|Year : 1966 | Volume
| Issue : 3 | Page : 117-123
ERG in diseases
AM Gokhale, SR Limaye
Department of Ophthalmology, Topiwala National Medical College, Bombay, India
|Date of Web Publication||16-Jan-2008|
A M Gokhale
Department of Ophthalmology, Topiwala National Medical College, Bombay
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Gokhale A M, Limaye S R. ERG in diseases. Indian J Ophthalmol 1966;14:117-23
All the living tissues -the nervous tissue in particular, show a property of electrical changes occurring when a resting phase is switched on t o an active phase. Retina is a very active nervous tissue and shows typical electrical changes.
To serve the visual function, radiant energy of light has to be absorbed by the visual pigment, where it is trans-formed into neural energy. When the light is absorbed by the visual pigment it triggers two series of responses
1) Degeneration and resynthesis of the visual pigment.
2) Neural conduction along the intra-retinal pathway.
The exact nature of connection between the two is still a matter of controversy.
There exists, independent of illumination a resting potential between the electro positive cornea and electro negative fundus. A steady current of 6-8 millivolts flows from the cornea to the fundus. This is the resting potential. The record of the changes modulating this resting potential in response to any sudden change of surrounding illumination is Electroretinogram. This action potential may be recorded by placing an electrode on the cornea and another on the forehead which is in continuity with the posterior pole of the eye, provided suitable amplifier and recorder are used. This electrical record is the sum total of electrical activities occurring throughout the retina. The pattern of the record may vary depending upon the position of the active electrode, whether on the cornea, or in the vitreous or intraretinal.
The ERG is described by a scientist both as a praise and a pain: praise, because records are obtained due to the sum total of the changes in the electrical potential in the absence of which perhaps records would not be obtained: pain, because localised changes of a particular part of the retina cannot be recorded. Moreover, excitation in one part brings about inhibition in the other part, and what we obtain is only the sum total.
1) Machine: We have an ERG machine manufactured by 'Elema' of Stockholm. The main peculiarity of the machine is an extremely sensitive pointer consisting of a glass tube thinner than a hair through which ink is pumped on the recording paper so that there is no friction while writing. Calibration of the machine is done every time before starting the record. Stimulus is flash light from a 25 Watt bulb kept at a distance of about 1 meter (approx.) which gives illumination of about 80 lux at the eye. The duration of the stimulus is marked on the paper.
2) Room : The ERG room should preferably be air-conditioned and sound-proof besides being completely dark.
3) Patient : Eye is anaesthetised with Anethain. The contact lens is placed on the cornea and all the connections are made. The patient is dark adapted for at least 20 minutes but preferably for 30 minutes. We present below many interesting records that we have obtained.
| Uses or ERG|| |
1) When the optical media of the eye are not clear e.g. leukoma, cataract or vitreous opacities, the condition of the retina cannot be visualised with an ophthalmoscope. The retinal function, to a certain extent can be assessed by ERG as it is the record of the change in the electrical potential from rest to excitation of rods and cones-the receptor of the light stimulus, for example ERG No. 5.
2) To differentiate between the inflammatory conditions and the abiotrophic degenerative conditions ERG may be useful in certain cases. In abiotrophic degenerative condition the essential pathology is in the sensitive layer of the retina i.e. the layer of rods and cones. If the rods degenerate, the electrical potentials are markedly reduced and ERG pattern is changed considerably in spite of absence of visible fundus changes e.g. in retinitis pigmentosa sine pigmento, ERG shows diminished scotopic `b' wave. In cases of inflammantery lesions affecting primarily the choroid there is not much destruction of the sensitive layer of the retina and hence rods would be spared and the ERG may be normal in spite of presence of the visible fundus lesions.
3) By decrease in the amplitude of scotopic 'b' wave, diagnosis of retinitis pigmentosa or congenital absence of rods can be made by ERG (No. 9, 10, 11). Similarly cases of vitamin A deficiency can be distinguished from xeroses without vitamin A deficiency and response to the treatment with vitamin A can be well assessed (e.g. No. 7 and 8).
4) In congenital colour blind patients there is decrease of photopic 'b' wave response.
5) ERG is also useful to find out malingeres complaining of night blindness. In all real cases of night blindness scotopic 'b' wave is decreased or absent.
6) It is very useful for assessing the prognosis in cases of retinal detachment. Very small 'b' wave indicates very poor viability of the retina as a whole.
7) It is also useful for differentiating primary optic atrophy from consecutive optic atrophy. The ERG reflects the activity only of inner layers of the retina and contributes no information regarding the state of the optic pathways or visual cortex. Thus in case of optic atrophy though the patient has no vision the ERG is within normal limits. But in cases of consecutive optic atrophy there is some destruction of the sensitive layer of the retina, and thus the electrical response is subnormal.
8) When high intensity light is used the ascending limb shows wavelets and these are known as oscillatory potentials. Oscillatory potentials are greatly reduced or absent in diabetic retinopathy but present in arteriosclerosis.
9) For experimental physiological work it will, in the long run be useful to determine the function of different retinal elements.
| Summary|| |
The practical application of ERG in differentiating different retinal conditions and determine prognosis is illustrated by examples.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18]