Clinical Skills

Tonometry & IOP Measurement

2 lessons·Intermediate·AAO/IJCAHPO Standards

Principles of Tonometry

Intraocular pressure (IOP) is the fluid pressure inside the eye, normally ranging 10-21 mmHg. IOP measurement is essential for glaucoma screening and management. The Imbert-Fick principle states that the pressure inside an ideal sphere equals the force needed to flatten its surface divided by the area flattened (P = F/A). Goldmann applanation tonometry (GAT) is the gold standard, flattening a 3.06mm diameter area of the cornea. Central corneal thickness (CCT) affects IOP readings: thicker corneas overestimate and thinner corneas underestimate true IOP.

Key Points

Normal IOP: 10-21 mmHg
Imbert-Fick principle: P = F/A
GAT flattens 3.06mm diameter corneal area
CCT affects readings: thick = overestimate, thin = underestimate
Average CCT: ~545 μm

Proper Goldmann applanation tonometry mire alignment — inner edges of the upper and lower fluorescein semicircles just touching — Proper mire alignment: the inner edges of the upper and lower fluorescein semicircles just touch — read the IOP off the force drum at this exact moment.