In automated perimetry (Humphrey visual field test), the 'mean deviation' (MD) reflects:
- A Variability of patient responses at threshold measurement
- B The average sensitivity loss across the entire test field compared to age-matched normal values ✓
- C The number of test points that fell outside normal limits
- D A focal scotoma pattern independent of diffuse loss
Correct answer: B. The average sensitivity loss across the entire test field compared to age-matched normal values
Explanation
Mean deviation (MD) in Humphrey perimetry is the average difference in decibels between the patient's measured sensitivity and age-matched normal values across all tested points, providing an overall measure of diffuse sensitivity loss. A very negative MD indicates global depression of the visual field. Pattern standard deviation (PSD) captures localized irregularities. Short-term fluctuation captures variability. MD alone cannot distinguish diffuse from focal loss without considering PSD.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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