Etiology and Types
I、Introduction
Cataract is the leading cause of blindness and visual impairment throughout the world.
Cataract is a very common cause of visual impairment in older people. The prevalence
of cataracts is about 50% in people between the ages of 65 and 74 and it increases to about
70% in those over the age of 75.
II、Healthy lens
The lens is a transparent, biconvex structure whose functions are to maintain its own
clarity, to refract light, and to provide accommodation. The lens can grow
continuously throughout our life. The index of refraction of lens will decrease with
age, probably as a result of the increasing presence of insoluble protein particles.
III、Types of Cataract
Congenital cataract:
The term congenital cataract refers to a lens
opacity found at birth.
Traumatic cataract:
Traumatic lens opacity may be caused by mechanical
injury, physical forces and osmotic influences.
Secondary cataract:
Lens opacity induced by some inflammatory diseases
in eyes.
Metabolic cataract:
Systemic metabolic diseases induced cataract; such
as diabetes mellitus, galactosemia, hypocalcemia, Wilson disease, and myotonic
dystrophy.
Drug-induced cataract:
1. Long-term use of corticosteroids,
phenothiazines, miotics, or amiodarone may cause a specific type of cataracts.
2.
Some steroid-induced cataract in children may be reversible with cessation of the drug.
Senile
cataract:
l. The pathogenesis of age-related cataracts in
multifactorial and not completely understood.
2. There are three main types of
age-related cataracts: nuclear, cortical and posterior subcapsular cataracts.
3. In the
early stages, the progressive hardening of the lens nucleus commonly causes an increase in
the refractive index of the lens and thus a myopic shift in refraction.
4. In some cases, the myopic shift transiently
enables other wise presbyopic individuals to read without spectacles, a condition referred
to as second sight. |