János Németh:

Tear film and IOLMaster measurements

Dear Editor,

I read with great interest the recent article of K. Sander & W. Bissmann and also that of M. Rütz, both on the reproducibility of keratometry performed using the IOLMaster instrument.

Our own experiences confirm the reported high precision of IOLMaster corneal power measurements. In our comparative study (1) we found that the corneal refractive power measurements of a Javal-type keratometer and those from the IOLMaster were highly correlated (included one eye for each individual, r = 0.955; p < 0.001), with a mean difference of only 0.17 diopters. This difference, although statistically significant (p < 0.001), is unimportant clinically.

Figure 1. Correlation between the corneal power (K) measured by Javal type keratometer and by the IOLMaster (both eyes included, r=0.936, p<0.001).

The other interesting point is the possible effect of tear-film alterations on the keratometric measurements. These two ULIB articles showed that there is no need for artificial tear drops before performing IOLMaster keratometry, and indeed that without tear drops the measurements seem to be more accurate. It would however be very interesting to examine with the IOLMaster another important aspect relating to the tear film, namely whether the elapsed time since the last blink has any significant effect on the measured keratometric values.

The tear film is not a stable structure, and it alters rapidly during the interval between two successive blinks. At the end of a blink, as the upper lid moves upwards it spreads the tear film over the exposed corneal and conjunctival surface. But after the lid becomes stationary, the tears still move rapidly upwards during a period of about 1 second; and even after this an extra 2-6 seconds are needed for the tear film to attain its most regular surface. In a recent publication (2), we have described this phenomenon and named it the "tear film build-up process". At a later time (around 10 seconds after blinking, in healthy eyes), the well known tear film break-up starts to occur.

I would like to call attention to our findings that in the case of corneal topographic measurements, the length of the elapsed time since the last blink significantly influenced the surface regularity indices, because of the above mentioned rapid build-up changes in the tear film (although we found no significant effect on the simulated keratometric values).

We have published two articles providing evidence of significant tear film alterations just after blinking in the inter-blink period (2,3). We have suggested that it is desirable to try to optimise the time point at which corneal topography measurements are made. It would appear that the optimum time for measurement might be the time period after the end of the tear film build-up process (2) but before the tear film break-up starts, i.e. measurements should be made typically between ca. 3-7 seconds after the last blink (3).

I believe that such tear-film alterations might influence IOLMaster keratometry as well, and therefore it may be important to examine the effect of such alterations on IOLMaster keratometric readings during the different phases of the interblink period.

Congratulations on the very useful and interesting ULIB published articles.

Best regards

Prof. János Németh MD, PhD
1st Dept. of Ophthalmology
Semmelweis University
Tömő u. 25-29.
H-1083 Budapest, Hungary
Tel: + 36 30 952-3310
Fax: + 36 1 303-8974
email: nj@szem1.sote.hu

Literature:

(1) Németh J., Fekete O., Pesztenlehrer N.: Comparison of optical and ultrasound measurement of axial eye length and anterior chamber depth for intraocular lens power calculation. J Cataract Refract Surg 29: 85-88, 2003

(2) Németh J., Erdélyi B., Csákány B., Kahlesz F., Gáspár P., Lang Zs.: High-speed video topographic measurement of tear film build-up time. Invest Ophthalmol Vis Sci 43: 1783-1790, 2002.

(3) Németh J., Erdélyi B., Csákány B.: Corneal topography changes after a 15 second pause in blinking. J Cataract Refract Surg 27: 589-592, 2001.

For copies of the above articles please contact the author via nj@szem1.sote.hu.