Tears and tear ducts
Tears are a vital fluid produced by the lacrimal gland located beneath the upper eyelid, serving essential functions for eye health. They consist of three layers: an outer oily layer that prevents evaporation, a middle watery layer rich in nutrients and oxygen, and an inner mucous layer that helps spread the tears evenly across the eye. Tears not only keep the eye lubricated and clean but also provide a barrier against infections due to the presence of antibacterial enzymes. The drainage of tears occurs through a system that leads from the eyes to the nasal cavity, which is why excessive tear production often results in a runny nose.
Common issues related to tears and tear ducts include excessive tearing and dry eyes, both of which can arise from various factors such as infections, allergies, or environmental irritants. Tear duct obstructions can also occur, particularly in infants, leading to symptoms like watery eyes. Recent research has begun to explore the potential diagnostic uses of tears, investigating their ability to provide insights into overall health and even detect certain diseases like cancer through biomarkers. This area of study highlights the promising future of tears beyond their traditional roles.
Tears and tear ducts
Also known as: Lacrimal ducts, nasolacrimal ducts, tear film
Anatomy or system affected: Eyes, head, nose
Definition: Fluid produced by the eye and the channel that carries it from the eye to the nasal cavity
Structure and Functions
The lacrimal gland is located beneath the upper eyelid, on the outer edge of each eye, and its key function is to produce tears. Tears flow constantly across the conjunctiva, which is the front surface of the eye, to keep it clean and lubricated. Blinking spreads tears across the eye. This constant flushing of the eye surface also supplies oxygen and nutrients to the cornea. Tears provide an important barrier to infection as they contain the antibacterial enzyme lysozyme, which destroys microorganisms on the eye surface. As new tears are produced, old tears drain from the eye at its inner edge, called the "canthus," by being drawn into two small holes, the upper and lower puncta, through capillary action. Tears flow through the canaliculus, entering the lacrimal sac, then to the tear duct (called the "nasolacrimal duct"), and at last into the nasal cavity. This is the reason that a surplus of tears results in a runny nose.
Tears, or tear film, are made up of three distinct layers: an outer oily layer to prevent drying, which is produced by the meibomian glands; a watery middle layer, which contains the oxygen and nutrients; and an inner mucous layer, produced by the conjunctival goblet cells, reducing the surface tension of the tears and allowing them to be spread evenly on the surface of the eye.
The body naturally produces tears to maintain and protect the eye, but in humans, tears are also produced in response to emotion in the form of crying. During a normal twenty-four-hour day, 0.75 to 1.1 grams of tears are secreted; however, this rate declines with age. Also, approximately 10 to 15 percent of tear secretion is lost to evaporation, dependent on the environment.
Disorders and Diseases
The most common disorder associated with tears is excessive tearing. This can occur for many reasons and may require evaluation by an ophthalmologist. An excessive amount of tears can be caused by infection, environmental irritants, glaucoma, certain medications, allergic reaction, eyestrain, dry eyes, foreign material in the eye, scratch on the surface of the eye, or other eyelid or eyelash disorders.
Dry eye, also described as “scratchy eye,” can occur when too few tears are being produced, or they are draining from the eye too rapidly or evaporating too quickly. Symptoms of dry eye may also include photosensitivity, blurred vision, redness, and an itching or burning sensation. Often the treatment is to instill drops of artificial tears or lubricating gel, to apply compresses, and to avoid potential irritants such as excessively dry or warm rooms to prevent evaporation. If the cause of dry eye is that tears are draining too quickly, then a surgeon may modify the tear duct to slow or limit tear drainage.
Another common disorder associated with tear ducts is obstruction. If there is an obstruction of the tear duct (nasolacrimal duct), then tears are not able to drain easily into the nasal cavity. Blockage of the tear duct can occur in early childhood as a congenital blockage or later in life as a result of chronic infection or irritation, injury, or tumor. To test the tear duct for obstruction, a drop of fluorescein dye will be instilled into the lower lid and then the patient will be observed at regular intervals to determine if the dye has traveled through to the nasal cavity. Treatment varies depending on the underlying cause.
A congenital blockage occurs in 6 to 20 percent of infants when a membrane remains between the tear duct and nasal cavity. This results in the inability for the eye to drain, and the eye appears excessively watery or filled with mucus. Often the membrane will rupture spontaneously before one year of age, thus resolving the obstruction, but sometimes a probe will need to be inserted by an ophthalmologist.
Tear duct obstruction may cause an acute infection of the lacrimal sac, located at the upper, wider end of the nasolacrimal duct. This form of infection is called "dacryocystitis," and patients normally have an overflowing of tears (epiphora) and a swollen painful mass over the area of the lacrimal sac. Dacryocystitis may be treated with warm compresses and gentle massage but may require systemic antibiotics and drainage of the mass.
Perspective and Prospects
Research is being conducted to determine if tears not only provide nutrients and moisture to the eye but also have diagnostic uses. Tears are created from blood that has been filtered; therefore, tears can provide medical clues as to things happening in the blood. Studies are being done to determine if biological changes in the eye and eye health can be measured through small variations in the level of inflammatory proteins found in tears. Also, in 2000, a group of contact lens researchers in Australia found that tears contain protein markers that may be used to detect certain types of cancer. Research is continuing in this area in the hope that tears may provide useful information in future diagnoses. In 2020, Kobe University and System Instruments Co., Ltd. announced their recent development, TearExo, which built on previous research to use biomarkers in tears to detect breast cancer using exosomes. This noninvasive method of detection was a great advancement in noninvasive cancer prevention methods.
Bibliography
"Blocked Tear Duct." Mayo Foundation for Medical Education and Research, 8 Mar. 2023, www.mayoclinic.org/diseases-conditions/blocked-tear-duct/symptoms-causes/syc-20351369. Accessed 20 July 2023.
Cohen, Adam, Michael Mercandetti, and Brian Brazzo, eds. The Lacrimal System: Diagnosis, Management, and Surgery. Springer, 2016.
"Dry Eye Syndrome." MedlinePlus, medlineplus.gov/ency/article/000426.htm. Accessed 20 July 2023.
Howson, Alexandra, and Christopher Cheyer. "Lacrimal Duct Stenosis." Health Library, 4 June 2012.
Kohnle, Diana, and Michael Woods. "Dacryocystitis." Health Library, 24 Nov. 2012.
Sullivan, David, Darlene Dartt, and Michele Meneray, eds. Lacrimal Gland, Tear Film, and Dry Eye Syndromes 2: Basic Science and Clinical Relevance. Plenum Press, 1998.
Van Haeringen, N. J. “Aging and the Lacrimal System.” British Journal of Ophthalmology, vol. 81, 1997, pp. 824–826.