Aqueous Humor Dynamics Flashcards
What are the two ways in which aqueous humor leaves the eye
- 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%)
Unconventional pathway for AH outflow
- 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
Conventional outflow of AH
- 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
Two models of conventional pathway
Bulk flow model
Pumping method
Bulk flow method
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
Pumping model of conventional pathway of AH outflow
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
AH produced from
Pars plicata
Conventional pathway and IOP
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
TM and AH outflow
- 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
Occlusive angle in diabetes
Neovascularisation from proliferating diabetic retinopathy
Occlusive angle in uveitis
Inflammatory cells and peripheral anterior synechiae
Unconventional pathway of AH outflow and IOP
IOP independent
Hypema and occlusive angel
Blood accumulates in the anterior chamber
Occlusive angle and pseufoexfoliative glaucoma
Aging epithelial cells of the iris and lens capsule can release pigment and pseudoexfoliative material
Occlusive angle and pigment dispersion glaucoma
Pigment is released from the posterior layer of the iris due to posterior bowing of the iris against the lens zonules
In humans, 75% of the resistance to the AH outflow is localized to the _____ and 25% occurs beyond ______
TM
Schlemms canal
Fuchs heterochromic iritis
Results in chronic inflammation that can cause TM permanently damaged
Glaucomatocyclitic crisis
Acute inflammation of the TM
Long term influence of IOP
Genetics
Gender
Refractive error
Short term influence of IOP
Diurnal Postural variation Lid and eye movement Systemic conditions Environmental conditions Food and drugs
Genetics and IOP
Hereditary influence
Gender and IOP
- equal between men and women 20-40
- in older ages, the apparent increase in mean IOP with age is more in women
Refractive error and IOP
- a positive correction between IOP and both axial length and incfressing degrees of myopia
- myopic people also habe a higher incidence of OAG
Diurnal variation in IOP
- fluctuates throughout the day
- ranges from 3-6mmHg
- highest in the morning
Peak IOP is in the ___ hours
Morning
Postural variation in IOP
- 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
Lid and eye movement and IOP
- 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
Systemic conditions and IOP
- 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
Environmental conditions and IOP
Cold reduces IOP
Reduced gravity increases IOP
Foods and drugs and IOP
- Alcohol, heroin, and marijuana lowers it
- caffeine and tobacco increases it
- corticosteroids increase it
Which pathway is the major pathway of the AH outflow
Corneascleral pathway
Which pathway is IOP dependent
Uveoscleral pathway
In uveoscleral pathway, AH passes through SC
False
Which condition most likely causes Ah outflow obstruction, mild DM or hyphema?
Hyphema
What model is a transient increase in IOP
Conventional pathway: pumping model
Constant high IOP
Schlemms canal can collapse on itself and obstruct entry into venous system
Injury to TM
Causes outflow obstruction
