Aqueous Humor Lecture PDF

PHYSIOLOGY OF AQUEOUS HUMOR & FACTORS MAINTAINING NORMAL IOP AQUEOUS HUMOR Clear, colorless fluid that fills the anterior and posterior chambers of the eye. Physiological properties Volume 0.31ml Refractive index 1.333 PH 7.2 Rate of formation 1.5 to 4.5...

PHYSIOLOGY OF AQUEOUS HUMOR & FACTORS MAINTAINING NORMAL IOP AQUEOUS HUMOR Clear, colorless fluid that fills the anterior and posterior chambers of the eye. Physiological properties Volume 0.31ml Refractive index 1.333 PH 7.2 Rate of formation 1.5 to 4.5 µl/min  Composition Water constitutes 99.9% of Normal Aqueous Proteins (5-16mg/100ml) concentration in Aqueous is less than 1% of its plasma concentration Glucose – 75% of the plasma concentration. Electrolytes: – Na+ similar in plasma and aqueous – Bicarbonate ion: Concentration in PC & in AC – Cl ion concentration than plasma and phosphate concentration than plasma Ascorbic acid concentration is very high in aqueous. Various components of the coagulation and anticoagulation pathways may be present in human aqueous humor.  Functions of aqueous humor Brings oxygen and nutrients to cells of lens, cornea, iris Removes products of metabolism and toxic substances from those structures Provides optically clear medium for vision Inflates globe and provides mechanism for maintaining IOP High ascorbate levels protect against ultraviolet-induced oxidative products, e.g., free radicals Facilitates cellular and humoral responses of eye to inflammation and infection The blood–aqueous barrier Barriers to the movement of substances from the plasma to the aqueous humor. In the ciliary body the barriers include – Vascular endothelium – Stroma – Basement membrane – Pigmented and non-pigmented epithelium. Zona occludens  The blood–aqueous barrier is responsible for differences in chemical composition between the plasma and the aqueous humor. Aqueous humor dynamics Secreted by ciliary epithelium lining the ciliary processes Enters the posterior chamber. It then flows around the lens and through the pupil into the AC. It leaves the eye by two pathways at the anterior chamber angle: – Through the TM, across the inner wall of Schlemm's canal into its lumen, and thence into collector channels, aqueous veins, and the episcleral venous circulation – the trabecular or conventional route – Across the iris root, uveal meshwork, and the anterior face of the ciliary muscle, through the  Aqueous humor formation Aqueous humor is produced from pars Lens plicata along the crests of the ciliary processes. Aqueous humor is derived from plasma within the capillary network of the ciliary processes. Pars plicata Three physiologic processes contribute to the formation and chemical composition of the aqueous humor: – Diffusion – Ultrafiltration – Active secretion. Diffusion Diffusion is the movement of substance across a membrane along concentration gradient. As aqueous humor passes from the PC to Schlemm’s canal, it is in contact with ciliary body, iris, lens, vitreous, cornea, and trabecular meshwork. There is diffusional exchange, so that the AC aqueous humor resembles plasma.  Ultrafiltration The process by which fluid and its solutes cross semipermeable membrane under pressure gradient is called ultrafiltration. As blood passes through capillaries of the ciliary processes, about 4% of the plasma filters through capillary wall into the interstitial spaces between the capillaries and the ciliary epithelium. In the ciliary body, fluid movement is favored by the hydrostatic pressure difference between the capillary pressure and the interstitial fluid pressure and is resisted by the difference between the oncotic pressure of the plasma and the Active transport Active transport is energy-dependent process that selectively moves substance against its electrochemical gradient across a cell membrane. It is postulated that majority of aqueous humor formation depends on active transport. It is done by non-pigmented epithelial cells Basic Physiologic Processes Accumulation of Plasma Reservoir – Most plasma substances pass easily from the capillaries of the ciliary processes, across the stroma, and between the pigmented epithelial cells before accumulating behind the tight junctions of the nonpigmented epithelium. – This movement takes place primarily by diffusion and ultrafiltration.  Transport across Blood-Aqueous Barrier – Active secretion is a major contributor to aqueous humor formation. – Selective transcellular movement of certain cations, anions, and other substances across the blood-aqueous barrier formed by the tight junctions between the nonpigmented epithelium. – Aqueous humor secretion is mediated by transferring NaCl from ciliary body stroma to PC with water passively following.  Biochemistry of aqueous humor formation The structural basis for aqueous humor secretion is the bilayered ciliary epithelium.