Myasthenia Gravis: 5 Things to Know

Myasthenia gravis (MG) is a rare autoimmune neurologic disorder that occurs when the transmission between nerves and muscles is disrupted. It is caused by autoantibodies against acetylcholine receptors (AChRs), which results in muscle weakness that is often fatigable and affects various muscles in the body, including those that move the eyes, eyelids, and limbs. Ocular MG affects only the muscles that move the eyes and eyelids, whereas generalized MG (gMG) affects muscles throughout the body. When MG occurs with a thymoma, it is called thymoma-associated MG and is considered a paraneoplastic disease. In severe cases of MG, patients can experience a myasthenic crisis (MC), during which respiratory muscles weaken and necessitate mechanical ventilation. Diagnosis of MG is based on clinical examination, and laboratory tests are used to confirm the diagnosis. Treatment options include cholinesterase enzyme inhibitors and immunosuppressive agents, which aim to either reduce symptoms or cause nonspecific immunosuppression, respectively, but do not target the pathogenetic autoantibodies that characterize the disease.

1. The most common age at onset of gMG is the second and third decades in women and the seventh and eighth decades in men.

MG has an annual incidence of approximately  four to 30 new cases per million population. Prevalence rates range from 150 to 200 cases per million population, and they have steadily increased over the past 50 years. This increase in prevalence is probably the result of better disease recognition, aging of the population, and an increased life span in patients.

MG can occur at any age; however, onset is more common in females in the second and third decades and is more common in males in the seventh to eighth decades. Before age 40 years, the female-to-male ratio is 3:1, and after age 50 years, the female-to-male ratio is 3:2.

2. gMG commonly weakens muscles responsible for eye movement, facial expressions, and functions such as chewing, swallowing, and speaking.

gMG typically manifests as muscle weakness that worsens with repeated use. Patients often report that their function is best in the morning, with more pronounced weakness at the end of the day. Permanent muscle damage is rare, however, and maximal muscle strength is often good.

Extraocular muscles are more commonly affected, as twitch fibers in these muscles develop tension faster, have a higher frequency of synaptic firing than limb muscles, and have fewer AChRs, making them more susceptible to fatigue. Patients present asymmetrically; intermittent drooping of the upper eyelid (ptosis) and double vision (diplopia) are the most common symptoms.

Muscles innervated by the cranial nerves (bulbar muscles) are involved in 60% of patients with gMG and can lead to fatigable chewing, reduced facial expression, speech difficulties (dysarthria), and weakness of swallowing (dysphagia). Up to 15% of patients initially present with bulbar muscle involvement, including dysarthria and painless dysphagia.

3. Emotional stress can trigger an MC.

MC is a complication of MG characterized by worsening muscle weakness that results in respiratory failure and necessitates mechanical ventilation.

MC is often the result of respiratory muscle weakness but can also be due to bulbar weakness with upper airway collapse. MC can occur in 15%-20% of patients within the first 2-3 years of the disease; however, it can also be the first presentation of MG in 18%-28% of cases.

MC can be triggered by multiple causes, including emotional or physical stress. The most common precipitant is infection; other precipitants include surgery, pregnancy, perimenstrual state, certain medications, tapering of immune-modulating medications, exposure to temperature extremes, pain, and sleep deprivation. Approximately one third to one half of patients with MC may have no obvious cause.

4. High levels of anti-AChR antibodies strongly indicate MG, but normal levels do not rule it out.

All patients with a clinical history suggestive of MG should be tested for antibodies for confirmation. Most patients have anti-AChR antibodies (~85%), and those without have anti–muscle-specific kinase (MuSK antibodies) (6%) and anti–lipoprotein receptor-related protein 4 (LRP4) antibodies (2%).

The sensitivity of anti-AChR antibodies varies depending on whether the antibody is binding, modulating, or blocking the AChR. Binding antibody is the most common, and when combined with blocking antibodies, has a high sensitivity (99.6%) and is typically tested first. Higher AChR antibody titers are more specific for the diagnosis of MG than are low titers, but they do not correlate with disease severity.

