Intraocular pressure and aqueous dynamics
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Transcript of Intraocular pressure and aqueous dynamics
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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