-IBIS-1.5.0-
tx
nervous system
myasthenia gravis
diagnoses

definition and etiology

definition: An acquired autoimmune disorder recognized by easy muscle fatiguability and weakness, especially in the muscles innervated by the cranial nerves, that improves with the administration of cholinesterase-inhibiting medications.

etiology: The incidence of myasthenia gravis is 2-5 per year per million. Although the disease may strike anyone at any age the two most affected patient groups are adolescent and young adults, especially females, and adults over 40, who also have a higher risk of associated thymoma. The MG problem seems to lie in a deficiency of the acetylcholine receptors at the motor endplate of the muscles: apparently the release of acetylcholine by the nerve is normal, but lack of receptors decreases the muscle response. The disease presents either specifically with external ocular muscle problems or with generalized cranial nerve involvement. In one standard classification of the disease there are three subtypes:
• MG with thymoma: No association with sex or age of onset. The disease is usually severe. There is no HLA correlation.
• No thymoma, onset before 40 years old: This presentation is associated with females, and there is HLA correlation to several antigens.
• No thymoma, onset after 40 years old: This presentation is associated with males, and there are other specific HLA correlations.

The onset of the disease is purely ocular in 40% of patients, generalized in 40%, involves bulbar and/or ocular muscles in 10%, and affects only the extremities in 10% (proximal limbs are more severely affected than distal muscles). As the disease progresses, the weakness often spreads over the entire body. After the first year with MG, almost all patients have ocular involvement, and in the patients with whom the disease will become generalized, 90% have already done so. In only 16% of patients do the symptoms remain purely ocular. During the first three years the progression of the disease is rapid and most deaths occur during this time. Although remissions are seen (in about 25% of patients), they do not usually last long.

Only 10-15% of MG patients have a thymoma, however, two thirds of the patients have thymic hyperplasia. 10% of patients also exhibit the signs and symptoms of other autoimmune diseases such as SLE, RA, sarcoidosis, ulcerative colitis, pemphigus, etc.

signs and symptoms

signs and symptoms:
The onset can be sudden or insidious.
The most common initial presentation includes:
• Diplopia.
• Dysphagia.
• Dysarthria.
• Limb weakness.
• Weakness of the respiratory muscles.
• Symptoms are better from rest, worse exertion.

lab findings:
• Anticholinesterase drug administration (edrophonium): if symptoms improve, test is (+).
• Electromyography: decreased amplitude of the muscle action potential.
• Serologic: Positive acetylcholine receptor antibodies.

course and prognosis

Conventional treatments: anticholinesterase drugs, thymectomy, and prednisone.

The disease is progressive and involvement of the respiratory muscles can become life-threatening. A myasthenia gravis crisis occurs when the patient suffers increasing weakness with increasing amounts of anticholinesterases: this requires hospitalization.

differential diagnosis

• Transient neonatal myasthenia.
• Eaton-Lambert Syndrome.
• Neurasthenia.
• Oculopharyngeal dystrophy.
• Intracranial lesions compressing cranial nerves.
• Drug-induced myasthenic disorders.
• Psychogenic fatigue states.
• ALS.
• Peripheral neuropathy.
• Polymyositis.
• Thyrotoxicosis.


footnotes