Maintenance of intraocular pressure

The intraocular pressure (IOP) refers to the pressure
exerted by intraocular fluids on the coats of the
eyeball. The normal IOP varies between 10 and 21 mm
of Hg (mean 16 ± 2.5 mm of Hg). The normal level of
IOP is essentially maintained by a dynamic
equilibrium between the formation and outflow of the
aqueous humour. Various factors influencing
intraocular pressure can be grouped as under:
(A) Local factors
1. Rate of aqueous formation influences IOP levels.
The aqueous formation in turn depends upon
many factors such as permeability of ciliary
capillaries and osmotic pressure of the blood.
2. Resistance to aqueous outflow (drainage). From
clinical point of view, this is the most important
factor. Most of the resistance to aqueous outflow
is at the level of trabecular meshwork.
3. Increased episcleral venous pressure may result
in rise of IOP. The Valsalva manoeuvre causes
temporary increase in episcleral venous pressure
and rise in IOP.

4. Dilatation of pupil in patients with narrow
anterior chamber angle may cause rise of IOP
owing to a relative obstruction of the aqeuous
drainage by the iris.
(B) General factors
1. Heredity. It influences IOP, possibly by
multifactorial modes.
2. Age. The mean IOP increases after the age of 40
years, possibly due to reduced facility of aqueous
outflow.
3. Sex. IOP is equal between the sexes in ages 20-
40 years. In older age groups increase in mean
IOP with age is greater in females.
4. Diurnal variation of IOP. Usually, there is a
tendency of higher IOP in the morning and lower
in the evening (Fig. 9.7). This has been related to
diurnal variation in the levels of plasma cortisol.
Normal eyes have a smaller fluctuation (< 5 mm
of Hg) than glaucomatous eyes (> 8 mm of Hg).
5. Postural variations. IOP increases when
changing from the sitting to the supine position.
6. Blood pressure. As such it does not have longterm
effect on IOP. However, prevalence of
glaucoma is marginally more in hypertensives
than the normotensives.
7. Osmotic pressure of blood. An increase in plasma
osmolarity (as occurs after intravenous mannitol, oral
glycerol or in patients with uraemia) is associated
with a fall in IOP, while a reduction in plasma
osmolarity (as occurs with water drinking
provocative tests) is associated with a rise in IOP.
8. General anaesthetics and many other drugs also
influence IOP e.g., alcohol lowers IOP, tobacco
smoking, caffeine and steroids may cause rise in
IOP. In addition there are many antiglaucoma
drugs which lower IOP.