Pupillary light reflex alters corneal refraction

By Stefanie Petrou Binder MD

Heidelberg - How much does corneal refraction change after light-nduced pupillary constriction?

The answer to that question is the subject of a new study and has important implications for the development of accommodative lenses and wavefront ablation.

"The corneal surface curvature changes in response to contraction of the pupillary sphincter muscle.

"We were interested in determining the extent of the refractive difference, if any, before and after pupillary constriction," explained Michael Martin MD, Department of Ophthalmology, Heidelberg University.

He noted that because there are many studies currently underway concerning accommodative lens implantation, it is of great importance at this early stage in accommodative lens surgery that each factor involved be examined.

Dr Martin and colleagues analysed 13 normal eyes in the same number of healthy, young patients. They measured the corneal surface curvature and refraction of each eye using the Technomed C-Scan® .

Dr Martin and his colleagues took six measurements in each eye. They took three measurements for reference, under normal corneal topographical conditions, after the eyes adapted to dark. They then shone a light in each partner eye to elicit a pupillary light reflex and measured corneal refraction three additional times. The researchers compared these last values to the reference measurements.

The team divided the corneal topography into four meridians and evaluated refraction at predefined points on the 0°, 90°, 180° and 270° meridians, using diameters of 3.0 mm, 5.0 mm and 6.6 mm.

Up to 0.5 D changes in refraction
Contraction of the pupillary sphincter muscle due to light induction increased corneal refraction by 0.25 D to 0.5 D, depending on the meridian and diameter evaluated.
The corneal periphery in particular showed the greatest differences in refraction. This peripheral area was represented by the outer 6.6 mm zone.

Refractive changes were not noted in the 3.0 mm or 5.0 mm zones of most eyes, while slight differences were noted within these zones in the 180° meridian.
He noted that the observed differences in refraction were clinically irrelevant to the overall refraction, as the peripheral corneal zone is shielded from light by the iris when the pupil is narrowed. Four eyes showed no curvature changes at all.

The average keratometry measurements in each meridian showed definite differences in refraction in response to the light-induced pupillary reaction.
All, however, were statistically insignificant (p>0.1).

Dr Martin explained that the biomechanics of the cornea and the refractive changes that take place in the cornea under the influence of mechanical and pharmacological stimuli are of interest to the development of accommodative lenses and wavefront ablation.

The changes in corneal topography and the resultant differences in refractive indices serve to help optimise the ongoing developments in accommodative lens implantation.
Contraction of the pupillary sphincter muscle alters corneal topography and thereby also the refraction of the light passing through it. An increased steepening of the limbal area is responsible for the changes noted. These correspond to a tightening of the pupillary sphincter muscle and a corresponding widening of the chamber angle.

Dr Martin's research received the Best Poster award at this year's meeting of the DGII (German-Speaking Organisation of Intraocular Lens Implantation and Refractive Surgery).