There are many conditions that cause the eyeball to appear large - this happens because the eye protrudes outside the bony socket or the orbit. This condition is typically known as proptosis. One of the most common conditions to cause proptosis is thyroid eye disease. Here the muscles that move the eye known as the extraocular muscles become larger and bulkier in size and as a result the eye is pushed out of the bony socket and the eyes appear larger.

To treat this condition, it is important for eye doctors to measure how much is eye protruding compared to normal position. Exophthalmometry is a technique that measures the amount of protrusion of the eye giving the eye doctor an idea of the surgical procedures as well as the medical treatment that is needed to correct the condition. Eye doctors who treat these eye diseases are called as Oculoplasty (eyeplasty) specialists.

The instrument that is used to measure the amount of protrusion of the eye is called an exophthalmometer. The most reliable and sensitive exophthalmometer is the Hertel’s exophthalmometer which is available at best of the eye hospitals like the Advanced Eye Hospital and Institute at Sanpada, Navi Mumbai. 

HERTEL'S EXOPHTHALMOMETER

The differences between the different types of exophthalmometers are as follows:

1. Hertel exophthalmometer: measurement is taken from the lateral orbital rim (outside rim of the bony socket of the eye) to the corneal apex. If a patient presents with an orbital fracture or after lateral orbitotomy, the use of Hertel exophthalmometer may be complicated because the lateral orbital rim serves as a reference point for this instrument. Consideration should be given to the use of the Naugle exophthalmometer in these cases.

2. Naugle exophthalmometer uses fixation points slightly above and below the superior and inferior orbital rims (cheek bones and forehead). Naugle exophthalmometer measures the difference in proptosis between the two eyes rather than absolute measure with the Hertel method. 

3. Luedde exophthalmometer fixes on lateral orbital wall and uses a transparent ruler to measure the amount of protrusion.

Also, the exophthalmometer can also measure how much the eye has sunk in in compared to the normal eye. This condition known as enophthalmos can often occur after a fracture of the bones of the orbit where the eye appears smaller in comparison to the normal eye. This condition also known as enophthalmos and can also be detected using the exophthalmometer.

If you or anyone you know is suffering from any form of Thyroid disease it is important to have a baseline evaluation of your eyes once a year and this would include exophthalmometric evaluation. Besides thyroid eye disease there are many other conditions like eye cancers, vascular anomalies of the orbit, eye trauma, orbital fractures which can require these measurements as part of treatment planning and prognostication.