Yes, There Is a Documented Correlation Between Thyroid Eye Disease and Myasthenia Gravis
The incidence of myasthenia gravis is increased in patients with thyroid eye disease, and clinicians should assess for coexistence of myasthenia gravis when indicated by clinical examination findings. 1
Epidemiology and Clinical Association
A small percentage of patients with thyroid eye disease have co-existent myasthenia gravis, establishing a recognized clinical correlation between these two autoimmune conditions. 1
The association appears particularly strong with ocular myasthenia gravis (OMG), where patients with Graves' disease and ocular MG show a higher frequency of thyroid-associated ophthalmopathy (84.6%) compared to those with generalized MG or no MG. 2
Patients with Graves' disease and myasthenia gravis demonstrate a significantly greater prevalence of euthyroid ophthalmopathy (12.9%) compared to Graves' disease patients without MG (3.1%), suggesting preferential association between the ocular manifestations of both diseases. 2
Clinical Characteristics of the Overlap
Myasthenia gravis associated with autoimmune thyroid diseases has a milder clinical expression, with preferential ocular involvement (41.0% ocular MG) compared to MG without thyroid disease (21.4% ocular MG). 3
Patients with both conditions show lower frequency of thymic disease (26.7%) compared to MG patients without thyroid disease (59.7%), and lower acetylcholine receptor antibody positivity (35.7% vs. 57.6%). 3
Diplopia is significantly more frequent in OMG patients with TED (84.6%) compared to OMG patients without TED (44.8%), and this diplopia is notably difficult to treat, with 69.2% still experiencing diplopia after treatment with bromostigmine and glucocorticoid. 4
MG patients with TED have earlier age at onset and milder clinical symptoms compared to MG patients without hyperthyroidism. 4
Diagnostic Challenges and Clinical Pitfalls
Diagnosing ocular myasthenia gravis when thyroid eye disease is present can be very challenging because both conditions share overlapping clinical features including ptosis, diplopia, and extraocular muscle dysfunction. 5, 6
A multimodal diagnostic approach using clinical, serologic, imaging, and electrodiagnostic data is typically required when both conditions are suspected, as securing a diagnosis of OMG may not be possible on the basis of a single positive test in patients with coexisting TED. 5
The key distinguishing feature is that ptosis dramatically resolves after treatment with pyridostigmine bromide in myasthenia gravis, while restrictive myopathy from TED does not respond to anticholinesterase therapy. 6
Forced duction testing is critical: positive forced ductions (mechanical restriction) indicate TED, while negative forced ductions with fatigable weakness suggest myasthenia gravis. 1
Pathophysiologic Basis
The correlation may be due to immunological cross-reactivity against common autoimmune targets in the eye muscle as well as a common genetic background predisposing to both autoimmune conditions. 2
Both conditions represent autoimmune disorders with orbital tissue involvement, though TED causes mechanical restriction from inflammatory edema and fibrosis, while MG causes neuromuscular junction dysfunction. 7, 8
Clinical Management Implications
Because the incidence of myasthenia is increased in patients with TED, assessment for the coexistence of myasthenia is suggested if indicated by findings on clinical examination, particularly when clinical features don't fully align with expected TED patterns. 1
Clinicians should maintain high suspicion for overlapping disease when patients with thyroid disease display both ptosis and eye movement dysfunction, especially if only the ptosis responds to anticholinesterase medication. 6
The presence of fatigable ptosis, variable diplopia, or symptoms that worsen with sustained gaze should prompt evaluation for myasthenia gravis even in confirmed TED patients. 5
Treatment planning becomes more complex with coexisting disease, as diplopia in patients with both OMG and TED is very difficult to treat and may not respond adequately to standard therapies for either condition alone. 4