Diagnosis of Myasthenia Gravis

       A careful physical examination should be performed to establish the diagnosis of a myasthenia gravis. Muscle weakness can occur in many different degrees, usually descend the proximal portion of the body and symmetrical on both left and right limbs. Deep tendon reflexes are usually still in the normal range. Myasthenia gravis is usually always accompanied by a weakness in the facial muscles. Bilateral weakness of facial muscles will cause a mask-like face in the presence of ptosis, and horizontal smile.

      Bulbar muscle weakness are also common in patients with myasthenia gravis. On physical examination, there was weakness of the muscles of the palate, which causes the sound of people like being in the nose (nasal Twang to the voice) and regurgitation of food, especially the character of liquid into the nose of the patient. In addition, patients with myasthenia gravis will have difficulty in chewing and swallowing food, so it can happen that causes fluid aspiration and choking cough sufferers when drinking. Weakness of the muscles of the jaw in patients with myasthenia gravis caused hard to shut his mouth, so that the patient should continue sustained chin by hand. Neck muscles also experience weakness, so that interference occurs during flexion and extension of the neck.

      The muscles of the limbs can experience more often than the weakness of the muscles of the body to another, in which the muscles of the upper limb more often experience muscle weakness than lower limbs. Deltoid and extension function of the wrist muscles and fingers frequently experience weakness. Tricep muscles more often affected than the biceps. In the lower extremities, weakness often occurs during hip flexion, and perform dorsiflexion of the toes compared to toes plantarflexion .

      Weakness of the respiratory muscles can lead to acute respiratory failure, which makes it an emergency intubation and rapid action is needed. Weakness of the intercostal muscles and diaphragm can cause retention of carbon dioxide that would result in the occurrence of hypoventilation. Pharyngeal muscle weakness can cause the collapse of the upper airway, a watchful eye on respiratory function in patients with myasthenia gravis acute phase is needed.

    Usually extraocular muscle weakness occurs asymmetrically. Weakness often affects more than one extraocular muscles, and not just limited to the muscles innervated by the cranial nerves. This is a very important sign for the diagnosis of a myasthenia gravis. Muscular weakness in the lateral and medial rectus will cause a pseudointernuclear ophthalmoplegia, characterized by the limited ability of the adduction of one eye with nystagmus of the eye that does abduction.
       

       For the diagnosis of myasthenia gravis, can be examined as follows:

1. Patients were assigned to count, in a loud voice. Eventually it will sound the voice grow weak and become less bright. Patients become anarthria and aphonia.
2. Patients assigned to blink his eyes constantly. Eventually there will be ptosis. After the patient became hoarse voice or looks no ptosis, the patient was told to rest. Then, it appears that his voice would come back better and ptosis also does not appear anymore.

To confirm the diagnosis of myasthenia gravis, several tests can include:
1. Tensilon test (Tensilon chloride)
       To test tensilon, 2 mg tensilon injected intravenously, if there is no reaction then injected again tensilon much as 8 mg intravenously. Immediately after tensilon injected should be noted that weak muscles such as eyelid ptosis show. If that's true weakness caused by myasthenia gravis, ptosis then it will soon disappear. In this uiji weak eyelid should be considered very carefully, because the effectiveness of tensilon very short.

2. Prostigmin Test
       Injected 3 cc or 1.5 mg intramuscularly prostigmin merhylsulfat (if necessary, also given atropine ¼ or ½ mg). If that's true weakness caused by myasthenia gravis then symptoms such as ptosis, strabismus or other flaws will vanish soon.

3. Quinine test
     Quinine given 3 tablets of 200 mg each. 3 hours later again given 3 tablets (each 200 mg per tablet). If that's true weakness caused by myasthenia gravis, the symptoms such as ptosis, strabismus, and others will gain weight. For this test, you should also be prepared prostigmin injection, so that miasthenic symptoms, not getting  worse.

4. Examination Support for Devinitife Diagnosis.
Laboratory
• Anti-acetylcholine receptor antibodies
The results of this examination can be used to diagnose a myasthenia gravis, where there postitif results in 74% of patients. 80% of patients with generalized myasthenia gravis and 50% of patients with pure ocular myasthenia shows the test results of anti-acetylcholine receptor antibody positive. In patients with thymoma without myasthenia gravis occurs frequently false positive anti-ACHR antibody.
The average antibody titer to anti-acetylcholine receptor inspection antibody, conducted by Tidall, conveyed in the following table

Classification: R = remission, I = ocular only, IIA = mild generalized, IIB = moderate generalized, III = acute severe, IV = severe chronic

In that table shows that higher antibody titers in patients with myasthenia gravis in grave condition, although the titer can not be used to predict the degree of disease myasthenia gravis.
• Antistriated muscle (anti-SM) antibody
Anti-SM is one of the important tests in patients with myasthenia gravis. This test shows positive results in about 84% of patients suffering from thymoma in less than 40 years of age. In patients without thymoma with over 40 years of age, anti-SM Ab can show positive results.
• Anti-muscle-specific kinase (Musk) antibodies.
Nearly 50% of patients with myasthenia gravis showing the results of anti-ACHR Ab negative (seronegative myasthenia gravis), showed positive results for the anti-Musk Ab.
• Antistriational antibodies
In the serum of patients with myasthenia gravis few showed antibody binding in cross-striational patterns in skeletal muscle and heart muscle of patients. These antibodies react with epitopes on protein titin and ryanodine receptor (RyR). Antibody is always associated with thymoma with myasthenia gravis patients at a young age. Detection of titin / RyR antibodies is a strong suspicion of the existence of thymoma in young patients with myasthenia gravis.

CT scan of chest showing an anterior mediastinal mass (thymoma) in a patient with myasthenia gravis.

5. Imaging
• Chest x-ray (thoracic x-ray photo)
Can be performed in anteroposterior and lateral positions. In thoracic x-ray, thymoma can be identified as an anterior mediastinal mass. A negative x-ray results can not necessarily rule out the thymoma small size, so sometimes it is necessary to chest Ct scan to identify all cases of thymoma in myasthenia gravis, especially in patients with old age.
• MRI of the brain and orbit should not be used as a routine. MRI can be used when the diagnosis of myasthenia gravis can not be enforced by other investigations and to find the cause of the deficit in brain neurons.

6. Electrodiagnostic Approach
Electrodiagnostic approach can reveal defects in neuromuscular transmission through two techniques:
• Repetitive Nerve Stimulation (RNS)
In patients with myasthenia gravis are decreasing the number of acetylcholine receptors, so that the RNs there is no presence of an action potential.
• Single-fiber electromyography (SFEMG)
Using single-fiber needle, which has a small surface to record the muscle fibers of patients. SFEMG can detect a jitter (variability in the interval between 2 or more interpotensial single muscle fibers in the motor units of the same) and a fiber density (number of action potentials from single muscle fibers that can be recorded by the recording needle). SFEMG detect any defect in neuromuscular transmission in fiber include increased jitter and fiber density were normal.



RELATED ARTICLES

•    Myasthenia Gravis
•    Pathophysiology of Myasthenia Gravis
•    Clinical Manifestations of Myasthenia Gravis
•    Medical Treatment and Therapy of Myasthenia Gravis


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1. Picture: http://www.myasthenia.org/HealthProfessionals/ClinicalOverviewofMG.aspx