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Goldmann Applanation Tonometry our Golden Standard? Measure Central Corneal Thickness?
Michael Diestelhorst
There are observer-related reading errors and equipment-related reading errors.
The Goldmann Tonometer was calibrated for standard corneal thickness (520µm), standard radius of curvature (r = 7mm) and also standard corneal "rigidity". GAT is based on the Imbert-Fick law. The 4 prerequisites of this law are not fulfilled by the human eye:
- the cornea is not ball shaped and perfectly spherical
- the surface is not an endlessly thin membrane without rigidity
- during applanation aqueous shifts to the trabecular meshwork and the posterior chamber
- tear film adhesion forces occur during measurements
It is known that the variances related to corneal thickness (470 790 µm) exceed those of curvature variances (6.4 - 8.5 mm). To date a considerable number of research projects over many decades have attempted to quantify these variances. The main outcome has been a concentration on the issue of CCT. The range of estimates of error related to corneal thickness vary from 0.19 mmHg per 10µm to 0.7 mmHg per 10µm variance.
In their original paper in Ophthalmologica in 1957 Goldmann and Schmitt mentioned that there are numerous possibilities for faulty readings resulting in a 2 mmHg step scale.
To date the observer related reading errors still seem to outweigh the equipment related one's by far: corneal astigmatism, keratitis, scarring, keratoconus, corneal surgery, microphthalmos, buphthalmos, nystagmus, blepharospam, chronic treatment with preservatives leading to desquamation of corneal epithelium cells and the stroma, blinking, ventilation, spread of fluorescein during measurements. Not to mention the inter-observer variabilities.
In healthy eyes, IOP varies by 3-6 mmHg; in glaucoma by 6 20 mmHg within 24 hours. We measure IOP for about two seconds every three to five months and claim to know our the IOP of our patients. It does not need a statistician to tell us that our information on IOP is very low indeed. However, based on these very few readings, we decide on the success or failure of medical treatment or surgery.
It may very well be that CCT does have an influence on the response to treatment - with thin corneas appearing to respond better to presumably the same degree of IOP lowering than thick corneas. It may also be that we have over-treated OH patients, due to the fact that they had thick corneas, but that otherwise, everything else was normal. However, the conclusion that we should measure CCT in each patient seems to be misleading. Since lowering the IOP is the only reliable treatment option we have, it is high time to concentrate our research on newer IOP reading techniques. Until we have a telemetric system that gives reliable data from inside the eyeball over a period of 24 hours at ~ 40 Hz, we should try to improve the Goldmann system to the best of our ability. Fluorescein-free and automated measurements would be a good start and are already available (Ocuscan A, EPSA GmbH, Germany).
Why we should change from Goldmann:
- CCT induces clinical signs, errors in the IOP readings of GAT
- OH who have normal IOPs, but thick corneas
- Normal-tension glaucoma patients who have high IOPs, but thin corneas
- Undiagnosed people at risk with high IOP but thin corneasLASIK patients - how are we going to monitor their IOP in the future?
and:
- After ~ 50 years, Hans Goldmann would have liked us to have improved his device!
