Pharmacology

Question 0

Echothiophate iodide

Answer 0

Echothiophate iodide (brand name: phospholine iodide) is the only indirect parasympathomimetic agent available on the market as of 2011. Indirect agents work by inhibiting the enzyme acetylcholinesterase, thereby enhancing the availability of naturally-released acetylcholine. Direct agents act directly on (and activate) acetylcholine receptors. Note: carbachol has both direct and indirect actions although its primary effect is via the direct route.

Question 1

Contraindicated in acute angle closure

Answer 1

This patient has the classic presentation of acute angle closure glaucoma. All of the medications listed are acceptable in order to lower her intraocular pressure except perhaps for apraclonidine (brand name: Iopidine). This medication belongs to the “relatively-selective” adrenergic agonists (the other medication being brimonidine). According to the BCSC Glaucoma book, apraclonidine may exacerbate angle closure via its dilation and iris ischemia effects. The BCSC states that this problem occurs more with apraclonidine than brimonidine because the former has more significant alpha-1-adrenergic activity. Note: “relatively-selective” adrenergic agonists means preferentially selective for the alpha-2-receptor (but apraclonidine is less selective for the alpha-2-receptor than brimonidine).

Other glaucoma medications that should be avoided are pilocarpine formulations with a higher concentration than 2% (e.g. pilocarpine 4%) since these higher concentrations may increase vascular congestion or further shallow the anterior chamber.

Question 2

Anesthetic gas high IOP

Answer 2

When diagnosing or following up a patient with congenital glaucoma, it is important to obtain an accurate IOP measurement. In infants, it may be difficult to measure IOP accurately without the aid of general anesthesia. The physician should remember that most general anesthetic agents will lower IOP thus obscuring the diagnosis of congenital glaucoma.

Therefore, it is important to measure IOP immediately after induction of general anesthesia when you perform an EUA. The exception to this rule is the anesthetic ketamine which will actually raise IOP.

Question 3

What is the proposed mechanism for the iris darkening side effect of latanoprost?

Answer 3

The side effect of iris darkening, which occurs uniquely with the prostaglandin analogs (PGAs), happens most frequently with eyes that are green-brown (79%) and yellow-brown (“hazel”, 85%) in color. In contrast, this iris darkening side effect is rare in pure blue irises. Unfortunately, this increased iris pigmentation is considered permanent.

This unique side effect is due to an increased number of melanosomes within the melanocytes of the iris. There is no evidence that this increased iris pigmentation is associated with an increased rate of carcinogenesis, iris inflammation, or pigment dispersion glaucoma.

Other side effects of the PGAs include conjunctival hyperemia, trichiasis, distichiasis, hypertrichosis, and hyperpigmentation of the eyelid skin. These side effects are considered reversible with discontinuation of the PGA. Other often-tested side effects of PGAs include increasing the risk of a herpes simplex keratitis flare-up and perhaps cystoid macular edema.

Question 4

Factors affecting IOP?

Answer 4

Alcohol, cannabinoids (decrease)
Anesthesia (decrease, except succinycholine & ketamine which increase)

Caffeine, LSD (increase)
Laying down (increase)
DM (increase, even without POAG)

Exercise, fluid intake, time of day