Flashcards in Terminology, Epidemiology, Heredity, IOP and Aqueous Dynamics Deck (32):
location of outflow obstruction in POAG
Top two leading causes of blindness worldwide?
#1 cataract, #2 glaucoma
race with highest relative risk of primary angle closure glaucoma?
Inuit (20-40 x higher than whites)
gene and locus for juvenile and adult open angle glaucoma?
TIGR/MYOC gene @ locus GLC1A
gene and locus for congenital glaucoma?
CYP1B1 @ GLC3A
gene for normal tension glaucoma?
gene for Axenfeld Rieger syndrome
gene for pseudoexfoliation syndrome
what is the Goldmann equation
P = (F-U)/C + Pv
F: aqueous humor formation
U: uveoscleral outflow
C: outflow facility (reflects trabecular outflow)
Pv: episcleral venous pressure
Where is aqueous fluid produced?
the inner nonpigmented epithelium of the ciliary body
Name and describe the 3 methods of aqueous fluid formation. Which accounts for the majority of aqueous production?
Active secretion (ATP-dependent; produces majority of aqueous), simple diffusion (passive movement of ions based on charge and concentration), ultrafiltration (pressure dependent)
Compared to plasma, what is the relative concentration of the following substances in aqueous fluid: hydrogen, chloride, ascorbate, bicarbonate, protein.
What is the relative pH of aqueous?
-Excess of hydrogen, chloride, and ascorbate
-Deficiency of bicarbonate and protein (flare represents protein, and flare is an abnormal finding on exam because there is very little natural protein in aqueous)
-acidic (more hydrogen and less bicarb = more acidic, about 7.22 pH compared to 7.40 of plasma)
1. What is the average production rate of aqueous fluid?
2. What is the average volume of the anterior chamber?
3. What is the average volume of the posterior chamber?
4. How long does it take to turnover all of the aqueous fluid in the eye?
1. 2.5-3.0 microliters/minute.
2. Anterior chamber: 250 microliters
3. Posterior Chamber: 60 microliters
4. About 100 to 120 minutes: (250uL+ 60uL) / (2.5 to 3.0 uL/min) ~= 103 to 124 min
What is a normal C value in the Goldmann equation (facility of outflow)? Would is the relative C value of a POAG patient?
0.22 to 0.3 uL/min/mmHg. LOWER in POAG (less outflow)
What are the two major outflow pathways for aqueous fluid? Which one is pressure-dependent?
trabecular and uveoscleral. trabecular is pressure dependent
Where is the primary site of resistance in aqueous outflow?
the juxtacanalicular trabecular meshwork
What are the three layers of the trabecular meshwork?
uveal, corneosceral, and juxtacanalicular
Describe the trabecular outflow pathway of aqueous fluid, starting at the site of production.
Produced in posterior chamber by inner nonpigmented ciliary body epithelium, travels through the pupil into the anterior chamber, through the trabecular meshwork into Schlemm's canal, into episcleral veins, into the anterior ciliary and superior ophthalmic vein, and into the cavernous sinus.
At what IOP will you see blood in Schlemm's canal?
IOP less than episcleral venous pressure
Describe the general pathway of nontrabecular (uveoscleral) aqueous outflow
pressure independent outflow, mostly through the ciliary body into the supraciliary and suprachoroidal space
Which medications increase uveoscleral outflow? Which meds decrease?
- Cycloplegics (paralyzing CB leads to less tense, more leaky muscle)
- Adrenergic agents (adrenergics impede CB contraction/accommodation making muscle less tense and more leaky, where parasympathetics increase CB contraction/accommodation)
-Prostaglandin analogs (inflammatory mediators lead to increased muscle tissue permeability)
Decrease: miotics, parasympathetics
What is tonography?
A method used to measure the facilty of aquoues outflow
What is normal episceral venous pressure?
What are the mean and standard deviation of IOP as determined by pooled data from large Western epidemiologic studies?
What statistical distribution does IOP follow?
15.5 and 2.6 (2 standard deviations away from the mean is 20.7, therefore 21 is often used as the cutoff for ocular hypertension)
Non-Gaussian with a skew towards higher IOPs
Indicate whether the following conditions/factors increase or decrease IOP:
valsalva, blepharospasm, exercise, pregnancy, increased age, alcohol, LSD, ketamine, heroin, marijuana, succinylcholine
Increase: valsalva, blepharospasm, LSD, ketamine, increased age
Decrease: exercise, pregnancy, alcohol, heroin, marijuana, succinylcholine
What is a normal IOP fluctuation in a 24-hour period?
What fluctuation would suggest glaucoma?
10 mmHG or greater fluctuation suggests glaucoma
When is the peak IOP in most patients?
early morning hours
What is the Imbert-Fick principle, and how does it apply to glaucoma?
States that the pressure in a dry, thin walled sphere is equal to the force necessary to flatten its surface area divided by the area of flattening (P = F / A). This principle is used in applanation tonometry, where the force applied by the tonometer divided by the area of the tonometer head is equal to the IOP.
What is the diameter of a Goldmann applanation tonometer?
What are the effects of each of the following on applanation readings: Wide and narrow mires, corneal edema, corneal scar, soft contact lens, scleral buckle, CCT
Wide mires, corneal scars, and thick CCT overestimate IOP
Narrow mires, corneal edema, SCL, scelral buckles, and thin CCT underestimate IOP
Name 2 ways to accurately applanate a patient with marked corneal astigmatism
1. Rotate the prism so that the red mark on the prism holder is aligned with the least curved meridian of the cornea (along the negative axis).
2. Take 2 measurements 90 degrees apart and average them