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IOP Measurement
High YieldTonometry methods including Goldmann, rebound, pneumatic, and non-contact tonometry
IOP remains the only modifiable risk factor in glaucoma, making accurate tonometry central to every management decision. Goldmann applanation is the reference standard, but its readings are only as reliable as your understanding of the Imbert-Fick principle and corneal thickness artefacts. This section covers applanation and rebound tonometry with their sources of error and clinical corrections.
Key Points
- GAT is the gold standard — based on Imbert-Fick principle (force to flatten 3.06 mm diameter corneal area = IOP)
- CCT correction: thin cornea (<520 µm) underestimates IOP; thick cornea (>570 µm) overestimates — each 10 µm deviation ≈ 0.5 mmHg correction
- Normal IOP: 10-21 mmHg (population mean ~15.5 mmHg) — but normal-tension glaucoma occurs at <21 mmHg
- Icare rebound tonometry: no anaesthesia or fluorescein needed — ideal for children, screening, and post-LASIK corneas
- Diurnal variation: IOP is highest in early morning (supine position + cortisol peak); phasing (multiple measurements through the day) may reveal peaks missed on single measurement
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