Corneal Edema

Osher has proposed that corneal edema can cause a transient negative dysphotopsia.s. Literatur ^{,​ 10} His survey of 250 patients found that, although just over 15% reported temporal shadowing on postoperative day 1, that number was greatly reduced within just a few weeks. The great majority of these patients appeared to experience a temporary or transient from of negative dysphotopsia. He postulated that the symptoms coincided well with the healing of the temporal corneal incision commonly used during cataract surgery. After the incision, the cornea will thicken and develop haziness from the corneal edema. A thicker and more opaque area of the temporal cornea then leads to blockage of light passage and a shadow effect in the temporal visual field. This same shadow effect was not observed in eyes receiving a superotemporal incision that was covered by the upper eyelid after surgery.

There have been case reports by other authors, however, that negative dysphotopsias can occur after cataract surgery even when only superior incisions have been used.s. Literatur The theory is also complicated by the fact that negative dysphotopsias have not been reported in patients after corneal surgeries, such as a penetrating keratoplasty or flaps during a laser-assisted in situ keratomileusis (LASIK) procedure, which could also induce corneal edema. The transient nature of most negative dysphotopsias with ultimate symptomatic resolution, therefore, may simply be the result of neuroadaptation.

Anatomical Factors

Several different anatomical optical features of patients reporting negative dysphotopsias have been implicated as predisposing factors to visual shadows, including corneal curvature, pseudophakic anterior chamber depth, IOL power, and axial length. ^{2 ,​ 12}

Holladay et al ⁹ suggested that pupil size, the distance behind the pupil to the implant, and functional nasal retina that extends anterior to the region of shadowing caused by the IOL were all correlated with an increase in reported negative dysphotopsia after uncomplicated cataract surgery. It is proposed that all of these anatomical conditions increase the amount of retinal shadowing from an IOL placed within the capsular bag. This retinal shadow is thought to be related to IOL designs, IOL placement, or possibly the edge of the anterior capsulotomy.

IOL Design

Patients have reported negative dysphotopsias after implantation of many different IOL types and materials, including three-piece hydrophobic acrylic IOLs,s. Literatur one-piece hydrophobic acrylic IOLs, ¹³ one-piece yellow-tinted hydrophobic acrylic IOLs, ¹⁴ a hydrophilic acrylic IOL, ¹² and a three-piece silicone IOLs. ^{15 ,​ 16} Some smaller studies have suggested that silicone lenses do not cause negative dysphotopsias as frequently as acrylic lenses due to the rounded IOL edge design, ³ but there have not been large studies to evaluate for a significant difference.

Holladay et al ⁹ explored the effect of the IOL edge design on light dispersal using ray-tracing models. They found that light passing through the IOL edge refracted, and different angles left an area behind the IOL devoid of any light penetration (Fig. 14.2). The area left in shadow was most pronounced in IOLs with sharp or squared edges. Round edges, such as those found in the silicone lenses tested, had a smaller area left in shadow behind the lenses. They proposed that the edge design of IOLs was the cause of chronic negative dysphotopsias. This supported the clinical observation in small studies that silicone lenses were less likely to cause chronic dysphotopsia. This claim has been disputed, however, because exchange of problematic IOLs with others of different material or edge design has not always been successful. ¹⁷

IOL Placement/Anterior Capsule

Masket and Fram ⁷ observed that covering the edges of the anterior capsulotomy created during cataract surgery with an IOL seems to alleviate the symptoms of negative dysphotopsias. It had been previously observed that IOL exchange within the capsular bag did not improve symptoms, but that IOL exchange with placement of the second IOL in the ciliary sulcus did alleviate symptoms. ¹² Similarly, improvement of symptoms was observed with secondary piggyback IOL implantation on top of an IOL in the capsular bag, as well as reverse optic capture of the original IOL. All of these techniques place an IOL over the edges of the anterior capsulotomy. It has therefore been proposed that negative dysphotopsias may be the result of an abnormal reflection of light from the anterior capsulotomy edge and IOL surface to the peripheral retina. This theory would explain why negative dysphotopsias are not observed in anterior chamber or sulcus-fixated IOLs.