is frequently retained into adult life.

Despite the severity of the weakness, and progression of disability during childhood, the condition tends to stabilise in early adulthood and during adult life progression is extremely slow. A proportion of people with the milder forms will require nocturnal non-invasive ventilation. All of those with the intermediate form, and most of those with the juvenile form, will be wheelchair-dependent in adult life.

Further Evidence

The General Practitioner, Hospital Specialist, Specialist Nurse, Physiotherapist or other Health Care Professional will be able to provide further information.

Care and Mobility Considerations

Because of the range of disabilities caused by these conditions, it is impossible to generalise. Needs arise as muscle groups degenerate, and the type and level of need in any individual will depend on which muscles are involved and what stage has been reached in the disease process. Either, care or mobility needs may be the first to arise and in the more severe forms of these conditions, both may be extensive.

It is important that these people are kept under regular review by medical, orthopaedic and physiotherapy agencies. It is essential to maintain good posture, mobility either independently or with aids, and as good a quality of life as possible.

Duration of Needs

All these conditions are progressive, and cause gradually increasing disability.

Needs, once established do not abate. The level of need will increase as the disease progresses.

All information must be taken into account when considering the duration of disabling effects and the duration of disabling effects must be based on the particular circumstances of the individual claimant.

Adapted from the DHB – July 2011

Amended October 2011