Encyclopedia of Health and Medicine

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refraction test A diagnostic procedure to measure
REFRACTIVE ERRORS of the EYE, such as MYOPIA,
HYPEROPIA, and ASTIGMATISM. An optometrist or
ophthalmologist conducts the test using a device
called a refractor. The refractor fits against the face
somewhat like a flattened pair of binoculars, with
a chin rest and forehead pad to support the head
in proper position. After covering one eye, the eye
professional applies combinations of lenses to the
eye piece. The person looks through the eye piece
and lens at a rendition of the SNELLEN CHART. The
eye professional examines first one eye with the
other eye covered, switches eyes, and finally
examines both eyes together to confirm the
appropriate refractive correction.
See also CORRECTIVE LENSES; OPHTHALMIC EXAMINA-
TION.
refractive errors Vision disorders in which a
defect of the EYE does not allow proper refraction
of light. When the eye is longer than normal from
front to rear, the lightwaves entering the eye focus
short of the RETINA, resulting in MYOPIA (nearsightedness).
When the eye is shorter, the lightwaves
that enter the eye focus behind the retina, resulting
in HYPEROPIA (farsightedness). An irregular surface
or shape to the cornea may produce
distortions of refraction, resulting in ASTIGMATISM.
The optometrist or ophthalmologist measures
refractive errors using a refraction test. Treatment
is CORRECTIVE LENSES (eyeglasses or contact lenses)
or REFRACTIVE SURGERY. Refractive errors may
change frequently during childhood, though usually
stabilize by early adulthood. Severe refractive
errors may be uncorrectable, notably myopia,
resulting in functional or legal blindness.
See also PRESBYOPIA; VISION IMPAIRMENT.
refractive surgery Operations to correct REFRAC-
TIVE ERRORS of vision such as MYOPIA (nearsightedness),
HYPEROPIA (farsightedness), and ASTIGMATISM
(irregularity of the CORNEA). Refractive surgery
became an option for permanent refractive correction
in the United States in the 1980s, following
its introduction and rapid growth in popularity in
Europe. Now, about 1.5 million Americans
undergo refractive surgery operations each year.
Surgical Procedure
There are numerous refractive surgery techniques
in use today. They fall into the general categories
of those that use lasers, those that use microkeratomes
(specialized blades), and those that use
implants to alter the eye’s natural structure. There
are five commonly performed refractive correction
operations:
• LASIK (laser-assisted in situ keratomileusis) has
become the standard procedure for most refractive
corrections. The EYE surgeon makes a small
flap to expose the inner portion of the cornea,
uses an excimer (cool) laser to remove microscopically
thin layers of corneal tissue, and
replaces the corneal flap. Because the surface of
the cornea, which contains NERVE endings,
remains intact, there is almost no postoperative
discomfort and results are immediately apparent.
LASIK is most effective for hyperopia,
astigmatism, and moderate myopia.
• Photorefractive keratectomy (PRK) was the
original refractive LASER SURGERY. The eye surgeon
uses an excimer laser to reshape the surface
of the cornea. Results are not apparent
until the cornea heals, which takes several
weeks. There is some discomfort during the
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