R

refraction test A diagnostic procedure to meas-
ure REFRACTIVE ERRORSof 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 EYEdoes 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(nearsight-
edness). When the eye is shorter, the lightwaves
that enter the eye focus behind the retina, result-
ing in HYPEROPIA(farsightedness). An irregular sur-
face 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 usu-
ally 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 ERRORSof vision such asMYOPIA(nearsighted-

ness), HYPEROPIA(farsightedness), and ASTIGMATISM

(irregularity of the CORNEA). Refractive surgery

became an option for permanent refractive correc-

tion 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 microker-

atomes (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 refrac-
  tive corrections. The EYEsurgeon makes a small
  flap to expose the inner portion of the cornea,
  uses an excimer (cool) laser to remove micro-
  scopically 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 appar-
  ent. LASIK is most effective for hyperopia,
  astigmatism, and moderate myopia.


• Photorefractive keratectomy (PRK) was the
  original refractive LASER SURGERY. The eye sur-
  geon uses an excimer laser to reshape the sur-
  face 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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