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Diagnostic Testing
Patients visiting the Pretoria Eye Institute for a comprehensive eye exam may undergo one or more of the following tests or hear some of these terms:
6/6 or 20/20: Used to describe “normal vision”. If you can read the 6/12 or 20/40 line, that means that at 20 feet or 6 meters, you can see what a person with normal vision can see at 40 feet or 12 meters.
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Amsler grid: a simple screening test used to assess central vision, at the macula. The Amsler grid looks like graph paper – a series of small squares – with a dot in the centre of the grid. The patient is instructed to stare at the dot and notice if any lines appear wavy or missing.
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Corneal topography:
produces detailed information about the
curvature of the cornea. Computer
software is used to measure and analyze
the surface of the cornea and generate a
colour map from the data. The data is
helpful to evaluate and plan to correct
astigmatism, monitor corneal disease and
detect irregularities in the shape of
the cornea. Accurate measurement of
astigmatism is important for refractive
surgery, contact lens fitting and
calculating the power of intraocular
lenses.
Fluorescein angiogram:
is useful for evaluating various disease
effects on the retina. The test requires
an injectable dye (fluorescein),
specialized camera with filters, and
timing. The dye is injected into the
patient’s arm; within seconds, the dye
travels to the blood vessels inside the
eye. Photographs are taken to document
any fluid leakage as the dye circulates
through the eye. A digital camera is
often used which allows the
ophthalmologist to interpret the results
immediately.
Fundus photography: may
be ordered by your ophthalmologist to
document the status of the optic nerve,
macula, retina, blood vessels and the
vitreous. Specialized cameras are used
to document the progression of diseases
such a macular degeneration, glaucoma
and diabetic retinopathy.
Gonioscopy: used to
examine the anterior chamber angle, the
portion of the eye that directs aqueous
humor (the fluid produced by the eye).
Gonioscopy is a standard part of a
glaucoma evaluation. A special mirrored
contact lens is used during this
evaluation.
Indocyanine
Green Dye (ICG): is used to
evaluate the choroidal circulatory
system, just behind the retina. ICG
reacts to light with a longer wavelength
than fluorescein dye, allowing the
ophthalmologist to determine exactly
where, and if, the vessels are leaking.
The ICG dye takes about 15 to 20 seconds
to travel through the bloodstream to the
eye. The digital photos are interpreted
by the ophthalmologist. If treatment is
needed, the photos indicate where the
laser treatment needs to be directed.
Keratometry:
the measurements of the steepest and the
flattest surfaces of the cornea.
Accurate measurements are easily
obtained form most eyes with a
keratometer. Corneal topography is used
to obtain more corneal details, or when
reliable keratometry measurements cannot
be obtained. Corneal measurements are an
important component in formulas used to
calculate and determine the power of
intraocular lenses and in the fitting of
contact lenses.
Ophthalmoscopy: is performed
with an ophthalmoscope and allows the
ophthalmologist to examine the retina
and vitreous. Ophthalmoscopy is usually
performed with dilated pupils, to allow
the best view inside the eye. Two types
of ophthalmoscopes may be used: direct
and indirect. The direct ophthalmoscope
has a battery powered light source and
is hand-held. Multiple lenses may be
selected with a dial, which allows the
doctor to focus on and view the optic
nerve and the central retina. The
periphery, or entire retina, may be
viewed by using an indirect
ophthalmoscope, which is worn on the
doctor’s head. A lens is placed in front
of the patient’s eye while the doctor
looks through the magnifying glasses in
the headgear. The combination of the
instrument and the lens allows a good
view of the retina.
Optical Coherence Tomography:
The technique of optical coherence
tomography (OCT) produces high
resolution, high speed, non-invasive,
cross-sectional images of body tissue.
The technology is best compared to
ultrasound, except that it employs light
rather than sound and thereby achieves
clearer, sharper resolution.
Non-invasive OCT examinations produce
real-time cross-sectional images of
retinal tissue, in ophthalmic
applications, and are usually
accomplished in less than 10 minutes.
Thanks to OCT’s high resolution, which
is 10 times greater than magnetic
resonance imaging (MRI) or ultrasound,
microscopic early signs of disruption in
tissue can be detected and treated. The
ophthalmic applications have already
benefited patients with glaucoma,
retinal and macular diseases and those
considering corneal and refractive
surgery.
Phoropter (or
refractor): Device, placed in
front of a patient’s eyes, as they are
asked to read an eye chart on the wall.
The phoropter contains various lenses
that are changed by the examiner until
the best corrected vision is determined.
The eye chart on the wall used to be
placed 20 feet or 6 meters in front of
the patient, which meant the room had to
be at least 20 feet or 6 meters long.
Today, the distance has been modified by
using mirrors to reflect the images, but
they still appear to be 20 feet away.
Slit lamp examination:
performed using a slit lamp, which is an
instrument with a high-intensity light
source that can be focused and narrowed.
The structures in the front section of
the eye are examined: the eyelids, the
cornea, the anterior chamber, the
surface of the sclera, the iris and the
crystalline lens.
Tonometry: A tonometer
or a tonopen may be used to measure the
intraocular pressure (pressure within
the eye). It is used to screen for
glaucoma. It is a good idea to have the
intraocular pressure checked routinely
after the age of 40, or if you are at a
high risk for developing glaucoma.
Ultrasound: is used
in ophthalmology when a cloudy cornea, a
dense cataract or blood in the vitreous
obscures a view into the eye with an
ophthalmoscope. Standardized ultrasound
also may be used to evaluate the extent
and location of a retinal detachment, to
determine the exact borders and height
of intraocular or orbital tumors; to
determine the axial eye length (the
distance form the outer cornea to the
macula); and the thickness of extra
ocular muscles. B-scan ultrasound
produces a two-dimensional,
cross-sectional image; A-scan produces a
one-dimensional view. It is appropriate
to obtain an A-scan on patients before
cataract surgery to determine the
appropriate power of the intraocular
lens implant.
Visual acuity testing:
although it is a simple exam, it is an
important component of a comprehensive
eye exam. An eye exam measures a
person’s ability to see an object at a
specified distance. The results of an
eye exam determine if a person has
normal vision or if a prescription for
glasses or contact lenses would help put
things into proper focus.
Visual field testing:
monitors peripheral vision. Visual
fields are obtained to monitor visual
changes that may be caused by specific
eye diseases, such as glaucoma, as well
as the neurological function of the
retina, optic nerve and brain. There are
different types of visual field exams,
usually they require the patient to
focus on one spot and respond to
flashing lights by pressing a button.
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