Goldmann applanation tonometry in patients with regular corneal astigmatism.

By averaging vertical and horizontal applanation readings from the Goldmann tonometer, we developed a simple technique that accurately measures intraocular pressure in patients with regular corneal astigmatism. This procedure eliminates the need for keratometry and oblique alignment of the applanator. When the applanator is oriented normally, with the mires displaced horizontally, intraocular pressure is underestimated for with-the-rule corneal astigmatism and overestimated for against-the-rule corneal astigmatism. The error is approximately 1 mm Hg for every 4 diopters of astigmatism. For oblique axis corneal astigmatism, the error is smaller and approaches zero when the axis is either 45 or 135 degrees.