Multimodal mechanisms of lens may account for near vision performance

At the annual meeting of the American Society of Cataract and Refractive Surgery, Kevin L. Waltz, OD, MD, and Steven J. Dell, MD, provided evidence that accommodative arching contributes to increased focusing power of the crystalens accommodating IOL.

"These studies suggest that both the crystalline lens of the prepresbyopic patient and the accommodating IOL increase their focusing power in part by changing their radius of curvature," said Dr. Waltz, a private practitioner in Indianapolis. "This arching may play an important role in the accommodative mechanism of action of the accommodating IOL."

In all of the phakic subjects as well as in pseudophakic patients who had the accommodating IOL implanted, the wavefront was relatively flat at distance, reflective of a type of monofocal vision, whereas it exhibited asymmetry, representing multifocality, in response to accommodation. However, there were age-related changes in the accommodative response of the phakic patients.

In younger subjects (< 35 years old), the multiple foci seen in the wavefront map were more generalized and tended to cover the entire pupil. With increasing age, there was a decrease in the intensity of the accommodative response with a tendency for the area of near focus to become smaller and more centrally localized. Among persons in their 50s, the accommodative response seen on the wavefront maps was essentially extinguished.

"This process rewards the smaller pupil of an accommodating eye out of proportion to what you would expect," Dr. Waltz said. "As the area of the change becomes smaller, the constriction of the pupil covers a more significant area of the pupil that is not focused at near, and that improves the vision."

Measuring anterior movement

To investigate the mechanisms underlying the near vision outcomes with the accommodating IOL, Dr. Dell began by performing studies to measure its anterior movement in response to accommodation. Using pilocarpine and cyclopentolate for pharmacologic stimulation and paralysis of accommodation, he determined that the accommodating IOL moved anteriorly by up to 0.84 mm.

He contrasted those findings with reports from other investigators. For example, Luciano Burratto, MD, determined the accommodating IOL moved anteriorly by about 1.44 mm based on measurement of change in anterior chamber depth after pilocarpine stimulation. Obtaining measurements with non-contact laser interferometry, Di Chiara et al. found the lens moved forward up to 1.1 mm between distance and near fixation. However, it has also been reported that there is a posterior shift of about 0.12 to 0.15 mm in response to pilocarpine.

"It is clear that some patients have near acuity that is surprisingly better than the level expected from the degree of movement found in these three studies," observed Dr. Dell, who was an investigator in the FDA trial of the accommodating IOL and is a private practitioner in Austin, TX. "In fact, I and several other surgeons have observed patients achieving very good near acuity despite implantation of an accommodating IOL of relatively low dioptric power that would be expected to give minimal levels of near vision.