exophthalmometry

Exophthalmometry is the clinical measurement of forward ocular projection, or exophthalmos, using an exophthalmometer. It provides a quantitative estimate of how far the eyeball sits anterior to the orbital margin. This measurement is commonly used to detect and monitor proptosis associated with conditions such as thyroid eye disease, orbital tumors, inflammatory orbitopathy, and trauma, and to track changes over time or in response to treatment.

The most widely used instrument is the Hertel exophthalmometer, though other designs exist. These devices measure the distance from the corneal apex to the lateral orbital rim, with the base adjusted to the interpupillary distance. The examiner places the instrument on the lateral orbital rims and asks the patient to fixate straight ahead; readings are obtained for each eye in millimeters and recorded along with the base setting.

Normal values vary by population and age, but adult ranges are typically around 12 to 20 millimeters, with averages near 15 to 18 mm. Proptosis is suggested by readings above the normal upper limit or by notable asymmetry between eyes. A difference of about 2 millimeters or more between eyes is often considered clinically significant and may prompt further evaluation.

Limitations include inter-examiner variability, differences in instrument base, and individual orbital anatomy; measurements can be influenced by corneal contour and globe size. Exophthalmometry is a screening and monitoring tool rather than a standalone diagnosis, and abnormal results usually lead to further assessment with imaging and relevant clinical tests.