Myasthenia gravis is an autoimmune disease affecting the myoneural junction, and is characterised by varying degrees of involuntary skeletal muscles weakness, involuntary muscles which are responsible for carrying out different functions like breathing, movement of the body parts including arms and legs. The word myasthenia gravis comes from both greek and latin where greek word mys is “muscles”, asthenia is “weakness” and gravis comes from the latin word gravis “serious or grave”. Women are affected more frequently than men, and they tend to develop the disease at an earlier age (20 to 40years of age, versus 60 to 70 years for men) (mazzoni, et al., 2006).
Generally, a chemical impluse precipitates thhe release of acetylcholine from vesicles on the nerve terminal at the myoneural junction. The acetylcholine attaches to receptor sites on the motor endplate and stimulates muscle contraction. Continues binding of acetylcholine to the receptor site is required for muscular contraction to be sustained. In this disorder, antibodies directed at the acetylcholine receptors sites impair transmission of impulses accross the myoneural junction. Therefore, fewer receptors are available for stimulation, resulting in voluntary muscle weakness. These antibodies are found in 80% to 90% of people diagnosed with myasthenia gravis. Almost 80% of the people with myasthenia gravis have either thymic hyperplasia or a thymic tumor, and the thymus gland is believed to bee the site of antibody production. In patients who are antibody negative, researchers believe that the offending antibody is directed at a portion of the receptor site rather than the whole complex.
- The initial symptoms of myasthenia gravis in two thirds of patients involves the ocular muscles Diplopia ( double vision) and ptosis (drooping if eyelids) are common.
- Many of them also experience weakness of face and throat muscles i.e bulbar symptoms and generalised weakness.
- Weakness of the facial muscles results in a bland facial expression.
- Laryngeal involvement produces dysphonia (vocal impairment) and increases the risk of choking and aspiration.
- Generalised weakness affects all the extremities and the intercoastal muscles, resulting in decreasing vital capacity and respiratory failure.
- Myasthenia gravis is a motor disorder with no effect on the sensation or coordination.
- An acetylcholinesterase inhibitor test is used to diagnose myasthenia gravis. The test stops the breakdown of acetylcholine, thereby increasing availability at the neuromuscular junction.
- The thymus gland, a site of acetylcholine receptor antibody production, may be enlarged in myasthenia gravis, which may be identified by MRI scan.
- EMG detects a delay or failure of neuromuscular transmission and is about 99% sensitive in confirming the disorder.
- The most common precipitator is respiratory infection; otherw include medication change, surgery, pregnancy and medications that exacerbate myasthenia
- A cholinergic crisis caused by overmedication with cholinesterase inhibitors is rare; atropine sulfate should be on hand to treat possible bradycardia and respiratory distress.
- Neuromuscular respiratory failure is the critical complication in myasthenic and cholinergic crisis.
- Respiratory muscle and bulbar weakness combine to cause respiratory compromise. An adequate cough and an impaired gag reflex caused by bulbar weakness result in poor airway clearance.
- Cholinesterase inhibitors are stopped when respiratory failure occurs and gradually restarted after the patient shows improvement with a course of plasmapheresis.
Treatment and management of myasthenia gravis is directed at improvement of function, reducing and removing circulating antibodies. There is no cure for myasthenia gravis; where treatment do not stop the production of the acetylcholine receptor antibodies. The treatment includes pharmacological approach, plasmapheresis and surgical approach.
A number of medications are contraindicated for patients diagnosed with myasthenia gravis because they exacerbate the condition and aggravate symptoms. The physian and the patient should weigh risks and benefits before any new medications are prescribed.
Pyridostigmine bromide is an anticholinesterase medication, is the first line drug of therapy. It provides symptomatic relief by inhibiting the breakdown of acetylcholine and increases the relative concertration of available acetylcholine at the neuromuscular junction. Thhe adverse effects include abdominal pain and distension, diarrhoea, fasciculations and increased oropharyngeal secretions.
If these lines of drug do not improve muscle strength and control fatigue then the next agents given are immunosuppressive drugs. The goal of immunosuppressive is to minimize the production of the antibody.
Cytotoxic medications are given to treat myasthenia gravis if there is inadequate response to steroids. Its serious adverse effects include leucopenia, hepatotoxicity and nephrotoxicity, so monthly evaluation of liver enzymes and white blood cell count is mandatory.
Plasmapheresis is plasma exchange technique used to treat exacerbations. The patient’s plasma and plasma components are removed through a centrally placed catheters. The blood cells and antibody containing plasmma are separated after which the cells and plasma substitute are infused. The number of treatment and the number of treatments is determined by the patient’s response.
Thymectomy is a surgical removal ofvthe thymus gland. It can produce antigen-specific immunosuppressive and result in clinical improvement. The entire gland must be removed for optimal clinical outcomes.