IOP and Aqueous Dynamics Flashcards
What is the pathway of Aqueous outflow?
Produced in non pigmented CB epithelium -> enters PC -> AC -> TM ->Schlemm Canal -> Collector Channel -> Venous plexus -> Suprachoroidal space
What is the Goldmann Equation?
Allows for IOP to calculated
= (rate of aqueous formation/facility of outflow) + episceral venous pressure
How does resistance to aqueous outflow relate to facility of outflow?
Resistence to outflow equals the inverse of the facility of outflow
Where is the aqueous produced?
Aqueous is thought be produced in the numerous mitochondria and microvilli in the non pigmented epithelium of the CB
How does the aqueous humor enter the posterior chamber (PC) from the CB?
Active Transport
Ultrafiltration
Simple Diffusion
How does the active transport of aqueous humor function?
The exact mechanism is unknown, but is thought to involve Na, Cl, and HCO3.
Accounts for majority of aqueous that enters PC
What is ultrafiltration?
a pressure-dependent movement along a pressure gradient
How does the aqueous humor compare to plasma in terms of ionic concentration?
It contains excess of H+ and Cl- ions, ascorbic acid
It has a deficit of HCO3
It is protein free
Why is the aqueous essentially devoid of proteins?
The lack of protein allows for optical clarity, and is a product of an intact blood-aqueous barrier
What is the rate of aqueous humor production?
2-3 uL/min
What is fluorophotometry?
it is the most common method by which the rate of aqueous humor formation is measured
What volume of aqueous humor is turned over per minute?
1% per minute
What factors affect the formation of the aqueous humor?
- integrity of blood-aqueous barrier
- blood flow to CB
- neurohormonal regulation of vascular tissues and ciliary epithelium
What is the average facility of outflow in the eye (relating to aqueous humor dynamics)? What affect does age have on it?
0.22-0.3 uL/Min/mm Hg
It decreases with age
How many parts make up the trabecular meshwork (TM)?
three parts.
1- uveal portion
2- corneoscleral portion
3- juxtacanalicular
Describe the uveal portion of the TM
located adjacent to the AC
arranged in bands that extend from iris root/CB to peripheral cornea
Describe the corneoscleral portion of the TM
consists of sheets of trabeculum that extend from scleral spur to the lateral wall of the scleral sulcus
Describe the juxtacanalicular portion of the TM
located adjacent to, and actually forms the inner wall of schlemm canal
thought to be the major site of resistance to outflow
What is the function of the TM?
the TM functions as a one-way valve controlling the outflow of aqueous humor from the eye
It is the site of pressure-dependent outflow
What is the Schlemm Canal?
It is a single canal completely lined by endothelium with a diameter of 370 uM and traversed by tubules
Connected to the episcleral venous plexus via a complex system of channels
Describe the outflow of aqueous via the TM pathway
TM -> Schlemm Canal -> collector channels -> Episcleral Venous Plexus -> Anterior Ciliary and Superior Ophthalmic Veins -> Cavernous Sinus
Describe uveoscleral outflow from the eye
Any outflow not occurring via the TM
Fluid exits the eye via intact sclera or along nerves/vessels that penetrate it (emmisaria?)
What are some factors that affect uveoscleral outflow?
Increased Uveoscleral outflow: - Cycloplegia
- adrenergic agents - prostaglandin analogs - complications of surgery
Decrease Uveoscleral Outflow: - Miotics
What is Tonography?
Measures the facility of outflow
What is the normal range of episcleral venous pressure?
8-10 mm Hg
What is the relationship of episcleral venous pressure (EVP) to IOP according to the Goldmann equation?
According to the equation, every 1 mm Hg increase in the EVP causes a 1 mm Hg increase in IOP
What effect does high EVP have on aqueous outflow?
Abnormally high EVP causes schlemms canal to collapse and increase the resistance to aqueous outflow
What is average IOP? What kind of distribution does IOP have among the population?
Average IOP= 15.5 mm Hg
There is a non-Gaussian distribution with a skew toward higher pressures
What are factors that affect IOP?
Time of day (2/2 natural diurnal variation) Heart Rate Respiration Exercise Fluid Intake Systemic/Topical Rx
What affect does EtOH have on IOP? Cannabis?
EtOH transiently lowers IOP
Cannabis decrease IOP, but has a very short duration of action
What is the normal diurnal variation for IOP? What level of variation is suggestive of glaucoma?
2-6 mm Hg
Variations >10 mm Hg are suggestive of glaucoma
What principle is tonometry (measure of IOP) based off of?
the Imbert-Fick Principle (= pressure inside an ideal dry thin-walled sphere is equal to the force necessary to flatten its surface, divided by the area of flattening)
What is the Imbert-Fick Principle?
states that the pressure inside an ideal, dry, thin-walled sphere is equal to the force necessary to flatten its surface divided by the area of flattening
What does the Golmann Applanation Tonometer measure?
It measures the force needed to flatten the cornea with a diameter of 3.06 mm
It is the most valid and reliable method
What kind of staining pattern occurs in a patient with astigmatism?
an ellipsoid pattern of staining occurs
How do you obtain accurate pressures in astigmatic patients?
to obtain an accurate reading, you rotate the prism so the red mark is aligned with the least curved median of the cornea.
Or, average two readings taken 90 degrees apart
What affect does scleral rigidity have on IOP measurements? Give an example of when this would occur
Decreased scleral rigidity causes an artificially low reading.
This would occur in a patient with a scleral buckle
What affect does central corneal thickness (CCT) have on IOP?
In patients with thick CCT, it causes an artificially elevated IOP
According to the Ocular Hypertension Treatment Study, what relationship does CCT have with Glaucoma?
The study found that decreased CCT is a predictive factor for the development of glaucoma in patients with ocular hypertension
Patients with CCT< 535 uM have a 3x increased risk of developing glaucoma
