New diagnostics: making surgery easier and safer

New developments in diagnostic technology should enable better screening and follow-up of refractive surgery patients, said Dan Z. Reinstein, MD, delivering a keynote address during the refractive surgery subspecialty day preceding the annual meeting of the American Academy of Ophthalmology in November 2007.

Dr Reinstein provided an update on some of the most recent advances in diagnostic instrumentation that are relevant to refractive surgeons, focusing on improved methods for measuring intraocular pressure (IOP) after refractive surgery, instrumentation for measuring quality of vision, developments in corneal topography, and new indices for keratoconus screening.

Overcoming the limitations of other instruments

The CH-measuring instrument is a modified air-puff tonometer that provides an IOP measurement that is virtually independent of corneal thickness. "While it does not directly measure pressure in the eye, [this instrument] does a very good job of approximating the value," Dr Reinstein told Ophthalmology Times. In addition to its role for IOP measurement, the device provides two measures of the biomechanical properties of the cornea: CH and the corneal resistance factor (CRF). These indices are calculated by analyzing the signal derived from the inward and outward applanation events induced by the air-puff.

The contact tonometer uses the Pascal principle instead of applanation to eliminate systematic errors due to corneal properties such as thickness, rigidity and elasticity. The instrument directly measures pressure within the anterior chamber through a pressure-coupling concept that can be understood using the Pascal principle. The performance of this instrument has been validated in multiple studies, including an investigation involving direct cannulation of the anterior chamber, Dr Reinstein noted.

Studies have shown that, in contrast to Goldmann tonometry, the preoperative IOP value and the post-LASIK IOP are similar. A study presented by Jay Pepose MD, at the 2006 American Society of Cataract and Refractive Surgery annual meeting demonstrated that the IOP measurements obtained by those instruments were relatively unaffected by corneal thickness.

Quantity and quality

Dr Reinstein advised of a new diagnostic tool (Optical Quality Analysis System; Visiometrics) that uses a double-pass measurement technique and records the image of a point source after reflection on the retina and double pass through the optics of the eye. The instrument generates two- and three-dimensional maps of the point spread function (PSF) and the modulation transfer function (MTF). It also provides an objective scatter index (OSI). The device, therefore, is helpful in discriminating ocular scatter from wavefront error.

"In a normal eye, the MTF value from the [instrument] will be the same as that calculated from the wavefront measurement obtained with a Hartmann-Shack aberrometer. However, in an eye where some lens scattering elements are present, the MTF [value] from the [device] will be more affected, because [the instrument's] measurement incorporates scatter as well as higher-order aberrations (HOAs), whereas a Hartmann-Shack aberrometer only includes the HOAs." noted Dr Reinstein.

"This instrument might provide another tool for quantifying lens opacity and selecting patients for cataract surgery. For the refractive surgeon, the [device] also could be useful when considering phakic IOL versus clear lens exchange," he said.

Another device (COAS/WASCA Aberrometer; AMO) has an add-on module for measuring accommodation and changes in ocular wavefront during accommodation. This module has been used to demonstrate interesting changes in refraction and HOAs, particularly spherical aberration, during accommodation. This technology is promising for studying accommodative IOLs, Dr Reinstein said.