Summary
Ocular hypertension refers to an IOP > 21 mmHg without apparent glaucomatous damage, (no optic nerve damage or visual field loss).
Background
The mean IOP is 16 mmHg, with the normal range being 11-21 mmHg (two standard deviations either side.) However, 4-10% of the population above the age of 40 is likely to have an IOP > 21 mmHg, without any glaucomatous damage. In these cases, if there is no angle closure or any other cause of secondary glaucoma, we have ocular hypertension (OHT).
The exact cause has not been determined, however the Ocular Hypertension Treatment Study (OHTS) through a longitudinal trial, suggested various risk factors at play.
These include:
- Raised IOP
- Older age
- Central corneal thickness: Eyes with a low CCT are at greater risk to higher - possibly in part due to under and over estimation of the IOP respectively
- Cup/disc ratio: The greater the C/D ratio, the higher the risk (optic nerve heads with a large cup may be structurally more vulnerable)
- Pattern standard deviation: Visual field loss in glaucoma is highly irregular, and therefore a measure which quantifies irregularities is required. Pattern standard deviation is a way of measuring irregularities in perimetry, and a larger value may represent a greater risk for developing OHT.
Clinical features
- Raised IOP, generally defined as > 21 on 2+ visits.
- Open anterior chamber angle, with normal appearance, and normal anatomy on gonioscopy
- Apparently normal optic nerve, retinal nerve fibre layer, and visual field
Diagnosis
Ocular hypertension should be investigated in the same way as POAG. However, extra consideration may be given to whether any systemic medication is being taken that could reduce (beta blockers) or increase (steroids) IOP.
Differential diagnoses