Cataract Surgery and Intraocular Pressure
Video Lecture: https://youtu.be/6AjLsSMsrig
Cataracts and glaucoma are the first and second leading causes of blind-ness worldwide. Because both diseases usually occur after the fifth decade of life, it is not surprising that these 2 diseases occur simultaneously in many patients. Intraocular pressure (IOP) reduction after cataract surgery has been shown in many studies, and the most recent data indicates that IOP reduction after cataract surgery is more significant and sustained than previously thought.
Lowering IOP is the mainstay of glaucoma treatment. Traditional glau-coma surgeries such a trabeculectomy and tube shunts work well to lower IOP and decrease progression of glaucoma; however, these procedures carry significant morbidity. Many patients with glaucoma have concurrent cata-racts, and some studies have suggested that glaucoma itself is a risk factor for cataract development. Certainly glaucoma-filtering procedures, periph-eral iridotomy, and some glaucoma medications increase the risk of cataract formation. Historically, patients with moderate to advanced glaucoma with concurrent cataracts would have either a combined procedure or a 2-stage surgery. Surgeons have traditionally felt that cataract surgery lowers IOP in open-angle glaucoma (OAG) only slightly and temporarily, despite a paucity of robust data. In contrast, current data demonstrate a greater and more sus-tained IOP reduction. As such, cataract surgery may be a safe alternative to glaucoma surgery in some patients and could shift the surgeon’s approach in treating concurrent cataract and glaucoma, especially in the early or moderate stages of glaucoma.
Summary of Various Studies
The earliest studies of IOP after cataract surgery showed little if any reduction of IOP. However, these results probably do not apply today because of advances in surgical technique and intraocular lens (IOL) technology. As phacoemulsification became the standard, studies began to demonstrate more sustained IOP lowering following cataract surgery. The conventional wisdom that cataract surgery lowers IOP by 2 to 4 mm Hg for a couple of years was partially confirmed by the only meta-analysis of the topic; however, nearly all of the studies showed only mean IOP change and did not stratify patients based on preoperative IOP, which has critical significance in data interpretation because only a subset of patients had an elevated and, therefore, modifiable IOP. Those studies that did stratify patients based on preoperative IOP clearly demonstrated that patients with higher preoperative IOP enjoy the greatest reduction of IOP after cataract surgery. IOP reduction is maintained in 75% to 85% of patients at 5 years, and IOP can be controlled in 20% of patients with OAG without drops after cataract surgery.
The method of cataract extraction may influence the reduction of IOP. Phacoemulsification (particularly clear corneal phacoemulsification) seems to lower IOP more than manual extracapsular cataract extraction. The type of OAG may also influence IOP reduction. Pseudoexfoliation patients may have an even greater long-term decrease in IOP than primary OAG patients although IOP often rises on postoperative day 1. However, the early IOP variability following cataract surgery rarely has clinical consequence. Many other factors, such as pressurization at the time of surgery, immediate postoperative medications (especially corticosteroid), and viscoelastic type, contribute to short-term IOP fluctuations following cataract surgery.
Pathophysiology of Reduced Intraocular Pressure After Cataract Surgery
Although the physiological reasons for decreased IOP after cataract surgery remain speculative, the facility of outflow is known to increase after cataract surgery. Angle width changes less than one would expect following cataract surgery, suggesting improved function of the trabecular meshwork itself rather than improved aqueous access to the trabecular meshwork. Three or more different mechanisms may contribute to the observed reduction in IOP after cataract surgery.
Lens-Induced Changes to Outflow Pathway
As the eye ages, the crystalline lens increases significantly in volume. This may initiate a series of anatomical changes that ultimately leads to the increase in IOP observed with aging. As the lens grows, the anterior lens capsule is displaced forward causing the zonules to place anteriorly directed traction on the ciliary body and uveal tract, which in turn compresses the canal of Schlemm and the trabecular meshwork. Because the anterior ten-dons of the ciliary muscles contribute to the architecture of the trabecular meshwork, as the ciliary body is displaced forward by the enlarging lens, the tendons relax and the space between trabecular plates becomes narrowed.
Inflammation Induced by Cataract Surgery
Phacoemulsification typically induces a low-grade inflammation in the immediate postoperative period. It is plausible that the induced inflamma-tion lowers IOP by either decreasing aqueous production of the ciliary body, as seen in uveitis, or it could increase outflow similar to the mechanism of selective laser trabeculoplasty or prostaglandin analogues. Although these options seem plausible, little experimental data exist currently to support these hypotheses.
Fluidics of Phacoemulsification
Another possible for lower IOP after cataract surgery is that high flow of fluid and high IOP (up to 90 mm Hg) experienced during cataract surgery forces fluid through the trabecular meshwork into the canal of Schlemm and the episcleral veins. Forcing this large amount of fluid through the drainage system may increase patency and promote flow. Again, little evi-dence exists to support or refute this hypothesis.
Is Cataract Surgery a Glaucoma Surgery?
Cataract surgery is a very common, successful, highly refined surgery with a favorable risk–benefit profile including improved visual acuity and visual field. The widespread general belief that cataract extraction alone lowers IOP 2 to 4 mm Hg is slowly evolving toward an understanding of a larger and more sustained IOP reduction, especially in patients with higher preoperative IOP. Although cataract surgery alone lowers IOP, com-bined glaucoma/cataract surgery lowers IOP more with fewer postopera-tive pressure spikes. Surgeons should carefully monitor IOP after cataract surgery to prevent a postoperative pressure spike that could “snuff” the nerve, especially in patients with pseudoexfoliation syndrome. Moreover, the influence of corticosteroid on IOP must be monitored. It often takes weeks or months following surgery to achieve the new postoperative base-line. During this period, IOP should be managed with the knowledge that temporary factors, such as typical postsurgical inflammation and topical steroid, may cause IOP to be elevated. The clinician should not interpret the early postoperative IOP as the new postsurgical baseline until all inflam-mation has subsided and steroids discontinued. Nonetheless, cataract surgery seems to be emerging as a safe way to lower IOP in patients with mild to moderate glaucoma while avoiding the morbidity of traditional glaucoma surgery.
Although the mechanism is unclear, cataract surgery appears to lower IOP in glaucoma patients. Traditionally, cataract surgery has been thought to lower IOP about 2 mm Hg; however, emerging data indicate that while the average IOP reduction after cataract surgery is 2 mm Hg, the IOP reduc-tion is greater in those with an elevated IOP. Additionally, the IOP-lowering effect appears to be sustained at 5 years. Given the favorable risk–benefit profile of cataract surgery and its low morbidity, cataract surgery provides a good IOP-lowering option for patients with mild to moderate glaucoma.
- Cataract surgery alone lowers IOP.
- The IOP reduction of cataract surgery appears sustained at 5 years.
- IOP is reduced more in patients with a higher baseline IOP.
- Because cataract surgery has a favorable risk–benefit profile, it may be a good option to lower IOP in mild to moderate glaucoma.
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