INTRAOCULAR PRESSURE AND AQUEOUS DYNAMICS

It is a clear, colourless, watery fluid filling the anterior chamber and posterior chamber of the eyeball.

Volume: 0.31ml Anterior chamber- 0.25ml Posterior chamber- 0.06ml Refractive Index: 1.336 Density : 1.025-1.040 ( greater than water) pH : 7.2 (acidic) Rate of formation: 2.3µl/min

Aqueous Humour

Composition: constituents of normal aqueous humour are Water (99.9%) Solids (0.1%) which includes

1. Proteins (5-16mg%)2. Amino acid (5mg/kg of water)3. Non-colloid constituents –a) Glucose (6.0 millimols/kg)b) Urea (7 millimols/kg)c) Ascorbate (0.9 millimols/kg)d) Lactic acid (7.4 millimols/kg)e) Inositol (0.1 millimols/kg)f) Sodium (144 millimol/kg)g) Potassium (4.5 millimols/kg)h) Chloride (10 millimol/kg)i) Carbonates (34 millimol/kg) Oxygen ( in dissolved state)

Aqueous Humour

Composition of aqueous is similar to plasma except:

High concentration of : Ascorbate, pyruvate and lactate.

Low concentration of: Proteins, urea and glucose.

The composition of aqueous in anterior chamber differs from that in posterior chamber because of metabolic interchange:

Aqueous Humour: Anterior chamber versus Posterior chamber

Anterior chamber

Posterior chamber

HCO3- Low High

Cl- High Low Ascorbate Low High

It maintains proper intraocular pressure. It plays an important metabolic role by

providing nutrients and by removing metabolites from avascular cornea and lens.

It maintains optical transparency. It also acts as lymph in the eyeball.

Functions of Aqueous Humour

Aqueous is derived from plasma within the capillary network of:

1. Posterior segment2. Ciliary body3. Iris

The normal aqueous production rate is 2.3µl/min.

Production of Aqueous Humour

The system of semipermeable membranes separating the blood from the ocular cavity is known as blood-aqueous barrier.

Aqueous humour is mainly derived from plasma within the capillary network of ciliary processes.

The following processes are involved in the production of aqueous humour:

1. Ultrafilteration2. Secretion3. Diffusion

Mechanism of formation of Aqueous Humour

Diurnal variation Blood pressure Plasma osmotic pressure Intraocular pressure Role of adrenergic innervation, vasopressin

and adenylcyclase

Control of Normal Aqueous Formation

Aqueous flows from posterior chamber into the anterior chamber through the pupil.

In the anterior chamber, there exist a convection current which results from temperature gradient between anterior and posterior parts of anterior chamber.

Dynamics of aqueous humour

Aqueous flows from posterior chamber into the anterior chamber through the pupil.

From anterior chamber it is drained out by two routes:

1. Trabecular outflow2. Uveoscleral outflow

Drainage of Aqueous Humour

It is the main outlet (90%) for aqueous drainage.

It consists of :i. Trabecular meshworkii. Schlemm’s canaliii. Collector channels

Trabecular (Conventional) outflow

It is sieve like structure. It consists of three portions

1. Uveal meshwork2. Corneoscleral meshwork3. Juxtacanalicular(endothelial) meshwork

Trabecular Meshwork

This an endothelial lined canal present circumferentially in the sclearl sulcus.

The endothelial cells present on its inner wall are irregular, spindle shaped and contains giant vacuoles.

The outer wall contains smooth flat cells and contains opening of collector channels.

Schlemm’s Canal

These are also called intra-scleral aqueous vessels.

They are about 25-35 in number. They leave the Schlemm’s cannal at oblique

angles to terminate in the episcleral veins. They do not have valves. They are divided into two systems:

1. Direct system2. Indirect system

Collector Channels

It is responsible for 10% of aqueous drainage.

Aqueous passes across the ciliary body into the suprachoroidal space and is drained by the venous circulation in the ciliary body, choroid and sclera.

Uveoscleral outflow is approximately around 0.3µl/min.

Uveoscleral (Unconventional) Outflow

Most of the aqueous drains into the episcleral veins.

These veins ultimately drain into the cavernous sinus via the anterior ciliary and superior ophthalmic veins.

Episcleral Veins

IOP is the pressure exerted by the intraocular contents on the coats of the eyeball.

Normal IOP : 10-21 mm of Hg (mean 16 ± 2.5 mm of Hg)

IOP is essentially maintained by the dynamic equilibrium between formation and outflow of aqueous humour.

INTRAOCULAR PRESSURE (IOP)

a) Local factorsb) General factors

Factors influencing IOP

1. Rate of aqueous formation2. Resistance to aqueous outflow3. Increased episcleral venous pressure4. Dilation of pupil

Local factors

1. Hereditary2. Age3. Sex4. Diurnal variation5. Postural variation6. Seasonal variation7. Blood pressure8. Osmotic pressure of blood9. Effects of Drugs 10. Effects of general anesthesia11. Systemic hyperthermia12. Refractive error13. Mechanical pressure on globe

General factors

1. Manometry2. Tonometry

Measurement of IOP

Manometry

It is an indirect method of measuring IOP using a specialised instrument called tonometer.

There are two types of tonometry:1. Indentation or Impression tonometry2. Applanation tonometry

Tonometry

It is based on the principle that a plunger will indent soft eye more than a hard eye.

When tonometer is placed on the cornea, W weight of the tonometer acts on A area of cornea and indents it, displacing a volume Vc. The tensile force T sets up in the outer coats of the eye tangentially to the corneal surface opposing W so that an additional T is added to baseline or resting IOP (P0) which is artificially raised to a new value P1.

Tonometer measures the artificially raised IOP P1.

Indentation tonometry

Schiotz Tonometer

Technique of Schiotz Tonometry

Errors inherent in the instrument Errors due to contraction of extraocular

muscles Errors due to accomodation Errors due to ocular rigidity Errors due to variation in corneal curvature Errors in scale reading Blood volume alteration Moses effect

Errors of Indentation tonometry

It is based on Imbert-Fick law which states that pressure inside a sphere P is equal to the force W required to flatten its surface divided by area of flattening A

P= W/A Two types:

1. Fixed Area (variable force) commenly used.2. Fixed force (variable area).

Applanation Tonometry

Goldmann Tonometer

Other commenly used tonometers:1. Perkin’s applanation tonometer2. Pneumatic tonometer3. Air-puff tonometer ( Non contact )4. Pulse air tonometer5. Tono pen

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