Atlas of pathophysiology, 2 Edition

Part II - Disorders

Sensory Disorders

Cataract

A cataract is a gradually developing opacity of the lens or lens capsule. Light shining through the cornea is blocked by this opacity, and a blurred image is cast onto the retina. As a result, the brain interprets a hazy image. Cataracts commonly occur bilaterally, and each progresses independently. Exceptions are traumatic cataracts, which are usually unilateral, and congenital cataracts, which may remain stationary.

Age Alert

Cataracts are most prevalent in people older than age 70. The prognosis is generally good; surgery improves vision in 95% of affected people.

Causes

·   Aging

·   Trauma, foreign body injury

·   Exposure to ionizing radiation or infrared rays

·   Exposure to ultraviolet radiation

·   Drugs that are toxic to the lens, such as prednisone, ergot alkaloids, dinitrophenol, naphthalene, phenothiazines, pilocarpine

·   Genetic abnormalities

·   Infection such as maternal rubella during the first trimester of pregnancy

·   Maternal malnutrition

·   Metabolic disease, such as diabetes mellitus or hypothyroidism

·   Myotonic dystrophy

·   Uveitis, glaucoma, retinitis pigmentosa, retinal detachment

·   Atopic dermatitis

Pathophysiology

Pathophysiology may vary with each form of cataract. Congenital cataracts are particularly challenging. They may result from chromosomal abnormalities, metabolic disease, intrauterine nutritional deficiencies, or infection during pregnancy (such as rubella). Senile cataracts show evidence of protein aggregation, oxidative injury, and increased pigmentation in the center of the lens. In traumatic cataracts, phagocytosis of the lens or inflammation may occur when a lens ruptures. The mechanism of a complicated cataract varies with the disease process—for example, in diabetes, increased glucose in the lens causes it to absorb water.

Typically, cataract development goes through these four stages:

·   immature—partially opaque lens

·   mature—completely opaque lens; significant vision loss

·   tumescent—water-filled lens; may lead to glaucoma

·   hypermature—lens proteins deteriorate; peptides leak through the lens capsule; glaucoma may develop if intraocular fluid outflow is obstructed.

Signs and symptoms

·   Gradual painless blurring and loss of vision

·   Milky white pupil

·   Blinding glare from headlights at night

·   Poor reading vision caused by reduced clarity of images

·   In central opacity—vision improves in dim light; as pupils dilate, patients able to see around the opacity

Age Alert

Elderly patients with reduced vision may become depressed and withdraw from social activities rather than complain about reduced vision.

Diagnostic test results

·   Indirect ophthalmoscopy and slit-lamp examination show a dark area in the normally homogeneous red reflex.

·   Visual acuity test confirms vision loss.

Treatment

·   Extracapsular cataract extraction of anterior lens capsule and cortex:

§  phacoemulsification to fragment the lens with ultrasonic vibrations

§  aspiration of pieces

·   Intracapsular cryoextraction of entire lens; rarely performed today

·   Laser surgery to restore visual acuity if a secondary membrane forms in the intact posterior lens capsule after an extracapsular cataract extraction

·   Discission (an incision) and aspiration possibly still used in children with soft cataracts

·   Contact lenses or lens implantation after surgery to improve visual acuity, binocular vision, and depth perception

P.397

CATARACT

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