Myasthenia Gravis
What is Myasthenia Gravis?
Myasthenia Gravis Is a neuromuscular disorder characterised by weakness and fatiguability of skeletal muscles due to decrease in number of Acetylcholine receptors at the neuromuscular junction due to antibodies. It is an autoimmune disorder.
What are the Symptoms and Signs of Myasthenia Gravis
1: Onset is insidious or subacute, rarely acute
2: External ocular movements are weak. There may be unilateral or bilateral ptosis. Pupils are always Spared.
3: Facial: Weakness of orbicularis oculi is quite constant. Retractors of the angles of the mouth suffer more than elevators resulting in snarling smile.
4: Other cranial nerves: There is palatal palsy. Involvement of the masseters prevents closure of mouth (hanging jaw sign). There may be nasal speech and ultimately respiratory paralysis.
5: Limbs: Proximal weakness initially in the shoulder girdle, later on maybe generalized.
6: The weakness is more in the evening and disappears after a night's rest. Usually there is no wasting or loss of deep reflexes.
7:Muscle groups commonly involved in decreasing order are bulbar, neck, limb-girdle, distal limb and trunk.
8: If respiratory muscles are affected, mechanical ventilation may be required and patient is said to be in a "Myasthenia crisis".
What causes myasthenia gravis?
1: Myasthenia gravis is an autoimmune disease, which means the immune system which normally protects the body from foreign organisms mistakenly attacks itself.
2: Myasthenia gravis is caused by an error in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction the place where nerve cells connect with the muscles they control.
3: Neurotransmitters are chemicals that neurons, or brain cells, use to communicate information. Normally when electrical signals or impulses travel down a motor nerve, the nerve endings release a neurotransmitter called acetylcholine that binds to sites called acetylcholine receptors on the muscle. The binding of acetylcholine to its receptor activates the muscle and causes a muscle contraction.
4: In myasthenia gravis, antibodies (immune proteins produced by the body’s immune system) block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction, which prevents the muscle from contracting. This is most often caused by antibodies to the acetylcholine receptor itself, but antibodies to other proteins, such as MuSK (Muscle-Specific Kinase) protein, also can impair transmission at the neuromuscular junction.
The thymus gland
The thymus gland controls immune function and may be associated with myasthenia gravis. It grows gradually until puberty, and then gets smaller and is replaced by fat. Throughout childhood, the thymus plays an important role in the development of the immune system because it is responsible for producing T-lymphocytes or T cells, a specific type of white blood cell that protects the body from viruses and infections.
In many adults with myasthenia gravis, the thymus gland remains large. People with the disease typically have clusters of immune cells in their thymus gland and may develop thymomas (tumors of the thymus gland). Thymomas are most often harmless, but they can become cancerous. Scientists believe the thymus gland may give incorrect instructions to developing immune cells, ultimately causing the immune system to attack its own cells and tissues and produce acetylcholine receptor antibodies—setting the stage for the attack on neuromuscular transmission.
How is Myasthenia Gravis Diagnosed?
1: Clinically
- Breath Holding Time: Breath counting is done to assess vital capacity,
- Forward arm abduction(> 5 mins),
- On sustained upward gaze, increased ptosis may occur,
- Single breath count.
2: ACH receptor Antibodies : Generalised: 90% positive. Occular : 50% positive
3: Anti MUSK Antibodies (muscle-specific kinase) positive is 40% of negative cases.
4: Tensilon test: Edrophonium 2 mg is given IV. If muscle power is improved within 30 seconds and sustained improvement for 2-3 minutes test is positive. It can be repeated with 8 mg if required.
5: CT scan (To look for thymoma) Thymus is enlarged in 70% of MG cases.
6: Electrophysiological study.
- Repetitive nerve stimulation - decremental pattern.
- Single fibre EMG - Increased variability of interpotential interval.
What is the Treatment of Myasthenia Gravis?
1: Pyridostigmine Orally 30 - 120 mg every4-8 hour, titrated individually for each patient.
2: Corticosteroids : IV or oral.
3: Plasma Exchange: Removes antibodies, used in crisis.
4: Immunomodulators: Azathioprine, cyclophosphamide, mycophendlate mofetil, tacrolimus.
5: Thymectomy: In patients under 60 years, improves symptoms.
Drugs contraindicated in MG
Antibiotics: Aminoglycosides, Quinolones, Muscle relaxants: pancuronium, D Tubocuan,
Beta-blockers, local anaesthetics, quinine, worsen the muscle weakness.
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