Myathenia Gravis-أ.د.عمرو حسن الحسني أستاذ المخ والأعصاب

Myasthenia gravis (MG) is a relatively rare acquired, autoimmune disorder caused by an antibody-mediated blockade of neuromuscular transmission resulting in skeletal muscle weakness and rapid muscle fatigue. The autoimmune attack occurs when autoantibodies form against the nicotinic acetylcholine postsynaptic receptors at the neuromuscular junction of skeletal muscles (see the image below). [1, 2] Although the chief target of the autoimmune attack in most cases is the skeletal muscle nicotinic acetylcholine receptor (nAChR), other antigenic targets that are components of the neuromuscular junction (NMJ) have also been implicated.
Normal neuromuscular junction showing a presynapti
Normal neuromuscular junction showing a presynaptic terminal with a motor nerve ending in an enlargement (bouton terminale): Synaptic cleft and postsynaptic membrane with multiple folds and embedded with several acetylcholine receptors.
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Signs and symptoms

The presentation of MG has the following characteristics:

The clinical hallmark of MG is the presence of fluctuating fatigable muscle weakness that worsens with activity and improves on rest

50% to 85% of patients with MG present with ocular symptoms with or without generalized weakness

50% to 60% of patients with MG who initially present with isolated ocular involvement go on to develop generalized weakness, often within 3 years after onset of symptoms

The disease remains exclusively ocular in only 15% to 25% of patients throughout their course [1, 2]

Approximately 20% of patients with MG may present with prominent bulbar symptoms

Bulbar muscle weakness is also common, along with weakness of head extension and flexion

Limb weakness may be more severe proximally than distally

Isolated limb muscle weakness is the presenting symptom in about 5% of patients [3]

Weakness progresses from mild to more severe over weeks or months, with exacerbations and remissions

The following factors may trigger or worsen exacerbations:

Warm weather

Surgery

Immunization

Emotional stress

Menstruation

Intercurrent illness (eg, viral infection)

Tapering of immunosuppression

Pregnancy and postpartum period

Worsening of chronic medical illnesses (cardiac, renal, autoimmune, etc.)





The anti–acetylcholine receptor (AChR) antibody test for diagnosing MG has the following characteristics:

High specificity (up to 100% [4] )

Positive in as many as 85% of patients who have generalized MG [4]

Positive in only 50% of patients who have purely ocular MG [4]


Assays for the following antibodies may also be useful:

Anti-MuSK antibody (present in about half of patients with negative results for anti-AChR antibody)


Chest computed tomography is important to identify or rule out thymoma or thymic enlargement in all cases of MG


Electrodiagnostic studies (repetitive nerve stimulation and single-fiber electromyography)

See Workup for more detail.
Management


Therapy for MG includes the following:

Symptomatic therapy

Anticholinesterase (AchE) inhibitors

Pyridostigmine is used for maintenance therapy [6, 7]

Neostigmine is generally used only when pyridostigmine is unavailable

Rapidly acting or short-term immunomodulating agents:

Intravenous immune globulin (IVIg)

Plasmapheresis
Efgartigimod alfa is a human IgG1 antibody fragment that binds to neonatal Fc receptor (FcRn). It is used for generalized MG in adults who are anti-acetylcholine receptor (AChR) antibody positive.

Long-term immunosuppression
Prednisone is the most important immunosuppressant and provides short- and long-term benefit.
Azathioprine (AZA) is a first-line steroid-sparing agent
Mycophenolate mofetil (MMF) is also a first-line steroid-sparing agent. It is widely used and less toxic than azathioprine.





Thymectomy

The Randomized Trial of Thymectomy in Myasthenia Gravis, a landmark study, has shown that trans-sternal thymectomy improves MG and even induces remission, or lessens the requirements or perhaps removes the need for prednisone and other immunosuppressants over a period of 3 years. [15]