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Intraocular pressure.pdf

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Mohan

The document discusses intraocular pressure (IOP), including: 1. Normal IOP ranges from 10.5 to 20.5 mm Hg and is maintained by a balance of aqueous humor formation and outflow. 2. IOP is created by aqueous humor formation from blood pressure and ciliary body tissue pressure as well as osmotic pressure from ion secretion. 3. IOP can be measured directly via manometry or indirectly via tonometry methods like indentation and applanation tonometry.

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ANATOMY AND PHYSIOLOGY Intraocular Pressure
Features of normal Intraocular pressure: • The intraocular pressure refers to the pressure exerted by intraocular contents on the coats of the eyeball. The normal level of IOP is essentially maintained by a dynamic equilibrium between the aqueous humour formation, aqueous humour outflow and episcleral venous pressure. • IOP is distributed evenly throughout the eye, so that the pressure is always the same in the posterior viterous as it is in the aqueous humour. • The intraocular pressure is important in maintaining the shape of the eyeball and thus also the optical integrity. • Normal IOP varies between 10.5 and 20.5mm Hg with a mean pressure of 15.5 ± 2.57 mm Hg.
• The intraocular pressure is created by aqueous formation which has two components: 1. A hydrostatic component from the arterial blood pressure and ciliary body tissue pressure. 2. An osmotic pressure induced by the active secretion of sodium and other ions by the ciliary epithelium. • Normal IOP is pulsatile, reflecting in part its vascular origin and effects of blood flow on the internal ocular structures. • The IOP is a dynamic function.Any single measurement of IOP is just a momentary sample and may or may not reflect the average pressure for the patient in that hour, day or week.
Factors influencing the intraocular pressure: • Factors causing long-term changes in IOP. • Factors causing short-term changes in IOP. Relations of patients with primary open angle glaucoma tend to have higher IOP. Heredity influences IOP, possibly by multifactorial modes. After the age of 40, there occurs a slight increase in the mean IOP and standard deviation after each decade. This probably occurs due to age related reduction in aqueous outflow facility, despite a concomitant reduction in the aqueous production.
IOP is equal between the sexes in ages of 20-40 yrs. In older age group, increase in mean IOP with age is greater in females than males. Through population based studies in different ethnic groups do not show significant difference in mean IOP, the race may occasionally influence IOP distribution. For example, full blood Indians in a New Mexican tribe were found to have significantly lower mean IOP than a control population. Myopes tend to have slightly higher IOP as compared to emmetropes.
The IOP is generally not affected by physiological changes in the arterial blood pressure, sudden large swings may affect the IOP accordingly. Changes in SVP can cause a profound effect on IOP by affecting ipsilateral venous pressure, for about 1mm Hg rise in episcleral venous pressure raises the IOP by 0.8mm Hg.
Mechanical pressure on the globe from outside initially raises the IOP by indentation but after some time due to acceleration of aqueous outflow the IOP returns to normal and by prolonged pressure decreases below the initial levels. This forms the basis of occular massage to lower the IOP. Plasma osmolarity affects the IOP profoundly. When the total concentration of solute molecules in the blood exceeds, the water from the eye(viterous and aqueous) is withdrawn, lowering the IOP. This effect is used clinically to lower the IOP by use of hyperosmotic agents like mannitol.
