The sudden, noticeable difference in the size or appearance of one eye compared to the other, known as ocular asymmetry, can be alarming. While a subtle difference is common in human facial features, a swift or dramatic change should prompt an immediate medical evaluation. The appearance of a “bigger” eye can stem from a physical change to the eyeball or surrounding tissues, or it can be a visual trick created by the position of the eyelids. This article focuses on the underlying causes of this sudden asymmetry.
Actual Enlargement Versus Apparent Size Difference
It is important to determine whether the asymmetry is due to the eyeball itself being physically larger, or if the eye is simply pushed forward. True physical enlargement of the eye globe, a condition called buphthalmos, is extremely rare in adults. It is typically a result of high intraocular pressure (glaucoma) during early childhood when the eye tissues are still pliable. In an adult, the rigid structure of the bony orbit prevents the eyeball from expanding significantly.
The far more common cause of a seemingly larger eye is proptosis, also known as exophthalmos, which is the forward protrusion of a normal-sized eyeball. This protrusion occurs when something increases the volume of the contents behind the eye within the confined space of the bony eye socket. When the eye is pushed forward, it can show an asymmetry of more than 2 millimeters compared to the other eye.
Conditions Causing Eye Protrusion
The most frequent cause of eye protrusion in adults is Thyroid Eye Disease (TED), often associated with Graves’ disease, an autoimmune condition. In TED, the immune system mistakenly attacks the tissues within the orbit, particularly the extraocular muscles and the orbital fat. This process involves the stimulation of fibroblasts to produce hydrophilic molecules, which then draw water into the tissues, causing them to swell.
The swelling of the eye muscles and the expansion of the fatty tissue increase the overall volume behind the eyeball. Since the bony orbit cannot expand, this increased volume forces the eye forward, resulting in proptosis. Although Graves’ disease frequently affects both eyes, it can present unilaterally, or affect one eye more severely than the other.
Protrusion can also be caused by a space-occupying lesion, such as a tumor or cyst, within the orbit. These orbital masses displace the eye slowly over time. Vascular abnormalities, like orbital varices or a carotid-cavernous fistula, can also cause protrusion, sometimes leading to a pulsating sensation.
A rapidly developing, painful protrusion may be a sign of a severe infection called orbital cellulitis. This condition involves an infection of the fat and muscle tissues behind the orbital septum. Orbital cellulitis frequently stems from a bacterial sinus infection that spreads into the orbit, causing inflammation, pain, and fever.
Eyelid Issues That Create the Appearance of Size
Sometimes, the eye that appears “bigger” is normal, and the asymmetry is created by an eyelid issue. Eyelid retraction occurs when the upper eyelid is positioned higher than normal, or the lower eyelid is pulled down. This exposes more of the white part of the eye (sclera), giving the illusion that the eye is wider and larger than the other.
Eyelid retraction is a common finding in thyroid disease, where it can result from inflammation and fibrosis of the small Müller’s muscle in the eyelid. It can also be a result of sympathetic nervous system overactivity, or a consequence of previous surgery or trauma. The opposite situation, known as ptosis, or eyelid drooping, can also create the illusion of a size difference.
In cases of ptosis, the normal eye may appear bigger because the other eye looks smaller due to the upper lid partially covering the pupil. Ptosis can occur due to aging, muscle fatigue, or nerve damage. A specific type of mild ptosis, often accompanied by a smaller pupil, is a sign of Horner syndrome, caused by a disruption to the sympathetic nerve pathway.
When to Seek Immediate Medical Attention
A sudden or rapidly progressing change in eye size or appearance should always be evaluated by a medical professional immediately. Certain accompanying symptoms are red flags that may indicate a sight-threatening or life-threatening emergency. You should seek immediate care if the protrusion is accompanied by severe eye pain or a headache that develops rapidly.
Any sudden changes in vision, such as double vision (diplopia), blurred vision, or outright vision loss, require emergency attention. The inability to move the eye normally or pain upon attempted eye movement can indicate nerve or muscle compression, or a serious infection. Fever, particularly when combined with a bulging, red, or tender eye, is a strong indicator of orbital cellulitis, which necessitates urgent treatment.