Notes on Ophthalmic and Otic Medications from Fundamentals of Pharmacology for Veterinary Technicians

Chapter 18: Ophthalmic and Otic Medications

Basic Ocular System Anatomy and Physiology

Function of the Ocular System
- Responsible for vision.
- Comprises the eyes and adnexa (accessory structures).
Components of the Globe
  - Consists of three layers:
    - Sclera: The outermost layer of the eye.
    - Choroid: The middle layer containing blood vessels.
    - Retina: The innermost layer responsible for light detection.
Adnexa
  - Surrounding structures include:
    - Orbit: The bony cavity housing the eye.
    - Eye Muscles: Control the movement of the eye.
    - Eyelids: Protect the eye and distribute tears.
    - Eyelashes: Protect the eye from debris.
    - Conjunctiva: A mucous membrane covering the eye.
    - Lacrimal Apparatus: Contains glands and ducts for tear production and drainage.

Ophthalmic Drugs

Drug Penetration
- Ophthalmic medications must be absorbed into the anterior chamber for effectiveness.
Frequency and Ease of Drug Application
- Application frequency varies based on whether the drug is in ointment or solution form.
- Medications must be easy to administer to ensure client compliance.

Routes of Ophthalmic Drug Administration

Topical Solutions and Suspensions (Drops)
  - Effects: Affects cornea, conjunctiva, anterior uvea, eyelids, and nasolacrimal system.
  - Solutions: Least irritating but with the shortest contact time, requiring more frequent application.
  - Suspensions: Require shaking prior to use and may get caught in the delivery device.

Advantages of Topical Applications

Easy to administer to small animals.
Minimal visual impairment.
Lower incidence of contact dermatitis.

Disadvantages of Topical Applications

Difficult to administer for large animals.
Dosing imprecision may occur.
Limited contact time necessitates more frequent applications.
May be diluted by tear production.
Potential for systemic absorption.
Ointments
   - Advantages:
     - Longer contact time leading to less frequent administration.
     - Protects cornea from drying.
     - Generally more cost-effective than drops.
     - Not influenced by tear dilution.
   - Disadvantages:
     - Causes temporary blurring of vision.
     - More challenging to administer than drops.
     - May cause contact dermatitis.
     - Not suitable for use with penetrating corneal wounds.
     - Difficult to determine precise dosages.
Injections
  - Types:
    - Subconjunctival: Delivers drugs to the cornea and anterior uvea.
    - Retrobulbar: Targets the vitreous chamber and optic nerve.
    - Intracameral: Administers drugs into the anterior or vitreous compartment.
    - Intravitreal: Directly within the vitreous compartment.
Systemic Administration
- Allows drug penetration to areas such as eyelids, vitreous chamber, optic nerve, anterior uvea, and nasolacrimal system.
- Risks potential systemic toxicity and limited corneal penetration.

Ophthalmic Diagnostic Drugs

Topical Anesthetics
- Examples: Proparacaine, tetracaine.
- Used to facilitate comprehensive eye examinations and remove foreign materials.
Fluorescein Sodium
  - Applied to the cornea with sterile saline.
  - Used for assessing corneal defects (stain appears orange until adhering to corneal defects, turning green).
  - Must wash the stain from the eye post-examination.
Schirmer Tear Test (STT)
- Utilizes a filter paper strip marked with 1 mm increments, wetted by tears to diagnose keratoconjunctivitis sicca (KCS).
- The strip is placed in contact with the eye for 60 seconds.

