Myasthenia gravis is arare autoimmune disease, in which the function of the myoneural junction of the voluntary muscles is impaired
Symptoms include variable degrees of fatigue and weakness in voluntary musculature, which is aggravated by strain - often towards evening - and relieved by rest.
Two main types:
Ocular, limited to the eye muscles (ca. 15% of the cases)
Generalised; the symptoms are usually most prominent in nuchal muscles and in the proximal muscles of the shoulder and pelvic region.
Treatment is the domain of neurology units.
Patients with myasthenia are very sensitive to drugs: check always the compatibility with myasthenia when starting a new drug for a myasthenic patient!
In myasthenia gravis, antibodies are formed, most typically, against the acetylcholine receptors (AChR) of the postsynaptic membrane in the neuromuscular junction, whereby the number of receptors is reduced and impulse transmission is impaired in the synaptic cleft.
In the generalised disease there are often AChR antibodies (in 80% of the patients), in the ocular form only in about one half of the patients.
Anti-MuSK (muscle specific kinase) antibodies are found in about 4-5% of the patients.
Thymic hyperplasia is found in ca. 70% of patients; ca. 10% have a benign thymoma. The onset of the disease may not occur until several years after the discovery of the thymoma.
Symptoms
Double vision, uni- or bilateral ptosis
Bulbar symptoms: nasal tone of speech and tiredness when speaking, difficulty in swallowing
Facial muscle weakness: hypomimia, grimacing/whistling are not possible
Muscle fatigue in neck muscles and proximal muscles of the limbs and pelvis
Clinical symptoms vary.
The patient may be asymptomatic in the morning or after rest; symtoms are often more prominent towards evening.
Accompanying disorders
About 15% of the patients have also another autoimmune disease.
None of the currently available pharmacotherapies for myasthenia have been studied with modern, controlled research methods, but they have been found effective and safe in decades' use.
Plasmaphaeresis or intravenous immunoglobulin as needed
After the crisis, a permanent immunosuppressive medication is started (see above)
Prognosis
Following thymectomy, the disease is relieved in ca. 70% of all patients, but a complete remission is rare.
Special problems
Infections
These often aggravate the disease symptoms
Influenza vaccination and other vaccinations belonging to a national immunization programme are indicated, irrespective of pharmacotherapy for myasthenia.
Vaccinations containing live attenuated viruses should not be used during immunosuppressive medication.
Do not use expectorants or respiratory centre-depressing antitussives. Teophylline or ephedrine(special permit may be required) often relieves bulbar and respiratory muscle weakness during infections.
Limitations concerning most common medicines
Absolutely contraindicated: botulinum toxin, intravenous magnesium, macrolides and fluoroquinolones
To be used with caution (because they may aggravate symptoms): beta blockers, calcium blockers, macrolides, sertraline, citalopram, lithium, amisulpiride, gabapentin, contrast media containing iodine and chloroquine, aminoglycosides, statins
Glucocorticoids in different forms (oral, as an infusion) can temporarily worsen symptoms, but they are used for acute and long-term treatment of myasthenia.
Pregnancy and parturition require monitoring by a specialist. Breastfeeding can be undertaken in the normal manner.
The newborn may develop a transient neonatal myasthenia gravis (the anti-acetylcholine receptor antibodies are transferred from the mother to the foetus through the placenta). The symptoms appear in 3 to 6 days after birth and disappear in about 3 weeks. A mother with myasthenia gravis should stay in hospital about 6 days after delivery.
When anaesthesia is planned, the staff must absolutely be informed if the patient has myasthenia gravis. Slow recovery from anaesthesia may actually be the first symptom of myasthenia.
If there are severe myasthenia symptoms despite medication, the patient should receive dental care in hospital.
References
Narayanaswami P, Sanders DB, Wolfe G ym. International Consensus Guidance for Management of Myasthenia Gravis: 2020 Update. Neurology 2021;96(3):114-122. [PubMed]
Gronseth GS, Barohn R, Narayanaswami P. Practice advisory: Thymectomy for myasthenia gravis (practice parameter update): Report of the Guideline Development, Dissemination, and Implementation Subcommittee of the American Academy of Neurology. Neurology 2020;94(16):705-709. [PubMed]
Wolfe GI, Kaminski HJ, Aban IB ym. Randomized Trial of Thymectomy in Myasthenia Gravis. N Engl J Med 2016;375(6):511-22. [PubMed]