Phakic IOL could contribute to faulty IOL power calculations

September 1, 2004

San Diego-Phakic anterior chamber IOLs affect the biometric measurement of axial length, which results in clinically significant errors in IOL power calculations in patients who need to undergo cataract surgery. Gerald Zaidman, MD, described such a patient and offered suggestions for managing this complicated problem here at the American Society of Cataract and Refractive Surgery.

"IOL power calculations can depend on keratometry, axial length, and the A constant of the IOL that is implanted. Keratometry usually is stable throughout life but can change after refractive surgery or in the presence of corneal disease such as keratoconus. The axial length is usually stable in adult eyes, but it also can change after retinal diseases requiring a scleral buckle. Biometry can vary, for example, in pseudophakic eyes and eyes with silicone," Dr. Zaidman discussed.

He recounted a case in which he had to determine if the presence of a phakic IOL affects ocular biometry and IOL calculations. A 60-year-old man with +3.0 D of hyperopia had undergone implantation of a phakic anterior chamber IOL and astigmatic keratotomies performed bilaterally in the right eye in 1997 and the left eye in 1999 at another institution.

The left eye had 20/30 uncorrected vision, a phakic anterior chamber IOL, no cataract, and normal cell count and pachymetry.

Dr. Zaidman, who is professor, New York Medical College, Westchester Medical Center, Valhalla, NY, debated doing a one-stage surgery in which he removed the IOL, extracted the cataract, and dealt with a large incision, a small fixed pupil, a low endothelial cell count along with the questionable A scan under these conditions before surgery. A two-stage surgery would involve removing the IOL, wait, and then perform standard phacoemulsification with perhaps more reliable biometry done through a cataractous eye.

Two-stage approachHe opted to perform the two-stage surgery. In March 2003, before the IOL was removed the axial length was 22.18 mm, which would require a 23-D posterior chamber IOL. Before performing phacoemulsification in July 2003, after the IOL was removed, the axial length was 22.84 mm, which requires a posterior chamber IOL of 22 D. Following surgery, the patient had 20/30 vision with some astigmatism; the corrected vision was 20/20.

"The axial length measurements taken before and after removing the phakic IOL differed substantially. There are almost no data in the literature on this problem," Dr. Zaidman commented.

An article by Jorge Alio, MD, published in the Journal of Cataract and Refractive Surgery in 2000, reported nine patients who had phakic anterior chamber IOLs implanted and then developed cataracts. Dr. Alio showed the axial length measurements before implantation of the phakic IOLs and the axial length measurements before cataract surgery with the phakic IOL in place.

"In the fourth patient in that series, there was only a 0.4-mm difference, but in patient 7 there is almost a 2-mm difference in the axial length measurements. The mean axial length was 31.15 mm in the nine patients in the series before the phakic IOL was implanted; the average axial length measurement was 30.29 mm in the pres-ence of the phakic IOL, which was a highly significant difference," Dr. Zaidman emphasized.

"The data indicate that measurement of an IOL power will be problematic in patients with phakic IOLs. Kenneth Hoffer, MD, addressed this problem in the Journal of Cataract and Refractive Surgery in 2003 and reported that all phakic IOLs, that is, anterior chamber IOLs, iris clip IOLs, and posterior chamber IOLs, create a condition of biphakia that affects axial length measurement. The presence of a phakic IOL affects A-scan biometry," Dr. Zaidman said.