Dry-eye disease is a disorder of the ocular surface and tear film in which the tears are insufficiently produced, evaporate excessively, have abnormal composition or fail to maintain a stable protective layer over the eye. The medical term includes a broad group of conditions involving tear-film instability, increased osmolarity, ocular-surface inflammation and abnormal sensory signalling. Symptoms include dryness, burning, grittiness, irritation, redness, light sensitivity, excessive reflex tearing and fluctuating or blurred vision. Paradoxically, watery eyes can occur because an irritated ocular surface triggers reflex tear production, but those tears may not contain the appropriate balance of oils and other components needed for sustained lubrication. Dry eye is therefore not simply an absence of tears.
Tears form a multilayered film over the cornea. The aqueous component provides water and dissolved substances, mucins help the tear film spread over the hydrophilic ocular surface, and the lipid layer produced largely by the meibomian glands helps reduce evaporation. A healthy tear film protects the cornea, provides nutrients and antimicrobial factors, supports epithelial health and contributes to optical quality.
When the film breaks up too quickly between blinks, exposed areas of the cornea can become increasingly concentrated in solutes and trigger inflammatory and sensory responses. This can produce both discomfort and visual fluctuation. Because the cornea is highly innervated, disturbances of the ocular surface can produce substantial symptoms even when visible signs are relatively modest.
Aqueous-deficient dry eye occurs when the lacrimal glands do not produce enough aqueous tears. Ageing can reduce tear production, while autoimmune diseases such as Sjögren's syndrome can severely affect lacrimal and salivary glands. Other systemic diseases, hormonal changes, dehydration and some medications can contribute. Medicines associated with dry-eye symptoms include some antihistamines, antidepressants, blood-pressure medicines and other drugs with anticholinergic or drying effects, although the relationship varies by individual. Damage to the sensory nerves of the eye can also impair the normal feedback system that stimulates tear production.
Evaporative dry eye is commonly associated with meibomian-gland dysfunction. The meibomian glands in the eyelids secrete lipids that slow evaporation from the tear film. When these glands become blocked or dysfunctional, the lipid layer may be inadequate, allowing tears to evaporate more rapidly. Blepharitis and ocular-surface inflammation can accompany this process. Environmental conditions such as wind, low humidity, smoke and air conditioning can increase evaporation. Prolonged screen use and concentrated visual tasks can reduce blink frequency and increase tear-film breakup. The result may be fluctuating symptoms that improve temporarily after blinking.
Contact lenses can contribute to dry-eye symptoms by altering the tear-film environment and reducing comfort during wear. Some people are more susceptible because of lens material, fit, wearing time, environmental conditions or pre-existing ocular-surface disease. Eye surgery, including certain refractive procedures, can temporarily or persistently alter corneal nerve function and tear-film regulation. Damage to corneal sensory nerves can disrupt the normal neural feedback loop between the ocular surface and lacrimal glands. Dry eye can also accompany eyelid abnormalities that prevent complete closure or alter the distribution of tears across the cornea.
Hormonal and systemic factors can influence dry eye. Symptoms become more common with increasing age and are more frequent in women, with hormonal changes thought to contribute. Autoimmune diseases can produce severe aqueous deficiency, while thyroid-related orbital disease and other conditions can affect eyelid closure and ocular exposure. Nutritional factors are less commonly responsible in developed settings but severe vitamin A deficiency can impair the ocular surface and cause serious eye disease. Neurological disorders affecting facial sensation or eyelid movement can also disturb the tear system. The presence of dry eye alongside dry mouth, joint symptoms, skin disease or other systemic features can be relevant when considering an underlying systemic disorder.
Symptoms can be amplified by inflammation and altered sensory processing. Tear-film instability exposes the ocular surface to environmental stress and can stimulate inflammatory pathways. Repeated stimulation of corneal nerves may alter how discomfort is perceived, meaning symptom severity does not always correspond closely to visible ocular-surface damage. Some people have significant symptoms with relatively few objective findings, while others have substantial tear-film abnormalities but little discomfort. This is one reason dry-eye diagnosis considers both symptoms and clinical findings. Tests may include tear-break-up time, ocular-surface staining, tear production measurements such as the Schirmer test and assessment of meibomian-gland function.
Treatment and prevention depend on the mechanism. Environmental measures, regular blinking, appropriate screen breaks and avoidance of direct airflow can reduce evaporation. Lubricating eye drops can supplement the tear film, while management of blepharitis or meibomian-gland dysfunction may address an important underlying cause. Severe or inflammatory dry eye may require prescription anti-inflammatory treatments or specialist procedures. Contact-lens use may need modification. Persistent pain, significant light sensitivity, reduced vision or a suspected corneal problem should be assessed professionally because these features can indicate something more serious than uncomplicated dry eye.
