Glaucoma
October 2008
by Tony Realini, M.D.
Inter-eye IOP differences may have diagnostic and prognostic significance
It is generally assumed that IOP in fellow eyes is symmetrical. IOP is determined by the relative rates of aqueous formation and outflow. Asymmetry of IOP in subjects with glaucoma may indicate asymmetry of the underlying disease process, resulting in asymmetric outflow impairment and asymmetric optic nerve damage. The results of studies on this topic have been mixed, with some finding remarkable asymmetry of IOP between fellow eyes, and others reporting striking concordance of IOP. Several new studies reported at the 2008 meeting of the Association for Research in Vision and Ophthalmology in Fort Lauderdale, Fla., have provided additional information on this important topic.
EyeWorld Factoid
Source: Glaucoma Research Foundation
A “Research to prevent blindness survey” found that 30% of those surveyed had never heard of glaucoma.
“Asymmetric findings between fellow eyes, including asymmetry of IOP, have long been considered a hallmark of glaucoma,” said George L. Spaeth, M.D., Wills Eye Hospital, Philadelphia. “However, there are relatively few studies specifically addressing the amount of IOP asymmetry associated with glaucoma.”
“Asymmetry of IOP can arise due to real differences in IOP between the eyes, or due to measurement error, asymmetry of central corneal thickness, or other structural differences,” he said. “If asymmetry of IOP were an accurate indicator of the presence of glaucoma, then recognizing such asymmetry might be a useful way to diagnose glaucoma, and noting symmetry of IOP might help rule out the presence of glaucoma.”
To explore this hypothesis, Dr. Spaeth and his colleagues undertook a post hoc evaluation of 100 normal subjects and 98 untreated glaucoma subjects who had previously undergone standardized IOP assessment while participating in various prospective clinical trials. “In each case, IOP was measured three times in masked fashion by a trained observer,” he said.
His results were surprising. “No subject with glaucoma had equal IOP in both eyes, whereas 34% of normal subjects had the same IOP in both eyes,” he said.
The inter-eye difference was 1 mm Hg or greater in 76% of normal subjects and 100% of glaucoma subjects; 2 mm Hg or greater in 41% of normal subjects and 82% of glaucoma subjects; and 3 mm Hg or greater in 13% of normal subjects and 57% of glaucoma subjects.
“No normal subject had IOP asymmetry exceeding 6 mm Hg, while 17% of glaucoma subjects had asymmetry exceeding 6 mm Hg,” said Dr. Spaeth.
Considering IOP asymmetry as a risk factor for glaucoma, an inter-eye IOP difference of 2 mm Hg or more was associated with a six-fold risk of having glaucoma, while an inter-eye difference of 3 mm Hg or more carried an eight-fold risk of having glaucoma.
“Few patients with glaucoma appear to have symmetric IOP, and few patients without glaucoma have asymmetric IOP,” he said. “IOP asymmetry, especially when absent or marked, can help with the detection of glaucoma. There is no question in my mind whether asymmetric IOP has diagnostic value. The more interesting question is whether symmetric IOP has diagnostic value.”
| Corneal Thickness | Goldmann IOP (mmHg) | DCT IOP (mmHg) | P-value |
|---|---|---|---|
| Thin (<520 microns) | 13.2 | 15.9 | 0.009 |
| Average (520–580 microns) | 15.9 | 16.0 | 0.91 |
| Thick (>580 microns) | 17.4 | 17.4 | 0.95 |
Source: Tony Realini, M.D.
Symmetry or asymmetry of IOP may not always be obvious, according to Samin Hong, M.D., and colleagues at Yonsei University in Seoul, South Korea.
“The precise patterns and characteristics of diurnal IOP fluctuation in glaucoma patients, especially those with asymmetric IOP, have not yet been fully described,” she said. “To more fully explore this issue, we investigated postural differences in IOP asymmetry and their relationship to visual field indices in a group of 53 patients with primary open-angle glaucoma controlled with the same medical regimen in both eyes.”
All 53 subjects underwent a 24-hour IOP curve both in the sitting and supine positions. IOP was considered asymmetric if sitting IOP between fellow eyes differed by 3 mm Hg or more at any time point, or if supine IOP differed by 5 mm Hg or more at any time point.
“Eleven of the 53 subjects had asymmetric IOP in the sitting position,” she said. “But interestingly, 16 of the 42 subjects—that’s 38%—with symmetric sitting IOP had asymmetric supine IOP.” She termed this condition “latent IOP asymmetry” and said, “There were no significant differences in age, gender, diagnosis, number of medications, or central corneal thickness among those who did and did not exhibit latent IOP asymmetry.”
But visual field parameters did differ significantly between the groups. “The mean deviation and pattern standard deviation in the higher IOP eye was significantly worse in subjects with latent IOP asymmetry compared to subjects with symmetric sitting and supine IOP,” she said.
“It appears that latent IOP asymmetry condemns the eye with the higher IOP to wider diurnal IOP fluctuation and probably as a consequence, to a worse visual field compared to the fellow eye,” said Dr. Hong. “In clinical practice, latent asymmetric IOP should be suspected if one eye shows worse visual field indices than the fellow eye, even though sitting IOP measurements are symmetrical. In these patients,” she said, “a more aggressive treatment regimen should be considered for the eye with the higher latent IOP.”
Editors’ note
Drs. Hong and Spaeth have no financial interests related to their comments.
Contact Information
Hong: samini@yuhs.ac
Spaeth: gspaeth@willseye.org
