Etiology and mechanisms of development
By nature, myasthenia gravis is an autoimmune disease. Its pathogenesis is based on an immune system malfunction where the body begins producing autoantibodies against its own structures.
The main target for these antibodies is nicotinic acetylcholine receptors, located on the postsynaptic membrane of the neuromuscular junction. As a result of constant autoimmune attack, there is a sharp decrease in the number of normally functioning receptors. It is this deficit of available receptors that disrupts the transmission of nerve impulses to the muscle fiber, clinically manifested as muscle weakness.
Epidemiological data show that this pathology occurs with a frequency of 10 to 15 cases per 100,000 population.
Role of the thymus gland
The pathogenesis of the disease is closely related to the state of the thymus gland (thymus). The vast majority of patients exhibit various pathological changes in this organ that potentiate and sustain autoimmune disorders:
- Hyperplasia (enlargement) of the thymus gland is diagnosed in 60% of patients.
- A tumor of the organ — thymoma — is detected in 10% of patients.
Clinical presentation and course
The basic and most characteristic symptom of myasthenia gravis is pathological muscle fatigability. A crucial diagnostic feature is that the weakness of individual muscle groups or the entire musculature rapidly increases with repetitive contractions and physical exertion.
Diurnal fluctuation
The disease is characterized by marked diurnal dynamics of symptoms (fluctuation):
- After night sleep, in the morning hours, symptoms may be completely absent — the patient feels healthy.
- During the day, as well as against the background of physical exertion, weakness begins to increase significantly.
- After a period of rest, the severity of clinical manifestations noticeably decreases.
Overall, the disease is characterized by the inconstancy of clinical signs and a wave-like course, in which periods of exacerbations are replaced by remissions.
Disease forms and prognosis
The manifestation of the disease most often begins with damage to the extraocular muscles. This stage or variant of the course is called the ocular form of myasthenia gravis. Its main manifestations include drooping of the eyelids (ptosis) and intermittent double vision (diplopia), which may worsen, for example, when reading.
Further development of the pathology can follow two scenarios:
- In 15% of patients, the process stabilizes and remains local, limited only to the ocular form.
- In 85% of patients, the disease steadily progresses. New and new muscle groups are gradually involved in the pathological process, and the disease transitions into the generalized form.
Life-threatening conditions: The greatest danger to the patient's life is the involvement of the respiratory muscles in the process. Pronounced weakness of the respiratory muscles can lead to critical disorders of pulmonary ventilation.
Briefly: answers to topical questions
What is the main function of the thymus in myasthenia gravis?
The main function of the thymus in myasthenia gravis is the induction of the autoimmune process and the potentiation of autoimmune disorders.
This mechanism is based on the expression of antigen on myoid cells of the thymus gland, which carry acetylcholine receptors (AChR) on their surface. Organ pathology is detected in the majority of patients:
- Thymic hyperplasia — occurs in 60% of patients (especially often in young patients).
- Thymoma — a tumor of the thymus gland, detected in approximately 10% of patients.
In MuSK-positive myasthenia gravis, thymus pathology is usually not detected.
What treatment methods are used for myasthenia gravis?
Treatment of myasthenia gravis includes drug and non-drug methods aimed at improving neuromuscular transmission and suppressing the autoimmune response.
The following methods are used:
- Anticholinesterase drugs — symptomatic therapy (neostigmine, pyridostigmine bromide, ambenonium chloride).
- Immunosuppressive therapy — glucocorticoids and cytostatics (e.g., azathioprine).
- Monoclonal antibodies — eculizumab, ravulizumab, and rituximab (used in severe cases).
- Specific therapy — intravenous administration of normal human immunoglobulins.
- Non-drug methods — plasmapheresis.
- Thymectomy — surgical removal of the thymus gland.
Drug dosage is selected strictly individually, and patients must categorically avoid self-medication.
To remember the essence of myasthenia gravis, keep the rule of the "Three Fs" (Morning, Fatigability, Falling receptors): Morning (best of all), Fatigability (increases with work), Falling receptors (due to antibodies).
Class and exam questions
Which receptors are affected in myasthenia gravis?
In myasthenia gravis, autoantibodies attack nicotinic acetylcholine receptors located on the postsynaptic membrane of the neuromuscular junction.
What symptoms do most diseases start with?
The disease most often manifests with damage to the ocular muscles. Patients complain of drooping eyelids (ptosis) and intermittent double vision (diplopia).
What thymus pathology accompanies myasthenia gravis?
Thymic hyperplasia is detected in 60% of patients, and a tumor (thymoma) in 10%.
What is the most dangerous complication of myasthenia gravis?
The most severe and life-threatening condition in the generalized form is respiratory muscle weakness.
What else to study on this topic
- Mechanisms of nerve impulse transmission in normal and pathological synapses
- Differential diagnosis of ptosis and diplopia in neurology
- Methods for assessing pathological muscle fatigability
- Generalized form of myasthenia gravis: sequence of muscle involvement
- Pathophysiology of the thymus gland in autoimmune diseases
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