Pupil Size

Pupil size measurement has been an important factor in preoperative evaluations, prompted by observations that some patients with large pupils (>8 mm) reported difficulties with night vision after undergoing keratorefractive surgery. Typical symptoms included the appearance of glare, starbursts, and halos; decreased contrast sensitivity; and poor overall quality of vision. Night-vision problems tended to occur in patients with both large pupils and small treatment zones (≤6 mm). The algorithms used in third-generation lasers, however, incorporate larger optical and transition zones, enabling surgeons to perform refractive procedures on patients with larger pupils. Use of these algorithms has dramatically decreased the incidence and severity of night-vision problems.

Many surgeons use default ablation zones during excimer procedures. The accepted standard transition zone between ablated and unablated cornea is 0.5–1.0 mm larger than the pupil; use of this zone helps minimize night-vision problems. To conserve corneal tissue, smaller optical zones are typically used in higher myopic corrections. In patients who require such corrections, the incidence of night-vision problems increases in part because of the mismatch between the size of the pupil and that of the optical zone. Modern transition zones aim to minimize this mismatch.

Patients with extremely large pupils (≥8 mm) should be identified and counseled about the potential for increased risk of complications. Spherical aberration may be increased in these patients. Clinical management of postoperative night-vision problems includes the use of a miotic such as brimonidine (0.2%) or pilocarpine (0.5%–1%).

• Chan A, Manche EE. Effect of preoperative pupil size on quality of vision after wavefront-guided LASIK. Ophthalmology. 2011;118(4):736–741.

• Myung D, Schallhorn S, Manche EE. Pupil size and LASIK: a review. J Refract Surg. 2013; 29(11):734–741.

Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.