Patient Assessment, Instrumentation, and Clinical Procedures
This domain tests technique and interpretation: whether the measurement you recorded actually means what you think it means.
Core procedures
- Visual acuity: Snellen or logMAR, with pinhole to separate refractive from pathological loss; contrast sensitivity for subtle dysfunction.
- Tonometry: Goldmann applanation flattens a 3.06 mm diameter area; calibration and corneal thickness both affect readings. Non-contact and rebound tonometers are screening tools.
- Perimetry: 24-2 or 30-2, with reliability indices (fixation losses, false positives, false negatives) checked before interpretation.
- Slit lamp: diffuse, optic section, direct and indirect illumination, retroillumination, sclerotic scatter.
- Pupil testing: swinging flashlight for a relative afferent pupillary defect.
- Imaging: OCT for RNFL and macula, topography for shape, biometry for IOL power, fundus photography for documentation.
Worked example
Goldmann applanation flattens an area of 7.35 mm² (diameter 3.06 mm). A force of 1.0 g gives 1.0/7.35 = 0.136 g/mm², which converts to about 1.33 kPa, i.e. 10 mmHg. The instrument's drum is scaled so that 1 g corresponds to 10 mmHg - which is why calibration weight checks and a clean biprism matter more than they appear to.
Traps
Interpreting fields without checking fixation losses; accepting non-contact tonometry in keratoconus or after refractive surgery; recording acuity without the pinhole when vision is reduced; and forgetting that pachymetry shifts the meaning of an applanation reading.