Saturday, May 3, 2014
Electrophysiology Services at ICARE Eye Hospital, Noida
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.
• 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.
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.