Dynamic contour tonometer accurate in eyes with thin contact lenses, study find

The presence of contact lenses did not skew measurements, which are independent of corneal thickness and rigidity, investigator says.

Dynamic contour tonometry yielded precise IOP measurements on normal corneas fitted with thin hydrogel contact lenses, a study found.

The Pascal dynamic contour tonometer (Ziemer Ophthalmic Systems) measures IOP independent of corneal thickness and rigidity, Daniela Sonja Nosch, MSc, MCOptom, the study’s corresponding author, told Ocular Surgery News.

“The message basically is that this tonometer gives you higher accuracy across the range with all kinds of different corneas, and that a thin contact lens does not have a statistically significant impact on the measurements,” Dr. Nosch said.

The dynamic contour tonometry (DCT) device showed the accuracy of IOP measurements in cases in which contact lenses could not be removed, Dr. Nosch said.

“[If a clinician] in Europe is not able to use diagnostic drugs, this study now enables him to be sure that the measurements he takes with this tonometer remain accurate when he measures over a contact lens,” she said. “For those … who use therapeutic lenses, they can take a measurement without taking the lens out.”

Patient selection considerations

Dr. Nosch said that the study included only healthy corneas with no irregularities. She also noted that patients undergoing measurement with DCT must maintain ocular fixation for 4 or 5 seconds.

“This is how long it takes to obtain a reading,” Dr. Nosch said. “On patients with difficulties to keep up fixation for this length of time, it is not easy to use the DCT. Hence, people who have nystagmus are not suitable. Any movement of the eye, of course, has a negative effect.”

DCT may be ideal for patients with keratoconus or previous refractive surgery because it does not rely on corneal thickness. The Goldmann applanation tonometer is prone to error because it is calibrated for a central corneal thickness of 520 mm, Dr. Nosch said.

“You are going to underestimate the intraocular pressure in eyes after refractive surgery or in those with keratoconic corneas,” she said. “Equally, if you’re dealing with a thicker cornea than average, you are going to overestimate the intraocular pressure with Goldmann tonometry.”

IOP and pulse amplitude

The prospective, randomized study, published in Contact Lens & Anterior Eye, included 46 eyes of 46 patients with a mean age of 43 years. Investigators used DCT to take IOP and ocular pulse amplitude (OPA) measurements on corneas with and without thin daily disposable hydrogel contact lenses (Filcon IV, –0.5 D).

The order of measurements with and without contact lenses in situ was random.

Study results showed that the average IOP measurements were 16.1 mm Hg in eyes with contact lenses and 16.51 mm Hg in eyes without contact lenses. The mean difference of 0.41 mm was statistically insignificant.

Average OPA measurements were 2.08 mm Hg in eyes with contact lenses and 2.2 mm Hg in eyes without lenses. The mean difference was statistically significant (P = .025).

OPA measurements made with contact lenses in place were 5.45% lower than those without lenses.

Reasons for the difference in OPA measurements are unknown, Dr. Nosch said.

“It is possible that the sensor wasn’t able to measure this amplitude as accurately through the contact lens,” she said. “Clearly, this finding has to be further investigated in future studies.”

The DCT also had a low standard error of 1.51 mm Hg in IOP measurement, Dr. Nosch said.

Data on the accuracy of DCT measurements in eyes fitted with contact lenses of varied powers, thicknesses and materials would be helpful, Dr. Nosch said.

“It would be interesting to use the silicone hydrogel lens with higher rigidity to work out if the measurements remain accurate,” she said. “In the next stage, it would also be interesting to carry out a study with differently powered lenses to find out if that still remains accurate if you take measurements over those lenses.” – by Matt Hasson

Reference:

• Nosch DS, Duddek AP, Herrmann D, Stuhrmann OM. Dynamic Contour Tonometry (DCT) over a thin daily disposable hydrogel contact lens. Cont Lens Anterior Eye. 2010;33(5):219-224.

• Daniela Sonja Nosch, MSc, MCOptom, can be reached at the University of Applied Science, Institute of Optometry, Aarauerstrasse 30, 4600 Olten, Switzerland; +41-44-272-78-18; fax: +41-44-275-90-30; e-mail: daniela.nosch@yahoo.de.
• Disclosure: Dr. Nosch has no direct financial interest in the products discussed in this article, nor is she a paid consultant for any companies mentioned.