Systemic acidosis lowers the IOP. Bietti has postulated that it is the metabolic acidosis induced by carbonic anhydrase inhibitation that is responsible for their pressure lowering effect. Like many other biological parameters, the IOP also fluctuates cyclically throughtout the day. The most common pattern is for the pressure to be highest in the early morning and lowest in the late evening. The mean amplitude of daily fluctuation is usually less than 5mm Hg in normal individual. A diurinal variation in IOP of more than 8mm is considered pathognomonic of glaucoma. The exact mechanism of
diurinal IOP variation is uncertain, however, it has been related to a diurinal variation in the level of plasma cortisol.
Seasonal variation in IOP has also been described with the highest pressure recorded in winter and lowest in summer. It has been shown to cause an increased IOP in rabbits. Similarly, a drop in body temperature will cause a decrease in the IOP, probably by inhibition of aqueous secretion. It has been widely studied. IOP may be affected under general anasthesia by premedication, induction agents, muscle relaxants, inhalation anaesthetics and other drugs administered during preoperative period.
In addition to the well established antiglaucoma drugs and effect of general anaesthetic agents as discussed, many other drugs also affect the IOP. Alcohol, heroin, systemic vasodilators have been reported to lower the IOP. While tobacco smoking, caffeine, LSD, corticosteroids may raise the IOP. It has a profound effect in raising the intraocular pressure. Such a block may occur at two places: 1. At the pupil where the flow of fluid from the posterior to the anterior chamber may be impeded. 2. At the angle of the anterior chamber.
Control of Intraocular pressure: Although there are minor physiological variation of IOP there exists more or less a constant steady level of IOP, in spite of the fact that there is continuous drainage of the aqueous humour or in other words there is continuous leakeage from the eye. This steady level of IOP is the result of a dynamic equilibrium between the aqueous humour formation, the normal resistance offered by the drainage channels for the outflow of the fluid from the eye and the episcleral venous pressure. The homeostatic mechanism locally help in maintaining the steady state level of IOP. For example,
When intraocular pressure rises, there occurs a decrease in the aqueous inflow (pseudo-facility) maintaining the equilibrium. 1. Manometry 2. Tonometry Manometry is the only direct measure of IOP. In this method, a needle is introduced either into the anterior chamber or into the viterous, which is then connected with a suitable mercury or water manometer to measure the IOP.
Not a practical method for routine in human beings. Needs general anasthesia which has its other effects on the IOP. Introduction of the needle produces breakdown of blood-aqueous barrier and release of prostaglandins which alter the IOP. Manometer is of the greatest and perhaps of the only use for a continous measurements over time and recording the changes in IOP induced by physiological and pharmacological manipulations in the experiment research work or animal eyes.
Intraocular pressure.pdf
It is an indirect method of measuring the IOP with the help of specially designed instruments known as tonometer. Tonometry is broadly of two types, 1. Indention or Impression tonometry 2. Applanation tonometry

