Tearing and the Lacrimal System




Oculoplastics




  • 1.

    What are the causes of tearing?


    Tearing, also known as epiphora, occurs when there is an increase in the amount of tears produced or when there is a problem with the tear drainage system. We produce too many tears when the cornea is irritated. This tearing is adaptive, because if there is a foreign body present, it will wash it away. Acute corneal irritation typically results from mechanical irritants, such as an eyelash, or chemical irritants, such as the fumes from a freshly cut onion. Chronic tearing from irritation may also result from mechanical irritation as in entropion and trichiasis. However, it often occurs as a result of tear-film deficiencies (seen in dry eye syndrome and blepharitis), exposure keratopathy, or allergic conjunctivitis. When the production of tears is normal, tearing indicates an inadequate drainage of tears. A blockage at any point in the tear drainage system can cause tearing. Eyelid malpositions, such as ectropions and punctal ectropions, will also reduce the drainage of tears, as will lower eyelid laxity, which interferes with the eyelid’s ability to pump the tears naturally through the lacrimal drainage system. For many patients, tearing is multifactorial.



    Key Points: Common Causes of Chronic Tearing




    • 1.

      Ocular irritation



      • a.

        Dry eyes


      • b.

        Allergies


      • c.

        Computer vision syndrome



    • 2.

      Tear-drainage dysfunction, such as from lower eyelid laxity


    • 3.

      Blockage of the lacrimal drainage system, such as from a nasolacrimal duct obstruction




  • 2.

    Describe the normal path of tear drainage in the eyelids


    The most important function of our tears is to lubricate the surface of the eye. Our tears travel across the cornea and conjunctiva, keeping them moist. Gravity then guides most tears to rest on the margin of the lower eyelid. Here, they are carried medially to the puncta, small openings in the eyelid located approximately 6 to 7 mm lateral to the medial canthal angle. Once inside the puncta, the tears enter the canaliculi, mucosa-lined ducts approximately 10 mm in length that carry the tears to the lacrimal sac. The first portion of each canaliculus is a 2-mm dilated, vertical segment called the ampulla. Distal to the ampulla, the canaliculus bends acutely and runs parallel to the eyelid margin toward the medial canthus. In 90% of the population, the upper and lower canaliculi join together, forming the common canaliculus before merging with the lacrimal sac. However, in 10% of the population, each canaliculus merges with the lacrimal sac independently.


  • 3.

    Where do tears go after leaving the eyelids?


    The tears exit the canaliculi and enter the lacrimal sac, a mucosa-lined structure lying in a bony fossa in the medial orbit formed by the maxillary and lacrimal bones. The superior portion of the sac extends a few millimeters superior to the medial canthal tendon. It extends inferiorly approximately 10 mm and then continues as the nasolacrimal duct.


    Tears travel from the sac to the nasolacrimal duct. The first 12 mm of the duct lies in a bony canal in the maxillary bone. The duct then continues inferiorly for an additional 3 to 5 mm before opening into the inferior meatus of the nose. The tears exit the duct through its ostium into the nasal cavity. The ostium of the nasolacrimal duct can be found 30 mm posterior to the external nares in an adult. In young children, this distance is approximately 20 mm.


  • 4.

    What is the tear pump?


    The tear pump is a muscular “pump” that drives the tears through the drainage system by peristalsis. Tears first enter the puncta by capillary action. During a blink, the orbicularis oculi muscle contracts, closing the puncta, shortening the canaliculi, and moving them medially, while dilating the lacrimal sac. As the sac dilates, it creates a vacuum, drawing in the tears from the canaliculi. When the orbicularis muscle relaxes, the lacrimal sac collapses, the canaliculi lengthen, and the puncta reopen. The valve of Rosenmüller sits between the canaliculi and the sac, preventing the tears from reentering the canaliculi. Thus, tears are forced to continue their course down the nasolacrimal duct into the nose.


  • 5.

    How does lower eyelid laxity affect tear drainage?


    Normal drainage of tears requires normal structure and function of the eyelids. The pretarsal orbicularis muscle surrounds the canaliculi and attaches to the wall of the lacrimal sac. Contraction and relaxation of this muscle help draw the tears into the canaliculus and the sac and eventually force the tears down the nasolacrimal duct. When lower eyelid laxity is present, contraction of the orbicularis muscle does not compresses the canaliculi or force open the lacrimal sac, and the lacrimal pump mechanism cannot function adequately.


  • 6.

    How can you tell if a patient has lower eyelid laxity?


