Dry Eyes

Accessory Structures and Anatomy of the Eye

• Eyelid

  • These consist of two thin, movable folds located in front of the eye.

  • Key functions include protecting the eye from injury by closing and spreading tears over the ocular surface.

• Lacrimal Apparatus

  • This system includes the lacrimal glands and their excretory ducts.

  • Lacrimal Glands and Excretory Ducts: These function to secrete and convey tears to the surface of the eye.

  • Lacrimal Ducts, Lacrimal Sac, and Nasolacrimal Duct: These components convey fluid into the nasal cavity.

  • Associated structures include the lacrimal punctum, the lacrimal canal, and the nasolacrimal duct.

• Conjunctiva

  • These are transparent mucous membranes that line the inner surface of the eyelids and the outer surface of the eye.

  • Functions: The conjunctiva secretes oils and mucous to moisten and lubricate the eye.

• Extrinsic Muscles: These muscles control the movement of the eye.

The Tear Layer: Composition, Physiology, and Function

• Multilayer Film Structure

  • Outer Lipid Layer: This layer is secreted by the Meibomian glands. Its primary roles are to reduce evaporation and maintain the optical properties of the eyes.

  • Middle Aqueous Layer: This layer is secreted by the lacrimal gland and is largely responsible for the wetting properties of the tear film.

  • Inner Mucous Layer: This layer is secreted by the conjunctiva. It allows the aqueous and lipid layers to maintain constant adhesion across the cornea and conjunctiva, ensuring tears spread evenly on the eye surface.

• Tear Physiology

  • Functions: Tears keep the ocular surface lubricated and maintain the health of the cornea by supplying nutrients and flushing away waste products.

  • Normal Production Rate: Tears are produced at a rate of $1-2\,mL/min$.

  • Tear Volume: The volume of tear fluid on the ocular surface at any given time is approximately $7-10\,mL$.

  • Reflex Tearing: This occurs during ocular irritation, during which tear production can increase to $> 300\%$ of the non-stimulated rate.

Etiology and Clinical Manifestations of Dry Eye Disease

• Causes of Dry Eyes

  • The natural aging process.

  • Interior lid or corneal defects.

  • Various collagen diseases, such as rheumatoid arthritis.

  • Systemic medications, specifically antihistamines.

  • Environmental conditions, including dry or dusty working environments, heating systems, and air-conditioning.

  • Inadequate production of tears.

• Symptoms

  • Mildly red eyes.

  • A sandy or gritty feeling.

  • Foreign body sensation (feeling like something is in the eye).

  • Blurred vision.

• Complications

  • Failure to properly diagnose and treat dry eyes can lead to severe damage to eye tissue, particularly the corneal surface.

Management Strategies for Dry Eyes

• Treatment Goals

  • Alleviate and control the dryness of the ocular surface.

  • Relieve symptoms of irritation.

  • Prevent potential tissue and corneal damage.

• Non-Pharmacologic Therapy

  • Avoid environments that increase the evaporation of the tear film.

  • Utilize humidifiers to increase moisture in the air.

  • Reposition workstations away from heating and air-conditioning vents.

  • Avoid prolonged viewing of computer screens.

  • Avoid windy outdoor environments unless wearing eye protection.

• General Pharmacologic Therapy Overview

  • Management primarily involves the use of ocular lubricants.

  • Product Selection: Products vary by viscosity based on their ingredients. Less viscous products are used for mild cases, while more viscous products are reserved for severe cases. Ointment preparations are used for severe cases and for bedtime application.

  • Advanced Treatment: In severe cases, ocular lubricants may be combined with immunomodulating agents such as topical cyclosporin. Dry eye disease may be linked to a T-cell mediated inflammatory process.

Artificial Tear Preparations: Classification and Composition

• Demulcents

  • These agents form a protective layer over mucous membranes to relieve inflammation and irritation.

  • Examples: Hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylcellulose (CMC), and methylcellulose.

  • Polyvinyl Alcohol (PVA): These are hydrophilic polymers that coat and protect the eye. They crosslink upon contact with the ocular surface to increase tear clearance times. These are typically found in older dry eye preparations.

• Emollients

  • These are oily or fat-based agents used to soften and protect tissues by sealing in moisture.

  • Examples: Mineral oil, lanolin, paraffin, and petrolatum.

