dr. reema thomas aqueous dynamics 18 1-17

AQUEOUS HUMOUR

DYNAMICS

FUNCTIONS• Aqueous humor is a clear, colourless, watery solution continuously

circulated from the posterior chamber throughout the anterior chamber.• Maintains the intraocular pressure.• Maintains the shape and the internal structural arrangement of the

eye.• Cornea takes glucose and oxygen from the aqueous.• Lens uses glucose, oxygen and amino acids from the aqueous.

PHYSICOCHEMICAL PROPERTIES• VOLUME 0.31 ml• (o.25 ml in anterior chamber and 0.06 ml in posterior chamber)• Refractive index 1.336• Density greater than water• Hyperosmotic ( compared to plasma) 3-5 mOsm/l• pH: acidic• Rate of formation: 2.3 µl/min.• Protein content less.

FACTORS AFFECTING COMPOSITION OF AQUEOUS HUMOUR• BLOOD AQUEOUS BARRIER• Protein and large molecular size substances are largely prevented

from entering the cavities. This maintains the clarity of the media of the eye.• Formed by the tight junctions between the cells of epithelium of the

ciliary body and endothelium of iris capillaries.

EFFECT OF BREAKDOWN OF BLOOD AQUEOUS BARRIER ON AQUEOUS HUMOUR COMPOSITION

• Proteins and antibodies in aqueous equilibrate with those in plasma to form plasmoid aqueous (secondary aqueous).• Recognised clinically in slit lamp microscope as Tyndall Beam.• Fibrinogen appears in aqueous, which may allow the aqueous actually

to clot.• Ionic composition of the aqueous approches same as plasma.

Aqueous humor production• Active secretion by double-

layered ciliary epithelium.• Primarily derived from the

plasma within the capillary network of the ciliary processes.

Aqueous humor• Secretion depends on:• Integrity of blood aqueous barrier• Blood flow to ciliary body• Neurohormonal regulation of blood and ciliary body• Adrenergic innervation

Aqueous humor outflow

• Trabecular out flow (90%)

• Uveoscleral out flow (10-20%)

Trabecular outflow channelPosterior chamber

Anterior chamber Angle of anterior chamber ( Trabecular meshwork)

Schlemm’s canal

Collector channels

Episcleral vein, aqueous vein

UVEA SCLERAL OUTFLOW• Pressure independent outflow• Responsible for 10-20 % of outflowCiliary body

Suprachoroidal space

Venous circulation in the ciliary body, choroid, sclera

INTRAOCULAR PRESSURE• Refers to the pressure exerted by the intraocular contents on the

coats of the eyeball.• IOP is maintained by the a dynamic equilibrium between the aqueous

humour formation, aqueous humour outflow, and episcleral venous pressure.• Normal IOP is 10 – 21 mm Hg.

Factors affecting IOP

• Rate of aqueous production

• Resistance to aqueous outflow

Pretrabecular , trabecular and post trabecular

• Level of episcleral venous pressure.

Factors influencing IOP HEREDITARY

• Age more than 40

• Time of the day: pressure highest in the morning, lowest in the evening

Diurnal variation of More than 8 mm is considered pathognomonic of glaucoma.

• Systemic venous pressure- external pressure of jugular veins, compression of superior vena cava by tumour,

cavernous sinus thrombosis.

• General anaesthesia decreases IOP (except nitrous oxide, ketamine)

• Topical drugs

• Blockage of the circulation of aqueous – angle of AC, pupillary block

MEASUREMENT OF IOP

VISUAL FIELD• Portion of space in which objects are simultaneously visible to the

steadily fixating eye.

VISUAL FIELD EXAMINATIONScreening tests• Confrontational visual field testing • Amsler grid (assesses the central 10° the visual field ) .

Quantitative measurements using manual or automated perimetry

VISUAL FIELD DEFECTS• Relative paracentral scotoma

• Roenne’s nasal step

• Seidel scotoma

• Arcuate scotoma

• Double arcuate / ring scotoma

• End stage / near total field defect

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