The sudden appearance of one eye looking noticeably bigger or smaller than the other often causes immediate concern. While the difference may sometimes be harmless, a sudden change in eye symmetry can signal a serious underlying health condition requiring prompt medical attention. The appearance of asymmetry can be traced to three primary areas: the position of the eyelid, the position of the entire eyeball, or the size of the pupil.
Asymmetry Caused by Eyelid Position
One of the most common reasons an eye appears smaller is due to a drooping upper eyelid, a condition known as ptosis. This occurs when the levator muscle, responsible for lifting the eyelid, is weakened or the nerve controlling it is damaged. The sudden onset of ptosis can be caused by mechanical issues directly affecting the eyelid or by more complex neurological problems.
Mechanical Causes
Mechanical causes involve a direct physical change to the eyelid tissue, such as swelling from trauma, infection, or an allergic reaction. An infection like a stye, a tender red bump on the eyelid edge, can cause inflammation that physically weighs the eyelid down. An insect bite or a sudden allergic reaction can also cause rapid swelling, making the eye opening appear significantly smaller.
Neurological Causes
Neurological causes are more concerning and involve damage to the nerve pathways that control the eyelid muscles. For instance, a third cranial nerve palsy, which affects the oculomotor nerve, can cause a sudden eyelid droop, often accompanied by double vision and a fixed, dilated pupil. Horner syndrome results from a disruption of the sympathetic nerve pathway, presenting with a mild droop alongside a small, constricted pupil.
Asymmetry Caused by Eyeball Protrusion
If one eye suddenly looks bigger or seems to be bulging forward, the cause is likely related to the eyeball being physically displaced, a condition called proptosis. This displacement suggests that something is taking up extra space within the bony socket behind the eyeball, pushing it forward.
Graves’ disease, an autoimmune condition causing an overactive thyroid, is the most common cause of eyeball protrusion in adults. In this condition, immune cells attack the tissues behind the eye, causing the fat and muscles to swell and enlarge, which pushes the eye forward.
More acute and dangerous causes include infections or sudden bleeding within the eye socket. Orbital cellulitis is a serious bacterial infection of the tissues behind the eye, leading to rapid swelling, pain, and protrusion. A retrobulbar hemorrhage, or bleeding behind the eye following trauma, is a time-sensitive emergency because the rapid pressure increase can threaten vision. Tumors or inflammatory masses growing behind the eye can also cause protrusion.
Sudden Differences in Pupil Size
In some cases, the difference in appearance is due to a change in the dark center of the eye, the pupil, rather than the eyelid or eyeball position. A sudden, significant difference in pupil size between the two eyes is medically termed anisocoria. While a slight difference is normal for about 20% of the population, a sudden change can make the eye with the larger pupil look darker and bigger overall.
Pathological anisocoria results from a problem with the nervous system controlling the iris muscles. Conditions like Horner syndrome cause the pupil to become smaller and slower to dilate due to sympathetic nerve pathway disruption. Conversely, a sudden third cranial nerve palsy can cause the pupil to become large and unreactive to light due to parasympathetic nerve damage.
Exposure to certain medications can also cause an immediate, temporary difference in pupil size. Accidental contact with eye drops or transfer of medication from the hands to one eye can cause the pupil to fully dilate. Because discerning a benign cause from a serious neurological event is impossible without medical testing, any sudden, new, or unexplained difference in pupil size requires evaluation.
Warning Signs Requiring Emergency Care
Although some causes of sudden eye asymmetry are benign, many indicate conditions that pose a threat to vision or life. Recognizing accompanying symptoms is imperative for timely action. Any sudden change in eye appearance combined with other symptoms warrants an immediate trip to the emergency room or urgent care.
Immediate medical attention is needed if the change is combined with sudden vision loss or significant blurring. The presence of double vision, or diplopia, which indicates a problem with eye alignment or movement, is also a serious sign. Sudden-onset eye pain, especially if accompanied by redness or a high fever, suggests a rapidly progressing infection or inflammation.
Additional symptoms requiring emergency evaluation include an inability to move the affected eye in all directions or a sudden headache. If the asymmetry is associated with signs of a stroke, such as slurred speech, weakness on one side of the body, or confusion, emergency medical services should be contacted immediately.