Dry eye syndrome is often thought of as a condition that equally affects both eyes, but it is entirely possible to experience dryness, grittiness, or burning in only one eye, a presentation known as unilateral dry eye. Dry eye occurs when the eyes do not produce enough tears, or when the tears produced are of poor quality and evaporate too quickly. While systemic conditions typically affect both eyes, symptoms confined to one side point toward a localized, rather than body-wide, problem.
Understanding Unilateral Dry Eye
The tear film covering the eye is a complex, three-layered structure, consisting of an oil layer, a watery layer, and a mucus layer. A compromise to any of these layers in one eye can lead to unilateral dry eye symptoms. When dry eye is caused by an underlying health issue or general aging, it tends to manifest in both eyes. Small anatomical differences can predispose one eye to poor tear distribution or faster evaporation.
A unilateral presentation suggests the cause is specifically impacting the tear-producing or tear-spreading mechanisms of that single eye. This distinction directs the diagnostic process toward localized factors rather than systemic ones. Symptoms in one eye may include a persistent foreign body sensation, redness, light sensitivity, or excessive watering as a reflex response to the irritation. The focus of investigation shifts away from generalized tear deficiency to a specific structural or environmental trigger.
Localized Causes Affecting Only One Eye
One of the most common localized causes involves issues with the meibomian glands, which produce the oily layer of the tear film. Meibomian gland dysfunction (MGD), an underlying factor in most dry eye cases, can be more pronounced or exclusively present in one eyelid. When these glands are blocked or inflamed, the tears on that side evaporate much faster, leading to localized evaporative dry eye.
Structural problems with the eyelids can also lead to unilateral symptoms. Conditions like ectropion (eyelid turns outward) or entropion (eyelid turns inward) can affect only one eye, preventing the proper spread of tears across the ocular surface. Localized nerve damage, such as from Bell’s palsy, can weaken the muscles controlling blinking or eyelid closure on one side, resulting in poor tear distribution and subsequent dryness. Past trauma or surgery, including procedures like LASIK, can disrupt the nerves and glands in the affected eye, causing unilateral dry eye.
Environmental factors can also act unilaterally, especially during sleep or while driving. Sleeping with one eye facing a circulating fan or air conditioning vent increases the rate of tear evaporation from that eye, leading to morning dryness confined to that side. Driving with a window open on only one side exposes that eye to increased wind and irritants, potentially causing localized symptoms.
Conditions That Mimic Single-Eye Dryness
When dryness or discomfort is strictly unilateral, eye care professionals must investigate conditions that present similar symptoms but are not classic dry eye syndrome. A corneal abrasion (a scratch on the eye’s surface) can feel like the grittiness of dry eye but is caused by a physical injury. A foreign body, such as debris trapped under the eyelid, will also cause intense irritation and tearing in only one eye. Localized inflammation, such as conjunctivitis or unilateral blepharitis, can mimic dry eye due to associated redness and discomfort. An issue with the tear drainage system, like a blocked tear duct, can paradoxically cause tearing alongside dryness symptoms, as the lack of proper drainage leads to a reflex overproduction of poor-quality tears.
Seeking Professional Diagnosis and Treatment
A persistent symptom of dryness in only one eye warrants a consultation with an eye care professional, such as an optometrist or ophthalmologist. The unilateral nature of the complaint signals that a detailed differential diagnosis is necessary to isolate the specific cause. The professional will perform a comprehensive eye examination, including a review of your medical history and lifestyle.
Diagnostic tools are used to examine the tear film and ocular surface in detail, including a slit lamp examination to inspect the eyelids, meibomian glands, and the cornea for damage or inflammation. Specific diagnostic stains, such as fluorescein, may be applied to measure how quickly tears evaporate (tear breakup time) and to identify localized damage. Treatment is then targeted at the underlying localized cause, such as resolving inflammation or correcting an eyelid position, rather than simply applying general artificial tears.