Veterinary diagnostic ophthalmology

1. Dr. Urfeya Mirza

2. Introduction
History
Indications
Types of ERG
Technical aspects
Clinical applications
Factors influencing ERG
Limitations of ERG
Commercial ERG devices
Conclusions

3. Electroretinography (ERG) is an electrophysiological and functional test of
the retina which provides an objective and quantitative measure of the
retinal function in response to light stimulation.
(Schaeppi and Liverani, 1987; Samuels, I. et al., 2017)
• .
It is a mainstay of clinical ophthalmic diagnostic testing which establishes
the function of the structures visualized by ophthalmoscopy, a property
that is not shared by the other electrodiagnostic techniques.
(Sims, 2000)
• .
It is a useful tool for the differential diagnosis of various ophthalmic
diseases in animals.
(Aguirre and Rubin, 1992; Aguirre, 1995; Sims, 2000; Oriá et al., 2004; Bianca Brüggen et al., 2016)

4. Eye
Optic Tract
Optic Chiasm
Optic Nerve
Lateral Geniculate
Nucleus
Occipital Cortex

6. The retina is the photosensitive lining of the posterior segment of the globe.
The retina has basically two layers: the retinal pigment epithelium and the
neuroretina.
The neuroretina is disposed in nine layers: the visual cell layer (rod and cone
layer), the outer limiting membrane, the outer nuclear layer, the outer
plexiform layer, the inner nuclear layer, the inner plexiform layer, the ganglion
cell layer, the nerve fiber layer and the inner limiting membrane.
There are 2 types of light-sensitive cells (photoreceptors): cones and rods.
The macula, composed mainly of cones, is used primarily for color vision
(photopic vision) while the remaining retina, composed mainly of rods is used
primarily for night (scotopic) vision.
Veterinary Ophthalmology (4th Ed.
2007) by Kirk N. Gelatt.

10. Inherited abnormalities Trauma
Metabolic disturbances Generalized infections
Tumors Blood disorders
High blood pressure Nutritional deficiencies

11. Electroretinogram
Multifocal Electroretinogram
Pattern Electroretinogram
Multifocal Visual Evoked
Potential
Electrooculogram
ERG mERG
pERG mVEP
EOG
Electrophysiological Tests
In small animal practice, ‘Electroretinography’ is most useful
than other diagnostic techniques for the assessment of retinal
function.
Liapis, I.C. 2004

12. The beginnings of visual electrophysiology date back to the 19th century when
DuBois-Reymond (1849) discovered the resting potential between the anterior
and posterior pole of the un-stimulated eye.
Holmgren (1870) demonstrated differences in the eye electrical potential
generated by light stimulation.
Einthoven and Jolly, (1908) recorded detailed records of the frog’s eye with a
string galvanometer .
Kahn and Lowenstein (1924) presented ERG wavelets recorded in humans with a
string galvanometer.
In the mid-20th century, Karpe (1945) reported that electroretinography was an
examination used to determine retinal function.
In veterinary medicine, Parry et al. (1953, 1955) were the first to perform ERG in
healthy and diseased dogs, respectively.
Rubin (1963) examined blind cats and was the first to justify the usefulness of
ERG in evaluation before cataract surgery.

13. Diagnosis and evaluation of retinal function in animals when the optic
system is not transparent due to cataract, glaucoma, intoxications and
progressive retinal atrophy (PRA).
Differential diagnosis between the sudden acquired retinal degeneration
(SARD) and optic neuritis.
Diagnosis of retinal diseases in cats, such as non-inflammatory
retinopathies, hereditary retinal degenerations and central retinal
degenerations due to dietary taurine deficiency.

14. ERG
Full field
ERG
Pattern
ERG
Focal ERG
Multifocal
ERG

15. Full-Field ERG:
Retinal potential elicited by a brief flash of light, recommended to be
about 5 ms in duration, that evenly illuminates the entire retina.
Pattern ERG:
It mainly represents inner retinal activity. Recorded with full correction of
refractive errors as visualization of stimulus for extended time is essential
for recording.
Focal ERG:
Used for detecting small focal lesions or pathologies. A small stimulus of 4⁰
size is projected on area of retina to be tested. Mostly used in research
setting than in clinical setting.
Multifocal ERG:
The stimuli consists densely arranged black or white hexagonal elements
displayed on CRT monitor. These hexagonal elements change from light to
dark independently and this change results into recording of mfERG.

