Diagnostic Procedures, Instrumentation and Low Vision
Concepts
Know the principles of tonometry (Goldmann applanation, non-contact, rebound), perimetry (Humphrey 24-2, 30-2, 10-2), gonioscopy (Shaffer grading), and OCT (macular and RNFL). Understand the difference between screening and diagnostic tests, and the role of visual field indices (MD, PSD, VFI). For low vision, know magnification principles (relative distance magnification, relative size magnification, angular magnification), and the types of aids (magnifiers, telescopes, CCTV, filters).
Traps
Examiners test the Goldmann tonometer's calibration (1 g force = 10 mmHg) and the effect of corneal thickness (thick cornea overestimates IOP). Watch for the difference between MD (mean deviation, overall depression) and PSD (pattern standard deviation, localised loss). Also know that a 10-2 field is used for macular disease, not glaucoma screening.
Worked rule of thumb
For low vision magnification: M = F/4, where F is the dioptric power of the magnifier. A +20 D magnifier gives 5× magnification. For a patient needing 4× magnification, a +16 D lens is required.