Nasolacrimal Ducts and Disorders

Tear Duct

Lacrimal apparatus
Lacrimal apparatus. Dorling Kindersley/Getty Images

The nasolacrimal duct is part of Lacrimal Apparatus, which is our body's system for secretion and drainage of tears in our eyes. The nasolacrimal duct is part of the drainage system that allows for drainage of tears from the lacrimal sac to the nose. Problems with the proper functioning of tear drainage can lead to other health problems.

Tears

Secretion of tears involves a process of tear production from the primary and accessory lacrimal glands.

The glands are situated just above the level of your eye on the opposite side of your nose behind part of your skull. The glands work together to produce your tears which are made up of three distinct layers, which are referred to as tear film:

  • mucinous layer which is closest to the surface of your eye
  • aqueous layer which is the middle layer
  • lipid layer or the outer last layer of tears

The mucinous layer is made up of glycoproteins (proteins that are also made up of carbohydrates) call mucin. By nature, you eye is hydrophobic (doesn't like water). This layer makes your eye hydrophilic instead, so that the tears do not irritate the eye, and can cover the surface of your eye. This layer is actually released from your conjunctiva, which covers the surface of the white part of your eye (sclera).

The aqueous layer is secreted by the lacrimal glands. This is the largest layer of your tear. This layer is made up of many substances.

Proteins like albumin and immunoglobulins (proteins associated with your immune system) are present and helps to protect your eye from infections. Your tears are also made up of electrolytes like potassium, sodium chloride (which is why teardrops have a salty taste), and glucose. The average pH (acid-base balance) of this layer is 7.35.

Liquids that get into your eyes outside of this range will cause irritation to occur. Irritation of the eye surface and emotion is responsible for stimulating tears from being released from the lacrimal glands.

The lipid layer is the outermost layer of your tears and is secreted by glands in your eyelids. This layer serves to create a watertight seal when your eyelids are closed. The lipids also help to slow the rate of evaporation and subsequent drying of your eyes.

How Tears Drain

Your tears are constantly being replenished because of normal evaporation and drainage. Your tears are drained in the corner of your eyes on the same side as your nose. When you blink, your eyelids actually function like a zipper. Your eyelids start to close at the corner closest to your ear and push tears to the opposite corner of your eye. If you look in a mirror at the upper and lower corners of your eyelids, you can see 2 tiny openings.

These openings in your eyelids are referred to as lacrimal puncta. Tears are collected at the lacrimal puncta and transported to the lacrimal sac by canals called canaliculi.

Once the tears are in the lacrimal sac, the tears drain down the nasolacrimal duct into your nose by gravity. Folds within the duct function like one-way valves prevent air or fluid from the nose travel back up toward the eye. The nasolacrimal duct exits at the level of the inferior turbinate, which is the lower part of the nose where humidification of air you breath in occurs.

Nasolacrimal Duct Disorders

One of the most common problems associated with the lacrimal apparatus is an infection of the lacrimal sac. This is almost always due to an obstruction occurring with the nasolacrimal duct. Blockages of the nasolacrimal duct cause a backup of fluid in the lacrimal sac which is why this is the predominant location for an infection to occur.

Infants can have blockages at birth due to the nasolacrimal duct not being completely canalized from the lacrimal sac to the nose. In these cases, the eye will look goopy. Generally, this will resolve on its own by the end of the first month. The other common reason for obstruction in infants is one of the valves in the nasolacrimal duct, the valve of Hasner, having a membrane that blocks the flow. In most cases, either manual pressure or another procedure called probing can be used to establish patency of the duct. Probing is approximately 75% effective on the first try, however, if needed a second probing usually can complete the job. Surgery is rarely required.

In adults, you will be at highest risk for having a nasolacrimal duct obstruction if you are a post-menopausal woman. This is believed to be a result of inflammation that causes the duct to stiffen through fibrosis. To repair a nasolacrimal duct obstruction in adults, a dacryocystorhinostomy is performed. This surgery makes a fistula, or artificial connection, between the lacrimal sac and the nose.

Sources:

Vagefi, M.R., Sullivan, J.H., Corrêa, Z.M. & Augsburger, J.J. (2011). Vaughan & Asbury's General Ophthalmology. 18th ed. Chapter 4. Lids & Lacrimal Apparatus. Accessed on 2/21/2016 from http://accessmedicine.mhmedical.com (Subscription Required)

Nijm, L.M., Garcia-Ferrer, F.J., Schwab, I.R., Augsburger, J.J. & Corrêa, Z.M. (2011). Vaughan & Asbury's General Ophthalmology. 18th ed. Chapter 5. Conjunctiva & Tears. Accessed on 2/21/2016 from http://accessmedicine.mhmedical.com (Subscription Required)

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