Myasthenia Gravis

SX of Myasthenia Gravis

MG presents as skeletal muscle weakness and fatigability. Fatigability is usually worse at the end of the day or after repeated activity.
The weakness of the eyelids and the extraocular muscles, which causes a characteristic asymmetric ptosis and diplopia. The neck extensors are often weak and the head droops.
Weakness of facial and bulbar muscles may result in a facial snarl when a patient attempts to smile, nasal or dysarthric and low-volume dysphonic speech, and dysphagia, which can result in regurgitation and choking.
Proximal skeletal muscle weakness and fatigue produce difficulty in walking, climbing steps, combing hair, and carrying objects.
Respiratory muscle weakness can be significant. Symptoms fluctuate and are often better in the morning. If a patient is asked to keep the arms abducted, a gradual decline is noted. If a patient is asked to talk continually, the voice may become husky, nasal, slurred, and finally inaudible.
Sensation and cognition are normal. Tendon reflexes, especially from nonatrophic muscles, tend to be brisk or normally active. The toes are downgoing. The symptoms worsen during or before the menstrual period and during viral or bacterial infections.

DX of Myasthenia Gravis

Physical exam that shows fatigue and weakness on examination.
Diagnosis be confirmed by an edrophonium or neostigmine test.
Repetitive nerve stimulation studies demonstrate a rapid reduction (greater than 12 percent) in the amplitude of the MAP.
Single-fiber EMG demonstrates increased jitter and blocking in more than 90 percent of patients.
An MRI of the chest should be performed to search for thymic hyperplasia or thymoma.
The serologic diagnosis of MG is made by detecting AChR antibodies with a radioimmunoprecipitation assay. These antibodies are present in as many as 90 percent of patients with generalized MG and in as many as 70 percent of patients with ocular MG. Such antibodies are also found in as many as 30 percent of patients with autoimmune liver disease, in 10 percent of patients with pernicious anemia, and in as many as 13 percent of patients with LEMS. Generally, AChR antibody titers do not correlate with clinical severity.

Differential diagnosis of Myasthenia Gravis
MG must be differentiated from other diseases or agent-induced disorders that present similar clinical pictures, such as botulism, organophosphate poisoning, d-penicillamine toxic reaction, mitochondrial myopathy, compressive lesion affecting cranial nerves, LEMS, and congenital myasthenic syndromes.

RX of Myasthenia Gravis

The initial choice in mild cases is generally an anticholinesterase drug every four hours while the patient is awake.
Thymectomy is done in most centers through the transsternal approach in patients beyond puberty and up to 60 years of age.
Prednisone is the first-line immunotherapeutic drug therapy. The preferred dosage is 60 to 80 mg as a single daily dose in the morning for an initial period of three to four weeks. Then, over an eight-week period, the dose on alternate days is gradually reduced by 10 mg a week (more often if side effects are severe) until the lowest possible dose is reached that controls the disease. Some physicians prefer to start with lower dosages of prednisone (i.e., 40 mg/day) and gradually increase the dosage to prevent an occasionally noted transient worsening.
Plasmapheresis or intravenous Ig is generally reserved for crisis and severe cases and to strengthen a patient before thymectomy.
Cyclosporine or azathioprine may be used as a second-line therapy. Azathioprine, for example, given orally in doses up to 3 mg/kg, is an effective immunosuppressant.