(pigmented epithelium & non- pigmented epithelium ) The active process of aqueous secretion is mediated by two enzymes present in the NPE: Na+- K+-ATPase and carbonic anhydrase AQUEOUS HUMOR OUTFLOW The aqueous humor leaves the eye at the anterior chamber angle through trabecular meshwork, the Schlemm’s canal, intrascleral channels, and episcleral and conjunctival veins. This pathway is referred to as the conventional or trabecular outflow. In the unconventional or uveoscleral outflow, aqueous humor exits through the root of iris, between the ciliary muscle bundles, then through the suprachoroidal - scleral tissues. Trabecular outflow accounts for 70% to 95% of the aqueous outflow. And remaining 5% to 30% by uveoscleral outflow. FACTORS EXERTING LONG-TERM INFLUENCE ON IOP Genetics Age Gender Refractive Error Ethnicity  Genetics The IOP is under hereditary influence. IOP tends to be higher in individuals with enlarged CDR & in those who have relatives with open-angle glaucoma.  IOP increases with age. Studies indicate that children have lower pressures than the rest of the normal population, But tonometric measurements may be influenced by the level of cooperation of the child, tonometer used, use of general anesthesia or a hypnotic agent. Age There may be a positive independent correlation between IOP and age & may be related to reduced facility of aqueous outflow & decreased aqueous production.  Gender – IOP is equal between the sexes in ages 20 to 40 years. – In older age groups, the apparent increase in mean IOP with age is more in women. Refractive Error – A positive correlation between IOP and both axial length of the globe and increasing degrees of myopia – Myopes also have a higher incidence of COAG Blacks have been reported to have slightly higher pressures Ethnicity than whites.  FACTORS EXERTING SHORT-TERM INFLUENCE ON IOP Diurnal Postural Variation Exertional Influences Lid and Eye Movement Intraocular Conditions Systemic Conditions Environmental Conditions General Anesthesia Foods and Drugs  Diurnal Variation IOP shows cyclic fluctuations throughout the day. Ranges from approximately 3 mm Hg to 6 mm Hg. Higher lOP is associated with greater fluctuation, and a diurnal fluctuation of greater than 10 mm Hg is suggestive of glaucoma. The peak IOP is in the morning hours Primary clinical value of measuring diurnal IOP variation is to avoid the risk of missing a pressure elevation with single readings. Postural Variation The IOP increases when changing from the sitting to the supine position, average pressure differences of 0.3 to 6.0 mm Hg. The postural influence on IOP is greater in eyes with glaucoma and persists even after a successful trabeculectomy.  Exertional Influences Exertion may lead to either a lowering or an elevation of the IOP, depending on the nature of the activity. Prolonged exercise, such as running or bicycling, has been reported to lower the IOP. The magnitude of this pressure response is greater in glaucoma patients than in normal individuals. Straining, as associated with the Valsalva maneuver, or playing a wind instrument, has been reported to elevate the IOP. May be due to elevated episcleral venous pressure and increased orbicularis tone. Lid and Eye Movement Blinking has been shown to rise the IOP 10 mm Hg, while hard lid squeezing may raise it as high as 90 mm Hg. Contraction of extraocular muscles also influences the IOP. There is an increase in IOP on up-gaze in normal individuals, which is augmented by Graves' thyroid ophthalmopathy. Intraocular Conditions Elevated IOP is with associated glaucoma IOP may be reduced in Anterior uveitis, Rhegmatogenous retinal detachment Systemic Conditions Positive correlation between systemic hypertension, Systemic hyperthermia has been shown to cause an increased IOP. IOP may increase in response to ACTH, glucocorticoids, and growth hormone and it may decrease in response to progesterone, estrogen, chorionic gonadotropin, and relaxin. It is significantly reduced during pregnancy, may be due excess progesterone. IOP is lower in hyperthyroidism and higher in hypothyroidism. In myotonic dystrophy, the IOP is very low, which may be due to reduced aqueous production & increased outflow. Diabetic patients have higher pressures than the general population, while a fall in IOP is seen during acute hypoglycemia. Patients with HIV have lower than normal mean IOPs  Environmental Conditions – Exposure to cold air reduces IOP, apparently because episcleral venous pressure is decreased. Reduced gravity causes a sudden, marked increase in IOP. General Anesthesia – General anesthesia reduces the IOP, – Exceptions are trichloroethylene and ketamine which elevate the ocular pressure. – In infants and children GA can mask a pathologic pressure elevation. – Depolarizing muscle relaxants, such as succinylcholine and suxamethonium cause a transient increase in IOP, possibly due to a combination of extraocular muscle contraction and intraocular vasodilation. – Tracheal intubation may also cause an IOP rise. – Elevated pCO2causes an increase in IOP, whereas reduced pCO2 or increased concentration of O2 is associated with an IOP reduction. Foods and Drugs Alcohol has been shown to lower the IOP, more so in patients with glaucoma. Caffeine may cause a slight, transient rise in IOP. A fat-free diet has been shown to reduce IOP, which may be related to a concomitant reduction in plasma prostaglandin levels. Tobacco smoking may cause a transient rise in the IOP, and smokers have higher mean IOPs than nonsmokers Heroin and marijuana lower the IOP, while LSD(lysergic acid diethylamide) causes an IOP elevation. Corticosteroids may also cause IOP elevation. THA NK U