OCULAR HYPERTENSION TREATMENT STUDY

Eyes with ocular hypertension (OHT) are at increased risk of developing primary open angle glaucoma (POAG). Prompt diagnosis and treatment of OHT may prevent the development of POAG and visual disability. OHT is defined as IOP greater than 21 mmHg without any evidence of optic nerve damage and visual field loss. There should be no ocular and systemic cause for the raised IOP levels.  

Ocular Hypertension Treatment Study (OHTS)¹  is a long term, multicenter, randomized clinical trial started in 1994. It has helped in understanding the natural course of OHT, role of ocular hypotensives in its treatment and significant risk factors contributing towards its progression to POAG. OHTS has contributed in better understanding and early identification of high risk individuals thereby, reducing the ocular morbidity due to glaucoma.  

OBJECTIVE-

1.     To determine whether ocular hypotensive agents are helpful in delaying the onset of glaucomatous optic nerve damage and visual field defects in subjects those who are at moderate risk of developing POAG.

2.     To produce natural history data to assist in identifying patients at most risk of developing POAG and whether they are benefited by early treatment or not.

3.     To quantify risk factors for developing POAG among ocular hypertensives. 

STUDY DESIGN-  OHTS is a long term, multicenter, randomized clinical trial started in 1994 with 1636 participants at 22 different clinical centers. The eligible candidates had no evidence of glaucomatous damage, aged between 40 to 80 years with IOP between 24-32 mmHg in one eye and 21-32 mmHg in other eye. The eligible candidates were randomized in equal proportion to either the medication group or observation group.²

Goal in medication group was to decrease IOP by 20% or more from the observed baseline readings and to attain target IOP of 24 mmHg or less. Topical medication was changed and/or added until both of these goals were met or the participant was receiving maximum tolerated topical medical therapy.

RESULTS-  At 60 months, the cumulative probability of medication group of developing POAG was 4.4% as compared to 9.5% in observation group (hazard ratio, 0.40; 95% confidence interval, 0.27-0.59; p<0.0001). There was little evidence of increased systemic/ocular risk associated with ocular hypotensive medication.

CONCLUSION- The OHTS has shown that topical ocular hypotensive medication is effective in reducing the incidence of glaucomatous optic nerve damage and visual field loss in individuals with elevated IOP between 24-32 mmHg.

RISK FACTORS PREDICTED BY OHTS FOR CONVERSION OF OHT TO POAG-

1.     CENTRAL CORNEAL THICKNESS (CCT) – CCT was found to be a powerful predictor for the development of POAG.³ IOP assessed by applanation tonometry may be overestimated or underestimated in thicker and thinner corneas, respectively. CCT less than 555µ were found to be at greater risk than eyes with CCT more than 588µ. The relative risk of POAG increased by 81% for every 40µ decrease in CCT. 

2.     IOP - Studies have revealed the normal IOP range of 10-21 mmHg.⁴ Although, IOP readings may show considerable variations among glaucoma patients, IOP reading more than 22 mmHg is a positive predictive factor for the development of POAG.³

3.     AGE – Age is an independent risk factor for the development of POAG. Individuals with older age had a greater risk for conversion to glaucoma. OHTS found an increased risk of POAG with age (per decade), of 43% in the univariate analysis and 22% in the multivariate analysis.³

4.     PATTERN STANDARD DEVIATION (PSD) - Although the patients with ocular hypertension may not have visual field defects on Standard Automated Perimetry (SAP), OHTS found that greater PSD on SAP correlated with increased risk of progression to POAG. With 0.2dB increase in PSD, 22% increase in relative risk was found in OHTS.³

5.     OPTIC NERVE – Although OHT patients have no apparent glaucomatous disc changes, increased vertical and horizontal cup-disc ratio is a risk factor for progression to POAG. Increase in cup-disc ratio by 0.1 leads to 32% and 27% increase in relative risk in vertical and horizontal cupping, respectively.³