Chronic ocular hypertension alters local retinal responsiveness.

Abstract

Electrophysiological responses of the retina and visual cortex to a series of grating stimuli (6-768 minutes of arc) were recorded in seven sessions using normal beagles, 21 sessions using beagles afflicted with inherited ocular hypertension, and 12 sessions using rhesus monkeys. A 150 field centred around the animal's area centralis or fovea was used to stimulate the central retina. A 30°field, centred on the same spot, was then used to stimulate the larger area. Two recording series were completed on each animal, with both field sizes presented in each recording session. The first recording took place 30 minutes after and the second 2 hours after the injection of thiamylal sodium. Only the signals from the toroidal 150 of the retina of the hypertensive dogs were remarkably larger during the second recording (p=0001). No significant differences were found between the two recordings from the retinas of normal dogs or monkeys, nor were there any significant differences between the two recordings from above the cortex in any group. Several hypotheses are proposed to explain the basis for the interaction of thiamylal with the more peripheral retinal function in clinically glaucomatous dogs. (BrJ Ophthalmol 1993; 77: 502-508) Thiamylal sodium is the thiobarbiturate analogue of secobarbitone. It is an ultra short acting drug, commonly used for induction of general anaesthesia for several short term veterinary surgical procedures. In the dog the drug is metabolised rapidly by the liver,' and has no apparent cumulative effect.23 The duration of anaesthesia in the dog is 10-45 minutes,2 with complete recovery after 113 minutes.3 In mixed breed dogs, disposition of thiamylal is characterised by a rapid distribution phase (half life 38-9 minutes) and an elimination phase with a half life of 666 minutes.4 The total plasma clearance of the drug in dogs is 3-21 ml/min/kg. 18 Despite this wealth of information on the multifactorial effect of anaesthetic drugs and increased IOP on visual electrophysiology, we find no information on the interactions between anaesthesia and OHT in the glaucomatous retina. This is surprising, considering the fact that ERG recordings are used in many institutions to diagnose and monitor the progress of glaucoma in non-verbal patients such as children, mentally handicapped people, and animals.'9-22 They may be used to diagnose glaucoma at early stages, before the appearance of funduscopic changes and visual field losses.'ll22 Frequently at this stage the visual deficits are not entirely diffuse, with the greatest losses in the more peripheral, paracentral regions of the visual field. 19 21 23 We report on interactions between thiamylal and different retinal regions of normal and ocular hypertensive dogs. Materials and methods EXPERIMENTAL ANIMALSSeventeen beagle dogs and seven rhesus monkeys (Macaca mulatta) were used in the study. Of the 17 dogs, 13 were hypertensive and four were normal. Some ofthe animals were used for more than one recording session. In such cases, the minimum inter...

Progressive changes in ophthalmic blood velocities in Beagles with primary open angle glaucoma

“…Past studies in the Beagle POAG model have suggested impaired circulation to the optic nerve head and retina when IOP is elevated above normal limits. Pattern electroretinograms of the central and peripheral fundus, 32 increased sensitivity of the peripheral populations of retinal ganglion cells to a thiobarbiturate (thiamylal sodium), 33,34 increased rate of loss of the large diameter retinal ganglion cells axons, 35 and elevated levels of intravitreal glutamate 36 all support this theory in this model.…”

Comparison of full-field electroretinogram in diabetic and non diabetic dogs with cataracts

“…ERG recordings are influenced by many factors, such as pupil size, dark adaptation time, electrodes, stimulus intensity, age, breed (Hérbert et al 1996, Marmor et al (Maehara et al 2007), glaucoma (Ofri et al 1993) retinal hereditary degenerations (Safatle et al 2005) or diabetic retinopathy (Bresnick & Palta 1987, Layton et al 2007. In this survey, studied animals did not present any other systemic or ophthalmic disorder then diabetes and mature or hypermature cataracts; each subject was submitted to the same protocol, including dark adaptation time, electrodes, ERG machine, stimulus intensity, sedation protocol and fully pupil dilatation.…”

Determinação dos valores normais do eletrorretinograma de campo total em cães da raça Poodle portadores de catarata de acordo com a faixa etária

“…Em cães, as indicações mais comuns para ERG são: avaliação pré-cirúrgica de pacientes com catarata, caracterização de distúrbios que causam cegueira como glaucoma, acromatopsia (KOMAROMY et al, 1998b;RUBIN, 1971;SIMS, 1999), displasia retiniana, retinopatias degenerativas (OFRI et al, 1993;SAFATLE et al, 2005), hipoplasia de nervo óptico, degeneração retiniana repentina adquirida e lipofuccinose ceroide neuronal (NARFSTRÖM et al, 2007). As avaliações eletrorretinográficas em cães propiciam também um importante modelo para o estudo da distrofia retiniana que acomete o homem (PETERSEN-JONES et al, 2006).…”