Scheimpflug device, ultrasound pachymetry both repeatable for CCT measurements

April 5, 2009

San Francisco-Both ultrasound pachymetry and a proprietary Scheimpflug imaging device (Galilei Dual Scheimpflug Analyzer, Ziemer Ophthalmic Systems) produced excellent repeatability of central corneal thickness (CCT) measurements, and the repeatability was strong for both "virgin" and post-refractive eyes, according to Mitchell P. Weikert, MD, MS, assistant professor of ophthalmology, Baylor College of Medicine, Houston.

San Francisco-Both ultrasound pachymetry and a proprietary Scheimpflug imaging device (Galilei Dual Scheimpflug Analyzer, Ziemer Ophthalmic Systems) produced excellent repeatability of central corneal thickness (CCT) measurements, and the repeatability was strong for both “virgin” and post-refractive eyes, according to Mitchell P. Weikert, MD, MS, assistant professor of ophthalmology, Baylor College of Medicine, Houston.

He presented data from a study of 77 eyes in 39 patients who were candidates for refractive surgery, and 39 eyes in 21 post-refractive surgery patients.

“Ultrasound pachymetry is considered the gold standard for CCT measurements, but it’s not without its problems,” Dr. Weikert said. “It requires the operator to place the probe directly in the center of the cornea, and there are other issues such as tissue compression. Scheimpflug imaging, on the other hand, is non-contact, repeatable, and can provide a measurement of a larger area of the cornea, not just the center.”

In this study, a single examiner made three consecutive measurements with each device in patients who had virgin corneas, postLASIK patients at least 3 months after surgery, and postPRK patients at least 6 months after surgery.

They found no statistically significant difference in the CCT measurements of virgin eyes taken with either instrument. In the post-refractive surgery patients, the mean corneal thickness as measured by the Scheimpflug device was 500.4 µm (± 44.8 µm), and it was 494.3 µm (± 50.3 µm) as measured by ultrasound pachymetry. The difference was small (–6.2 µm ± 9.9 µm) but statistically significant (p = 0.0004).

Dr. Weikert said both devices have high repeatability of measurements when used carefully by a single examiner. He added that the dual Scheimpflug instrument tended to measure thicker corneas in general.

Although he now uses it as the primary method for checking CCT before refractive surgery, other practitioners who choose to do so may wish to re-check thinner corneas with ultrasound pachymetry, Dr. Weikert concluded.

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