ANSWER 14
This woman’s generalized weakness is due to myasthenia gravis. Myasthenia gravis is
due to the presence of acetylcholine receptor antibodies causing impaired neuromuscular
transmission. It characteristically affects the external ocular, bulbar, neck and shoulder
girdle muscles. Weakness is worse after repetitive movements which cause acetylcholine
depletion from the presynaptic terminals. The onset is usually gradual. Ptosis of the upper
lids is often associated with diplopia due to weakness of the external ocular muscles.
Speech may become soft when the patient is tired. Symptoms are usually worse in the
evenings and better in the mornings. Permanent paralysis eventually develops in some
muscle groups. In severe cases respiratory weakness occurs.
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- Motor neurone disease: suggested clinically by muscle fasciculation and later by
marked muscle weakness. - Muscular dystrophies: selective muscular weakness occurs in specific diseases, e.g.
facioscapulohumeral dystrophy. There is usually a family history. - Dystrophia myotonica: this causes ptosis, wasting of the masseter, temporal and
sternomastoid muscles and distal muscular atrophy. There is a characteristic facial
appearance with frontal baldness, expressionless facies and sunken cheeks. There
may be gonadal atrophy and mental retardation. There is usually a family history.
The electromyogram (EMG) is diagnostic. - Polymyositis: this may have an acute or chronic onset. A skin rash and joint pains
are common. The creatine kinase level is raised and a muscle biopsy is diagnostic. - Miscellaneous myopathies: thyrotoxic, hypothyroid, Cushing’s, alcoholic.
- Non-metastatic associations of malignancy: thymoma is associated with myasthenia
gravis in 10 per cent of cases; the Eaton–Lambert myasthenic syndrome is
associated with small-cell lung carcinoma.
Differential diagnoses of generalized muscle weaknessThis patient should be investigated by a neurologist. Electromyography (EMG) will demon-
strate fatiguability in response to repetitive supramaximal stimulation. Intravenous injection
of edrophonium (Tensilon) will increase muscular power for a few minutes. Blood should
be assayed for acetylcholine receptor antibodies (present in 90 per cent). A computed
tomography (CT) of the thorax should be performed to detect the presence of a thymoma
or lung cancer. Corticosteroids are the drugs of first choice. Anticholinesterase drugs
greatly improve muscle power but have many side-effects. Thymectomy should be con-
sidered. It is most effective within 5 years of diagnosis and when there is no thymoma.
- Myasthenia gravis is a cause of abnormal muscular fatiguability.
- In its initial stages it affects certain characteristic muscle groups.