For patients who do not have anti-AChR antibodies but do have clinical features of MG, anti-MuSK antibodies and anti-LRP4 antibodies are measured to increase diagnostic sensitivity. For symptomatic patients who do not have any autoantibodies (seronegative), electrodiagnostic testing that shows evidence of impaired signal transmission at the neuromuscular junction is used to confirm the diagnosis of MG.

5. Studies suggest that over 75% of seropositive MG patients show distinct thymus abnormalities.

More than 75% of patients with AChR antibody–positive MG have abnormalities in their thymus, and up to 40% of patients with a thymoma have MG. Among those with thymic pathology, thymic hyperplasia is the most common type (85%), but other thymic tumors (mainly thymoma) can be present in up to 15% of cases. Thymomas are typically noninvasive and cortical, but in some rare cases, invasive thymic carcinoma can occur.

Given this overlap in presentation, it is recommended that patients with seronegative and seropositive MG undergo chest CT or MRI for evaluation of their anterior mediastinal anatomy and to detect the presence of a thymoma. For patients with MG and a thymoma, as well as selected (nonthymomatous) patients with seropositive or seronegative MG, therapeutic thymectomy is recommended.

Myasthenia gravis (MG) is a rare autoimmune neurologic disorder that occurs when the transmission between nerves and muscles is disrupted. It is caused by autoantibodies against acetylcholine receptors (AChRs), which results in muscle weakness that is often fatigable and affects various muscles in the body, including those that move the eyes, eyelids, and limbs. Ocular MG affects only the muscles that move the eyes and eyelids, whereas generalized MG (gMG) affects muscles throughout the body. When MG occurs with a thymoma, it is called thymoma-associated MG and is considered a paraneoplastic disease. In severe cases of MG, patients can experience a myasthenic crisis (MC), during which respiratory muscles weaken and necessitate mechanical ventilation. Diagnosis of MG is based on clinical examination, and laboratory tests are used to confirm the diagnosis. Treatment options include cholinesterase enzyme inhibitors and immunosuppressive agents, which aim to either reduce symptoms or cause nonspecific immunosuppression, respectively, but do not target the pathogenetic autoantibodies that characterize the disease.

1. The most common age at onset of gMG is the second and third decades in women and the seventh and eighth decades in men.

MG has an annual incidence of approximately  four to 30 new cases per million population. Prevalence rates range from 150 to 200 cases per million population, and they have steadily increased over the past 50 years. This increase in prevalence is probably the result of better disease recognition, aging of the population, and an increased life span in patients.

MG can occur at any age; however, onset is more common in females in the second and third decades and is more common in males in the seventh to eighth decades. Before age 40 years, the female-to-male ratio is 3:1, and after age 50 years, the female-to-male ratio is 3:2.

2. gMG commonly weakens muscles responsible for eye movement, facial expressions, and functions such as chewing, swallowing, and speaking.

gMG typically manifests as muscle weakness that worsens with repeated use. Patients often report that their function is best in the morning, with more pronounced weakness at the end of the day. Permanent muscle damage is rare, however, and maximal muscle strength is often good.

Extraocular muscles are more commonly affected, as twitch fibers in these muscles develop tension faster, have a higher frequency of synaptic firing than limb muscles, and have fewer AChRs, making them more susceptible to fatigue. Patients present asymmetrically; intermittent drooping of the upper eyelid (ptosis) and double vision (diplopia) are the most common symptoms.

Muscles innervated by the cranial nerves (bulbar muscles) are involved in 60% of patients with gMG and can lead to fatigable chewing, reduced facial expression, speech difficulties (dysarthria), and weakness of swallowing (dysphagia). Up to 15% of patients initially present with bulbar muscle involvement, including dysarthria and painless dysphagia.

