contact lens case reports

HVIDs: When Lens Size Matters

BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDR�, FCLSA
November 1999

L.M. is a 21-year-old female with a 2-month history of moderately successful soft contact lens wear. She presented to our office with chief complaints of fluctuating vision and lens awareness after 4-6 hours of wear.  L.M.'s spectacle prescription was -2.25 sphere OU, with keratometric readings of OD 45.00 sphere and OS 45.50 sphere. Her soft lenses were 8.4 -2.25 14.0 NewVues (CIBA Vision), with a measured sagittal height of 3.89. Her most striking features were her smaller than normal horizontal visible iris diameters (HVIDs) of 10.8mm.

Despite L.M.'s steep (45.00D) keratometric readings, her HVIDs resulted in an exceptionally shallow sagittal height (Fig. 1). Figure 2 illustrates a different patient with flatter central keratometric readings of 42.00D, however, this patient's more "normal" corneal diameter of 11.8mm resulted in a steeper sagittal height.

By comparing the two photographs, it is clear that the steeper, 45.00D cornea requires a lens with a minimal sagittal height value. We refit L.M. with an 8.8
-2.25 14.0 Surevue lens (Vistakon) with an exceptionally flat/shallow sagittal height of 3.50mm. Table 1 shows the sagittal height specifications of some commonly used soft contact lenses measured by Dr. John Ellison of Pacific University.

HVID Reference Guide

Additionally, we have developed a simple guide for selecting the appropriate soft lens base curve based on studies which have shown the average HVID to be 11.8mm.

If the patient's HVID is 1.00mm smaller than the average (10.8mm), subtract 5.00D from the flat K reading. In our example, L.M.'s 45.00D/10.8mm cornea now has an effective K value of 40.00D. Conversely, if the patient's HVID is 1.00mm larger than 11.8mm (12.8mm), add 5.00D to the flat K reading. In this case, a cornea of 42.00D/12.8mm has an effective K value of 47.00D.

L.M.'s case illustrates how misleading central K readings can be and how critical HVID is in fitting soft contact lenses.

Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.

Mark Andr� is director of contact lens services at the Oregon Health Sciences University.Source: Dr. John Ellison, Pacific University College of Optometry