IOP Control Index helps predict surgery results in chronic angle-closure glaucoma cases

A formula combines a patient’s preoperative IOP and number of medications to help determine postoperative outcomes following phaco alone or combined phacotrabeculectomy.

A quantitative parameter, the IOP Control Index, could assist surgeons in predicting postoperative results in chronic angle-closure glaucoma cases by combining preoperative IOP and number of medications as a single number, a glaucoma expert said.

Clement C.Y. Tham, FRCS, and colleagues devised the IOP Control Index to help determine postop IOP control for chronic angle-closure glaucoma patients with coexisting cataract who undergo either phacoemulsification or combined phacotrabeculectomy.

“Basically, we are trying to look for a single quantitative parameter that can reflect the quality of IOP control, either before or after surgery,” Prof. Tham said at a presentation at the World Glaucoma Congress

The IOP Control Index is the mean number of drugs, plus one, multiplied by the mean IOP number. One was arbitrarily added to the number of IOP-lowering drugs to avoid a difference in IOP being negated when the number of medications was zero, he said.

Prof. Tham and colleagues examined the IOP Control Index in a randomized clinical trial of 123 eyes of 123 patients with chronic angle-closure glaucoma and coexisting cataract. Patients were randomized into two groups: a phaco-alone group of 62 eyes and a combined phacotrabeculectomy group of 61 eyes. All patients had at least 24 months of follow-up, and in his presentation, the outcome measures up to 24-month follow-up were reported.

“We found that in 123 cases, the mean postoperative IOP Control Index was 25.8, and so good IOP control was defined as having an IOP Control Index of 25.8 or less,” Prof. Tham said. “On the contrary, poor IOP control was defined as having an IOP Control Index of greater than 25.8 in this study.”

Results

In the study, Prof. Tham and colleagues found that the preop IOP Control Index was significantly associated with postop control of IOP and the need for additional surgery in the combined group.

Factors significantly associated with postop hypotony in this group included lower preop IOP, fewer medications before surgery and a lower preop IOP Control Index, Prof. Tham said.

“From a clinical perspective, it may be safer to perform phacoemulsification, rather than combined phacotrabeculectomy, in those chronic angle-closure glaucoma cases with a low preop IOP Control Index, eg., 50 or below, in order to avoid postoperative hypotony,” he said.

In contrast, combined phacotrabeculectomy rather than phacoemulsification alone, may be considered in those chronic angle-closure glaucoma cases with a grossly elevated preop IOP or IOP Control Index, even though a clear cut-off point could not be identified in this study, Prof. Tham said.

However, a significant overlap of values in the groups studied and a significant rate of combined cases needing additional surgery after phacotrabeculectomy could affect that observation, he said. Overall, the study’s overlap of values played a role in the index’s use for selecting surgical options.

“We find that it’s a bit difficult to use the preoperative IOP or the IOP Control Index to guide us in deciding what surgery to perform, in the sense that there’s quite a significant overlap in values in the preoperative IOP Control Index between the groups that require subsequent trabeculectomy and the group that doesn’t,” he said.

Results of phaco-alone group

For the group receiving phaco alone, univariate analysis determined that a lower postop IOP Control Index was associated with plateau iris configuration, fewer preop IOP-lowering drugs, a lower preop IOP Control Index and a lower pattern standard deviation on Humphrey automated perimetry, Prof. Tham said.

With multivariate analysis, the only factor connected to final results was the preop IOP Control Index, he said.

Additional analysis determined that four phaco-alone eyes (6.5%) needed trabeculectomies after cataract surgery.

“We found that the factors associated with the need for subsequent trabeculectomy included a higher preoperative IOP and also a higher preoperative IOP Control Index,” he said.

Results of combined group

For the combined surgery group, univariate analysis found that a lower preop IOP and IOP Control Index were associated with a lower postop IOP Control Index, Prof. Tham said.

With multivariate analysis, preop IOP Control Index was the only factor associated with low postoperative IOP Control Index, as with the phaco-alone group, he said.

The most important result in the combined group was the finding that nearly 18% of combined surgery cases needed further surgery for IOP control, including subconjunctival 5-fluorouracil injections and needling.

“We realized that the higher preoperative IOP Control Index was significantly associated with the need for additional surgery to control IOP in this group of patients,” he said. – by Erin L. Boyle

References:

• Tham CC, Kwong YY, Leung DY, et al. Phacoemulsification versus combined phacotrabeculectomy in medically controlled chronic angle closure glaucoma with cataract. Ophthalmology. 2008;115(12):2167-2173.
• Tham CC, Kwong YY, Leung DY, et al. Phacoemulsification versus combined phacotrabeculectomy in medically uncontrolled chronic angle closure glaucoma with cataracts. Ophthalmology. 2009;116(4):725-731.

• Clement C.Y. Tham, FRCS, can be reached at Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, 147K Argyle St., Kowloon, Hong Kong; 852-2762-3196; fax: 852-2816-7093; e-mail: clemtham@hkstar.com.

Prof. Tham et al set out to create a new metric that could predict the postoperative outcomes in patients with chronic angle-closure glaucoma undergoing phacoemulsification or phacotrabeculectomy. They report that patients with higher ICI values (ie, those with higher preoperative IOP and/or those using more medications) were more likely to need more surgery to control their IOP levels. This would be expected. Other important factors such as type of cataract, status of the angle and length of medication use are not included in the ICI. It is unclear at this time what the clinical utility (if any) would be of the ICI.

– Malik Y. Kahook, MD
University of Colorado Denver School of Medicine