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Corneal hysteresis a factor in IOP measurement
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CHICAGO — Direct measurement of the mechanical properties of the cornea offers the possibility of more accurate IOP measurement, according to David F. Garway-Heath, MD, MBBS.
Applying a “correction factor” to IOP to compensate for the error induced by central corneal thickness is “inadvisable,” Dr. Garway-Heath said. Corneal biomechanical factors may have a greater impact on measured IOP than corneal thickness, he said, and corneal hysteresis is one way of measuring these factors.
Dr. Garway-Heath spoke here at the American Academy of Ophthalmology meeting in October about two new devices for measuring IOP that either try to take mechanical properties into account or to eliminate their influence on IOP.
He said the Pascal Dynamic Contour Tonometer, from Ziemer Ophthalmic Systems, attempts to eliminate corneal biomechanical properties during IOP measurement. The device provides direct transcorneal measurement of IOP using the principle of contour matching instead of applanation, according to the manufacturer.
Dr. Garway-Heath’s presentation focused on another new instrument, the Ocular Response Analyzer from Reichert Ophthalmics. The device measures the cornea’s response to indentation twice: first as the cornea flattens, and again as it returns to its normal shape. The difference between the first and second measurements, called corneal hysteresis, is an indication of the dampening effect of the cornea.
“Hysteresis is a direct measurement that may relate to IOP more strongly than central corneal thickness,” Dr. Garway-Heath said.
Early studies indicate that hysteresis is related to central corneal thickness and weakly associated with age, he said. IOP measurements with the Ocular Response Analyzer that take account of hysteresis are independent of corneal thickness, he said.
Dr. Garway-Heath said the cornea appears to stiffen with age. Factors responsible for the stiffening may include increased cross-linking of collagen fibrils in the cornea and decrease in the tear film with age, he said.