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3.6 Tips for the measurement
NOTICE
Only the shortest possible measuring time periods should
be carried out on each eye.
Should drying of the cornea epithelium be observed, then
vision and field of sight should be checked in advance.
The measuring procedure can be repeated several times. Ex-
cited or anxious patients often display a higher ocular pressure
during the first measurement.
For this reason, a reduction of tension is experienced du-
ring the first few minutes, as patients notice that tonometric
examinations cause no unpleasant effects. Patients will feel
absolutely nothing when well anaesthetised and with the eyes
wide open.
A test measuring procedure should therefore be first made
on each eye. These results can be disregarded. Thereafter,
three measuring procedures are to be carried out on each
eye. The readings will be correct when the pressure has been
stabilised. When measuring procedures have been correctly
carried out, divergences in the results will be only of the order
of ± 0.5 mm Hg.
When the measuring procedure lasts too long on one eye,
drying will occur on the cornea epithelium of both eyes. A ring
of fluorescein-positive deposits around the contact positions
of the cornea and the measuring element will occur on the eye
under examination.
On the other eye, map-like fluorescein-positive dried places
will occur, which will not allow a reliable measuring procedure.
Extensive drying phenomena disappear after a short time
without treatment becoming necessary. Focused vision will be
affected by such fine epithelium defects.
3.7 Astigmatism
If the cornea is spherical, measurements can be made on any
meridian, but it is most convenient to do it on the 0° meridian.
This is not so when eyes with higher corneal astigmatism
than 3 dioptres are examined, as the flattened areas are not
circular but elliptic.
It has been calculated that, in cases of larger corneal astig-
matisms, a surface of 7.354 mm
2
(ø 3.06 mm) is to be appla-
nated, when the measuring prism is at an angle of 43° to the
meridian of the greatest radius.
Example
If the corneal astigmatism amounts to
6.5 mm / 30°
= 52.0 D / 30° and
8.5 mm / 120°
= 40.0 D / 120°
the graduation value 120° of the prism is set at the red 43°
mark of the prism holder.
If there is a corneal astigmatism of
8.5 mm / 30°
= 40.0 D / 30° and
6.5 mm / 120°
= 52.0 D / 120°
the graduation value 30° is set at the red 43° mark. In other
words, set the axial position of the greatest radius, that is the
axis of a minus cylinder, on the prism graduation at the red
mark on the prism holder.
