Ophthalmology: Aqueous Humor Formation and Intraocular Pressure

Intraocular Pressure (IOP)

• IOP is the fluid pressure inside the eye.
• Normal eye pressure ranges from 12-22 mm Hg, and eye pressure greater than 22 mm Hg is considered higher than normal.

Aqueous Humor Formation and Drainage

• Aqueous humor is produced by the ciliary body.
• Most of the aqueous humor exits the eye at the angle formed by the junction of the iris and cornea.
• It exits primarily via the trabecular meshwork and the Schlemm canal, and to a lesser extent through the ciliary body face entering the uveoscleral pathway.

Composition of Aqueous Humor

• Aqueous humor resembles blood plasma in composition but contains less protein and glucose and more lactic acid and ascorbic acid.
• It also contains amino acids, electrolytes (pH = 7.4, sodium, potassium, calcium, magnesium, chloride, HCO3-, phosphate), and immunoglobulins.

Glaucoma

• Glaucoma is a multi-factorial, complex eye disease with specific characteristics such as optic nerve damage and visual field loss.
• While increased pressure inside the eye (IOP) is usually present, even patients with normal range IOP can develop glaucoma.

Types of Glaucoma

• Open-angle glaucoma and angle-closure glaucoma are the most common forms of glaucoma.

Drug Treatment of Glaucoma

• Reducing IOP in glaucoma patients includes:
  • Topical β-adrenergic antagonists (e.g., timolol, betaxolol)
  • Carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide)
  • Cholinergics drug (e.g., pilocarpine)
  • α-Adrenergic agonists (e.g., brimonidine)
  • Prostaglandins (e.g., latanoprost, travoprost)
  • Prostamides (bimatoprost)

Mechanisms of Action for Glaucoma Drugs

• β-Blockers:
  • Decrease IOP by blockade of sympathetic nerve endings in the ciliary epithelium, reducing aqueous production.
• Carbonic anhydrase inhibitors:
  • Lower IOP by reducing aqueous production.
• Cholinergic agonists:
  • Increase aqueous outflow through the trabecular meshwork by ciliary muscle contraction and stimulating the iris sphincter muscle.
• α-Adrenergic agonists:
  • Lower IOP by decreasing aqueous secretion and enhancing aqueous outflow through the uveoscleral route.
• Prostaglandins:
  • Lower IOP by enhancing uveoscleral outflow.
• Prostamides:
  • Lower IOP by increasing outflow of the aqueous humor via the trabecular meshwork and uveoscleral pathways.

Mydriatic Drugs

• Mydriasis is the dilation of the pupil, used during an eye examination to allow visualization of the retina and other structures deep within the eye or to treat inflammatory eye conditions such as iritis and cyclitis.

Iris Innervation

• The iris has two muscles that control light intensity:
  • Dilator pupillae, a longitudinal radial muscle that dilates the pupil (mydriasis) in the dark, innervated by the sympathetic nervous system.
  • Constrictor pupillae, a circular muscle that constricts the pupil (miosis) in places with good lighting, innervated by the parasympathetic nervous system.

Anticholinergic Drugs

• Atropine:
  • Blocks muscarinic receptors in the Constrictor pupillae and causes relaxing of circular muscles (mydriasis).
• Tropicamide and cyclopentolate:
  • Used as ophthalmic solutions for mydriasis and cycloplegia, with a shorter duration of action than atropine.

Intraocular Pressure (IOP)

• IOP is the fluid pressure inside the eye.
• Normal eye pressure ranges from 12-22 mm Hg, and eye pressure greater than 22 mm Hg is considered higher than normal.

Aqueous Humor Formation and Drainage

• Aqueous humor is produced by the ciliary body.
• Most of the aqueous humor exits the eye at the angle formed by the junction of the iris and cornea.
• It exits primarily via the trabecular meshwork and the Schlemm canal, and to a lesser extent through the ciliary body face entering the uveoscleral pathway.

Composition of Aqueous Humor

• Aqueous humor resembles blood plasma in composition but contains less protein and glucose and more lactic acid and ascorbic acid.
• It also contains amino acids, electrolytes (pH = 7.4, sodium, potassium, calcium, magnesium, chloride, HCO3-, phosphate), and immunoglobulins.

Glaucoma

• Glaucoma is a multi-factorial, complex eye disease with specific characteristics such as optic nerve damage and visual field loss.
• While increased pressure inside the eye (IOP) is usually present, even patients with normal range IOP can develop glaucoma.

Types of Glaucoma

• Open-angle glaucoma and angle-closure glaucoma are the most common forms of glaucoma.

Drug Treatment of Glaucoma

• Reducing IOP in glaucoma patients includes:
  • Topical β-adrenergic antagonists (e.g., timolol, betaxolol)
  • Carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide)
  • Cholinergics drug (e.g., pilocarpine)
  • α-Adrenergic agonists (e.g., brimonidine)
  • Prostaglandins (e.g., latanoprost, travoprost)
  • Prostamides (bimatoprost)

Mechanisms of Action for Glaucoma Drugs

• β-Blockers:
  • Decrease IOP by blockade of sympathetic nerve endings in the ciliary epithelium, reducing aqueous production.
• Carbonic anhydrase inhibitors:
  • Lower IOP by reducing aqueous production.
• Cholinergic agonists:
  • Increase aqueous outflow through the trabecular meshwork by ciliary muscle contraction and stimulating the iris sphincter muscle.
• α-Adrenergic agonists:
  • Lower IOP by decreasing aqueous secretion and enhancing aqueous outflow through the uveoscleral route.
• Prostaglandins:
  • Lower IOP by enhancing uveoscleral outflow.
• Prostamides:
  • Lower IOP by increasing outflow of the aqueous humor via the trabecular meshwork and uveoscleral pathways.

Mydriatic Drugs

• Mydriasis is the dilation of the pupil, used during an eye examination to allow visualization of the retina and other structures deep within the eye or to treat inflammatory eye conditions such as iritis and cyclitis.

Iris Innervation

• The iris has two muscles that control light intensity:
  • Dilator pupillae, a longitudinal radial muscle that dilates the pupil (mydriasis) in the dark, innervated by the sympathetic nervous system.
  • Constrictor pupillae, a circular muscle that constricts the pupil (miosis) in places with good lighting, innervated by the parasympathetic nervous system.

Anticholinergic Drugs

• Atropine:
  • Blocks muscarinic receptors in the Constrictor pupillae and causes relaxing of circular muscles (mydriasis).
• Tropicamide and cyclopentolate:
  • Used as ophthalmic solutions for mydriasis and cycloplegia, with a shorter duration of action than atropine.