Ocular Myasthenia Gravis in Monozygotic Twins with Mirror-image Myopic Anisometropia

Dear Editor, Mirror-image twinning with inverse laterality happens in 10% to 15% of monozygotic twins and can be useful in establishing that twinning events have occurred after the cells of the embryonic plate began to lateralize, but before formation of the primitive streak [1]. Previous reports on ocular mirror image included cases with myopic anisometropia, infantile esotropia, infantile nystagmus, optic glioma, optic nerve dysplasia, and retinoblastoma [2,3]. Herein we report a case of monozygotic twins with mirror-image myopic anisometropia who developed ocular myasthenia gravis (MG) and had remnant motility defects in mirror-image eyes. To the best of our knowledge, this is the first report of ocular MG in mirror-image twins. A pair of 26-year-old female twins presented with mirror-image asymmetry of monocular low vision and motility deficits. One (twin 1) had myopic anisometropia, a myopic disc, and an adduction deficit in the left eye, while the other (twin 2) exhibited the same conditions in the right eye. Anisometropia of -0.75 Dsph -1.00 Dcyl axis 90 (20 / 20) in the right eye and of -6.50 Dsph -1.50 Dcyl axis 90 (counting fingers) in the left eye, whereas the same measures for twin 2 were -8.00 Dsph -1.50 Dcyl axis 180 (counting fingers) in the right eye and -5.00 Dsph -1.50 Dcyl axis 180 (20 / 25) in the left eye. On fundus examination, a myopic tilted disc with parapapillary atrophy and tigroid fundus with reduction of the foveal reflex were observed in the left eye of twin 1 (Fig. 1A) and in the right eye of twin 2 (Fig. 1B). These twins had a history of ocular motility disturbances and a positive acetylcholine receptor antibody titer, which led to a presumptive diagnosis of ocular MG. They had previously had an eye examination at 19 years of age. On this initial examination, twin 1 exhibited left exotropia Korean J Ophthalmol 2016;30(5):392-393 http://dx.doi.org/10.3341/kjo.2016.30.5.392