An asymmetry between eyes on Hertel Exophthalmometry is considered abnormal when the inter-eye difference is

• A. 5 mm
• B. 4 mm
• C. 2 mm
• D. 3 mm

Answer: (D)

Hertel exophthalmometry compares how far each globe projects forward from the orbit. Small differences between eyes are common because of natural asymmetry and minor measurement variability, so a modest disparity isn’t alarming. When the inter-eye difference reaches about 3 millimeters, that level of asymmetry is considered abnormal, suggesting possible exophthalmos from conditions like thyroid eye disease or other orbital pathology. To assess accurately, take multiple readings with the head level and the patient looking straight ahead, and use the larger measurement as the basis for comparison. In this scenario, 3 mm is the threshold used to flag abnormal asymmetry.