Low-tension Glaucoma in Identical Twins

Abstract

Inquiries to Steven Ofner, M.D., Eugene Eye Care Associates, P.C., 1415 Pearl St., Eugene, OR 97401. Congenital glaucoma in identical twins has been observed, as has primary open-angle glaucoma in monozygotic twin pairs, but lowtension glaucoma in identical twins has not been well documented. In a large study of twins with chronic open-angle glaucoma, 29 monozygotic and 79 dizygotic pairs were studied. In another large study of the incidence of ocular disease in twins, low-tension glaucoma was not described. However, intraocular pressure criteria were probably part of the study. An autosomal dominant form of low-tension glaucoma has been previously described. We managed a case of low-tension glaucoma in identical twins. A 44-year-old woman was referred for examination after routine automated perimetry disclosed an abnormality in the visual field. Her ocular and medical history were noncontributory. Her identical twin sister was also healthy. Ocular examination showed a visual acuity of 20/20 in both eyes. The left eye had an afferent pupillary defect. There were inferior iris transillumination defects in both eyes without Krukenberg's spindles. Gonioscopy showed the iridocorneal angle to be open 360 degrees to the scierai spur with mild trabecular pigmentation. The optic disk had a cup/disk ratio of 0.7 in both eyes, with a splinter hemorrhage at the disk margin in the superotemporal portion of the left eye. Static threshold perimetry disclosed a dense inferonasal step in the left eye that corresponded with the disk hemorrhage. The visual field in the right eye showed an inferonasal depression of retinal sensitivity. The highest intraocular pressure measured was 21 mm Hg in both eyes during an all-day diurnal curve study. The lowest intraocular pressures were 16 mm Hg in the right eye and 17 mm Hg in the left eye. Intraocular pressure decreased to 13 mm Hg in the right eye and 15 mm Hg in the left eye while the patient was instilling timolol 0.5% twice a day in both eyes. Subsequently, the patient's asymptomatic, identical twin sister was examined. Results of her ocular examination were similar to those of her twin sister, except her pupillary responses were normal. No iris transillumination defects were noted. The optic disk showed a cup/disk ratio of 0.7, with some sloping temporally in both eyes. A large splinter hemorrhage was present superotemporally in the right eye, which was contralateral to the eye of her twin sister. Results of static threshold perimetry were normal in the left eye, but showed an inferonasal defect in the right eye. Results of magnetic resonance imaging of the brain and orbits with gadolinium contrast were within normal limits as were results of a complete physical examination and carotid artery duplex scanning for each twin sister. The highest intraocular pressures measured were 21 mm Hg in the right eye and 19 mm Hg in the left eye during an all-day diurnal curve study. The lowest intraocular pressures were 18 mm Hg in the right eye and 15 mm Hg in the left eye. The