Cataract Extraction and the Iris Configuration

Abstract

Mansberger et al1Mansberger S.L. Gordon M.O. Jampel H. et al.Ocular Hypertension Treatment Study GroupReduction in intraocular pressure after cataract extraction: the Ocular Hypertension Treatment Study.Ophthalmology. 2012; 119: 1826-1831Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar recently described a 16.5% reduction in intraocular pressure (IOP) after cataract extraction. We agree that “the exact mechanism of IOP lowering after cataract surgery is unknown.”1Mansberger S.L. Gordon M.O. Jampel H. et al.Ocular Hypertension Treatment Study GroupReduction in intraocular pressure after cataract extraction: the Ocular Hypertension Treatment Study.Ophthalmology. 2012; 119: 1826-1831Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar Relevant insights into the mechanism might come from anterior segment optical coherence tomography images before and after cataract surgery in recent studies of patients with open angles,2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar including our series of 77 patients.4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Before cataract surgery, (1) the iris midperiphery is frequently bowed anteriorly,2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar although not always. (2) The lens seems to touch the central iris on anterior segment optical coherence tomography,2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar because the height of the iris–lens channel is below the resolution of current imaging. After cataract surgery, (3) the iris profile is straight.2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar A significant reduction in the curvature of the posterior iris was demonstrated by Huang et al.3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar (4) Surgery is associated with substantial posterior movement of the central iris.2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar In our series,4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar the posterior central iris moved posteriorly on average 810 μm, with a standard deviation of 270 μm. (5) Despite the posterior iris movement, the iris–lens channel is clearly open.2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar In our series, postoperative anterior segment optical coherence tomography showed an obvious separation, on average 450 μm (standard deviation, 190 μm), between the posterior chamber intraocular lens and the iris. Images from these publications2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar (which are merely examples) as well as our 77-patient series4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar suggest that any deviations from findings 2–5 must be unusual. The postoperative iris configuration is easily explained. The intraocular lens does not touch the iris, and the iris–lens channel is open enough to prevent any pressure differential between posterior and anterior chambers.4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Therefore, the postoperative configuration of the iris must represent its intrinsic structural features. What accounts for the preoperative findings? The anterior location of the central iris, and the commonly observed anterior bowing of the iris midperiphery? The first observation could be explained if the lens pushes the central iris anteriorly. However, the delicate nature of iris tissue suggests that this phenomenon alone would not account for the smooth profile of anterior bowing. The lens cannot be directly pushing the iris mid-periphery anteriorly, as the 2 structures do not appear near each other on available imaging.2Zhou A.W. Giroux J. Mao A.J. Hutnik C.M. Can preoperative anterior chamber angle width predict magnitude of intraocular pressure change after cataract surgery?.Can J Ophthalmol. 2010; 45: 149-153Abstract Full Text PDF PubMed Scopus (18) Google Scholar, 3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Another (non-mutually exclusive) possibility is that aqueous pushes the midperipheral iris anteriorly. According to the theoretical model of Silver and Quigley, an iris-lens channel height of ≤3 μm may produce a substantial pressure differential between the posterior and anterior chambers.4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Available images are consistent with such narrowing, but do not have the resolution to definitively establish or exclude it. Huang et al3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar made an observation consistent with the importance of lens–iris interaction: The anterior vault of the central lens predicted the drop in IOP associated with surgery. Other mechanisms might play a role in lowering IOP, as well.4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Observations regarding the iris–lens channel are independent of the location of the scleral spur, which cannot be identified in a substantial minority of cases.3Huang G. Gonzalez E. Lee R. et al.Association of biometric factors with anterior chamber angle widening and intraocular pressure reduction after uneventful phacoemulsification for cataract.J Cataract Refract Surg. 2012; 38: 108-116Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar The posterior chamber–posterior segment pressure is not directly measured by corneal applanation tonometry, but there are mechanisms by which the former might influence the latter. For instance, elevated posterior pressure might impair uveoscleral outflow through the suprachoroidal space. The suprachoroidal space and the vitreous cavity are separated by nonrigid tissues, the retina and the choroid. When the vitreous pressure rises, the suprachoroidal space pressure must also rise, or the space collapses and uveoscleral outflow decreases. In other words, the suprachoroidal space may act as a Starling resistor. In the setting of relative pupillary block, the anterior chamber to suprachoroidal space pressure differential is lessened, and uveoscleral outflow is impaired. Although some might assume that patients with open angles have little limitation to flow through the iris–lens channel, direct observations of aqueous flow suggest that, at certain locations and times, resistance through the channel becomes critical and flow ceases. The fluorescein studies of Holm5Holm O. A photogrammetric method for estimation of the pupillary aqueous flow in the living human eye, I.Acta Ophthalmol (Copenh). 1968; 46: 254-277Crossref PubMed Scopus (27) Google Scholar have documented that aqueous flow may be localized at “various points of the pupillary border.” This behavior contrasts with an idealized model based on a fixed iris–lens channel height that predicts equal flow at all points around the pupil.4Pradhan S. Leffler C.T. Wilkes M. Mahmood M.A. Iris configuration and the intraocular pressure after cataract surgery.J Cataract Refract Surg. 2012; 38: 117-123Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar It is conceivable that the iris actually touches the lens in some locations, leaving space elsewhere for aqueous passage. The channel height may vary temporally as well. With miosis induced by the consensual light reflex, Holm described a delay of 5–10 minutes before aqueous “vesicles” flowed into the anterior chamber.5Holm O. A photogrammetric method for estimation of the pupillary aqueous flow in the living human eye, I.Acta Ophthalmol (Copenh). 1968; 46: 254-277Crossref PubMed Scopus (27) Google Scholar In conclusion, cataract surgery changes the iris–lens channel from apparently closed (on current imaging) to obviously open, and straightens the iris profile if there is preoperative anterior bowing of the midperiphery. These changes might be important clues to the mechanism of the IOP drop. Reduction in Intraocular Pressure after Cataract Extraction: The Ocular Hypertension Treatment StudyOphthalmologyVol. 119Issue 9PreviewTo determine the change in intraocular pressure (IOP) after cataract extraction in the observation group of the Ocular Hypertension Treatment Study. Full-Text PDF