Corneal astigmatism can affect Intraocular Pressure IOP measurements.
If there is more than 3.00 D of astigmatism the following is true:
1. results in an under estimation of IOP in with-the-rule astigmatism and an over estimation with against-the-rule astigmatism, with an error range of about -2.5 to +2.5 mmHg.
Two options exist to counteract this source of error:
1. Align tonometer head at 43 degree to axis of astigmatism ( in negative cylinder).
In other word: Set the axial position of the greater radius (that is the axis of a minus cylinder) on the prism graduation at the red mark on the prism holder of the tonometer head.
2. Average IOP readings at 0 and 90 degrees.