Managing dysphotopsia after cataract surgery
Understanding causes can help avoid or reduce impact.
Understanding the causes of post-cataract and refractive surgery visual disturbances, including positive and negative dysphotopsias and entopic phenomena, can go a long way toward preventing or reducing their impact, according to Jack T Holladay MD.
The first step in managing such symptoms is to determine if the visual disturbance is entopic or an optical dysphotopsia, said Dr Holladay, of Baylor College of Medicine, Houston, Texas, USA. Entopic phenomena typically are arcuate or central flashes caused by vitreous traction on the retinal periphery or macula and are visible in light and darkness.
“Telling if it’s entopic is simple – the patient sees the flash with his eyes closed,” Dr Holladay said.
Such phenomena are common after cataract surgery because the intraocular lens (IOL) takes up only about 6% of the crystalline lens volume, leaving plenty of room for the vitreous to move after surgery. He recommended referring these cases to a retinal specialist for evaluation and treatment.
Negative dysphotopsias, which generally are arcuate shadows or dark lines along the temporal edge of the lens, sometimes with light on both sides of the line, are fairly easy to see, Dr Holladay said. Using ray tracing, his research shows that the line results from a shadow cast by the edge of the lens when light coming in at steep angles – around 88 degrees – misses the edge of the lens.
These shadows are likely to be wider in eyes with smaller pupils, larger angle kappa, an equi-biconvex or plano-convex IOL, smaller axial distance from iris to IOL and with the anterior capsule overlying the nasal IOL. IOL edge design, material, power, diameter, decentration, tilt and aspheric surfaces also are factors (Holladay JT, Simpson MJ. J Cataract Refract Surg. Feb 2017;43:263-275).
Dr Holladay said negative dysphotopsias appear immediately after surgery in about 16% of cases. Of these, 80% resolve spontaneously.
“That’s because the peripheral capsule, as it opacifies, scatters the light.”
Other treatment or prevention options include removing the nasal overlapping capsule and reverse optic capture. Peripheral shadows also may be eliminated by an IOL with a concave surface around the posterior periphery, which spreads light across the shadow area on the retina (Erie JC et al. J Cataract Refract Surg. July 2019;45:1023-19).
Positive dysphotopsias range from snowballs, haloes, starbursts and streaks to nighttime arc flashes from headlights, and daytime crescents and partial rings from the sky. Dr Holladay noted that they are more common in IOLs with diffractive optics, a high index of refraction and truncated edges. Patient risk factors include a high apparent chord μ or high corneal higher order aberrations.
Chord μ is the distance between the apparent pupil centre and the light reflex, or first Purkinje image, Dr Holladay explained. If it is greater than 0.6mm, performance of diffractive lenses will degrade to the point that they should not be implanted. Most current biometry devices, including the Lenstar (Haag-Streit) and IOLMaster (Zeiss), report chord μ values, he said.
For smaller chord μ distances, centring multifocal diffractive IOLs halfway between the pupil centre and the visual axis minimises dysphotopsias, Dr Holladay said.
“It wants to be in both places, but it can’t, so the best compromise is halfway in between,” he advised.
Topographic measurement of wavefront error due to higher-order aberration is also a good indicator of refractive surgery success, Dr Holladay said.
“If it is over 1.0 micron over a 6.0mm zone, that patient is not going to see well. They’re going to complain of glare and haloes and that’s true whether it is post refractive surgery with a laser or cataract surgery.”
A normal corneal topography value is 0.38±0.14 microns HO RMS. Research shows that patients who were happy with their LASIK outcomes had a mean HO RMS of 0.58±0.21 microns compared with 1.31±0.58 microns for unhappy patients (McCormick GJ et al. Ophthalmology. Oct 2005; 112(10):1699-709).
New research classifying different patient-reported dysphotopsias will help evaluate and improve IOL designs, Dr Holladay concluded.