For
certain additional tests – to examine your general eye health and the
retina, optic nerve, and blood vessels – the doctor will need to dilate
your pupils using eye drops. These drops take about twenty minutes to
fully open the pupils, giving the doctor a much wider view of the inside
of your eyes than would be possible with constricted pupils.
After
dilation, your vision might be blurred and highly sensitive to light
for several hours. You won’t want to walk out into bright sunlight with
your eyes uncovered. If you don’t have sunglasses, most eye doctors will
give you disposable sunglasses to wear on the way home. Since there’s
no way of knowing how long it will take your eyes to return to normal,
the best course is to arrange for someone to drive you home.
Ophthalmoscopic Examination
An
ophthalmoscope is a specialized device through which your doctor can
inspect the blood vessels and the optic nerve at the back of the eye. He
or she will also examine the retina for detachment and tears, and the
small areas on the retina responsible for sharp vision (the fovea) and
central vision (the macula).
Slit-Lamp Examination
A
slit lamp, or biomicroscope, allows the doctor to see signs of
infection or disease at the front of the eye, including problems in the
eyelids, cornea, conjunctiva (the thin, transparent membrane that
protects the front of the eye), and iris. Using a higher-powered lens,
he or she can also see to the back of the eye, detecting macular
degeneration and other problems. During a slit-lamp exam, your head will
be comfortably stabilized on the lamp’s chin rest.
As
its name suggests, the slit lamp shines slits of light into the eye.
The size of the slit is adjustable, so the doctor can see very small
sections of the eye at very high magnification. The lens nucleus is
clearly visible, as are the lens position, the other layers of the lens,
and the degree of brunescence (browning of the lens), which is
responsible for some cataract patients’ inability to distinguish blues
and purples.
Visual Field Measurement
The
simplest way to test your field of vision – how far you can see to the
left and right out of the corners of your eyes – is for you to focus on
the doctor’s face while he or she moves a finger slowly to the side and
asks you to signal when you can no longer see it. An instrument called a
perimeter, which emits flashes of light, can be used to reveal blind
spots. You simply stare at an image and tell the doctor when you see a
flash.
Tonometry-lntraocular Pressure (lOP) Measurement
As
part of a routine eye exam, your eye doctor will probably use a
tonometer to screen your eyes for glaucoma. A noncontact tonorneter is
generally used for screening. It expels a puff of air toward the eye and
measures the resulting small, instantaneous indentation. The size of
the indentation indicates the intraocular pressure (IOP) inside the eye.
Other types of tonometers are placed directly on the cornea after the
eye is numbed with eye drops.
Potential Acuity Testing
If
you have cataracts, the doctor may perform potential acuity testing, a
measure of what your vision would be like if the cataracts were removed.
Potential
acuity testing is especially useful in determining how much of your
vision loss is due to cataracts. One way of testing potential acuity is
with a pinhole acuity meter, which projects an eye chart directly onto
the retina, bypassing the cataract.
Contrast Sensitivity Testing
If cataracts make it hard for you to differentiate shades of gray, you have low contrast sensitivity. Your
eye doctor might measure this by using a low-contrast visual-acuity
chart, or possibly a chart with different contrast levels on symbols of
the same size. To find out more, you can check out How To Improve Eyesight Without Surgery.