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Intraocular pressure.pdf

  • 2. Features of normal Intraocular pressure: • The intraocular pressure refers to the pressure exerted by intraocular contents on the coats of the eyeball. The normal level of IOP is essentially maintained by a dynamic equilibrium between the aqueous humour formation, aqueous humour outflow and episcleral venous pressure. • IOP is distributed evenly throughout the eye, so that the pressure is always the same in the posterior viterous as it is in the aqueous humour. • The intraocular pressure is important in maintaining the shape of the eyeball and thus also the optical integrity. • Normal IOP varies between 10.5 and 20.5mm Hg with a mean pressure of 15.5 ± 2.57 mm Hg.
  • 3. • The intraocular pressure is created by aqueous formation which has two components: 1. A hydrostatic component from the arterial blood pressure and ciliary body tissue pressure. 2. An osmotic pressure induced by the active secretion of sodium and other ions by the ciliary epithelium. • Normal IOP is pulsatile, reflecting in part its vascular origin and effects of blood flow on the internal ocular structures. • The IOP is a dynamic function.Any single measurement of IOP is just a momentary sample and may or may not reflect the average pressure for the patient in that hour, day or week.
  • 4. Factors influencing the intraocular pressure: • Factors causing long-term changes in IOP. • Factors causing short-term changes in IOP. Relations of patients with primary open angle glaucoma tend to have higher IOP. Heredity influences IOP, possibly by multifactorial modes. After the age of 40, there occurs a slight increase in the mean IOP and standard deviation after each decade. This probably occurs due to age related reduction in aqueous outflow facility, despite a concomitant reduction in the aqueous production.
  • 5. IOP is equal between the sexes in ages of 20-40 yrs. In older age group, increase in mean IOP with age is greater in females than males. Through population based studies in different ethnic groups do not show significant difference in mean IOP, the race may occasionally influence IOP distribution. For example, full blood Indians in a New Mexican tribe were found to have significantly lower mean IOP than a control population. Myopes tend to have slightly higher IOP as compared to emmetropes.
  • 6. The IOP is generally not affected by physiological changes in the arterial blood pressure, sudden large swings may affect the IOP accordingly. Changes in SVP can cause a profound effect on IOP by affecting ipsilateral venous pressure, for about 1mm Hg rise in episcleral venous pressure raises the IOP by 0.8mm Hg.
  • 7. Mechanical pressure on the globe from outside initially raises the IOP by indentation but after some time due to acceleration of aqueous outflow the IOP returns to normal and by prolonged pressure decreases below the initial levels. This forms the basis of occular massage to lower the IOP. Plasma osmolarity affects the IOP profoundly. When the total concentration of solute molecules in the blood exceeds, the water from the eye(viterous and aqueous) is withdrawn, lowering the IOP. This effect is used clinically to lower the IOP by use of hyperosmotic agents like mannitol.
  • 8. Systemic acidosis lowers the IOP. Bietti has postulated that it is the metabolic acidosis induced by carbonic anhydrase inhibitation that is responsible for their pressure lowering effect. Like many other biological parameters, the IOP also fluctuates cyclically throughtout the day. The most common pattern is for the pressure to be highest in the early morning and lowest in the late evening. The mean amplitude of daily fluctuation is usually less than 5mm Hg in normal individual. A diurinal variation in IOP of more than 8mm is considered pathognomonic of glaucoma. The exact mechanism of
  • 9. diurinal IOP variation is uncertain, however, it has been related to a diurinal variation in the level of plasma cortisol.
  • 10. Seasonal variation in IOP has also been described with the highest pressure recorded in winter and lowest in summer. It has been shown to cause an increased IOP in rabbits. Similarly, a drop in body temperature will cause a decrease in the IOP, probably by inhibition of aqueous secretion. It has been widely studied. IOP may be affected under general anasthesia by premedication, induction agents, muscle relaxants, inhalation anaesthetics and other drugs administered during preoperative period.
  • 11. In addition to the well established antiglaucoma drugs and effect of general anaesthetic agents as discussed, many other drugs also affect the IOP. Alcohol, heroin, systemic vasodilators have been reported to lower the IOP. While tobacco smoking, caffeine, LSD, corticosteroids may raise the IOP. It has a profound effect in raising the intraocular pressure. Such a block may occur at two places: 1. At the pupil where the flow of fluid from the posterior to the anterior chamber may be impeded. 2. At the angle of the anterior chamber.
  • 12. Control of Intraocular pressure: Although there are minor physiological variation of IOP there exists more or less a constant steady level of IOP, in spite of the fact that there is continuous drainage of the aqueous humour or in other words there is continuous leakeage from the eye. This steady level of IOP is the result of a dynamic equilibrium between the aqueous humour formation, the normal resistance offered by the drainage channels for the outflow of the fluid from the eye and the episcleral venous pressure. The homeostatic mechanism locally help in maintaining the steady state level of IOP. For example,
  • 13. When intraocular pressure rises, there occurs a decrease in the aqueous inflow (pseudo-facility) maintaining the equilibrium. 1. Manometry 2. Tonometry Manometry is the only direct measure of IOP. In this method, a needle is introduced either into the anterior chamber or into the viterous, which is then connected with a suitable mercury or water manometer to measure the IOP.
  • 14. Not a practical method for routine in human beings. Needs general anasthesia which has its other effects on the IOP. Introduction of the needle produces breakdown of blood-aqueous barrier and release of prostaglandins which alter the IOP. Manometer is of the greatest and perhaps of the only use for a continous measurements over time and recording the changes in IOP induced by physiological and pharmacological manipulations in the experiment research work or animal eyes.
  • 16. It is an indirect method of measuring the IOP with the help of specially designed instruments known as tonometer. Tonometry is broadly of two types, 1. Indention or Impression tonometry 2. Applanation tonometry