Exophthalmometry (Hertel)

Exophthalmometry is the simple bedside measurement of how far each eye protrudes forward from the bony rim of the orbit. The number is usually obtained with a small instrument called a Hertel exophthalmometer and is reported in millimeters per eye. The test is fast and painless, and the trend over time is one of the useful objective data points in the management of thyroid eye disease and other orbital conditions, where small changes in protrusion can affect treatment decisions.

Why Eye Protrusion Is Measured

A change in how far the eye sits in the orbit is a key indicator of orbital disease. The orbit is a closed bony cavity; anything that takes up space inside it - inflammation, fluid, fat, muscle, or tumor - pushes the eye forward. The amount of forward displacement (proptosis) is a quantifiable sign of how much volume is being added.

In thyroid eye disease, the orbital fat and the extraocular muscles enlarge, pushing the eyes forward. Exophthalmometry numbers track this enlargement objectively. With an orbital tumor or mass, one eye may be progressively pushed forward as the lesion grows. In other orbital inflammatory disorders or after orbital fracture or hemorrhage, the eye position changes too.

The advantage of exophthalmometry is that it provides a number - not a description - and that number can be compared from visit to visit and from examiner to examiner.

What the Test Looks Like

The Hertel Exophthalmometer

The instrument is a small horizontal bar with two cups that rest against the lateral edges of each orbital rim, plus angled mirrors and a millimeter scale. The patient sits facing the examiner. The cups are positioned snugly against the bony rim. Through a small mirror system, the examiner reads off the position of the front of each cornea against a millimeter scale.

A typical exam takes 30 seconds. The most important technical point is that the base measurement - the distance between the two lateral orbital rims, which is also recorded - must be the same on each subsequent visit so that the readings are comparable. A change of base width can artificially appear to change the protrusion number.

Other Instruments

• Naugle exophthalmometer - uses superior and inferior orbital rims rather than lateral; useful when lateral rims are altered (e.g., after orbital decompression surgery)
• Luedde exophthalmometer - a transparent ruler that rests against one lateral rim at a time; useful in selected situations and in some pediatric exams
• CT-based measurement - used for surgical planning and for objective comparison; performed on an axial CT slice by measuring the distance from the interzygomatic line (connecting the lateral orbital walls) to the posterior corneal surface

What the Numbers Mean

There is no single normal value. Typical adult upper-normal limits, with substantial individual variation:

• European ancestry: approximately 18-20 mm
• African ancestry: approximately 21-22 mm
• East Asian ancestry: approximately 16-18 mm

Means run roughly 2-4 mm lower than these upper limits in each group. These ranges are guidelines - individuals of the same background can vary by several millimeters and still be entirely normal.

The two most clinically meaningful findings are:

• Asymmetry between the two eyes greater than 2 mm - points to unilateral orbital disease and prompts a search for tumor, inflammation, infection, or a vascular anomaly
• Change from a previous baseline - even small changes (1-2 mm) over weeks to months matter when tracking active disease, particularly thyroid eye disease

Conditions Where Exophthalmometry Is Tracked

Thyroid Eye Disease

The single most common indication. Tracks orbital inflammation and helps decide:

• Whether disease is active or stable
• Whether orbital decompression surgery is needed
• Response to immunotherapy (teprotumumab, IV steroids)
• When the disease has reached a quiet, "burnt-out" phase

Orbital Tumors and Masses

Slow, asymmetric increase in protrusion of one eye is a classic sign. Repeat exophthalmometry helps determine whether watchful waiting or intervention is appropriate.

Orbital Inflammatory Disease

Including idiopathic orbital inflammation (formerly "orbital pseudotumor") - protrusion may rise quickly during a flare and respond to systemic corticosteroids.

Orbital Cellulitis

Acute increase in protrusion in the setting of redness, fever, and decreased eye movement. Measured at presentation and again as the infection responds.

Carotid-Cavernous Fistula

A connection between the carotid artery and the cavernous sinus produces pulsating, throbbing proptosis often with audible bruit. Exophthalmometry confirms and quantifies the pulsing protrusion.

After Orbital Trauma or Surgery

• Orbital fracture with herniation can produce enophthalmos (the eye sinks back) - a negative change on exophthalmometry
• After orbital decompression - measures the achieved reduction in protrusion

Pseudoproptosis

Sometimes the eye looks bulged but is not truly protruding. A high myopic eye is longer and can appear protruded. Exophthalmometry shows the actual protrusion is normal - useful for distinguishing real proptosis from optical illusion.

What to Expect

Before the Test

• No special preparation needed
• The test is performed at the same visit as the rest of the eye exam
• You can wear glasses but will remove them for the measurement

During the Test

• You sit upright facing the examiner
• The instrument is positioned with cups resting on each lateral orbital rim
• You are asked to look at the examiner's right eye, then left eye (or at a target light)
• The examiner reads the millimeter values for each eye and records the base width
• The whole measurement takes well under a minute

After the Test

• No restrictions, no drops, no recovery time
• The numbers are usually entered directly into the chart and compared to previous visits

Limitations and Pitfalls

• Interobserver variability - different examiners can produce slightly different numbers, often by 1-2 mm. Same-examiner trends over time are most reliable.
• Base width changes - using a different base width on different visits can artificially shift the apparent protrusion. Always recorded and ideally kept constant.
• Asymmetric orbital rim anatomy - if one orbital rim has been altered by surgery or trauma, Hertel readings on that side may be unreliable; alternative instruments or CT-based measurement may be preferred.
• Patient cooperation - children and patients with severe lid swelling may be hard to measure reliably.
• High myopia and pseudoproptosis - a long eye may look protruded and exophthalmometry confirms the eye is not truly forward.

When Imaging Is Added

Exophthalmometry is the bedside measurement; imaging gives the structural picture. The most common combinations:

• Asymmetric or progressive proptosis - MRI brain and orbits with contrast, often the first imaging study, sensitive for tumor and inflammation
• Suspected orbital fracture - CT of the orbits
• Acute orbital cellulitis - contrast CT of the orbits and sinuses
• Pulsatile proptosis - MRI/MRA or CTA looking for carotid-cavernous fistula or AV malformation
• Surgical planning for orbital decompression or tumor resection - high-resolution CT or MRI with detailed orbital protocols

Frequently Asked Questions

What is a normal Hertel measurement?

There is no single "normal" number. Approximate adult upper-normal limits are often cited around 18-20 mm in European-ancestry patients, 21-22 mm in patients of African ancestry, and 16-18 mm in East Asian populations, with substantial individual variation. A more useful question than "is my number normal?" is "how does my number compare to my own previous measurements and to my fellow eye?"

What does it mean if my measurements are different in each eye?

A difference of more than 2 mm between the two eyes is generally considered significant and prompts a search for unilateral orbital pathology - tumor, inflammation, thyroid eye disease presenting asymmetrically, infection, or vascular anomaly. The doctor will follow up with a careful history and usually imaging.

Will I need this measurement every visit?

If you have a condition that affects orbital volume - most commonly thyroid eye disease - yes. A trend over time is what tells the doctor whether things are stable, improving, or worsening, and a single measurement is much less informative than a pattern over months.

Why do they record a "base width" number?

The base width is the distance between the two lateral orbital rim cups when the instrument is positioned. The protrusion reading depends slightly on this base width, so it must be kept the same on subsequent visits to allow valid comparison. Many practices simply use each patient's previously recorded base width to standardize.

Is exophthalmometry uncomfortable?

The instrument cups press lightly against the bony lateral orbital rim. Most patients describe this as a mild pressure, not pain. The exam is over in under a minute.

References