Diagnostic Testing: Tonometry, Visual Fields, Imaging, and Ultrasound
Tonometry
Goldmann applanation measures the force needed to flatten a fixed corneal area, so it depends on corneal thickness and curvature: thick corneas read falsely high, thin corneas falsely low, which is why pachymetry accompanies pressure readings. Proper technique uses anesthesia, fluorescein, aligned mires, and slit-lamp magnification; non-contact and rebound tonometers trade accuracy for convenience.
Visual Fields
Automated perimetry such as the Humphrey 24-2 or 30-2 tests the central field; 10-2 targets macular disease. Reliability indices are fixation losses, false positives, and false negatives - high false positives suggest a trigger-happy patient and an unreliable field. Mean deviation shows overall depression; pattern standard deviation highlights localized defects. Always compare with the fellow eye and repeat abnormal tests.
Imaging and Ultrasound
OCT gives cross-sectional retinal and nerve fiber layer thickness. Fundus photography documents appearance over time. A-scan ultrasound measures axial length for biometry and IOL calculation; B-scan shows anatomy when media are opaque, such as dense cataract or vitreous hemorrhage. Immersion or optical biometry is preferred for accuracy before cataract surgery.
Traps: confusing which ultrasound mode measures versus images; ignoring corneal thickness when interpreting pressure; accepting a single abnormal field.
Rule of thumb: A-scan for amplitude and axial length, B-scan for a two-dimensional picture.
Worked example: pressure reads 20 mmHg but pachymetry is 620 microns; the true pressure is likely lower than measured because a thick cornea resists flattening.