Histopathologic study of extraocular muscles in thyroid-associated ophthalmopathy coexisting with ocular myasthenia gravis: a case report

BACKGROUND: Coexistence of thyroid-associated ophthalmopathy (TAO) and ocular myasthenia gravis (OMG) is very rare. Little is known about the orbital histopathology associated with this condition. The authors reported a case of TAO coexisting with OMG and explored the histopathologic changes in extraocular muscles. CASE PRESENTATION: A 32-year-old man complaint of bilateral proptosis for 2 years. The patient was documented with a history of OMG and was treated with blepharoplasty to correct ptosis 3 years prior to presentation. Physical examination revealed right upper eyelid retraction resulting from the eyelid surgery. Computed tomographic scan demonstrated bilateral enlargement of the extraocular muscles. Thyroid function test confirmed hyperthyroid status. The patient was diagnosed with TAO (clinical activity score = 2/7) coexisting with OMG. Orbital decompression surgery reduced proptosis but resulted in new onset of left upper eyelid retraction because of the increased motor impulses to sustain eyelid elevation. Extraocular muscles were sampled during surgery and subjected to histopathologic stain. The stain results were analyzed against samples from age-, gender- matched TAO and control (non-TAO non-OMG) subjects. The measurement of myofiber size and glycosaminoglycan/collagen-occupied area was repeated in 3 randomly chosen fields of each slide. The variation of myofiber size was larger in the TAO + OMG (289.9 ± 142.5 μm(2)) samples than the TAO (544.1 ± 160.6 μm(2)) and control (157.0 ± 47.7 μm(2)) samples. Glycosaminoglycan was more abundant in the TAO + OMG (48.8 ± 12.2%) samples than the TAO (28.4 ± 3.6%) and control (3.3 ± 0.8%) samples. Collagen fibers accumulated in the TAO (60.5 ± 6.4%) samples but not in the TAO + OMG (36.1 ± 4.3%) and control (33.9 ± 2.7%) samples. Typical OMG changes were observed in the TAO + OMG samples but not in the TAO and control samples. These changes included central nuclei, aggregation of mitochondria and fiber type grouping. The histopathologic findings of TAO + OMG were summarized as inhomogeneously enlarged muscle fibers and predominantly endomysial accumulation of glycosaminoglycan. CONCLUSION: This study highlights the possibility of TAO coexisting with OMG and demonstrates the histopathologic features in this rare condition.