Ophthalmic Pharmacological Categories

Mydriatics (Pupil Dilation)
- Drugs that dilate pupils; cycloplegics paralyze ciliary muscles to alleviate pain.
- Examples: Atropine, homatropine, phenylephrine (not a cycloplegic), tropicamide.
Miotics (Pupil Constriction)
- Constrict pupil size; treat open-angle glaucoma by enhancing aqueous humor outflow.
- Example: Pilocarpine.
Glaucoma
  - Defined as a group of diseases that elevate intraocular pressure; treatments aim to lower this pressure.
    - Pressure-Reducing Drugs:
      - Prostaglandins: Topically reduce intraocular pressure by improving aqueous humor outflow.
      - Examples: Latanoprost, bimatoprost, travoprost.
    - Carbonic Anhydrase Inhibitors: Reduce aqueous humor production by inhibiting carbonic acid formation.
      - Examples: Acetazolamide, dichlorphenamide, methazolamide, brinzolamide, dorzolamide.
    - Beta-Adrenergic Blockers: Decrease aqueous humor production.
      - Examples: Timolol maleate, betaxolol hydrochloride, carteolol, levobunolol, metipranolol.
    - Alpha-Adrenergic Agonists: Sympathomimetics that reduce aqueous humor secretion.
      - Examples: Apraclonidine, brimonidine.
    - Osmotic Diuretics: Reduce vitreous humor volume to quickly lower intraocular pressure.
      - Example: Mannitol.
Dry Eye Correctors
  - KCS leads to decreased tear production, causing conjunctivitis and potential corneal damage.
  - Treatment Drugs:
    - Immunomodulators: Affect interleukin production (e.g., cyclosporine, tacrolimus).
    - Lacrimogenics: Enhance tear production (e.g., pilocarpine).
    - Artificial Tears and Antibiotic-Glucocorticoid Preparations.

Other Ophthalmic Drugs for Ocular Diseases

Antibiotics:
- Gram Positive: Bacitracin, oxytetracycline with polymyxin B, aminoglycosides (gentamicin, tobramycin).
- Gram Negative: Polymyxin B, fluoroquinolones (ciprofloxacin, ofloxacin).
Antifungals:
- Examples: Natamycin, amphotericin B.
- Antivirals: Idoxuridine, trifluridine, acyclovir.
Corticosteroids:
- Used in combination with anti-infectives (e.g., prednisolone acetate, dexamethasone).
NSAIDs:
- Examples: Flurbiprofen, ketorolac.
Tear Supplements:
- Artificial tears, lubricants for symptomatic relief.

Basic Ear Anatomy and Physiology

Function of the Ear
- Sensory organ facilitating hearing and equilibrium maintenance.
Parts of the Ear
  - Outer Ear: Pinna and external auditory canal.
  - Middle Ear: Tympanic membrane, auditory ossicles, Eustachian tube, oval and round windows.
  - Inner Ear: Vestibule, cochlea, semicircular canals.
Otitis Interna
- Inner ear infection resulting in symptoms like head tilt, ataxia, nausea, and vomiting.

Otic Cleansing, Drying, and Dewaxing Agents

Cleansing/Drying Agents
- Often contain mild acids (glycolic, boric, malic, acetic) with possible soothing agents (e.g., aloe vera).
- Examples: Oti-Soothe, Epi-Otic, Oti-Clens, MalAcetic.
Topical Antiseptics
- Examples: Chlorhexidine, Mal-A-Ket Plus TrizEDTA Flush, Vetericyn Ear Rinse.
Ceruminolytic Agents
  - Types:
    - Mild (propylene glycol, glycerin)
    - Moderate (squalene)
    - Potent (docusate sodium, urea).
  - Examples: Milytic, Cerumene which may also contain benzyl alcohol or benzoic acid.

Otic Medications

Antibiotics:
  - Aminoglycosides: Gentamicin, neomycin.
  - Fluoroquinolones: Ciprofloxacin, ofloxacin.
  - Polymyxin B: Available in combination products.
  - Phenicols: Chloramphenicol, florfenicol.
Antiparasitics:
- Examples: Pyrethrins, Milbemycin Oxime, ivermectin, selamectin.
Antifungals:
- Examples: Clotrimazole, nystatin, miconazole, thiabendazole.
Corticosteroids (with anti-infectives):
- Examples: Fluocinolone, mometasone, dexamethasone, triamcinolone, hydrocortisone.

Ototoxicity

Describes damage to inner ear structures resulting in hearing impairment or vestibular dysfunction.
  - Can be due to ruptured tympanum or increased permeability through round window caused by infection.
  - Risk factors may include topical cleaners and medications as well as high doses of certain systemic drugs.
  - Potential for permanent damage to hearing.

Uses for various medications in both ocular and otic contexts, as well as considerations for administration, contraindications, and risks of systemic effects or infections, must be carefully evaluated and followed by veterinary technicians and practitioners.