Introduction: Graves disease (GD) is a common cause of thyrotoxicosis, Myasthenia Gravis (MG) is less common (incidence of 3-30cases per million). Between 5-10% of patients with MG also have thyroid disease. However, only 0.14% of patients with GD having MG. Both are autoimmune diseases sharing pathophysiological mechanisms. Co-existence, although rare, is well established. MG may mimic the neuromuscular signs of GD especially if these are subtle. We report a case of a 32year old female with ptosis who was later found to have GD and MG coexistence.
Case: A patient with no past medical history was referred to Ophthalmology with a partial left ptosis. Physical examination revealed fatigable ptosis of her left upper eyelid and a lid lag with possible right eyelid retraction. She had a good levator palpebrae superioris muscle function, and her visual acuity was 6/4 in both eyes. She did not report diplopia, and there was no proptosis. Although symptoms of thyrotoxicosis were absent, her presentation raised the possibility of Graves ophthalmopathy (GO) - but ptosis with fatigability is not common in GO. Hence, investigations were arranged to check thyroid function (TFT), autoimmune thyroid screen and autoantibodies against the acetylcholine receptor (AChR) and muscle-specific tyrosine kinase (MuSK). Initial TFT showed a suppressed TSH (<0.01;N=0.27-4.20miu/l) and marginally elevated FT4 (22.5;N=12-22pmol/l)and FT3 (8.3;N=3.1-6.8pmol/l). However, a month later, further results showed a significant deterioration, FT4 46.6pmol/l and FT3 26.9pmol/l. Carbimazole 30mg once daily was commenced with subsequent biochemical improvement. Coeliac screen was negative. Both TSH receptor and Ach Receptor antibodies were positive, establishing dual autoimmune pathologies. She is waiting for an Electromyography.
Discussion: GD and MG are autoimmune diseases mediated by autoantibodies targeting membrane receptors- thyroid stimulating hormone receptors and postsynaptic neuromuscular junction respectively. Although coexistence may occur, and ocular symptoms and muscle weakness may be a presenting clinical feature of both conditions ptosis is not a common feature of GO; hence its presence may indicate MG. Furthermore, the co-association of GD and MG can pose a treatment challenge for the clinicians. Carbimazole for GD may increase the risk of myasthenic crises. Additionally, should definitive surgical management be considered for GD there is evidence of poorer outcomes when MG is also present. This case highlights the need for clinicians to be aware of the fact that both GD and MG may co-exist and the impact dual pathology may have on treatment outcomes.