Intraocular pressure and aqueous dynamics
- 2. 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
- 3. 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)
- 4. Composition of aqueous is similar to plasma except: High concentration of : Ascorbate, pyruvate and lactate. Low concentration of: Proteins, urea and glucose.
- 5. The composition of aqueous in anterior chamber differs from that in posterior chamber because of metabolic interchange: Anterior chamber Posterior chamber HCO3 - Low High Cl- High Low Ascorbate Low High
- 6. 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.
- 7. Aqueous is derived from plasma within the capillary network of: 1. Posterior segment 2. Ciliary body 3. Iris The normal aqueous production rate is 2.3µl/min.
- 8. The system of semipermeable membranes separating the blood from the ocular cavity is known as blood-aqueous barrier.
- 10. 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. Ultrafilteration 2. Secretion 3. Diffusion
- 11. Diurnal variation Blood pressure Plasma osmotic pressure Intraocular pressure Role of adrenergic innervation, vasopressin and adenylcyclase
- 12. 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.
- 13. Aqueous flows from posterior chamber into the anterior chamber through the pupil. From anterior chamber it is drained out by two routes: 1. Trabecular outflow 2. Uveoscleral outflow
- 14. It is the main outlet (90%) for aqueous drainage. It consists of : i. Trabecular meshwork ii. Schlemm’s canal iii. Collector channels
- 16. It is sieve like structure. It consists of three portions 1. Uveal meshwork 2. Corneoscleral meshwork 3. Juxtacanalicular(endothelial) meshwork
- 18. 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.
- 20. 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 system 2. Indirect system
- 21. 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.
- 22. 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.
- 23. 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.
- 24. a) Local factors b) General factors
- 25. 1. Rate of aqueous formation 2. Resistance to aqueous outflow 3. Increased episcleral venous pressure 4. Dilation of pupil
- 26. 1. Hereditary 2. Age 3. Sex 4. Diurnal variation 5. Postural variation 6. Seasonal variation 7. Blood pressure 8. Osmotic pressure of blood 9. Effects of Drugs 10. Effects of general anesthesia 11. Systemic hyperthermia 12. Refractive error 13. Mechanical pressure on globe
- 29. It is an indirect method of measuring IOP using a specialised instrument called tonometer. There are two types of tonometry: 1. Indentation or Impression tonometry 2. Applanation tonometry
- 30. 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.
- 34. 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
- 35. 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).
- 38. Other commenly used tonometers: 1. Perkin’s applanation tonometer 2. Pneumatic tonometer 3. Air-puff tonometer ( Non contact ) 4. Pulse air tonometer 5. Tono pen
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