Which anatomic measurements should be recorded during the pre-fit GP lens examination?

• A. Axial length, corneal thickness, refractive error, iris color
• B. HVID, pupil diameter, palpebral aperture height/lid position, lid tension, blink rate
• C. Eyelash length, tear film breakup time, blink rate
• D. Scleral depth, iris diameter, lid margin position

Answer: (B)

Understanding how the lids, pupil, and iris dimensions relate to a gas-permeable lens is key to predicting how the lens will sit and move on the eye. The horizontal visible iris diameter helps estimate corneal size and guides choosing a lens diameter that covers the cornea with stable edge clearance, promoting good centration and reducing edge-related issues. Pupil diameter matters because the pupil changes with lighting; a lens that stays well-centered as the pupil dilates or constricts helps maintain clear vision in different lighting conditions. The height of the palpebral aperture and lid position describe how much lid tissue and lid margins will contact or interact with the lens edge during blinking, which influences edge alignment, movement, and tear exchange under the lens. Lid tension reflects how strongly the lids grip or ease away from the lens margin; this can affect how the lens settles after a blink and its overall stability during wear. Blink rate shows how often the lids interact with the lens and how tear flow under the lens occurs, both of which impact comfort and lens stability. Other measurements, like axial length or scleral depth, aren’t directly used to size or assess GP lens fit in the same way these lid and pupil-related parameters are, and tear film quality, while important for comfort, is less about the basic fit dynamics these measurements illuminate.