In their letter to the editor, Rao and coauthors report results of intraocular pressure (IOP) measured with both Goldmann applanation tonometry (GAT) and Tono-Pen tonometry after photorefractive keratectomy (PRK).1 They state that they doubt the accuracies of peripheral measurements for 2 reasons. First, the necessity to abduct the eye may cause a change in IOP. The very small abduction, however, and the change in pressure consequent to the deformation is negligible.2 Second, the thickness of the cornea is greater in the periphery. This is true, theoretically. However, in their study, IOP as measured in the periphery of the cornea with the GAT is very similar to that measured preoperatively in both the study and the control groups. This accords with our findings.3
The differences in IOP measured temporally and centrally in untreated eyes is much smaller than the difference between Tono-Pen and GAT values. Measuring IOP with GAT after myopic PRK or laser in situ keratomileusis (LASIK) in the periphery of the cornea is, therefore, probably more accurate than central measurements with the Tono-Pen. This is lower than in the periphery of the cornea, as shown in the letter by Rao and coauthors and in our recent paper.4 Moreover, almost every ophthalmologist uses GAT routinely, and measurement in the periphery of the cornea is a simple procedure. It is suggested that peripheral GAT measurements should be performed in every glaucoma suspect after myopic PRK or LASIK.
Isac Schipper MD
1. Rao SK, Vineet R, Padmanabhan P. How and where should intraocular pressure be measured after photorefractive keratectomy. J Cataract Refract Surg 1999; 25:1558-1560
2. Moses RA, Lurie P, Wette R. Horizontal gaze position effect on intraocular pressure. Invest Ophthalmol Vis Sci 1982; 22:551-553
3. Schipper I, Senn P, Thomann U, Suppiger M. Intraocular pressure after excimer laser photorefractive keratectomy for myopia. J Refract Corneal Surg 1995; 11:366-370
4. Schipper I, Senn P, Oyo-Szerenyi K, Peter R. Central and peripheral pressure measurements with the Goldmann tonometer and the Tono-Pen after photorefractive keratectomy for myopia. In press, J Cataract Refract Surg