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Chemical, biomechanical factors potential agents for phaco-reduced IOP
Reduction may stem from enhanced trabecular meshwork outflow, anterior chamber deepening or stress hormone release related to ultrasound exposure.
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NEW ORLEANS — Phacoemulsification reduces IOP in glaucomatous and non-glaucomatous eyes, although researchers and clinicians are not certain about the underlying causes, a surgeon said here.
During Glaucoma Subspecialty Day preceding the American Academy of Ophthalmology meeting, Douglas J. Rhee, MD, OSN Glaucoma Board Member, discussed various factors that may account for decreased IOP after phacoemulsification.
“We’ve known for many years, many decades, in fact, with the advent of extracapsular cataract extraction, that … phaco lowers the intraocular pressure,” Rhee said. “The conventional wisdom had been that the pressure is usually reduced by about 1 mm Hg to 4 mm Hg, and this can be sustained for about 3 to 5 years.”
One study, by OSN Glaucoma Board Member Bradford J. Shingleton, MD, supported the conventional theory with thorough sampling and analysis, Rhee said.
Douglas J. Rhee
“There are a number of articles, but I like Dr. Shingleton’s article because it has such great sample retention among glaucoma, glaucoma suspects and non-glaucomatous individuals over 5 years and is one of those papers that highlights the conventional wisdom,” he said.
Rhee highlighted another study, by OSN Glaucoma Section Editor Thomas W. Samuelson, MD, that compared phacoemulsification alone with phacoemulsification combined with implantation of the iStent (Glaukos). Eyes undergoing phacoemulsification alone had preoperative IOP of 25.4 mm Hg, and IOP was reduced by almost 9 mm Hg.
“That was the mean reduction. So, we know that phaco can reduce the pressure by quite a bit,” Rhee said.
Outflow, chamber depth
A significant body of data shows that reduced IOP after phacoemulsification can be attributed to enhanced trabecular meshwork outflow, not increased uveoscleral outflow nor decreased aqueous secretion, Rhee said.
He also noted that although phacoemulsification reduces IOP, there is no change in circadian fluctuation of IOP, except in some eyes with pseudoexfoliation.
“You’d think that it might [cause a circadian fluctuation of IOP] because most interventions like laser or other surgical procedures will actually flatten out the diurnal curve,” he said. “Phaco does not. There are a couple of papers on this that found the same thing.”
Shan C. Lin, MD, and colleagues studied a mechanical effect related to anterior chamber deepening, Rhee said.
“What they found was that the degree of intraocular pressure lowering correlates to the extent of anterior chamber deepening. So, the more you deepen the angle, the more you lower the pressure,” he said. “What they showed was that, in this study, the difference that you made in the angle … made more of an impact if you had narrow angles to begin with, something that was a little bit unexpected.”
Ultrasound and stress hormone
Rhee cited laboratory research by OSN Glaucoma Board Member Joel S. Schuman, MD, showing elevated interleukin 1-alpha levels in the trabecular meshwork endothelial cells of glaucomatous eyes exposed to ultrasound. Interleukin 1-alpha is a stress hormone and pathologic marker for primary open-angle glaucoma.
Schuman and colleagues exposed cultured trabecular meshwork cells from normal and glaucomatous cadaver eyes to phacoemulsification with an ultrasound power setting of 70, Rhee said.
“But what he found was that interleukin 1-alpha, which is a stress hormone in response to stress, was released from the glaucomatous [trabecular meshwork] cells but not from the normal cells. What this indicates is that the molecular mechanism in non-glaucomatous eyes remains elusive. But this may be a possible mechanism for those eyes with glaucoma,” Rhee said. – by Matt Hasson