INTRA-OCULAR PRESSURE

MOHD AFIF KHAN

AJMAL KHAN TIBBIYA COLLEGE,

ALIGARH, INDIA

INTRODUCTION:

Until the 20^th century, little was known about the intra-ocular pressure (IOP) due to the lack of suitable equipment in measuring this parameter.

FEATURES OF NORMAL IOP:

IOP refers to the pressure created by intraocular contents on the coats of the eyeball. The normal level of IOP is essentially maintained by a dynamic equilibrium between the aqueous humor formation, aqueous humor outflow and episcleral venous pressure.

IOP is distributed evenly throughout the eye, so that the pressure is always the same in the posterior vitreous as it is in the aqueous humor.

The IOP is important in maintaining the shape of the eyeball and thus also the optical integrity.

Normal IOP varies between 10.5-20.5 mmHg with a mean pressure of 15.5 +/- 2.57 mmHg.

IOP is created by aqueous formation which has two components: First, a hydrostatic component from the arterial blood pressure and ciliary body tissue pressure and second, an osmotic pressure induced by the active secretion of sodium and other ions by the ciliary epithelium.

IOP serves as the tissue pressure of the vascularized internal structures of the eye and is thus much higher than the tissue pressure elsewhere in the body.

Normal IOP is pulsatile, reflecting in part its vascular origin and the effects of blood flow on the internal ocular structures.

IOP is a dynamic function. Any single measurement 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 IOP:

• a)      Local factors.
• b)      General factors.

Local factors include:

• 1)      Rate of aqueous formation.
• 2)      Resistance to aqueous outflow.
• 3)      Increased episcleral venous pressure.
• 4)      Dilatation of the pupil.

General factors include:

1. Heredity.
2.  Age.
3. Sex.
4. Blood pressure.
5. Diurnal variation.
6. Postural variation.
7. Seasonal variation.
8. Effects of drugs.
9. Osmotic pressure of blood.
10. Effect of general anesthesia.
11. Systemic hyperthermia.
12. Refractive error.
13. Mechanical pressure on globe.
14. Blockage of aqueous circulation.

MEASUREMENT OF IOP:

1.  Manometry.
2. Tonometry.

FREQUENCY DISTRIBUTION OF IOP IN THE POPULATION:

Several population based studies have been done to comment upon the frequency distribution of the normal IOP.

The conclusions drawn from the studies regarding frequency distribution of IOP in the population are as follows:

1. The distribution of pressure observed resembled a Gaussian curve but was skewed towards the right.
2. It has been assumed that perhaps two different population groups account for the skewed distribution: a large “normal” group and a smaller group that was felt to be glaucomatous without optic nerve head damage.
3. The mean IOP of normal group was 15.5 +/- 2.57 mmHg.
4. 95% of the population had an IOP between 10.5 and 20.5 mmHg.

 Some of the important conclusions drawn from other population based studies are as follows:

1. A slight increase of mean IOP in each decade over 40 years.
2. A slight higher pressure exists in women than men in population above 40 years of age.
3. IOP difference between right and left eye rarely exceeds 4 mmHg.
4. Level of IOP is inherited as a polygenic multifactorial trait.

Some of the facts about effect of general anesthesia (GA) on IOP are as follows:

Acute change in the volume of the blood present in the eye due to increased venous pressure as a result of coughing, retching, vomiting and bucking on endotracheal tube cause a serious increase in IOP and is a factor of cardinal importance to anesthesia.

Hypertension occurring during laryngoscopy and intubation under GA for the same reason may cause temporary increase in IOP.

Hypercapnia and hypoxia during GA produces an increase in IOP.

Drugs used in GA produce a decrease in IOP as long as the hemodynamic parameters and blood gases are kept in normal limits.

Diazepam, morphine, pethidine and pentazocin reduce IOP.

Ketamine has little effect on IOP.

Suxamethonium may cause severe elevation in IOP.

TONOMETRY:

It is an indirect method of measuring IOP using a specialized instrument known as tonometer.

There are two types of tonometers:

1. Indentation or impression Tonometry: It is based on the principle that a plunger will indent soft eye more than a hard eye. It measures the artificially raised IOP.
2. Applanation tonometer: It is based on “Imbert-Fick Law”, which states that the 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