Clinical Visual Sciences and Diagnostic Testing
Clinical visual sciences and diagnostic testing
This domain tests the principles, limitations, and interpretation of psychophysical and imaging tests. Questions often ask which test answers a specific clinical question or why a result is unreliable.
Core rules
- Standard automated perimetry measures differential light sensitivity. Total deviation compares with age-matched normals; pattern deviation removes overall depression such as cataract.
- Reliability indices include fixation losses, false positives, and false negatives. High false positives can create an apparent field defect.
- OCT provides cross-sectional retinal and nerve fiber layer measurements; segmentation errors are common in poor signal strength or media opacity.
- Goldmann applanation tonometry depends on corneal thickness and curvature; it is not a direct pressure measurement.
- ERG measures retinal function, VEP measures visual pathway function, and fluorescein angiography shows vascular leakage and perfusion.
Worked example
A Humphrey field shows a general reduction in sensitivity but a normal pattern deviation. What is the likely explanation? A normal pattern deviation means the field loss is diffuse rather than focal. Cataract, small pupil, or uncorrected refractive error can depress the entire hill of vision. The pattern deviation adjusts for this overall depression, so a normal pattern deviation with abnormal total deviation points to a media or refractive cause rather than glaucoma. The reasoning depends on comparing the two displays, not on the raw mean deviation alone.
Traps
- Reading total deviation as focal loss without checking pattern deviation.
- Ignoring reliability indices.
- Treating OCT as immune to media opacity.
- Assuming applanation tonometry is independent of corneal biomechanics.