Aqueous Humor Dynamics

Question 1

What are the two ways in which aqueous humor leaves the eye

Answer 1

• unconventional pathways: through the ciliary muscle and other downstream tissues (minor 5-35%)
• conventional pathway: though the trabecular mesh work (TM) and Schlemm’s canal (SC) (major 65-95%)

Question 2

Unconventional pathway for AH outflow

Answer 2

• uveoscleral outflow
• minor route (5-35%)
• AH enters the connective tissue between the ciliary muscle bundles, through the suprachoroidal space, and out through the sclera
• to fairly constant amount not affected by IOP
• the outflow rate through this route tends to decrease with age

Question 3

Conventional outflow of AH

Answer 3

• corneoscleral pathway
• major route (65-95%)
• AH passes through the TM, across SC, into its lumen and into draining collector channels, aqueous veins and epislercal veins
• two models
• IOP dependent

Question 4

Two models of conventional pathway

Answer 4

Bulk flow model

Pumping method

Question 5

Bulk flow method

Answer 5

A simplest hydraulic model of conventional pathway

• classic Goldmann equation
• deltP=IOP-EVP
• Fout=Trab(IOP-EVP)+UVEO
• an acute rise in EVP results in a 1:1 ratio of increased IOP

Question 6

Pumping model of conventional pathway of AH outflow

Answer 6

The aqueous outflow pump receives power from transient increases in IOP such as occurs in systole of the cardiac cycle during blinking and during eye movement

Question 7

AH produced from

Answer 7

Pars plicata

Question 8

Conventional pathway and IOP

Answer 8

IOP dependent

• increase of IOP caus drainage increase
• with constant very high IOP, Schlemm’s canal can collapse on itself and obstruct entry into the venous system

Question 9

TM and AH outflow

Answer 9

• the TM is suspended between two fluid compartments (anterior chamber and Schlemm’s canal) at different pressures
• the TM can sense the pressure differential and strives to maintain these parameters within a homeostatic range

Question 10

Occlusive angle in diabetes

Answer 10

Neovascularisation from proliferating diabetic retinopathy

Question 11

Occlusive angle in uveitis

Answer 11

Inflammatory cells and peripheral anterior synechiae

Question 12

Unconventional pathway of AH outflow and IOP

Answer 12

IOP independent

Question 13

Hypema and occlusive angel

Answer 13

Blood accumulates in the anterior chamber

Question 14

Occlusive angle and pseufoexfoliative glaucoma

Answer 14

Aging epithelial cells of the iris and lens capsule can release pigment and pseudoexfoliative material

Question 15

Occlusive angle and pigment dispersion glaucoma

Answer 15

Pigment is released from the posterior layer of the iris due to posterior bowing of the iris against the lens zonules

Question 16

In humans, 75% of the resistance to the AH outflow is localized to the _____ and 25% occurs beyond ______

Answer 16

TM

Schlemms canal

Question 17

Fuchs heterochromic iritis

Answer 17

Results in chronic inflammation that can cause TM permanently damaged

Question 18

Glaucomatocyclitic crisis

Answer 18

Acute inflammation of the TM

Question 19

Long term influence of IOP

Answer 19

Genetics
Gender
Refractive error

Question 20

Short term influence of IOP

Answer 20

Diurnal
Postural variation
Lid and eye movement 
Systemic conditions 
Environmental conditions 
Food and drugs

Question 21

Genetics and IOP

Answer 21

Hereditary influence

Question 22

Gender and IOP

Answer 22

• equal between men and women 20-40

- in older ages, the apparent increase in mean IOP with age is more in women

Question 23

Refractive error and IOP

Answer 23

• a positive correction between IOP and both axial length and incfressing degrees of myopia
• myopic people also habe a higher incidence of OAG

Question 24

Diurnal variation in IOP

Answer 24

• fluctuates throughout the day
• ranges from 3-6mmHg
• highest in the morning

Question 25

Peak IOP is in the ___ hours

Answer 25

Morning

Question 26

Postural variation in IOP

Answer 26

• IOP increases when changing from sitting to the supine position, average pressure difference between 0.3-0.6mmHg
• patient with systemic HTN have greater IOP increase after 15 minutes in supine

Question 27

Lid and eye movement and IOP

Answer 27

• blinking has been shown to rise the IOP 10mmHg, while hard lid squeezing may raise it as high as 90mmHg
• there is an increase in IOP on up-gaze in normal individuals

Question 28

Systemic conditions and IOP

Answer 28

• thicker corneas cause artificially high readings while thinner corneas cause artificially low read gains
• reduced during pregnancy (progesterone)
• HIV have lower than normal mean IOPs

Question 29

Environmental conditions and IOP

Answer 29

Cold reduces IOP

Reduced gravity increases IOP

Question 30

Foods and drugs and IOP

Answer 30

• Alcohol, heroin, and marijuana lowers it
• caffeine and tobacco increases it
• corticosteroids increase it

Question 31

Which pathway is the major pathway of the AH outflow

Answer 31

Corneascleral pathway

Question 32

Which pathway is IOP dependent

Answer 32

Uveoscleral pathway

Question 33

In uveoscleral pathway, AH passes through SC

Answer 33

False

Question 34

Which condition most likely causes Ah outflow obstruction, mild DM or hyphema?

Answer 34

Hyphema

Question 35

What model is a transient increase in IOP

Answer 35

Conventional pathway: pumping model

Question 36

Constant high IOP

Answer 36

Schlemms canal can collapse on itself and obstruct entry into venous system

Question 37

Injury to TM

Answer 37

Causes outflow obstruction