Comprehensive Study Guide to Dry Eye Disease (Keratoconjunctivitis Sicca): Pathophysiology, Pharmacology, and Clinical Management

Introduction to Ocular Pharmacology and Course Philosophy

  • Instructor Philosophy: Dr. Sheena emphasizes an interactive teaching style to gauge student understanding and aims for a high level of efficiency so that students have minimal study effort outside of the classroom.
  • Learning Objectives (LOs): Students are instructed to keep the course learning objectives as the primary guide for examination preparation. If a student can answer all LOs, they are guaranteed to succeed on the assessments for the T3 and subsequent T4 trimesters.
  • Clinical Responsibility: Beyond assessment, the goal is to equip students to provide the best treatment options for patients in a pharmacy setting, specifically for self-care conditions like dry eye.
  • Future Coursework: Dr. Sheena will lead two courses in the T4 trimester, meeting with students on Tuesday, Wednesday, and Thursday.

Anatomy of the Ocular Surface

  • Ocular Surface Components: The ocular surface is collectively composed of the cornea, sclera, and conjunctiva.
  • Cornea:
    • Description: A circular, dome-shaped, transparent front part of the eye. It is shaped like a convex lens.
    • Function: It acts as a primary light-focusing organ, directing light onto the retina. It must remain clear for vision to function correctly.
    • Avascularity: The cornea is one of the few living tissues in the human body without blood vessels. If blood vessels were present, vision would be obstructed. It survives and maintains its decent size without direct vascularization.
  • Sclera: The white part of the eye visible from the outside, which constitutes the majority of the ocular globe fitting into the orbit.
  • Conjunctiva:
    • Definition: A thin, transparent membrane covering the inner part of the eyelids and the visible portion of the sclera.
    • Pathological Note: The conjunctiva sits on top of the sclera but never covers the cornea. If the conjunctiva begins to cover the cornea, it is considered a disease state.

Composition, Secretion, and Dynamics of the Tear Film

  • The Tear Film: A thin layer of fluid that coats the ocular surface, essential for maintaining health and vision.
  • Volumetric Data:
    • Normally Present Volume: Approximately 6−7 μL6-7\,\mu\text{L}.
    • Secretion Rate: Approximately 2 μL/min2\,\mu\text{L/min}.
    • Daily Basal Secretion: Approximately 2−3 mL2-3\,\text{mL} over 24 hours24\,\text{hours} (calculated as 120 μL/hour×24 hours120\,\mu\text{L/hour} \times 24\,\text{hours}). This refers to basal tears, not emotional tears, which overflow onto the cheeks.
  • Structural Layers:
    • Outer Lipid Layer: The outermost part of the tear film. It has a high melting point (analogous to butter vs. water) and serves to prevent the evaporation of the underlying aqueous bulk.
    • Aqueous-Mucus Layer: The bulk of the tear film, consisting of water and mucins. This used to be considered two separate layers, but is now categorized as a single gel-like structure.
  • Secretory Glands:
    • Lipid Layer Sources: Meibomian glands are the primary source (located inside the eyelids, 20+20+ on the upper eyelid and approx. 1818 in the lower). Every blink secretes a tiny amount of "meibum." Secondary sources include the glands of Zeiss and Mol.
    • Aqueous Layer Sources: Primarily the lacrimal glands, located on the temporal side of the eyes beneath the orbital bone. Secondary sources include the accessory glands of Krause and Wolfring.
    • Mucus Layer Sources: Secreted by Goblet cells, corneal epithelial cells, and conjunctival epithelial cells.
  • Tear Drainage: Tears are secreted from the temporal side and wash across the eye toward the nasal side, draining into the nose.

Pathophysiology and Clinical Presentation of Dry Eye Disease (Keratoconjunctivitis Sicca)

  • Medical Name: Keratoconjunctivitis Sicca.
  • Definition: A dysfunction of the tear film resulting from a loss of homeostasis. It is caused by either decreased tear secretion, excessive evaporation, or a combination of both.
  • Hyperosmolarity: When the water component decreases, the tear film becomes hyperosmolar. Persistent hyperosmolarity acts similarly to spending a day in the hyperosmolar ocean without drinking water: it leads to tissue dehydration, epithelial cell damage, and an inflammatory response.
  • Chronicity: Dry eye is typically a lifelong, chronic condition that requires ongoing management rather than a one-time cure.
  • Signs and Symptoms:
    • Early Stage Paradox: Excessive tearing can occur initially as the body tries to compensate for the dryness.
    • Sensation: Sandy, gritty sensation (foreign body sensation), burning, and itching.
    • Appearance: Redness and stringy mucus around the eye, especially in the morning.
    • Untreated Risks: Damage to the cornea can lead to scarring and permanent vision loss.
  • Laterality: One eye typically experiences more severe symptoms than the other.

