Tears look simple. They are not. Every blink spreads a thin, carefully balanced film across the eye surface that lubricates, nourishes, protects, and smooths the cornea optically. When that film becomes unstable, even slightly, the result can be burning, fluctuating blur, reflex tearing, or real corneal surface damage. Dry eye disease is one of the most common problems in eye care, yet many people still think of it as minor irritation rather than a vision-quality disorder. That is too dismissive.

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The anatomy of the tear system

The lacrimal gland

The main lacrimal gland sits in a shallow recess at the outer upper corner of the orbit, tucked beneath the upper eyelid. It produces the aqueous component of tears, which is the thickest layer of the tear film. These aqueous tears contain water, electrolytes, proteins such as lysozyme and immunoglobulin A, growth factors, and dissolved oxygen. That is a lot of work for something most people think of as just water.

The gland produces tears continuously at a low baseline rate, then increases production sharply when the eye is irritated, a foreign body is present, bright light hits, or emotion triggers crying. Basal tearing and reflex tearing are related, but not interchangeable.

The meibomian glands

About 25 to 40 meibomian glands sit within the upper lid and a similar number within the lower lid. They secrete meibum, a lipid-rich material that forms the outer tear film layer. This oily layer slows evaporation and helps keep the front surface of the tear film smooth. Without it, tears disappear too quickly and the cornea is left exposed between blinks.

Meibomian gland dysfunction, or MGD, is the most common cause of evaporative dry eye. The gland openings become obstructed, the oil becomes thickened or poor in quality, and the tear film destabilizes faster than it should. Many patients with watery or burning eyes turn out to have this problem rather than a lack of tear production itself.

Goblet cells and the mucin layer

Goblet cells live within the conjunctiva and produce mucins, large glycoproteins that form the innermost part of the tear film. This layer allows aqueous tears to spread evenly across the normally hydrophobic corneal surface. That detail matters. Without mucin, tears do not coat the surface properly, no matter how much aqueous tear is present.

Diseases that damage the conjunctiva, including cicatricial conjunctivitis, Stevens-Johnson syndrome, chemical injury, and severe vitamin A deficiency, can destroy goblet cells and produce severe mucin-deficient dry eye. Those cases are not the routine dry eye clinic cases. They are often much harder.

The tear drainage system

Tears leave the eye through tiny openings called puncta, one in the upper lid and one in the lower lid near the inner corner. Each punctum connects to a canaliculus, and the canaliculi usually join before entering the lacrimal sac. From there, the nasolacrimal duct drains tears into the nose. That is why crying makes the nose run and why some eye drops can be tasted in the throat.

Blockage anywhere along this pathway causes tears to overflow onto the cheek. In infants, nasolacrimal duct obstruction is common and often resolves on its own in the first year. In adults, persistent obstruction is less forgiving and more likely to need intervention.

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The tear film and vision

The tear film is not just lubrication. It is one of the eye’s most important optical surfaces. A smooth tear film creates a clean refractive surface over the cornea and helps keep vision crisp from blink to blink. When the film breaks up early, that optical surface becomes irregular and image quality degrades until the next blink restores it.

This is why one of the most characteristic symptoms of dry eye is fluctuating blur: vision clears just after a blink, then gradually worsens as the tear film destabilizes. Patients often describe it well before the surface staining looks dramatic, which is one reason dry eye can be underestimated if the history is not taken seriously.

Tear breakup time, or TBUT, is measured clinically with fluorescein dye. A TBUT shorter than about 5 seconds strongly suggests tear film instability. It is a useful metric, but like most dry eye tests, it works best when interpreted alongside symptoms rather than as a stand-alone verdict.

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Disorders of the tear system

Dry eye disease

Dry eye disease can result from reduced aqueous production, excessive evaporation, or, most commonly, some combination of both. Risk factors include female sex, older age, contact lens wear, prolonged screen use, certain medications, autoimmune disease such as Sjogren syndrome and rheumatoid arthritis, and prior eye surgery including LASIK.

Treatment is usually layered rather than magical. Lubricating drops, lid hygiene, and environmental changes come first. Prescription drops such as cyclosporine or lifitegrast may be added when inflammation is part of the problem. More advanced options include thermal pulsation for MGD, punctal plugs, autologous serum drops, and scleral lenses in severe disease. The evidence for many therapies is decent, though not every patient responds as neatly as brochures suggest.

Meibomian gland dysfunction

MGD is now recognized as the most common cause of dry eye in many clinical populations. Warm compresses and lid massage remain the foundation of treatment because heat softens the secretions and helps clear blocked gland openings. That advice sounds basic because it is basic. It still works when patients do it consistently.

In-office thermal pulsation procedures such as LipiFlow can provide more standardized heat and pressure to the glands and may improve symptoms for months. Helpful for some. Overmarketed for others. Patient selection matters.

Epiphora (watering eyes)

Epiphora has two broad causes: the eye is making too many tears, or the normal drainage system is not carrying them away. Excess tear production is usually reflex tearing caused by ocular surface irritation, including dry eye, blepharitis, corneal disease, or a foreign body. Impaired drainage may result from punctal stenosis, canalicular disease, or nasolacrimal duct obstruction.

The two feel similar to patients because both leave tears running down the cheek. Clinically, they are not the same problem at all. One needs the surface treated. The other may eventually need surgery.

Nasolacrimal duct obstruction

Nasolacrimal duct obstruction in adults causes persistent watering and may be complicated by recurrent conjunctivitis or frank dacryocystitis, an infection of the lacrimal sac. Dacryocystitis typically presents as a painful red swelling below the inner corner of the eye. It needs antibiotic treatment first. Once the acute infection settles, definitive treatment is usually dacryocystorhinostomy, or DCR, which creates a new drainage passage from the lacrimal sac into the nose.

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For further reading: Dry eye disease, American Academy of Ophthalmology and Eye conditions and diseases, National Eye Institute.