3. DRUGS ACTING ON THE EYE

Written and reviewed by Dr. N. Sujith Kumar | Pharm.D Graduate from JNTUK | D.Pharmacy Academic Content Creator

DRUGS ACTING ON THE EYE: MIOTICS, MYDRIATICS, AND DRUGS USED IN GLAUCOMA: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future pharmacists and healthcare professionals!

The eye is a complex sensory organ responsible for eyesight. Any injury or disease to the ocular system can result in vision loss. In ophthalmological practice, eye problems can be treated with drugs—like local anaesthetics, antimicrobials, anti-inflammatory agents, immunomodulating agents, and drugs for diagnostic purposes and allergies—including the treatment of inflammation, infection, and reduction of intraocular pressure. As a pharmacology educator with years of experience teaching ocular pharmacology, I have observed that students often find this topic challenging due to the various drug classes and their specific actions on the eye. Let me tell you: Understanding drugs acting on the eye is essential for safe and effective ophthalmological practice.

In this comprehensive guide, I will walk you through the classification, pharmacological actions, doses, indications, and contraindications of miotics, mydriatics, and drugs used in glaucoma. By the end of this article, you will have a thorough understanding of these essential ocular pharmacological agents. Let us begin!

Dpharmguru’s exam insights:

Drugs acting on the eye are frequently tested in pharmacology exams. Remember: Miotics cause miosis (pupil constriction) and are used in glaucoma. Mydriatics cause mydriasis (pupil dilation) and are used for eye examinations. Glaucoma is associated with increased intraocular pressure (IOP). Drugs used in glaucoma include miotics, beta-blockers, carbonic anhydrase inhibitors, prostaglandin analogues, and osmotic agents. These are classic exam questions!

MIOTICS

Miotics are the category of drugs which cause constriction of the pupil (miosis). The pupil is constricted by parasympathetic nerve stimulation or sympathetic nerve endings blockage.

Drugs Used as Miotics

  • Echothiophate iodide
  • Physostigmine
  • Neostigmine
  • Demecarium bromide
  • Acetylcholine
  • Carbachol
  • Pilocarpine
  • Dibenamine

Pharmacological Action of Miotics

  • Muscarinic Agonists: Pilocarpine and other cholinergic drugs stimulate the parasympathetic nerve endings in the circular muscle of iris and constrict the pupil. Contraction of circular muscles sets the eye for near vision and lowers the intraocular pressure. Physostigmine and neostigmine have the same effects by their anticholinesterase action.
  • Relaxation of Radial Muscle of Iris: Dibenamine (an α-adrenergic receptor blocker) blocks sympathetic nerves and can lead to miosis, without affecting accommodation and IOP (intraocular pressure). Thus, it can safely be used in glaucoma.

Dose of Miotics

  • Pilocarpine nitrate: Sterile ophthalmic solution available as 1%, 2%, 4% drops
  • Carbachol: Available as 0.75%, 3% drops
  • Physostigmine: Available as a solution of 0.25%, 0.5% and an ointment containing 2% of the drug

Indications of Miotics

  • Used in glaucoma
  • Used to counter the effects of mydriatics after refraction testing
  • Alternate miotic and mydriatic is used to prevent or break adhesion between the iris and lens or iris and cornea due to iritis, corneal ulcer, etc.

Contraindications of Miotics

  • Hypersensitivity reactions
  • Iritis
  • Anterior chamber inflammatory disease
  • Neovascular glaucoma
  • Uveitic glaucoma

MYDRIATICS

Mydriatic agents are used to dilate the pupil so that an easier and more detailed examination of structures in the posterior segment of the eye can be performed.

Drugs Used as Mydriatics and Their Pharmacological Action

  • Tropicamide Hydrochloride: Used as mydriatic in lower concentration of 0.5% in single-dose units. Pupil dilates within a few minutes, maximum in ~30 minutes. Complete recovery in ~6 hours. 0.25% tropicamide is effective for mydriasis.
  • Phenylephrine Hydrochloride: Direct-acting sympathomimetic agent. 2.5% solution required. Pupil dilates within 10–20 minutes, maximum effects in ~60 minutes. Complete recovery in ~6 hours.
  • Atropine: Used as 1% solution. Persistent effect for about 7–12 days.
  • Scopolamine: Used as 0.5% solution. Shorter duration as compared to atropine.
  • Homatropine: Used as 1% or 2% solution. Less potent than atropine. Effect lasting for 1–3 days; accommodation recovers in 1 day. Most commonly used mydriatic in adults.
  • Cyclopentolate: Used as 1–2% solution. Potent and rapidly acting mydriatic. Cycloplegia lasts for 1 day.

Dose of Mydriatics

In a healthy adult, the first choice for mydriasis is 0.5% tropicamide or 2.5% phenylephrine. A combination of the two agents may also be used for satisfactory mydriasis. The combination solution having 0.5% tropicamide and 1.25% phenylephrine is enough to dilate pupils for internal ocular inspection.

Indications of Mydriatics

  • Used to determine refractive error. Homatropine is the most commonly used drug for this purpose.
  • Used for thorough examination of retina and optic nerve diseases.
  • Used in acute iritis, iridocyclitis, and keratitis.
  • Alternate mydriatics and miotics are used to prevent the development of or break adhesions between the iris and lens.

Contraindications of Mydriatics

  • Glaucomatocyclitic crises
  • Hypersensitivity
  • Narrow-angle glaucoma
  • Patients taking MAO inhibitors, tricyclic antidepressants, reserpine, guanethidine, or methyldopa
  • Patients with underlying asthma

GLAUCOMA

Glaucoma is defined as a progressive optic neuropathy causing damage to optic nerves that result in visual field defects. The disorder is progressive in nature and is usually associated with an increase in intraocular pressure (IOP)—normal IOP ranges from 10–20 mmHg.

Drugs Used in Glaucoma and Their Pharmacological Action

1. Osmotic Agents

These agents reduce intraocular pressure by drawing fluid from the eye into the circulation via osmotic effect.

  • Mannitol: 20% concentration; administered by IV infusion; dose of 1.5 g/kg body weight
  • Glycerol: 50% concentration; administered orally; dose of 1.5 g/kg

2. Carbonic Anhydrase Inhibitors

These agents reduce intraocular pressure by decreasing the formation of aqueous humor. They inhibit the enzyme carbonic anhydrase non-competitively.

  • Acetazolamide: Oral, IV (250 mg to 1000 mg per day)
  • Dorzolamide: Topical
  • Brinzolamide: Topical

3. β-Adrenergic Blockers