Proptosis is the abnormal forward protrusion of one or both eyeballs from the eye socket. You may also see it called exophthalmos, which means the same thing. A difference of 2 mm or more in how far each eye protrudes is generally considered abnormal. Proptosis itself is not a disease but a sign that something is taking up space or creating pressure behind or around the eye.
How Proptosis Is Measured
Eye doctors measure protrusion using a device called a Hertel exophthalmometer, which sits against the outer rims of your eye sockets and reads how far forward each eye sits in millimeters. Normal values vary by ethnicity and sex. In studies of Black adults, mean protrusion measured 18.2 mm in men and 17.5 mm in women, with upper limits of normal reaching 24.1 mm and 22.7 mm respectively. For white adults, the commonly cited upper limit of normal is around 21 mm. This distinction matters: using a single cutoff for all patients can lead to misdiagnosis.
Beyond the absolute measurement, doctors look at asymmetry between the two eyes. Even if both eyes fall within the normal range, a 2 mm or greater difference between them raises concern.
What Causes It
The eye socket is a bony cone with limited room. Anything that adds volume behind the eye, whether it’s swollen tissue, fat, blood, or a mass, pushes the eyeball forward. The direction the eye shifts can hint at the cause. If the eye bulges straight ahead (axial proptosis), the problem is typically located directly behind the globe, within the cone of muscles that move the eye. If the eye shifts to one side, the mass is usually on the opposite side, pushing the eye away from it.
Thyroid Eye Disease
The most common cause of proptosis in adults is thyroid eye disease, also called Graves’ orbitopathy. An overactive immune response causes the muscles and fatty tissue behind the eyes to swell. This typically affects both eyes, though one side can look worse than the other. It develops most often in people with Graves’ disease, an autoimmune condition that overproduces thyroid hormone, but it can also occur in people whose thyroid levels test normal.
Infections
Orbital cellulitis, a bacterial infection of the tissue surrounding the eye, is a common cause of sudden, painful proptosis. It usually affects one eye and tends to come with redness, swelling, fever, and pain with eye movement. In children, it often starts as a sinus infection that spreads into the orbit. This is a medical emergency because the infection can threaten vision and, in rare cases, spread to the brain.
Tumors and Masses
Both benign and malignant growths can cause proptosis. In adults, common orbital tumors include lymphomas and benign vascular masses like cavernous hemangiomas. In children, the picture is different. Rhabdomyosarcoma, an aggressive soft tissue cancer, accounts for about 3% of childhood cancers and most commonly affects children between ages one and four. It often presents as a rapidly enlarging, painless bulge of one eye, sometimes with eyelid swelling or restricted eye movement. Because it can mimic benign conditions like a stye or cellulitis, diagnosis is sometimes delayed. Early biopsy is critical since treatment needs to start quickly to preserve both vision and life.
Other masses that can cause proptosis in children include dermoid cysts, blood vessel tumors, and, rarely, cancers that have spread from elsewhere in the body.
Other Causes
Bleeding behind the eye from trauma can cause sudden proptosis. Inflammatory conditions where the immune system attacks orbital tissue without a clear trigger (sometimes called idiopathic orbital inflammation) are another possibility. Vascular malformations, abnormal connections between arteries and veins near the eye, can also push the eye forward over time.
Symptoms and Complications
The most obvious sign is the eye appearing to bulge or stare. But proptosis creates a cascade of secondary problems depending on how severe it is and how quickly it develops.
When the eye protrudes far enough, the eyelids can no longer close completely during blinking or sleep. This exposes the cornea to air, leading to dryness, irritation, and eventually corneal ulcers if untreated. Double vision is common when the muscles controlling eye movement become swollen, displaced, or restricted. Many people also notice aching pressure behind the eye, tearing, or a gritty sensation.
The most serious complication is compression of the optic nerve, which carries visual information from the eye to the brain. When swelling or a mass presses on this nerve, vision can deteriorate gradually or suddenly. Signs of optic nerve involvement include blurred vision, dimming of colors, and a pupil that doesn’t react normally to light. This is a sight-threatening emergency.
How It Is Diagnosed
After measuring protrusion and testing eye movement, vision, and pupil reactions, imaging is the next step. CT scans are often the first choice because they show both bone and soft tissue clearly and can be done quickly in an emergency. MRI provides better detail of soft tissues and is especially useful for distinguishing between different types of tumors or evaluating optic nerve health.
What doctors look for on imaging depends on the suspected cause. In thyroid eye disease, the muscles behind the eye appear enlarged while their tendons stay normal-sized, a distinctive pattern. Tumors show up as defined masses with characteristic shapes and locations. Infections typically appear as hazy swelling without clear borders, often with involvement of the adjacent sinuses.
Blood tests for thyroid function and thyroid antibodies are standard when both eyes are affected. If a mass is found and its nature isn’t clear from imaging alone, a biopsy may be needed.
Treatment Options
Treatment targets whatever is causing the proptosis rather than the protrusion itself.
Thyroid Eye Disease
For mild cases, lubricating eye drops and managing thyroid hormone levels may be enough. For moderate to severe disease, a medication called teprotumumab has changed the treatment landscape. In a phase 3 clinical trial published in the New England Journal of Medicine, 83% of patients receiving this drug achieved at least a 2 mm reduction in proptosis after 24 weeks, compared with just 10% on placebo. On average, treated patients saw their eyes recede by nearly 3 mm. The drug also improved double vision in 68% of patients who had it. It’s given as an intravenous infusion every three weeks over a roughly five-month course.
Surgery
Orbital decompression surgery creates more room in the eye socket so the eye can settle back into a more normal position. Surgeons achieve this in two ways, often combined: removing some of the fat cushioning behind the eye, and removing portions of one or more bony walls of the orbit. Depending on how much space is needed, one, two, or three walls may be partially taken out. Newer techniques use incisions hidden inside the lower eyelid, reducing visible scarring. This surgery is used for thyroid eye disease that hasn’t responded to medication, for cosmetic improvement after the disease stabilizes, and as an emergency procedure when the optic nerve is being compressed.
For tumors, treatment depends on the type. Benign masses may be surgically removed or monitored. Malignant tumors like rhabdomyosarcoma require chemotherapy, often combined with radiation. Infections are treated with intravenous antibiotics, and if an abscess forms, it may need to be drained surgically.
One Eye vs. Both Eyes
Whether proptosis affects one eye or both is one of the most useful clues to its cause. Bilateral proptosis (both eyes) points strongly toward thyroid eye disease. Unilateral proptosis (one eye) has a broader list of possibilities: infection, tumor, bleeding, or inflammatory disease. Rapid onset over hours to days suggests infection or bleeding. Gradual progression over weeks to months is more typical of tumors or slowly worsening thyroid disease. Painless proptosis in a child that worsens quickly warrants urgent evaluation to rule out malignancy.