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Myasthenia Gravis

Diagnosis

If an individual displays any of the symptoms described in the previous section, a physician will run an initial evaluation for possible signs of myasthenia gravis. If the physician suspects the patient has the disease, a number of different diagnostic tests may be performed. Some of the tests include the following.

Peek Sign

In this test the patient simply closes their eyes. A normal response entails the eyelids remaining closed. In myasthenia gravis patients, the fatigable weakness of the eye muscles causes the eyelid to begin to open revealing the whites of the eye (Tavee 2010).

Ice Pack Test

An ice pack test is another simple, non-intrusive test. It is recommended for patients who have drooping eyelids and is not effective in those patients who do not display this symptom. A physician places an ice pack over a patient’s eye for two to five minutes. If the patient has improvement in double vision or drooping eyelids after the ice pack is removed, the patient likely has myasthenia gravis (Jayam Trouth 2012; Kearsey 2010). It is thought that reducing the temperature of the tissue causes a decrease in acetylcholinesterase activity. The ice pack test is fairly sensitive considering its simplistic nature, and one study indicated it is able to correctly diagnose myasthenia gravis in approximately 77% of patients (Kearsey 2010).

Tensilon (Edrophonium Chloride) Test

Myasthenia gravis can be caused by antibodies that disrupt the interaction between acetylcholine and its receptor on muscle cells. Therefore, agents that promote communication through this receptor can be used to provide diagnostic insight. Edrophonium chloride is an inhibitor of acetylcholinesterase. It prevents the breakdown of acetylcholine and allows it to work for longer periods of time at nerve-muscle junctions, resulting in improvement of muscle strength (NINDS 2012). It is administered intravenously and the patient is monitored for improvement in drooping eyelids or eye muscle function. If the patient displays temporary improvement in any affected muscle group, the patient most likely has myasthenia gravis. The tensilon test typically works better in people presenting with eye muscle weakness (Howard 2006). It is important to note that the tensilon test is not effective in cases of MuSK+ myasthenia gravis (Sieb 2014). 

The tensilon test can also be used to monitor myasthenia gravis patients’ medication and determine if they have overdosed on acetylcholinesterase inhibitor drugs. When there is an overdose of acetylcholine in the body, tensilon makes the person even weaker. This suggests that their acetylcholinesterase inhibitor dosage is too high and their treatment medication should be adjusted accordingly (Jasmin 2014). 

Immunoanalysis of Blood

The majority of myasthenia gravis patients (about 85%) have antibodies against the acetylcholine receptor. Approximately 8% of patients have antibodies against MuSK (Meriggioli 2012a). Physicians can perform immunoanalyses of blood samples to detect these self-reacting antibodies (NINDS 2012). It is important to note that the presence and concentration of anti-acetylcholine receptor antibodies in the blood does not predict severity of disease (Jayam Trouth 2012; Evoli 2003). It is possible that a patient tests negative for both of these antibodies, yet may still have the disease. As mentioned previously, these patients are called double seronegative (NINDS 2012; Howard 2006). Several other muscle-specific self-antibodies may also be detected through blood analysis, including antibodies to titin, myosin, actin, and ryanodine receptors. The presence of these antibodies usually indicates that the individual has myasthenia gravis with involvement of the thymus, or a thymic tumor without myasthenia gravis (Jayam Trouth 2012).

Electrophysiological Tests

There are two electrostimulatory tests commonly performed to diagnose myasthenia gravis: repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG). In RNS, a physician stimulates a patient’s nerve cells repeatedly with small pulses of electricity. If the patient has progressively weaker muscle responses during the test, myasthenia gravis is suspected (Jayam Trouth 2012; NINDS 2012). RNS is not a specific diagnostic test for myasthenia gravis and can only detect the disease in approximately 60% of patients (Howard 2006). In SFEMG, individual muscle fibers are stimulated with small electrical pulses. In the muscle fibers of patients with myasthenia gravis, there is more variability in the time that it takes the action potentials to be transmitted along a nerve and to reach the muscle fibers that it innervates, unlike muscle fibers of healthy individuals in which there is hardly any variability in the transmission time of action potentials along a nerve (Jayam Trouth 2012; NINDS 2012; Selvan 2011; Tanhehco 2003). SFEMG is more sensitive than RNS and almost all patients with myasthenia gravis can be diagnosed with this neuromuscular stimulation test (Howard 2006).

Diagnostic Imaging

Approximately 10–15% of patients with myasthenia gravis have a thymic tumor, which is generally benign, but may become malignant in some cases. Computed tomography (CT) or magnetic resonance imaging (MRI) scans of the chest are performed to detect the presence of these abnormalities (Jayam Trouth 2012; NINDS 2012).

Thyroid Testing

People who have myasthenia gravis are often diagnosed with overactive or underactive thyroid disease (Mayo Clinic 2013). In overactive thyroid disease (hyperthyroidism), the thyroid produces excessive amounts of thyroid hormones leading to symptoms including weight loss, increased appetite, rapid and irregular heartbeat, anxiety, and sensitivity to heat. People with an underactive thyroid (hypothyroidism) have low amounts of hormones produced by the thyroid and experience symptoms including weight gain, fatigue, sensitivity to cold, constipation, and joint or muscle pain. Blood tests and radioactive iodine uptake tests are used to measure levels of thyroid hormone to diagnose hypothyroidism or hyperthyroidism. In the radioactive iodine uptake test, a patient swallows a liquid or capsule containing a small amount of radioactive iodine. The thyroid uses the iodine to make thyroid hormones that exhibit radioactivity and can be detected by a probe that is placed over the thyroid. Low levels of radioactivity suggest hypothyroidism, while high levels of radioactivity suggest hyperthyroidism (Jonklass 2012).

Classification and Staging of Myasthenia Gravis

Myasthenia gravis is classified according to the following characteristics (Corse 2014):

Class I: Patient displays any eye muscle weakness with possible drooping eyelids, but all other muscle strength is normal

Class II: Patient may have eye muscle weakness and mild weakness of other muscle groups throughout the body

  • IIa: Muscle weakness predominantly in the arms/legs or trunk/head or both groups
  • IIb: Muscle weakness predominantly in the mouth/pharynx or respiratory muscles or both groups

Class III: Patient may have eye muscle weakness and moderate weakness of other muscle groups throughout the body

  • IIIa: Muscle weakness predominantly in the arms/legs or trunk/head or both groups
  • IIIb: Muscle weakness predominantly in the mouth/pharynx or respiratory muscles or both groups

Class IV: Patient may have eye muscle weakness and severe weakness of other muscle groups throughout the body

  • IVa: Muscle weakness predominantly in the arms/legs or trunk/head or both groups
  • IVb: Muscle weakness predominantly in the mouth/pharynx or respiratory muscles or both groups; the patients also requires the use of a feeding tube without assisted ventilation

Class V: Patient requires assisted ventilation