New Insights Into Treating Ocular Hypertension
In the past, when a patient's intraocular pressure (IOP) was elevated with normal visual fields and optic nerves, you faced a dilemma. On one hand, you could initiate IOP reduction therapy, knowing full well that the patient might never develop a problem from the elevated IOP, and would be incurring the expense and risks associated with therapy. On the other hand, you could watch for glaucomatous change before initiating therapy, knowing that 20% to 50% of the optic nerve fibers may be lost before you could make a conclusive diagnosis. Perhaps the visual field defect that developed would be a troublesome paracentral scotoma, or the initial damage might make the remaining fibers more susceptible to further damage.
This year we saw the publication of The Ocular Hypertension Treatment Study (OHTS) results and its companion piece. This study has greatly increased our understanding of ocular hypertension and the risk for developing glaucoma.
Notably, the study concluded that lowering IOP in patients with ocular hypertension reduced the risk of developing glaucoma in five years from 9.5% to 4.4%.1 Thus, IOP reduction in ocular hypertension did benefit some patients. However, it is also easy to see that initiating therapy on every patient with ocular hypertension would result in gross over-treatment.
OHTS also attempted to identify which patients would most likely benefit from treatment.2 There were some surprising results. While race (blacks) and family history were expected to be predictive of the development of POAG, they weren't strongly predictive. Surprisingly, the presence of diabetes seemed to protect patients from the development of glaucoma. Not unexpectedly, older age, larger initial cup-to-disc ratio, and higher IOP were predictive of glaucoma.
However, the factor that was most predictive was the presence of a thin central cornea. Patients with a central corneal thickness of 555µm or less had a three-fold greater risk of developing POAG than those with a central corneal thickness of 588µm or greater.
The theory holds that the rigidity of a thick cornea artificially elevates the Goldmann applanation measurement of IOP and a thin cornea consequently lowers the reading of the true IOP, though other unknown factors may contribute to this finding.
Central corneal thickness appears to be a powerful predictor of the progression from ocular hypertension to POAG. The study shows patients with thin central corneas are likely to benefit most from IOP reduction. Rarely are the conclusions of a landmark study so emphatic: At this time, measurement of central corneal thickness is necessary to accurately manage patients with ocular hypertension.
1. Kass MA, Heurer DK, Higginbotham EJ, et al. The Ocular Hypertension Treatment Study. A randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open angle glaucoma. Arch Ophthalmol 2002;120:701-13.
2. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open angle glaucoma. Arch Ophthalmol 2002; 120:714-20