Etiology and Risk Factors: Modifiable, Non-Modifiable, and Pharmacological Triggers

  • Prevalence: Ranges from 5%5\% to 28%28\% depending on self-reporting and diagnostic criteria.
  • Non-Modifiable Risk Factors:
    • Age: Risk increases with age; approx. 15%15\% of those over 65 years65\,\text{years} are affected.
    • Sex: Female sex is a major risk factor, particularly around the age of menopause due to hormonal shifts.
  • Modifiable/Systemic Risk Factors:
    • Autoimmune Diseases: Sjogren’s syndrome (very severe), Lupus, Rheumatoid Arthritis, and Relapsing Polychondritis.
    • Other Conditions: Diabetes, Graves’ disease (thyroid eye disease), Rosacea, and Multiple Sclerosis.
    • Local Conditions: Blepharitis and Meibomian Gland Dysfunction (MGD).
    • Environmental/Behavioral Factors: Dry/dusty environments, smoking, alcoholism, and excessive screen use (linked to a decreased blink rate).
  • Pharmacological Risk Factors (Drug-Induced Dry Eye):
    • Anticholinergics: These block cholinergic control of the lacrimal glands. Examples include Diphenhydramine, Atropine, Scopolamine, and Benztropine.
    • Antidepressants: Older Tricyclic Antidepressants (TCAs) like Imipramine and Desipramine.
    • Neurological/Psychiatric: Benzodiazepines, Morphine, and other opiates affect the neuronal inputs to the tear film.
    • Acne Therapy: Isotretinoin (Vitamin A derivative) interferes with meibomian gland lipid synthesis.
    • Anticancer: Aromatase Inhibitors (e.g., Letrozole, Anastrozole) interfere with cholesterol/lipid synthesis; Epidermal/Fibroblast Growth Factor (EGF/FGF) inhibitors interfere with epithelial cell health.
    • Cardiovascular: Diuretics and Beta-blockers (can dry out the body overall).

Clinical Assessment, Diagnostic Tools, and Exclusion Criteria

  • Diagnostic Tools (Clinical):
    • Schirmer Test: A paper strip is placed in the lower eyelid to measure basal tear production over time; a result of less than 10 mm10\,\text{mm} is considered abnormal.
    • Tear Film Breakup Time (TFBUT): Visualized under a slit lamp.
    • Ocular Surface Disease Index (OSDI): A patient questionnaire.
    • Staining: Fluorescein, Rose Bengal, or Lissamine Green stains identify areas of epithelial damage.
  • Exclusion Criteria (When to Refer (Red Flags)):
    • Sudden Onset: Acute symptoms starting within 24−48 hours24-48\,\text{hours} suggest infection or inflammation rather than chronic dry eye.
    • Pain and Blurred Vision: These are signs of serious conditions such as angle-closure glaucoma or Keratitis.
    • Medical Emergency: Keratitis (infection of the cornea).
    • Medical Urgency: Uveitis (autoimmune inflammation of the anterior segment; needs treatment within 1−2 days1-2\,\text{days}).

Management of Dry Eye: Non-Pharmacologic Strategies and Treatment Goals

  • Treatment Goals: Relieve clinical symptoms and stop tissue damage/inflammation.
  • Hydration: Patients must consume enough water (3.7 L3.7\,\text{L} for men and 2.7 L2.7\,\text{L} for women).
  • Environmental Control: Avoid directing fans, hair dryers, or air conditioning vents toward the eyes. Use humidifiers in dry winters.
  • Ergonomics: Position digital screens below eye level. Because the upper eyelid is larger than the lower, looking down ensures more of the eye is covered, reducing evaporation.
  • Blinking: Encourage frequent, conscious blinking, especially during screen time.
  • Protective Eyewear: Wraparound glasses can prevent wind-induced evaporation.
  • Nutritional Supplements: Omega-3 fatty acids (fish oil, nuts) are often recommended by clinicians, though recent National Eye Institute (NEI) trials show mixed effectiveness.

