General Notes; Astigmatism - Haag-Streit Perkins Mk2 Manual De Instrucciones

8.

General Notes

Note
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. Excited or anxious patients often
display a higher ocular pressure during the first measurement.
For this reason, a reduction of tension is experienced during the first few minutes, as patients
notice that tonometric examinations cause no unpleasant effects.
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.
8.

Astigmatism

1
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 that 3 dioptres are examined, as
the flattened areas are not circular but elliptic.
It has been calculated that, in cases of larger corneal astigmatisms, a surface of 7.354 mm
(ø 3.06 mm) is to applanated, when the measuring prism is at an angle of 43˚ to the meridian
of the greatest radius.
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2
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