Intraocular Fluid Flow - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 288

will eventually lead to blindness. However, under severe conditions, the intraocular pres-
sure can rise to 60 mmHg, which will lead to blindness within weeks. In most cases of
glaucoma, the rise in intraocular pressure is caused by a decrease in aqueous humor drain-
ing. Commonly, the decrease in drainage is due to a blockage of the trabeculae, and thus,
the aqueous humor either does not enter the canals of Schlemm or there is an overall
increase in the resistance to flow through the anterior cavity. The intraocular pressure rises
because the production of aqueous humor is continuous and the sclera is fairly rigid and
cannot accommodate large changes in fluid volume by distending its walls. Although not
discussed in previous sections, there is one weak point within the sclera. Along the poste-
rior section of the eye, the optic nerve and the central retinal artery and vein pass through
the three layers of the eye. This location is termed the optic disc. As the intraocular pres-
sure increases, the optic nerve is forced outward due to the increased pressure forces. This
induces irreversible damage to the nerve fiber. When the intraocular pressure increases to
twice of the normal intraocular pressure, the damage to the optic nerve interferes with
action potential signal transmittance. Therefore, as the action potential becomes blocked,
vision is lost.
Glaucoma is common, affecting over 2% of people over the age 35. In the United
States alone, this is over 2 million people. If caught early enough, glaucoma can be trea-
ted with a topical drug, but it may eventually need corrective surgery. There have been
a number of studies conducted on the cause of glaucoma. According to Equation 10.2 ,
there can be changes in either the episcleral venous pressure or the inflow/outflow rate
of the aqueous humor. In multiple species, it has been shown that the episcleral venous
pressure and the inflow flow rate of the aqueous humor remain relatively constant under
glaucoma conditions as compared to normal conditions. Therefore, as discussed above,
glaucoma is more than likely caused by changes in the outflow flow rate of the aqueous
humor.
10.6.2 Cataracts
The transparency of the lens depends on the exact balance between structural character-
istics of the lens proteins and the biochemical interactions of the proteins. Once this bal-
ance is disturbed, the lens will tend to lose its transparency. Cataract is the condition that
is associated with this process. There are many causes of cataracts, including aging, expo-
sure to radiation, drug reactions, but it is most commonly found in the elderly. A cataract
is defined as a cloudy or opaque region within the lens. During the early stages of cataract
formation, some of the lens proteins become denatured. In later stages, the proteins aggre-
gate together, forming the opaque regions.
Over time, it is natural for the lens to become yellow and lose its transparency. It is
then typical for the patient to require brighter lights to read and will tend to have “night
blindness.” When the lens becomes completely opaque, the patient is functionally blind
and will require the removal of the lens. The patient is only functionally blind because the
photoreceptors within the retina function properly when stimulated with light. After lens
removal, an artificial substitute can be put in its place, and eyeglasses are used to focus
light onto the retina properly to allow for vision.