17. Conducted in ambient light. Pre-exposure to strong light is
avoided. A dark-adaptation period of 1 hour is recommended.
The animal is fully anaesthetized in order to prevent artifacts
through involuntary muscle movement.
Proper oxygenation and ventilation is maintained. Body
temperature is kept stable at about 39⁰C.
Eyelids are kept open and pupils are fully dilated throughout
the examination.
Proper positioning of the pupil is maintained by the use of
sub-conjunctival stay sutures at the limbus to stabilize the
globe.

18. The use of full-field conditions, such as the Ganzfeld
stimulator, in order to obtain a uniform distribution of
light across the retina is recommended.
Light flashes are not more than 5 milliseconds long.
White light is used both for stimulus and background.
Neutral density filters are used in front of the light source to
attenuate the light and modify the light stimulus.

20. GANZFELD STIMULATION GLOBE

21. Reusable corneal contact lens electrodes with adequate
curvature are used.
A reference electrode is placed halfway between the
temporal canthus and the ear.
A similar subcutaneous ground electrode is placed at an
indifferent location, such as at the central top portion of
the head.
The equipment enables the amplification of the signal so
that recordings are evaluated with high accuracy.

22. (A)Positive electrode.
(B)Reference electrode located 1 cm caudally to the lateral eye canthus.
(C)Ground electrode positioned in the region of the occipital
protuberance.

23. Reusable corneal
contact lens
electrode with
adequate curvature

25. Electrode placement for ERG recording, with the use of Kooijman-Damhof
corneal electrodes with a built-in light source and a light diffusing contact lens.

26. Electrode placement for ERG recording, with the use of Jet mono-polar
corneal electrodes.

27. A-wave (initial negative wave) = photoreceptors
Reduced in retinitis pigmentosa (RP)
B-wave (large postive wave) = inner retinal layers
Decreased in retinoschisis, central retinal vein occlusion (CRVO),
central retinal artery occlusion (CRAO), congenital stationary night
blindness(CSNB)
Oscillatory potentials (a ripple of 3 or 4 wavelets at
the ascending limb) = amacrine cells
Attenuated in central retinal vein occlusion (CRVO), central
retinal artery occlusion (CRAO), central serous
retinopathy(CSR), congenital stationary night blindness(CSNB)
A report of the ERG recording includes the following waves:

29. Amacrine cells
Muller cells
and bipolar
cells
Rods and
cones

30. Amplitude :
a-wave measured from
the baseline to the trough
of a-wave.
b-wave measured from
the trough of a-wave to
the peak of b-wave.
Time sequences :
Latency:- time interval
between onset of stimulus and
the beginning of the a-wave
response. Normally it’s 2 ms.
Implicit time:- time from the
onset of light stimulus until
the maximum a-wave or b-wave
response.

46. Factors related to the conditions of the procedure:
Parameters of the stimulus
Retinal adaptation to the ambient light
Background luminance during the procedure
Type of recording electrodes
Electric properties of the stimulating and recording systems.
Factors related to the patient:
Species, breed and age of the animal
Oxygenation of the patient
Globe, eyelid or muscle movements during the procedure
Pupil size
Depth and type of anesthesia
Transparence of the refracting tissues of the eye.
Mentzer et al., 2005

47. Since ERG measures only the mass response of
the retina, isolated lesions like a hole
hemorrhage, a small patch of chorioretinitis or
localized area of retinal detachment can not be
detected by amplitude changes.
Disorders involving ganglion cells (e.g. Tay
sachs’ disease), optic nerve or striate cortex do
not produce any ERG abnormality.
Miller and Murphy, 2003

50. RETIport Animal System

52. Ganzfeld ERG Focal ERG

53. Photopic Type

54. Scotopic Type