Electroretinography (ERG) for Clinical Evaluation of Vision Abnormalities - A Clinical Study of 9 Dogs.
Electroretinography (ERG) is useful adjunct test for diagnosis of retinal dystrophies and for pre-operative evaluation of retinal functions in conjunction with cataract surgery. ERG recordings were done in 9 dogs with various vision abnormalities viz. cataract, retinal detachment, vitreous degeneration, retinal degeneration and glaucoma. The causative factor for vision abnormality in majority of cases was cataract (n=5, 55.55%) followed by retinal degeneration (n=2, 22.22%) and one dog each with glaucoma and chronic kidney disease (11.11% each). ERG was used to quantify the severity of retinal impairment in these animals.
Keywords: Dog; electroretinography; vision abnormalities
Retina is in the deepest portion of eye and difficult to examine without specialized instrumentation. The most widely used method of examination of retina involves indirect ophthalmoscopy to inspect morphology of retina combined with electroretinography (ERG) to assess retinal function.
It is a useful tool for differential diagnosis of various ophthalmic diseases in small animals (Gum et al., 1984). Obtaining an electroretinogram (ERG) is not more complex than obtaining an electrocardiogram. The only obstacles involved are understanding and interpretation of waves in relation to the wide gamut of retinal diseases (Carr and Siegel, 1990).
In dogs, ERG is mostly used for pre-operative evaluation of retinal function before cataract extraction as many dogs may be simultaneously affected with both progressive retinal degeneration (atrophy) and cataract, for characterization of retinal disorders in which no ophthalmoscopic abnormalities are evident such as glaucoma, retinal dysplasia, degenerative retinopathies, optic nerve hypoplasia, hemeralopia, Sudden Acquired Retinal Degeneration (SARD) and differentiating between retinal and post-retinal causes of blindness viz. cases of SARD and optic neuritis may present similarly with acute loss of vision, a normal-looking fundus and fixed, dilated pupils as well as early diagnosis of inherited photoreceptor atrophies (Ofri, 2006).
Hence it was envisaged to diagnose various ophthalmic disorders based on ERG.
Materials and Methods
The present clinical study was conducted on nine dogs with various vision abnormalities irrespective of sex. Prior to electrodiagnosis, all the animals underwent detailed ophthalmic examination which included complete history pertaining to visual abnormality and clinical assessment of visual functions by reflexes like menace, pupillary light, 4th purkinje and tapetal, different tests like fluorescein staining, schirmer's tear test, tonometry, complete ophthalmic examination with hand held slit lamp biomicroscope and binocular indirect ophthalmoscope and B-mode ultrasonography in appropriate cases. After anamnesis and visual examination of patient, dogs were subjected to electroretinography (ERG).
All animals were fasted for at least 12 hr before performing ERG. The electrodiagnosis of retina was performed under general anaesthesia using a mixture of Ketamine HCl (50mg/ml) and Midazolam (10mg/ml) in the ratio 2:1 IV for induction along with Atropine Sulphate @ 0.03 mg/kg SC as premedicant.
The dog was placed in sternal recumbency in an exclusive dark room. All electronic devices viz. mobiles, batteries, etc. were kept off to avoid electrical interference with ERG unit. Before ERG's were recorded, impedance and baseline tests were performed, the latter for evaluating the noise level in environment. Electrodes were placed as per guidelines for ERG in dogs by Narfstrom et al. (2002) and ERGy was conducted using a table top RETI port ERG and VEP unit (S&V Technologies AG, Germany).
The a-wave amplitude was measured from baseline to a-wave trough and b-wave amplitude was measured from a-wave trough to b-wave peak. Then b/a ratios of amplitude were calculated.
Results and Discussion
Out of nine dogs presented with vision abnormalities, five dogs (55.55%) were from the age group of 1-5 years and rest (44.45%) between 6-10 years.
In eight dogs (88.88%), there was bilateral vision abnormality and in one (11.12%) unilateral. The causative factor for vision abnormality in majority of cases was cataract (n=5, 55.55%) followed by retinal degeneration (n=2, 22.22%) and one dog each with glaucoma and chronic kidney disease (11.11% each).
