Muscle Diseases

Symptoms of muscle diseases include:

• Primary symptom is weakness, which is usually progressive, bilateral and often involving muscles close to the trunk, i.e. hip and shoulder muscles
• Difficulty in walking or running, climbing stairs and/or getting up from sitting
• Difficulty in carrying heavy loads, lifting or reaching above the shoulders. In infants and young children, this weakness may show up as loss of tone and delay in the ability to stand, walk or run

Quite often, the weakness is associated with muscle wasting. Although muscle aches and cramps may occur in muscle diseases, these symptoms are more commonly due to other causes such as muscle strain. Easy fatigue with weakness is typically associated with myasthenia gravis. Episodic paralysis is a distinctive symptom in the periodic paralyses.

The muscular dystrophies

The muscular dystrophies are a group of muscle diseases, each caused by a specific gene abnormality, with progressive muscle wasting, weakness and contractures. Although the abnormal gene is often inherited, it can also occur spontaneously. These diseases can arise even when no one else in the family is affected.

Some muscular dystrophies are very severe, while others cause only mild symptoms. In children, the most common is Duchenne muscular dystrophy, a severe muscle disease which usually only affect boys. Unlike girls, affected boys do not have a second X-chromosome to compensate for an X-chromosome carrying the abnormal gene. Weakness is noticed as they begin to walk or run. Characteristic signs include walking on tiptoe and large calves. The disease occurs in one out of every 3000 newborn male children.

Other relatively common forms of muscular dystrophy include limb girdle muscular dystrophy, facioscapulohumeral muscular dystrophy and myotonic dystrophy. Both sexes are affected, as the abnormal genes are not in the X-chromosome. These diseases may start in adulthood or childhood.

Besides weakness and wasting, myotonic dystrophy causes muscle stiffness with difficulty in relaxation called myotonia. In some muscular dystrophies, the heart may also be involved.

The inflammatory muscle diseases

When the patient’s immune system damages the body’s own muscle tissues, the result is an inflammatory muscle disease.

Three main types of inflammatory muscle diseases are identified:

1. Polymyositis
2. Dermatomyositis
3. Inclusion body myositis

These diseases usually affect adults, although dermatomyositis may also affect children.

Patients with these diseases develop progressive weakness of the hip and shoulder muscles over a few weeks or months, sometimes with difficulty in swallowing. In the case of dermatomyositis, a characteristic rash may occur. It is very important to correctly diagnose these disorders because polymyositis and dermatomyositis can be treated effectively with drugs that suppress the immune system.

Myasthenia gravis

In myasthenia gravis, the body’s immune system inappropriately injures a specialised muscle structure called the acetylcholine receptor. Damage to these receptors leads to the typical symptom of this disease, namely easy fatigue and increased weakness on exertion.

Often, the eyes are affected with drooping of the upper eyelids and double vision. Generalised weakness and difficulty with swallowing or speaking is common. This disease will also respond effectively to treatments that suppress the immune system.

Cover picture shows a combined cytochrome oxidase and succinic dehydrogenase stain: Mitochondrial myopathy showing COX negative fibres, including a “ragged red fibre”.