[Primary open angle glaucoma in cornea plana masked by false normal applanation tonometry (Goldman) - a case report]

Cornea plana is a rare syndrome, which leads to a flat corneal curvature due to a reduced sclero-corneal angle. Depending on the regularity of the corneal astigmatism the frequently resulting hyperopia may be difficult to compensate for. Because of the flatness of the cornea the anterior chamber is also flat, the anterior chamber angle can be restricted, and the applanatory intraocular pressures (IOPs) are measured too low. A primary open angle glaucoma is therefore not diagnosed for a long time, until changes of the optic nerve head occur. Patient and methods We report on a 66-year-old male with cornea plana. Although his intraocular pressure (IOP), measured with an applanation tonometer (Goldmann), had always been normal (< 21 mm Hg), he suffered from an increasing glaucomatous atrophy of the optic disc. We carried out a complete ophthalmological investigation, including keratometry and corneal topography analysis (TMS-1, Tomey). Furthermore, visual field was determined (G1, Octopus) and the optic nerve heads were documented and analysed by papillometry. A 48-hour tension profile was worked out for both eyes including applanation and Schiötz tonometry. Results The central refractive power of the cornea was 31 diopters and the cornea seemed to be flattened on slit lamp evaluation. The glaucomatous atrophy of the optic disc was more pronounced in the OD than in the OS (OD=neuroretinal rim loss in the upper part, at the bottom and in the lateral part of the optic disc, OS=laterally distinct neuroretinal rim loss). While the anterior parts of the eye were shortened (depth of the anterior chamber was OD/OS=1.9 mm), a macrophthalmus posterior was stated (axial length OD=25.78 mm, OS=25.72 mm). However, the IOPs were measured below 21 mm Hg by applanation during the entire tension analysis, comparable values measured with the Schiötz tonometer showed values above 21 mm Hg. We converted the applanatory IOP values according to the flat corneal power, as described in literature (addition of 1 mm Hg to the applanatory values per 3 diopters decreased corneal power). The tension analysis now showed increased values, as expected after observation of the glaucomatous excavated optic nerve head. Conclusion In patients with cornea plana applanatory IOPs are measured too low. Therefore in case of very flat corneas a mathematical correction of the applanatory IOP should be carried out, in order to diagnose a primary open angle glaucoma early enough.