    Stretching of the medial and/or lateral canthal tendon causes lower eyelid laxity. In the distraction test, if the lower eyelid can be pulled more than 6 mm from the globe, it is lax.


    Poor orbicularis oculi tone, most obvious in patients with seventh cranial nerve palsy, also causes laxity of the lower eyelid. This is best demonstrated with the “snap-back” test, in which the lower eyelid is pulled down inferiorly and allowed to snap back into place. If the eyelid returns to its correct position immediately, the muscle tone is good. If the patient must blink to place the eyelid back in its normal position, eyelid tone is poor.


  • 7.

    How do you correct lower eyelid laxity?


    If there is laxity of the lateral canthal tendon, a horizontal lid shortening procedure is performed to tighten the eyelid. This is typically accomplished with a lateral tarsal strip procedure. In this operation the inferior limb of the lateral canthal tendon is disinserted from the periosteum of the lateral orbital rim, a portion or the entire tendon is removed, and a new lateral canthal tendon is created from the lateral portion of the tarsus. The newly formed lateral canthal tendon is sutured back to the periosteum of the lateral orbital rim. This effectively shortens the lower eyelid, making the eyelid margin more stable and improving tear pump function.


  • 8.

    Why do patients with dry eyes complain of tearing?


    Patients tear when they have dry eyes for the same reason that they tear when cutting an onion. Onion fumes cause corneal irritation, which, in turn, causes reflex tearing.


    Likewise, abnormalities in the tear film coating the cornea and conjunctiva cause irritation. Tear-film abnormalities can be caused by a decrease in the overall production of tears or by an imbalance in the composition of the tears. Inadequacies in any of the components of the tears cause a tear-film deficiency that can result in tearing.


  • 9.

    What is computer vision syndrome?


    Computer vision syndrome refers to a group of symptoms including tearing, eyestrain, and pain experienced by computer users. According to the National Institute of Occupational Safety and Health, computer vision syndrome affects some 90% of the people who spend three hours or more a day at a computer. Many of the symptoms relate to corneal exposure and the resultant dryness that occurs when extended time is spent staring at a computer screen.


  • 10.

    Of what are tears composed?


    Tears are composed of three layers. Mucin, made by the conjunctival goblet cells found mainly in the conjunctival fornices, covers the epithelium, ensuring a smooth, uniform tear film. The middle aqueous layer, made by the main lacrimal gland and accessory glands of Krause and Wolfring, provides hydration, oxygen, and nutrients. On the surface is the lipid layer, made in the meibomian, Zeis, and Moll glands of the eyelids. It prevents rapid evaporation of the tears and provides a smooth surface for the eyelids to glide across the cornea with each blink.


  • 11.

    How can you determine if a patient produces enough tears?


    The volume of tears can be indirectly assessed by visualization of the tear meniscus, the tear layer resting on the lower eyelid adjacent to the globe, which should be approximately 1 mm in height. However, the tear meniscus is also affected by tear drainage. The Schirmer test directly tests production. Gently dry the palpebral conjunctiva with a cotton swab and then place the small, folded end of a 5-mm-wide strip of Whatman No. 41 filter paper into the inferior conjunctival fornix at the junction of the middle and lateral third of the lower eyelid. In 5 minutes, measure the amount of wetting of the filter paper. When performed on an anesthetized cornea, it measures basal tear secretion. A normal result is 10 mm or greater. When performed on a nonanesthetized cornea, it measures both basal and reflex tearing. In this instance, normal wetting is 15 mm or greater.


  • 12.

    How do you know if the tear composition is inadequate?


    A decrease in the tear break-up time or the presence of protein, mucus, or debris in the tears indicates a tear inadequacy. The tear break-up time is the time it takes after a blink to develop a dry spot on the cornea. It is measured by touching the palpebral conjunctiva with a moistened fluorescein strip and observing the tear film through the slit lamp with a cobalt-blue filter. It is important to avoid using other eyedrops mixed with fluorescein, because this will change the composition of the tear film you observe. Once the patient blinks, the time is measured until the tear film begins to break up on the cornea, forming a dry spot. Less than 10 seconds is considered abnormal.


  • 13.

    What are ectropion and entropion? How do they cause tearing?


    Ectropion is an outward rotation of the eyelid margin. Entropion is an inward rotation of the eyelid margin. When either is present, patients tear. This occurs because both can cause corneal irritation with its associated reflex tearing, and both displace the punctum, so tears do not enter the tear-drainage system.


Jul 8, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Tearing and the Lacrimal System

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