Occasionally
a person experiences visual changes that are difficult to completely understand
with only a clinical eye exam. These changes may be due to abnormalities in the
function of some of the cells in the retina, or there may be changes in the
nerve connections between the eye and the brain. Sometimes it is difficult to
find out the exact point of damage in the pathway of vision. Furthermore, there
are several diseases according to which the eye is displayed normal during eye
examination, while the eye cell function is impaired. Young infants and
children are often unable to communicate or describe in detail any problems
they might have with the way they see. The recording of visual function is
difficult in these patients, and clear information on the visual function maybe
impossible to obtain. All these situations call for alternative methods of
testing the visual function.
Electrophysiology
testing includes a battery of tests which can be used to provide information
about the visual system beyond the standard clinical examination of the eye.
Each image we see is interpreted to an electrical sign. It is possible to
detect this electrical response each time a light flashes or a stimulus moves
or suddenly appears. This response comes either from retina, the
light-detecting part of the eye (electro-retinogram, ERG) or the region of the
brain that is responsible for vision (visual evoked potentials, VEP). We have
the ability to detect these responses through electrodes placed at specific
parts. Full electrophysiologic testing is required to detect damage to any part
of the visual pathway.
ICARE
eye hospital is one of the few eye care centers in India that has set up
comprehensive clinical visual electrophysiology services and provides a full
complement of electrophysiological tests for visual function assessment in the
diagnosis, prognosis, and treatment of eye diseases.
Tests
offered
• Electroretinogram (ERG) Full field ERG, Pattern ERG.
An ERG
is useful in evaluating inherited (hereditary) and acquired disorders of the
retina such as retinitis pigmentosa, Leber's congenital amaurosis, cone
dystrophies, Stargardt disease, congenital stationary night blindness, juvenile
X-linked retinoschisis, achromatopsia, etc. It is often ordered if there is a
complaint of night blindness. Pattern ERG consists of a specialized examination
in cases where it is necessary to evaluate the central vision, ie the function
of the macula. The visual stimuli that are commonly used are consisted by
"boards" (black squares) of alternating form. The measurements are performed
separately for each eye.
• Multifocal ERG (mfERG)
One of
the recent advances in ERG technology is the multifocal electroretinogram
(mfERG). The mfERG provides a detailed assessment of the health of the central
retina (macula). It can hint towards the diagnosis in certain disorders where
the retina examination maybe near normal. It can also be used to follow up and
compare pre and post treatment in disorders such as diabetic retinopathy, age
related macular degeneration, retinal vascular occlusions, central serous
retinopathy, macular hole where it
provides additional information about the function of the retina.
• Visual Evoked Potentials (VEP) Flash VEP, Pattern VEP.
VEP
tests are used to evaluate optic nerve disorders such as optic neuritis, optic
tumors, demyelinating diseases such as multiple sclerosis and intracranial
lesions, including brain tumors.
• Electro-oculogram (EOG)
Ordered
in inherited retinal dystrophies (especially Best disease). ERG and VEP may be
ordered together in cases with unexplained visual loss, infants with
questionable vision, toxic drug exposure (for eg, hydroxychloroquine,
ethambutol), opaque media or trauma including intraocular foreign bodies.
All
these electrophysiological tests are conducted according to the standards of
the International Society for Clinical Electrophysiology of Vision (ISCEV) by
specially trained staff and are interpreted in conjunction with the entire
clinical picture, by an ophthalmologist specialized in this field. These tests
not only help to diagnose the cause of a visual problem but are also useful for
monitoring the progression of a visual disorder or the effects of any
treatment.
Procedure
Electrophysiological
testing is completely non invasive. Local anesthesia with drops is only
required. ERG examinations require dilation of the pupil. In case of children
the support and cooperation of parents is very important and helps to perform
these tests quickly and with the least possible inconvenience.
For ERG,
a thin wire with an electrode is placed on the eye after topical anaethesia,
while for VEP electrodes are attached to the head. Before inserting the
electrodes, we clean the skin with a cotton swab with a gel that has a slightly
grainy texture. This gel improves contact with the skin and helps to take good
quality signals. All the electrodes are easily removed at the end of the
examination. The patient then watches a screen with different lights to
stimulate the retina. The electrical impulses from the retina are recorded. To
get useful readings, the patient must first sit in the dark for a period of
time to become completely dark-adapted. For this reason, this test usually
takes about two hours.
