17 Calculated correction of tonometrically mea
s
ured IOP
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Instruction Manual Myopia Master® (G/68100/EN Rev04 0820)
17 Calculated correction of tonometrically mea
s
ured IOP
Independent studies have shown that tonometric measurements of
intraocular pressure (IOP) are influenced by the thickness and curvature
of the cornea in its center. This systematic measurement error can only be
eliminated by calculation. This holds regardless of whether measurements
have been performed with an applanation tonometer or a noncontact
tonometer.
The Myopia Master® software provides a means of saving a
tonometrically measured IOP value obtained in a seperate examination
and correcting the value on the basis of the corneal thickness of that eye
as measured with the Myopia Master®. This can be done using any of the
various correction formulas available for this purpose. These formulas are
described further below, and some are illustrated with diagrams.
17.1 IOP correction ba
s
ed on central corneal thickne
ss
The correction formulas of Shah and Ehlers as well as the Dresden
correction formula are estimates of the true IOP based on measured IOP
and corneal thickness.
Correction is achieved by adding a correction summand, which is referred
to as IOP change and is a function of corneal thickness, to the measured
IOP:
IOP
corrected
= IOP
measured
+ IOP - change
The calculation of this summand is based on a mean thickness value for
which the summand assumes the value zero, i.e. where the measured
pressure need not to be corrected. Within a range of approx. ±70µm
around this thickness value the correction summand changes linearly
with corneal thickness, while for thicknesses above or below this range a
linear relationship has not been confirmed.
