myasthenia gravis

myasthenia gravis
Name	Myasthenia gravis
Caption	The neuromuscular junction is the site of pathology.
Field	Neurology, Immunology
Symptoms	Muscle weakness, ptosis, diplopia, dysphagia
Complications	Myasthenic crisis, Respiratory failure
Onset	Any age, peaks in women <40, men >60
Duration	Chronic
Types	Ocular, generalized
Causes	Autoantibodies against AChR, MuSK, LRP4
Risks	Thymoma, other autoimmune disorders
Diagnosis	Edrophonium test, Electromyography, Serology
Differential	Lambert–Eaton myasthenic syndrome, Botulism, Amyotrophic lateral sclerosis
Prevention	None known
Treatment	AChE inhibitors, Immunosuppression, Thymectomy, Plasmapheresis
Medication	Pyridostigmine, Prednisone, Azathioprine, Rituximab
Prognosis	Manageable with treatment
Frequency	~20 per 100,000

myasthenia gravis is a chronic autoimmune disorder characterized by weakness and rapid fatigue of voluntary muscles. The condition results from a breakdown in communication between nerves and muscles at the neuromuscular junction. While the course is variable, modern treatments often allow for effective management of symptoms.

Signs and symptoms

The hallmark is fluctuating, painless muscle weakness that worsens with activity and improves with rest. Ocular symptoms like ptosis (drooping eyelids) and diplopia (double vision) are common initial presentations. Bulbar weakness can lead to dysphagia, dysarthria, and difficulty chewing. Generalized weakness may affect the limbs, neck, and respiratory muscles, potentially precipitating a life-threatening myasthenic crisis. Symptoms often exhibit diurnal variation, typically being least severe in the morning.

Causes and pathophysiology

Myasthenia gravis is caused by autoantibodies that disrupt signaling at the neuromuscular junction. In most cases, antibodies target the nicotinic acetylcholine receptor on the postsynaptic membrane. Less commonly, antibodies are directed against other proteins like muscle-specific kinase or LRP4. These antibodies accelerate receptor degradation, block acetylcholine binding, and activate complement, reducing the muscle's response to nerve signals. The disease is frequently associated with abnormalities of the thymus, including thymoma or thymic hyperplasia.

Diagnosis

Diagnosis involves a combination of clinical assessment and specialized testing. The Edrophonium test, using a short-acting acetylcholinesterase inhibitor, can provide rapid but temporary improvement in weakness. Serology to detect specific autoantibodies against AChR, MuSK, or LRP4 is confirmatory. Electromyography, particularly repetitive nerve stimulation and single-fiber EMG, demonstrates a decremental response. CT or MRI of the mediastinum is performed to evaluate the thymus.

Treatment

Treatment strategies aim to improve neuromuscular transmission and suppress the abnormal immune response. Symptomatic relief is achieved with acetylcholinesterase inhibitors like pyridostigmine. Immunosuppression is a mainstay, using agents such as prednisone, azathioprine, mycophenolate mofetil, or rituximab. Thymectomy is recommended for many patients, particularly those with thymoma. For acute exacerbations or crisis, therapies include intravenous immunoglobulin and plasmapheresis.

Prognosis

With modern treatment, the prognosis is generally good, and most individuals can achieve significant improvement or remission. Life expectancy is near-normal for the majority of patients. However, the course can be variable, with some experiencing periodic exacerbations. The most serious complication remains myasthenic crisis, requiring intensive care and ventilatory support. Long-term outcomes are improved with consistent immunosuppressive therapy and management of comorbidities.

Epidemiology

Myasthenia gravis has a bimodal age distribution, with an early peak in women under 40 and a later peak in men over 60. The estimated prevalence is approximately 20 cases per 100,000 individuals in the United States. It is the most common disorder of the neuromuscular junction. There is an established association with other autoimmune conditions such as Graves' disease, rheumatoid arthritis, and systemic lupus erythematosus.

Category:Autoimmune diseases Category:Neurological disorders Category:Neuromuscular diseases