F

farsightedness See HYPEROPIA.

flashes Visual phantoms that appear as spots of
light. An ophthalmologist should evaluate occur-
rences of flashes, as they can be symptoms of RETI-
NAL DETACHMENTor other conditions affecting the
RETINA. Flashes represent stimulation of the rods
and cones, the cells of vision carpeting the retina,
that occurs when the gelatinous fluid holding the
retina in place (vitreous humor) moves across
them. Because the NERVEsignals these cells send to
the BRAINencode patterns of light, the brain inter-
prets messages from them as light. A blow to the
head that causes a person to “see stars” has similar
effect when it is forceful enough to jostle the vit-
reous humor against the retina. Flashes may
appear as multiple spots of light, “light showers,”
or lightning-like streaks.
The ophthalmologist typically performs a full
OPHTHALMIC EXAMINATIONto assess the integrity of
the retina. Prompt treatment is necessary to inter-
vene with a retinal tear or retinal detachment, to
preserve vision. Isolated flashes of light generally
are harmless and may occur for various reasons.
Lines or waves of light that last 20 to 60 minutes
are common with migraine headaches and have
no significance for vision or the health of the eye.
See also FLOATERS; HEADACHE; VISION IMPAIRMENT;
VITREOUS DETACHMENT.

floaters Fragments of inner EYE material that
float through the vitreous humor, casting shadows
against the RETINAas entering light strikes them.

Floaters may take various shapes and sizes, and

typically move around the VISUAL FIELD, changing

position with blinking or eye movement. Most

floaters are harmless, though large floaters may

interfere with vision. Holding the eye still may

allow the floater to settle to the bottom of the eye,

out of the visual field. A sudden increase in the

number of floaters, or floaters that occur in com-

bination with FLASHES, can signal a retinal tear or

RETINAL DETACHMENT. Prompt intervention is neces-

sary to prevent further retinal damage and pre-

serve vision. Floaters may also indicate UVEITIS.

Large floaters may remain indefinitely; small

floaters may eventually break apart and become

absorbed into the vitreous humor. There is no

treatment for floaters.

See also VISION IMPAIRMENT; VITREOUS DETACHMENT.

fluorescein staining A simple procedure for

diagnosing CORNEAL INJURYor foreign objects in the

EYE. The ophthalmologist places a strip of paper

containing fluorescein at the edge of the eye. The

dye rapidly leaches into the tears. Some people

experience a slight burning sensation when the

dye washes across the eye for the first time. Blink-

ing disperses the tears across the CORNEA. With the

regular room lights turned off, the ophthalmolo-

gist shines a cobalt blue light on the eye. Any

breach in the eye’s surface shows as bright green.

The tears wash the fluorescein from the eye

within a few minutes.

See also OPHTHALMIC EXAMINATION; SLIT LAMP

EXAMINATION; TRAUMA TO THE EYE.

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