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Renin Angiotensin Mechanism.

Pharmacology of Drugs Acting on EYE.

Pharmacology of Drugs Acting on EYE.



  • Mydriatics: - “These are the drugs which when instilled topically in eye dilates the pupil. i.e. produces Mydriasis"  and are employed in the clinical examination of eyes and in certain inflammatory eye conditions.

  •   e.g. Anticholinergic- Tropicamide, Homatropine, Cyclopentolate
  •    Adrenergic- Phenylephrine, Ephedrine, Adrenaline

Mechanism:-
  • The pupil is supplied by parasympathetic & sympathetic nerve fiber.
  • The parasympathetic supply is through Occulomotor nerve & supplies to constrictor pupilae, while sympathetic supply is through superior cervical ganglion & supplies to dilator pupilae.
  • When Mydriatics are instilled in eye causes relaxation of constrictor pupilae. i.e. circular muscle fibres & causes tightening of dilator papillae i.e. radial muscle fibres.
  • This causes flattening of lens known as Mydriasis.
  • Because of Mydriasis the focal length is increased, i.e. persons can see the object which is far away but fails to see the object which is too near. The condition is called as "Cycloplegia".

Pupil -------> instillation of Mydriatics   ------->  relaxation of constrictor pupilae   & tightening of dilator pupilae   
  ------->       flattening of lense    ------->  Mydriasis  ------->  focal length increase  -------> cyclopegia.

  • Miotics: - “These are the drugs when instilled in eye, produces reduction in size of pupil. I.e. Miosis (constriction of pupil)
OR
  • “The drugs which cause constriction of pupil that are used to counteract Mydriatic effect & are used mainly in treatment of glaucoma are called as Miotics”
  •  E.g. Pilocarpine, Physostigmine.

Mechanism:-
  • The parasympathetic supply is through Occulomotor nerve & supplies to constrictor pupilae. i.e. circular muscle fibres.
  • When Miotics are instilled in eye causes contraction of constrictor pupilae i.e. circular muscle fiber & relaxation of dilator pupilae i.e. radial muscle fibre.
  •    This causes marked reduction in size of pupil which is referred as miosis.
  •    Because of Miosis, focal length of pupil is get reduced; person can see the objects which are near, but fails, to observe which are far away, this is termed as "spasm of accommodation."

Pupil ---->   instillation of Miotics -----> constriction of constrictor pupilae & relaxation of dilator pupilae ------->  reduction in size of pupil  -------> Miosis ------->  focal length decreases  ------->  spasm of accommodation

  • Cycloplegics: -“The drugs which induce mydriasis & are used to reduce spasm due to injury, inflammation or in surgery of eye are called as cycloplegics.
  • E.g. Cyclopentolate, Atropine, Hyoscine.

Classification of drugs acting on eye ball:-

              A) Mydriatics & cycloplegics

  1. Anticholinergic / Parasympatholytics
  • e.g.
  • Atropine, Homatropine, Cyclopentolate,
  • Tropicamide, Scopolamine, Hyoscine.
2. Sympathomimetics
  • e.g. Phenylephrine, ephedrine, adrenaline, cocaine.

               B) Miotics & Agents causing spasm of Accommodation

  1. Cholinergic / Parasympathomimetics.
  • e.g. Acetylcholine, Methacholine, Carbachol, Pilocarpine, Physostigmine, Edrophonium, Bethenechol.
2. Sympatholytic. e.g. Guanethidine, Tolazoline, Dibenamine

             C) Other Drugs

  1. Ophthalmic antimicrobials
  • Therapeutic uses- Fungal keratitis
  • Antifungal e.g. Miconazole, Ketoconazole, fluconazole,Polymixin B.
  • Therapeutic uses- various external bacterial infections of eye & eyelid
  • Antibacterial e.g.. boric acid, sulfacetamide, Silver sulphadiazine, antibiotics like Gentamycin, Tobramycin, Ofloxacin, Ciprofloxacin, Moxifloxacin, Framycetin.
  • Antiviral e.g. Acyclovir, Idoxuridine
  • Therapeutic uses- Herpes simplex keratitis
  1. Ophthalmic anti-inflammatory
  • e.g. Topical corticosteroids like Prednisolone,Hydrocortisone,Dexamethasone,Betamethasone
  • Therapeutic use – Inflammatory eye conditions like allergic conjunctivitis, Keratitis
  1. Ophthalmic antiallergic e.g. Sodium cromoglycate,Ketotifen
  • Therapeutic use – allergic conjunctivitis

Drugs used in the treatment of glaucoma           

1 Miotics e.g. Pilocarpine, Physostigmine
2) Beta blockers e.g. Timolol,Betaxolol,Levobunolol
3) Prostaglandin analog e.g. latanoprost.
4) Diuretics e.g. Acetazolamide,Mannitol,Dorzolamide
5) Adrenergic agonist e.g. Dipivefrine,Brimonidine
6) Ophthalmic irrigations-These are electrolyte solutions used for cleansing of eyes during & after eye surgery
(Sodium hyaluronate is used to keep eye moist in xerophthalmia)
7) Ocular Lubricants
e.g. Hydroxy propyl methyl cellulose (HPMC), Carboxy methyl cellulose (CMC)
Therapeutic uses- To relieve dryness & irritation of eyes

Glaucoma

  • Glaucoma-It is an ocular disease characterized by elevated intraocular pressure which causes damage to optic nerve there by producing visual loss.
  • The damage to optic nerve & visual loss is probably due to ischemia & may be due to direct pressure on optic nerve.
  • It is of two clinical forms….
  1. Open angle glaucoma
  2. Closed angle glaucoma

Open-angle glaucoma-

  • It occurs due to degeneration of trabeculae leading to diminished reabsorption of aqueous humour.
  • Treatment-It is treated by promoting drainage of aqueous humour or by reducing its secretion.

  1. Topical beta blockers e.g. Timolol,Betaxolol,Levobunol
  • These are 1St choice drug.
  • They cause reduction of IOP by decreasing the production of aqueous humor.
  • Ocular side effects are burning of eye, dryness of eye, blurred vision.
  1. Adrenergic agonist e.g. Brimonidine, Dipivefrine.
  • These are 2nd choice drug. They reduce intraocular pressure by decreasing aqueous humor production & enhancing outflow.
  • Carbonic anhydrase inhibitor e.g. Acetazolamide (dose 250mg BD orally)
  • It is alternative second choice drug to brimonidine.
  • It reduces aqueous humour production by limiting generation of bicarbonate ion.

Closed-angle glaucoma-

  • It occurs due to congestion in iridocorneal angle.
  • It is emergency condition needs to reduce intraocular pressure.
Treatment-
  1. Topical beta blockers – Timolol 0.5% solution  is instilled 8 hourly
  2. Miotic- Pilocarpine 1-2% is instilled every 10 minutes but produces spasm of accommodation (last for 2-3 days) & blurred vision.
  3. Hypertonic Mannitol 20% is infused to decongest eye by osmotic pressure along with this Acetazolamide (dose 500mg BD orally) started simultaneously.
  4. Brimonidine 0.2% instillation may be added in severe cases.
  5. Definitive treatment is surgery (Iridotomy) after termination of attack.

Drugs Contraindicated in glaucoma

  1. Anticholinergic like atropine, tropicamide, homatropine & cyclopentolate are contraindicated in glaucoma as they precipitate closed angle glaucoma.

 

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