Pharmacologic Management: Demulsants and Inactive Ingredients

  • Demulsants (Active Ingredients): Substances that protect and lubricate the eye.
    • Cellulose derivatives: Carboxymethylcellulose (CMC) found in Refresh Tears and TheraTears; Hypromellose (HPMC) found in Genteal.
    • Polyols: Polyethylene glycol (PEG) and Propylene glycol found in Systane.
    • Alcohols: Polyvinyl Alcohol (PVA) and Povidone found in Soothe (Bausch + Lomb).
    • Dextran 70: A weaker demulsant always used in combination with others.
  • Inactive Ingredients (Biological Activity):
    • Osmo-protectants: Trehalose, Carnitine, and Erythritol protect cells from hyperosmolar injury (e.g., Refresh Optive series).
    • Humectants: Sodium Hyaluronate or Guar Gum increase moisture retention (e.g., Systane Ultra).
    • Lipids/Oils: Castor oil, Flaxseed oil, or Mineral oil. These create an emulsion (milky appearance) to mimic the natural lipid layer.
  • Ointments and Gels: Used for severe cases or nighttime relief. CMC becomes a gel at a concentration of 1%1\%.

Preservatives and Delivery Systems

  • Vanishing Preservatives: These are active in the bottle but break down upon contact with the eye and light into innocuous components (CO2CO_2, H2OH_2O, or low-level H2O2H_2O_2). Examples: Purite (patented by Refresh/Allergan) and Sodium Perborate/DQST (patented by TheraTears).
  • Detergent Preservatives: Surfactants that kill microbes by breaking down cell walls.
    • Polyquaternium (Polyquad): Used by Alcon/Novartis in Systane/Genteal. Relatively safe but slightly more toxic than vanishing types.
    • Benzalkonium Chloride (BAK/BAC): Highly toxic to human ocular epithelial cells as well as microbes. Pharmacists should avoid substituting generics containing BAK for patients with dry eye.
  • Preservative-Free (PF) Single-Units: Small vials where the cap is snapped off for a single use. Recommended if a patient needs drops more than 4−64-6 times per day. Technically they should be discarded after one use, though some clinicians suggest placing the tip-protected vial in a clean container (like a tequila shot glass) for use within the same day.
  • Advanced Filters: Some brands (e.g., Oasis) use a 0.2 micron0.2\,\text{micron} filter on the bottle to deliver sterile, preservative-free drops from a multidose vial.

Clinical Selection Guidelines and Contraindicated Therapies

  • Step-wise Treatment Selection:
    • Step 1: Start with simple demulsants (CMC or Polyols) for 14 days14\,\text{days}. Incorporate non-pharmacologic changes.
    • Step 2: If insufficient, increase frequency up to 66 times/day or switch to single-unit preservative-free drops.
    • Step 3: Try formulations with lipids (emulsions) or advanced inactive ingredients (osmo-protectants).
    • Step 4: Use gels or ointments at bedtime. If still uncontrolled, refer to an optometrist/ophthalmologist.
  • Contraindicated Self-Care (Vasoconstrictors):
    • Avoid: Redness-clearing drops (e.g., Visine) containing Alpha-1 agonists like Tetrahydrozoline, Naphazoline, or Phenylephrine.
    • Risks: These cause rebound redness, potential capillary damage, and tolerance. They often contain BAK, which worsens dry eye.
  • Secondary Therapies: Lacrisert is a prescription HPMC insert placed in the conjunctival sac that releases moisture over time, but it is not widely popular.

Questions & Discussion

  • Inquiry 1: Identifying the Lacrimal Gland. Correct identification of the lacrimal gland is on the temporal side, while the red drainage canals are located on the nasal side.
  • Inquiry 2: Identifying Meibomian Glands. These are the vertical glands visible in the cross-section of the eyelid.
  • Inquiry 3: Secondary Lipid Sources. Glands of Zeiss and Mol produce small amounts of lipids and are associated with specific pathologies like hordeolum (styes).
  • Inquiry 4: Tear Film Layers. The consensus categorization is the outer lipid layer and the bulk aqueous-mucus layer.
  • Poll Response: Benzalkonium Chloride (BAK) is confirmed as the most toxic preservative. Purite is confirmed as the specific vanishing preservative patented by the Refresh brand.