Myasthenia Gravis
Definition And Pathophysiology
Myasthenia gravis is a chronic autoimmune disease of the postsynaptic neuromuscular junction that leads to abnormal neuromuscular transmission or blockade.
- Pathophysiology: Acetylcholine release into the synaptic cleft is normal, but the postsynaptic muscle membrane (motor end plate) is less responsive.
- Mechanism: Autoantibodies target the postsynaptic acetylcholine receptor, causing abnormal architecture, abnormal folding of the postsynaptic membrane, and a decreased number of available receptors.
- Autoantibodies: Acetylcholine receptor antibodies are most common, followed by muscle-specific tyrosine kinase, LRP4, and agrin antibodies.
- Thymic Involvement: The thymus plays a significant role in the autoimmune pathogenesis, with thymoma or thymic hyperplasia frequently implicated.
Classification And Clinical Features
The hallmark of myasthenia gravis is fatigable weakness of striated muscles, which worsens with sustained activity, improves with rest, and shows diurnal variation (least severe in the morning). Sensation and pupillary reactions remain normal, and fasciculations are absent.
Clinical Subtypes
| Subtype | Epidemiology & Pathogenesis | Key Clinical Features |
|---|---|---|
| Transient Neonatal Myasthenia | Occurs in 10-20% of infants born to myasthenic mothers due to transplacental transfer of maternal antibodies. | Onset within hours to 3 days of birth. Presents with poor sucking and swallowing, weak cry, severe hypotonia, lack of facial expression, and respiratory insufficiency. Usually resolves within 2 months. |
| Juvenile Autoimmune Myasthenia | Onset in childhood or adolescence; accounts for 20% of cases. Female predominance (1.5:1 prepubertal; 1:1 postpubertal). | Unilateral or bilateral asymmetric ptosis and extraocular muscle weakness are the earliest and most constant signs. Prepubertal onset is often ocular-predominant, whereas postpubertal onset is typically generalized. Progresses to bulbar weakness (dysphagia, nasal speech), neck flexion weakness, and proximal limb weakness. |
| Congenital Myasthenic Syndromes | Genetic disorders of the neuromuscular junction (presynaptic, synaptic, or postsynaptic). Seronegative for autoantibodies. | Onset at birth or infancy. Presents with hypotonia, ptosis, ophthalmoparesis, feeding difficulties, and arthrogryposis. Variable response to cholinesterase inhibitors (some improve, some worsen) and no response to immunosuppression. |
Investigations And Diagnosis
Diagnosis relies on pharmacological testing, electrophysiology, serology, and imaging.
Diagnostic Evaluation
| Investigation Category | Specific Test & Methodology | Findings & Interpretation |
|---|---|---|
| Pharmacological Tests | Edrophonium (Tensilon) test: 0.1-0.2 mg/kg intravenous. | Transient resolution of ptosis, ophthalmoplegia, or dysarthria within 10-120 seconds. Not recommended in infants due to high risk of cardiac arrhythmias. |
| Neostigmine test: 0.04-0.125 mg/kg intramuscular. | Clinical improvement observed in 10-30 minutes. Safer alternative for infants. | |
| Electrophysiological Studies | Repetitive nerve stimulation: Applied to distal and proximal muscles. | Decremental response of >10% in compound muscle action potential amplitude is diagnostic. |
| Single-fiber electromyography: Specialized needle electrode study. | Shows increased jitter or blocking; it is the most sensitive electrophysiological test. | |
| Serology | Autoantibody panels: Acetylcholine receptor and muscle-specific tyrosine kinase. | Acetylcholine receptor antibodies are positive in 70-80% of adolescents and 50-60% of prepubertal children. Muscle-specific tyrosine kinase antibodies are found in 40% of seronegative patients. |
| Imaging & Laboratory | Chest imaging: X-ray, computed tomography, or magnetic resonance imaging of anterior mediastinum. | Evaluates for thymoma or thymic hyperplasia. |
| Serum enzymes & Autoimmune screening: Creatine kinase, thyroid function, antinuclear antibodies. | Creatine kinase is strictly normal. Helps exclude associated autoimmune diseases like systemic lupus erythematosus or thyroid disorders. |
Differential Diagnosis
Distinguishing myasthenia gravis from other causes of childhood weakness is critical.
| Differential Diagnosis | Overlapping Features | Distinguishing Features |
|---|---|---|
| Spinal Muscular Atrophy | Approach to a floppy infant, muscle weakness, bulbar/respiratory involvement. | Absence of ptosis and facial weakness; absent deep tendon reflexes; presence of tongue fasciculations; genetics confirm survival motor neuron 1 deletion. |
| Congenital Myotonic Dystrophy | Approach to a floppy infant, facial weakness, ptosis, bulbar involvement. | Maternal history of myotonic dystrophy; electromyography shows myotonia without decremental response; genetic testing confirmatory. |
| Congenital Myopathies | Hypotonia, facial weakness, variable ptosis, bulbar involvement. | Characteristic muscle biopsy findings (e.g., nemaline rods, central cores); no decremental response on repetitive nerve stimulation. |
| Botulism | Acute onset bulbar weakness, ptosis, hypotonia, ophthalmoplegia. | History of honey consumption or soil exposure; dilated and poorly reactive pupils; constipation. |
Management Principles
Management involves a multidisciplinary approach combining symptomatic relief, immunomodulation, and surgical intervention.
Symptomatic Therapy
- Cholinesterase inhibitors: Pyridostigmine is the first-line treatment. It is administered orally at 1-7 mg/kg/day in 4-5 divided doses.
- Monitoring: Dosage must be carefully titrated to balance efficacy against cholinergic side effects (e.g., increased secretions, diarrhea, cramping).
Immunomodulatory Therapy
- Corticosteroids: Oral prednisolone at 0.5-1 mg/kg/day is used for immunosuppression, tapered gradually to the minimum effective dose.
- Steroid-sparing agents: Azathioprine, cyclosporine, mycophenolate mofetil, and cyclophosphamide are utilized for refractory cases or to minimize long-term steroid toxicity.
- Newer biologicals: Rituximab (anti-CD20 monoclonal antibody) is effective in refractory or muscle-specific tyrosine kinase-positive cases. Efgartigimod is an emerging targeted therapy.
Management Of Acute Crises
- Myasthenic crisis: Characterized by severe respiratory or bulbar failure requiring intensive care and ventilatory support.
- Acute rescue therapies: Intravenous immunoglobulin (0.4 g/kg/day for 3-5 days) or plasmapheresis are the treatments of choice to achieve rapid clinical improvement.
- Differentiation: It must be distinguished from a cholinergic crisis (caused by cholinesterase inhibitor overdose), which presents with predominant muscarinic hypersecretion. The edrophonium test or ice-pack test helps differentiate the two.
Surgical Intervention
- Thymectomy: Highly beneficial and indicated in acetylcholine receptor antibody-positive patients. Early thymectomy (within 2 years of onset) is preferred to maximize remission chances.
Contraindicated Medications
- Certain drugs impair neuromuscular transmission and must be strictly avoided.
- These include aminoglycosides, fluoroquinolones, macrolides (erythromycin), tetracyclines, beta-blockers, procainamide, and neuromuscular blocking agents (e.g., succinylcholine, pancuronium).