Clinical features

There is much variation in presentation and course of the disease depending on which groups of muscles become weak, and how severe the progression of the condition is. In milder cases localised groups of muscles are affected, while in more serious disease there is generalised weakness.

Characteristically people report that the degree of muscle weakness fluctuates during the day, becomes worse with repetitive movement (fatigability) and improves after rest of the affected part. The onset of the disease may be gradual with mild symptoms, and it is some time before the extent and degree of muscle weakness is apparent. Symptoms and signs of weakness are often least noticeable in the mornings and worsen throughout the day. Weakness may be exacerbated by exercise, hot temperatures, infectious illnesses, menstruation, surgery and certain types of drugs. Sensation and tendon reflexes are normal on examination.

The typical clinical presentations are described under four groupings:-

Ocular myasthenia

The weakness affects the muscles around the eyes (ocular muscles) causing drooping eyelids, (ptosis) and double vision (diplopia). One eye may be affected more than the other. People report tiredness of the eyes and difficulty in carrying out of everyday tasks such as reading, watching television and driving. In 10 – 30% of people the effects of myasthenia gravis remain confined to the ocular muscles. However 90% of people with more widespread disease have symptoms affecting the eyes.

Limb girdle weakness

The weakness affects the muscles of the limb girdles i.e. around the shoulders and pelvis (buttocks and thighs), where the muscles of the arms and legs respectively articulate with the main trunk. The muscles of the shoulders are affected more often then the muscles of the pelvic girdle. People report difficulty in lifting their arms up, reaching up and carrying heavier items. If the pelvic girdle is affected, difficulties in standing for prolonged periods, squatting down, rising from a chair and walking occur. Fine movements of the hands may be lost as the disease progresses. In long standing disease that has not responded well to treatment there may be wasting of affected muscles.

Bulbar weakness

Weakness involves the muscles of the face and neck causing a lack of facial expression and an inability to smile. Some people may present with difficulty in holding their neck up, and the jaw tends to drop open. The ability to chew, swallow, speak and breathe becomes affected. People report choking, nasal regurgitation and drooling of salvia. The speech is slurred, the voice is described as weak and it tends to falter with use. These types of signs and symptoms are described as bulbar.

Respiratory weakness

The disease process affects the muscles of respiration causing shortness of breath. In a minority of cases the shortness of breath becomes so severe that it causes a life threatening paralysis of respiration requiring artificial ventilation in intensive care. This may also be accompanied by severe paralysis of all four limbs. This is known as a myasthenic crisis and is often precipitated by an intercurrent infection.

Course of disease

In 10 -30% the condition affects the ocular muscles only. But in other cases the disease tends to progress affecting the limb girdles and then the facial, bulbar and respiratory muscles. The progression to more generalized disease may take place over some weeks or months. Within the group of people who have progressive disease, 90% will have developed generalised disease within one year of onset of the myasthenia.

Amended June 2008