3. Emotional stress can trigger an MC.

MC is a complication of MG characterized by worsening muscle weakness that results in respiratory failure and necessitates mechanical ventilation.

MC is often the result of respiratory muscle weakness but can also be due to bulbar weakness with upper airway collapse. MC can occur in 15%-20% of patients within the first 2-3 years of the disease; however, it can also be the first presentation of MG in 18%-28% of cases.

MC can be triggered by multiple causes, including emotional or physical stress. The most common precipitant is infection; other precipitants include surgery, pregnancy, perimenstrual state, certain medications, tapering of immune-modulating medications, exposure to temperature extremes, pain, and sleep deprivation. Approximately one third to one half of patients with MC may have no obvious cause.

4. High levels of anti-AChR antibodies strongly indicate MG, but normal levels do not rule it out.

All patients with a clinical history suggestive of MG should be tested for antibodies for confirmation. Most patients have anti-AChR antibodies (~85%), and those without have anti–muscle-specific kinase (MuSK antibodies) (6%) and anti–lipoprotein receptor-related protein 4 (LRP4) antibodies (2%).

The sensitivity of anti-AChR antibodies varies depending on whether the antibody is binding, modulating, or blocking the AChR. Binding antibody is the most common, and when combined with blocking antibodies, has a high sensitivity (99.6%) and is typically tested first. Higher AChR antibody titers are more specific for the diagnosis of MG than are low titers, but they do not correlate with disease severity.

For patients who do not have anti-AChR antibodies but do have clinical features of MG, anti-MuSK antibodies and anti-LRP4 antibodies are measured to increase diagnostic sensitivity. For symptomatic patients who do not have any autoantibodies (seronegative), electrodiagnostic testing that shows evidence of impaired signal transmission at the neuromuscular junction is used to confirm the diagnosis of MG.

5. Studies suggest that over 75% of seropositive MG patients show distinct thymus abnormalities.

More than 75% of patients with AChR antibody–positive MG have abnormalities in their thymus, and up to 40% of patients with a thymoma have MG. Among those with thymic pathology, thymic hyperplasia is the most common type (85%), but other thymic tumors (mainly thymoma) can be present in up to 15% of cases. Thymomas are typically noninvasive and cortical, but in some rare cases, invasive thymic carcinoma can occur.

Given this overlap in presentation, it is recommended that patients with seronegative and seropositive MG undergo chest CT or MRI for evaluation of their anterior mediastinal anatomy and to detect the presence of a thymoma. For patients with MG and a thymoma, as well as selected (nonthymomatous) patients with seropositive or seronegative MG, therapeutic thymectomy is recommended.

Myasthenia gravis (MG) is a rare autoimmune neurologic disorder that occurs when the transmission between nerves and muscles is disrupted. It is caused by autoantibodies against acetylcholine receptors (AChRs), which results in muscle weakness that is often fatigable and affects various muscles in the body, including those that move the eyes, eyelids, and limbs. Ocular MG affects only the muscles that move the eyes and eyelids, whereas generalized MG (gMG) affects muscles throughout the body. When MG occurs with a thymoma, it is called thymoma-associated MG and is considered a paraneoplastic disease. In severe cases of MG, patients can experience a myasthenic crisis (MC), during which respiratory muscles weaken and necessitate mechanical ventilation. Diagnosis of MG is based on clinical examination, and laboratory tests are used to confirm the diagnosis. Treatment options include cholinesterase enzyme inhibitors and immunosuppressive agents, which aim to either reduce symptoms or cause nonspecific immunosuppression, respectively, but do not target the pathogenetic autoantibodies that characterize the disease.

1. The most common age at onset of gMG is the second and third decades in women and the seventh and eighth decades in men.

MG has an annual incidence of approximately  four to 30 new cases per million population. Prevalence rates range from 150 to 200 cases per million population, and they have steadily increased over the past 50 years. This increase in prevalence is probably the result of better disease recognition, aging of the population, and an increased life span in patients.

