Does the high modulus of SiH affect the accuracy of tonometry taken directly over the lens?

Fabrizio Zerio, Luigi Lupelli, Luciana Zarrillu, Dipartimento di Optometria dell”Instituto “De Amicis” Via Galvani, Rome Italy

Purpose:  Several authors have reported that it is possible to measure the intraocular pressure (IOP) accurately and safely using Goldmann applanation tonometry over a thin soft hydrogel contact lens (CL).  This procedure could be very useful in case of continuous wear as it allows us to measure the pressure without removing the CL.  However silicone hydrogel (SH), the most common material for continuous wear, has a higher modulus that could affect the accuracy of the procedure.  The aim of this work was to compare the measure of IOP over the SH CL with the highest modulus available in the market versus a hydrogel CL.

Method:  Twenty-four healthy subjects (13 males and 11 females; mean age 40.4 year, SD 13.4) were studied.  Two measurements of IOP using Goldmann applanation tonometer were taken in each subject for both eyes (for a total of 48 eyes); one with a sph -1.50D lotrafilcon A and the other with a sph -1.50D hilafilcon A.  This latter kind of hydrogel daily disposable CL was chosen because, as previously assessed, it had given a good reliability respect to the standard procedure.  The sequence of the two measurements over the two CLs was always inverted in the eyes of the same subject.  A t-test for dependent sample was used.

Results:  No differences were found between the value of the IOP with hilafilcon A and with lotrafilcon A (t-0.46, p=0.64).

Conclusion:  The IOP recorded over a lotrafilcon A CL (the SH with the highest modulus available) was the same as that over the hilafilcon A CL.  Although in this case the higher modulus of the SH doesn’t seem to affect the final outcome, further studies are needed to verify if similar findings occur in case of positive or higher negative CL powers.