Pediatric and Teen CL Care

Determining Power When You Have No Refractive Information

BY CHRISTINE W. SINDT, OD, FAAO

Every once in a while, I am asked to design a lens when I don’t have corneal curvature readings nor a clear axis to determine power. In addition, it’s usually for a child who is pre-verbal or otherwise difficult to examine clinically.

Impossible you say? Well, it’s not as difficult as you may think.

A Case of PHPV

Little Baby J was born with persistent hyperplastic primary vitreous (PHPV, also known a persistent fetal vasculature) OD. In PHPV, the embryologic hyaloid vasculature fails to regress (Silbert and Gurwood, 2000). It is characterized by the presence of white vascularized retrolental tissue, microphthalmia, centrally dragged ciliary processes, severe intraocular hemorrhage, occasional retinal folds, and varying degrees of lenticular opacification. PHPV requires prompt cataract removal and optical rehabilitation.

Shortly after Baby J’s cataract extraction, a retro-iris pupillary membrane formed, which closed her pupil (Figure 1). Because time is crucial in these early cases, I determined the lens fit and power in the operating room at the time of the pupilloplasty surgery.

Determining Lens Parameters

I fit the GP lens using standard fluorescein evaluations (Sindt, 2010). Then I determined the axial eye length (AEL) and anterior chamber depth (ACD) using standardized immersion echography a-scan.

I entered the GP base curve as the K reading and the AEL into the IOL Master biometer (Carl Zeiss Meditec), which determined the theoretical IOL power. The Hoffer-q formula is used for AELs under 21mm (Hoffer, 1993), and the SRK-T formula is used for longer eyes (Straub, 2012). Although each formula uses variable constants specific to each brand of IOL, this small variation does not generate enough difference in power for our application. I then vertexed the theoretical IOL back to the corneal plane to determine the contact lens power. Easy, right?

Baby J’s AEL = 20.03mm, her ACD = 3.68mm, and the GP base curve was 50.00D. The Hoffer-q formula determined a theoretical IOL power of 26.5. The vertex formula is power new = power old/{1-(d)(power old)}, where d = distance in meters. In this case it is moving out of the eye (against the direction of light), so the sign on d is negative.

Figure 1. PHPV pupillary membrane post dilation, prior to pupilloplasty. The membrane effectively rendered the eye unrefractable.

So the contact lens power = +26.5/{1-(-.00368)(+26.5)} = +24.15D. Because Baby J is aphakic and does not have the ability to accommodate, an additional +3.00D was added to the power to focus her at near. Baby J’s final contact lens order was 9.8mm diameter (1mm smaller than her iris diameter) (Sindt, 2011), 50.00D base curve, and +27.00D power.

Easier Than You Think

A contact lens is Baby J’s only shot at developing vision in her PHPV eye. While determining power using the IOL Master is different from common retinoscopy, it can make a difficult task quite easy. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #213.

Dr. Sindt is a clinical associate professor of ophthalmology and director of the contact lens service at the University of Iowa Department of Ophthalmology and Visual Sciences. She is the past chair of the AOA Cornea and Contact Lens Council. She is a consultant or advisor to Alcon Vision Care and Vistakon and has received research funds from Alcon. You can reach her at christine-sindt@uiowa.edu.