Week 5: Pharmacology of the eye

Question 1

What are the effects of muscarinic agonists and antagonists on pupil diameter, close vision accommodation, and light reflex?

Answer 1

Muscarinic blocker (atropine or tropicamide)
-mydriasis
-absent light reflex
-cycloplegia: inability to focus on close objects bc accommodation is blocked
Muscarinic agonists
-miosis
-light reflex present
-can accommodate

Question 2

What are the effects of alpha-1 agonists and antagonists on pupil diameter, close vision accommodation, and light reflex?

Answer 2

alpha-1 agonist (phenyephrine)
-mydriasis
-light reflex present 
-no effect on accommodation
alpha-1 antagonist
-miosis
-light reflex present
-no effect on accommodation
Sympathetic plays no role in light reflex or accommodation

Question 3

Compare and contrast the use of tertiary and quaternary amine cholinesterase inhibitors for drugs that are applied to the eye.

Answer 3

• Tertiary amine ChE inhibitor can be used to allow Ach to compete with tropicamide (muscarinic antagonist) and reverse its effects
• tertiary is more effective that quaternary because tertiary are lipid soluble and can penetrate to act on iris and muscles supplying the lens

Question 4

Diagram fluid flow in the eye.

Answer 4

1. aqueous forms in the ciliary body
2. Fluid flows behind the iris and through the pupil into the anterior chamber
3. fluid enters the anterior chamber through the pupil and flows out through trabecular meshwork and drains through Canal of Schlemm

Question 5

What are the major differences between open angle and angle closure glaucoma

Answer 5

Open Angle
-trabecular meshwork is clogged–>fluid pressure increases
Angle closure
-small anterior chamber depth
-when pupil dilates, iris pushed forward against trabecular meshwork

Question 6

List the classes of drugs and their mechanism of action in treating open angle and angle closure glaucoma

Answer 6

Treatment of angle closure glaucoma
-pilocarbine and physosigmine to induce miosis to pull iris forward on lens and away from trabecular meshwork
Treatment of open angle glaucoma
1. parasympathomimetics e.g. Pilocarbine, and ChE inhibitor e.g. physostigmine: lower intraocular pressure by increasing outflow via stimulating ciliary muscle contraction–>traction on scleral spur–>increases trabecular network porosity (press on tear duct to prevent systemic absorption)
2. carbonic anhydrase inhibitors: inhibit aqueous formation
3. Beta blockers: unknown method
4. prostaglandin agonist (latanoprost): increases aqueous outflow, also causes longer eyelashes and iris pigmentation
5. alpha-2 agonists: decreases sympathetic
6. marijuana