The menace response, the pupillary light response and dazzle reflex was found negative in dogs with glaucoma and CKD. 4th Purkinje reflex was observed in six cases and tapetal reflex was found to be positive in seven cases. Intraocular pressure in all dogs was within normal range except the dog with glaucoma with IOP 90 mm Hg for left eye. Bilateral vitreous degeneration was noticed in one dog following ultrasonography with 18 MHz frequency linear probe. Vitreous degeneration was ultrasonographically seen as a large, irregular hyper-echoic structure in vitreous chamber. Bilateral retinal detachment was noticed in one dog with CKD following ultrasonography. Detached retina was seen as hyper echogenic membrane in vitreous body.
ERG of eyes that had mature and hypermature cataracts (Fig. 1) detected decrease in b-wave amplitude. Mean peak to peak amplitudes seemed to be smaller for each dogs with b/a ratio higher than the normal range; 3.74+0.97 for dog with mature cataract and 5.40+2.72 for dog with hypermature cataract and vitreous degeneration indicating affection of outer retinal layers. Cataracts may occur secondary to retinal degeneration. Cataractous dogs with inherited retinal disease must be differentiated from dogs without retinal pathology before surgical treatment (Chiu et al., 2009).The presumed reason for decreased ERG amplitude and prolonged implicit time in eyes with mature cataract is that cataract acts as a filter reducing stimulus strength (Maehara et al., 2007).
ERG of Labrador retriever indicative of congenital retinal degeneration (Fig. 2) revealed non-detectable scotopic rod responses and scotopic maximal response in flash ERG. Oscillatory potentials samplitude measured with the addition of OP wavelets was severely diminished. The ERG was able to quantify the severity of retinal impairment, confirming the retinal degeneration diagnosis in Pit Bull dog with severe visual impairment (Safatle et al., 2005).
For, ERG of eye with glaucoma (left eye) (Fig. 3) the scotopic b/a ratio was lower than normal range and photopic b/a ratio was higher than normal range. Similar findings were observed in a dog with sudden loss of vision, wherein scotopic and photopic b/a ratio of left eye was higher than normal range while scotopic and photopic b/a ratio for right eye was lower than normal indicative of SARD. There is no citation in the literature regarding ERG findings in dogs with glaucoma and sudden acquired retinal degeneration. In Veterinary clinics, ERG is used to evaluate retinal function in animals with cataract, glaucoma, PRA, SARD and other conditions (Narfstrom et al., 2002).
The established b/a ratio at intensity of 3 cds.s/[m.sup.2] in healthy Spitz dog is 2.33+0.73 (Kelawala, 2014). ERG in the dog with Chronic Kidney Disease (CKD) (Fig. 4) induced retinal detachment revealed rapid reduction in amplitude of b-wave. But b/a ratio for ERG were within normal range i.e. 2.02+0.38 indicating that detached retina is electrically active and non-invasive method could assess the recovery potential of detachments before surgery. In retinal detachments the scotopic ERG is generally more disturbed than photopic ERG; both are more disturbed than would be expected from visible detached retina. The disturbance is characterized by reduction of both the a-wave and b-wave. Furthermore, the photopic responses are clearly delayed when detachment extends over more than half of retina, giving a typical, even pathognomonic, wave form when the detachment covers more than three quarters of retina (Van Lith et al., 1981).
As many of the dogs with cataracts also had certain degree of retinal dysfunction in this study and in some dog breeds, ERG may detect changes in retinal function long before ophthalmoscopic or behavioral signs are observed, pre-surgical electro-retinographic evaluation of the retina is strongly advised.
We are grateful to the owners of dogs for their willingness and permission for conducting electroretinography. This work was supported by grants from Department of Biotechnology (DBT), Govt. of India.
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D.N. Kelawala (1), D.B. Patil (2), P.V. Parikh (3), K.R. Sini (4) and U.Y. Kothamdi (4)
Department of Veterinary Surgery and Radiology
College of Veterinary Science and Animal Husbandry
Anand Agricultural University (AAU)
Anand - 388001 (Gujarat)
(1.) Ph.D. Scholar
(2.) Director of Research and Dean (PGS), Kamdhenu University, Gandhinagar and Corresponding author. E-mail: firstname.lastname@example.org
(3.) Professor and Head
(4.) Post Graduate Scholar
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|Title Annotation:||Clinical Article|
|Author:||Kelawala, D.N.; Patil, D.B.; Parikh, P.V.; Sini, K.R.; Kothamdi, U.Y.|
|Date:||Jul 1, 2016|
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