Electroretinography in Cats (Felis) | Vetlexicon
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Electroretinography

ISSN 2398-2950

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Synonym(s): ERG

Introduction

  • Measures the potential difference generated across the retina when stimulated by light.
  • This provides a means of measuring retinal function.
  • The electroretinogram (ERG) consists of a negative 'a' wave, positive 'b' wave and a positive 'c' wave which correspond to the photoreceptors, Muller's cells and retinal pigment epithelium respectively.
  • Many modifications possible to distinguish between rod and cone response.
  • Pattern electroretinograhy is used in research settings to investigate retinal ganglion cell function within the inner retina in cases of glaucoma.
  • Visual evoked potentials are used in a research setting to investigate the central visual pathways.

Uses

  • The most common indication in clinical practice is evaluation of retinal function prior to cataract surgery, where fundus assessment is not possible due to the lens opacity.
  • Investigation of sudden onset blindness when the fundus looks normal; most often to distinguish sudden acquired retinal degeneration syndrome from retrobulbar (brain or optic nerve disease) blindness Blindness.
  • Investigation or early detection of various inherited retinopathies (dystrophy or degeneration) Retina: inherited degenerations Retina: degeneration (FPRD/FCRD comparison).

Advantages

  • Objective, repeatable test.
  • Non-invasive.
  • Equipment becoming more affordable and portable systems available.

Disadvantages

  • Specialized equipment and training required.

Technical problems

  • Specialist knowledge required.
  • Artifacts can arise, such as electrical noise interference from electrical equipment in the room.

Requirements

Personnel

Anesthetist expertise

  • Typically carried out in lightly sedated cats, in combination with topical anesthesia Local anesthesia: retrobulbar.
  • Measurement of visually evoked potentials requires deep sedation or general anesthesia.

Materials required

Minimum equipment

  • Electroretinogram unit and computer system.
  • Dark room.
  • 'Safe light'.

Stimulating system

  • Strobe light or optical bench with light and shutters.
  • Filters to alter light color and intensity.

Recording system

  • Corneal (active/positive) electrode.
  • Reference (negative) electrode.
  • Ground electrodes.
  • Amplifier.
  • Oscilloscope or computer recording system.

Minimum consumables

  • Sedative or general anesthetic (GA).
  • Mydriatic, eg tropicamide Tropicamide.

Preparation

Pre-medication

  • Apply mydriatic to eye.

Restraint

  • Sedation or general anesthesia generally required.
  • The advantage of general anesthesia is that it eliminates background muscle activity which will interfere with the measurement of the ERG.

Technique

Approach

Step 1 - Dark adaptation

  • Approach varies depending on whether a rod or cone response is being investigated.
  • Dark adaption for a minimum of 20 min.
  • Following dark adaption, a 'safe light' should be used to provide illumination for the remainder of the procedure.

Step 2 - Reference and ground electrodes

  • Reference electrode applied subdermally adjacent to the lateral canthus.
  • Ground electrode placed subdermally by the ear base or occiput.

Core procedure

Step 1 - Obtaining the ERG

  • Apply corneal electrode with a coupling agent such as carbomer gel.

Step 2 - Positioning

  • Position strobe light adjacent to test eye and part palpebrae with eyelid speculum.

Step 3 - Measurement

  • The impedance of the three electrodes should be checked to ensure that it is low before proceeding. If it is high, the electrodes should be repositioned and the impedance rechecked.
  • Trigger single strobe pulse and measure the ERG displayed on the oscilloscope.

Step 4 - Repeat

  • Repeat procedure if required.

Step 5 - Record and analyze

  • The computer records the signal.
  • The waveform is analyzed at the time of recording to determine if there are any artifacts with the traces obtained.

Aftercare

Immediate Aftercare

Special precautions

  • Keep patient in subdued environment until effects of the mydriatic and sedation have worn off.

Outcomes

Reasons for treatment failure

  • Poor dark adaptation (depends if measuring light adapted or dark adapted ERG).
  • Poor contact between corneal electrode and corneal surface.
  • Electrical interference.

Further Reading

Publications

Refereed papers