MG can occur at any age; however, onset is more common in females in the second and third decades and is more common in males in the seventh to eighth decades. Before age 40 years, the female-to-male ratio is 3:1, and after age 50 years, the female-to-male ratio is 3:2.

2. gMG commonly weakens muscles responsible for eye movement, facial expressions, and functions such as chewing, swallowing, and speaking.

gMG typically manifests as muscle weakness that worsens with repeated use. Patients often report that their function is best in the morning, with more pronounced weakness at the end of the day. Permanent muscle damage is rare, however, and maximal muscle strength is often good.

Extraocular muscles are more commonly affected, as twitch fibers in these muscles develop tension faster, have a higher frequency of synaptic firing than limb muscles, and have fewer AChRs, making them more susceptible to fatigue. Patients present asymmetrically; intermittent drooping of the upper eyelid (ptosis) and double vision (diplopia) are the most common symptoms.

Muscles innervated by the cranial nerves (bulbar muscles) are involved in 60% of patients with gMG and can lead to fatigable chewing, reduced facial expression, speech difficulties (dysarthria), and weakness of swallowing (dysphagia). Up to 15% of patients initially present with bulbar muscle involvement, including dysarthria and painless dysphagia.

3. Emotional stress can trigger an MC.

MC is a complication of MG characterized by worsening muscle weakness that results in respiratory failure and necessitates mechanical ventilation.

MC is often the result of respiratory muscle weakness but can also be due to bulbar weakness with upper airway collapse. MC can occur in 15%-20% of patients within the first 2-3 years of the disease; however, it can also be the first presentation of MG in 18%-28% of cases.

MC can be triggered by multiple causes, including emotional or physical stress. The most common precipitant is infection; other precipitants include surgery, pregnancy, perimenstrual state, certain medications, tapering of immune-modulating medications, exposure to temperature extremes, pain, and sleep deprivation. Approximately one third to one half of patients with MC may have no obvious cause.

4. High levels of anti-AChR antibodies strongly indicate MG, but normal levels do not rule it out.

All patients with a clinical history suggestive of MG should be tested for antibodies for confirmation. Most patients have anti-AChR antibodies (~85%), and those without have anti–muscle-specific kinase (MuSK antibodies) (6%) and anti–lipoprotein receptor-related protein 4 (LRP4) antibodies (2%).

The sensitivity of anti-AChR antibodies varies depending on whether the antibody is binding, modulating, or blocking the AChR. Binding antibody is the most common, and when combined with blocking antibodies, has a high sensitivity (99.6%) and is typically tested first. Higher AChR antibody titers are more specific for the diagnosis of MG than are low titers, but they do not correlate with disease severity.

For patients who do not have anti-AChR antibodies but do have clinical features of MG, anti-MuSK antibodies and anti-LRP4 antibodies are measured to increase diagnostic sensitivity. For symptomatic patients who do not have any autoantibodies (seronegative), electrodiagnostic testing that shows evidence of impaired signal transmission at the neuromuscular junction is used to confirm the diagnosis of MG.

5. Studies suggest that over 75% of seropositive MG patients show distinct thymus abnormalities.

More than 75% of patients with AChR antibody–positive MG have abnormalities in their thymus, and up to 40% of patients with a thymoma have MG. Among those with thymic pathology, thymic hyperplasia is the most common type (85%), but other thymic tumors (mainly thymoma) can be present in up to 15% of cases. Thymomas are typically noninvasive and cortical, but in some rare cases, invasive thymic carcinoma can occur.

Given this overlap in presentation, it is recommended that patients with seronegative and seropositive MG undergo chest CT or MRI for evaluation of their anterior mediastinal anatomy and to detect the presence of a thymoma. For patients with MG and a thymoma, as well as selected (nonthymomatous) patients with seropositive or seronegative MG, therapeutic thymectomy is recommended.