• Preservatives and Additives

  • Preservatives: Benzalkonium chloride (BAK), Purite, and Polyquad are used, usually in multidose bottles.

  • Inorganic Electrolytes: Used as buffering agents to maintain tonicity and pH.

  • Retention Agents: Ingredients such as HP-guar are included to increase the retention time of the drops on the eye.

• Cautions Regarding Preservatives

  • Preservatives can be toxic to ocular tissues and cause hypersensitivity.

  • Benzalkonium Chloride (BAK): It is advisable to avoid eye drops containing BAK as it may increase eye surface irritation and disease.

  • Preservative-Free Requirements: Avoid preservatives if you are sensitive to them, if the dry eye condition is severe, or if you must use drops more than four to six times a day.

Specialized Product Examples and Mechanisms

• Liquifilm Tears

  • Contains Polyvinyl alcohol $1.4\%$ (equivalent to $14\,mg/mL$).

  • Preserved with Benzalkonium Chloride $0.05\,mg/mL$.

  • Storage: Store between $15-25\,^{\circ}C$; discard one month after first opening.

  • Note: Not for use with soft contact lenses.

• Preservative-Free Formulations

  • Examples include combinations of Polyethylene Glycol 400 $0.4\%$ and Propylene Glycol $0.3\%$.

  • These may use Hydroxypropyl guar (HP-guar) as a gelling agent to increase viscosity and bioadhesive properties.

  • Specific product components seen in the market include Carboxymethylcellulose $0.5\%$ or $1\%$, and mixtures of Mineral oil $42.5\%$ with White petroleum $56.8\%$ (preserved with Chlorbutanol).

• Mechanism of Action for Artificial Tears

  • They form a hydrophilic layer on the corneal surface to mimic natural conjunctiva mucin.

  • They stabilize the tear film to retard tear evaporation and increase tear retention.

  • They promote the wetting of the ocular surface and increase viscosity.

  • They create an ocular shield to allow for epithelial repair.

• Non-Medicated Ophthalmic Ointments

  • White Petrolatum: Acts as a lubricant and ointment base.

  • Mineral Oil: Helps the ointment melt at body temperature.

  • Lanolin: Facilitates the incorporation of water-soluble medications and prevents evaporation.

  • Ointment Mechanism: Enhanced retention time in the eye and enhanced integrity of the eye film.

  • Side Effects: Ointments notably cause blurred vision upon application.

Dosage and Administration Guidelines

• Dosage Guidelines

  • Artificial Tear Solutions: For mild cases, apply $1-2$ times daily or as necessary. For severe cases, apply $3-4$ times daily or more frequently.

  • Ointments: Generally administered twice daily. They may be administered as often as every few hours or occasionally as needed.

• Steps for Eye Drop Administration

  1. Wash hands thoroughly.

  2. Tilt head back.

  3. Gently grasp the lower outer eyelid below the lashes and pull it away from the eye to create a pouch.

  4. Place the dropper over the eye, looking directly at it.

  5. Look up just before applying a single drop.

  6. Release the eyelid slowly once the drop is applied.

  7. Close eyes gently for $3\,minutes$. Minimize blinking or squeezing the eyelid.

  8. Use a finger to apply gentle pressure over the opening of the tear duct (punctal occlusion).

  9. Blot excessive solution from around the eye.

• Steps for Eye Ointment Administration

  1. Wash hands thoroughly and sit in front of a mirror.

  2. Unscrew the cap from the tube.

  3. Tilt head back and look upwards; gently pull the lower eyelid down.

  4. Hold the tube above the eye without touching the eye or eyelashes.

  5. Squeeze approximately $1\,cm$ of ointment along the inside of the lower eyelid.

  6. Close the eye and blink several times to spread the ointment.

  7. Wipe away excess with a clean tissue.

  8. Vision may be blurred temporarily after opening the eye; blink to clear it. Do not rub the eye.

  9. Replace the cap immediately.

• Multiple Therapy Protocols

  • Multiple Drops: If more than one drop is required, wait at least $5\,minutes$ before instilling the second drop to prevent the first drop from being washed out.

  • Suspensions: If using a suspension, it should be administered last.

  • Drops and Ointments: If both are indicated, instill the drops at least $10\,minutes$ BEFORE the ointment. This is because the ointment creates a barrier that prevents the drop from penetrating the eye surface if applied second.