Clinical Repercussions
MG Class Overview and Diagnosis:
Although all MG causes muscle weakness in all cases, the severity can vary based on the amount of acetylcholine receptors that are available and which muscle groups are involved. There are two broad categories which MG patients fall into. The categories are ocular and generalized. In ocular MG, the patient is only hampered by muscle weakness in the eyes, causing blurred and double vision and ptosis. The generalized form of MG, is associated with more than just the ocular muscles.
These two categories can be broken down future into 5 classes with subcategories within each class. These classes define the clinical repercussions of MG. The MG symptoms can come and go and are not always constant. The symptoms can range from ocular to life-threatening respiratory failure (MG Crisis). The below sections explain the clinical repercussions of MG based on class. These classifications are scored using the exam on the treatment page of this site. The class structure below was gathered from Trouth et al and The Myasthenia Gravis Foundation of America.[3,4] Figures 1-5 are examples of muscles that can be effected by MG.
For MG diagnosis, the antibody levels for anti-AchR and anti-MuSK are measured along with the symptoms and signs on the Symptom and Sign page. Another important consideration when diagnosed with MG is avoiding medications that can exacerbate the symptoms of MG. It is important to go over the list with your physician to ensure the right medication is given. This can cause complications in treating other ailments, but your physician and neurologist can weight the benefits and detriments. On the right is a link to a list of medications to typically avoid.[4]
Class 1:
This MG class involves the weakness of eye muscle and eye closure without any other muscles being involved.
Class 2:
This class may have ocular weakness, but also have mild weakness in other muscle groups. This class is broken into two subgroups:
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Limbs, axial, and low amount in oropharyngeal muscles.
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Mostly oropharyngeal and respiratory with some of limbs and axial muscles.
Class 3:
Class 3 is also split into two groups with possible ocular weakness and moderate weakness in other muscles. The two groups are:
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Limbs, axial, and low amount in oropharyngeal muscles.
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Mostly oropharyngeal and respiratory with some of limbs and axial muscles.
Class 4:
This class has the same effects as Classes 2 and 3 but with severe weakness.
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Limbs, axial, and low amount in oropharyngeal muscles.
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Mostly oropharyngeal and respiratory with some of limbs and axial muscles.
Class 5:
Class 5 MG requires the patient to have intubation which may or may not be accompanied by mechanical ventilation in the class subcategory. The second subcategory has no intubation for the feeding tube.
MG Epidemiology:
The incidence of MG in the US is approximately 20 in every 100,000 people, making it a rare disease. The amount of people with only ocular symptoms ranges from 10-40% of cases. In the other cases the symptoms progress to other muscle groups usually within the first 2 year.[1] The total estimated number of people in the US with MG is 36,000 to 60,000. Carr et al. performed a search of over 55 epidemiologic studies in 2010. That study compared all MG to anti-AchR to anti-MuSK. The findings were that 77.7 out of 1,000,000 people fell into the all MG category with ranges of 70.6 to 163.5 per million for anti-AchR and 1.9 to 2.9 per million for anti-MuSK. The conclusion showed an increased in overall incidence of MG per year.[2]
Figure 1: From Britannica, this shows the main muscles in the human body.
Figure 3: From StudyBlue.com, this shows the main pharyngeal muscles.
Figure 2: From Britannica, this shows the main eye muscles in the human body.
Figure 4: From Britannica, this shows the main neck muscles in the human body.
Thymoma:
Another clinical repercussion developed in some cases of MG is thymoma. Thymomas are present in approximately 15% of MG cases.[6] This is related to the effect on the thymus gland causes by MG and the CD4 T-cell stored in that location. This condition can lead to swelling and pain. This can lead to tumor growth and hyperplastic changes. The tumors are typically benign. The exact mechanism of how the thymus is related to MG is not proven. The thymus however is the location of interaction between helper T cells and B cells. It is also where self-regulation occurs.[1] Since, MG is caused by self destruction antibodies, the line of thought is that the regulation malfunctions and caused growth and swelling of the thymus. A common procedure in MG patients is to remove this found with a thymoma is removal of the thymoma. This has been shown to reduce the severity of and even alleviate the patient of all MG symptoms in some cases. Thymomas are typically diagnosed through computerized tomographic (CT) imaging.[6] Figure 6 shown below is an image of a thymoma caused by MG.
Figure 5: This image from Koeppen et al [5] shows the human respiratory muscles.
Figure 6: This is a CT scan pointing to a thymoma. Image taken from wphospital.org.
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If diagnosed with MG, it is important to understand which medications can exacerbate the symptoms of MG. Click below for a list of drugs to avoid and patient emergency cards.
