Page 2206 - Cote clinical veterinary advisor dogs and cats 4th

1097.e4 Electroretinogram

anesthesia only (and contact lens electrode), Preparation errors: • Dark adapt patient for minimum of 4
but the risk of artifact is increased. • Failure to dilate pupils fully minutes, optimum of 20 minutes. Before
VetBooks.ir ment (artifact source) and allows more precise • Failure to direct light stimulus directly into • Place topical anesthetic in eye.
performing ERG on second eye, dark adapt
Procedural errors:
• General anesthesia prevents animal move-
dog again for at least 4 minutes.
eye
direction of the light stimulus into the eye
by fixation of the eye.
contact lens.
eye while recording first eye; this decreases
• Sedation using one of these protocols IV or • Failure to prevent light spill into fellow • Place conducting gel on concave surface of
IM amplitude of second eye by bleaching retinal • Place contact lens in eye, ensuring that the
○ Acepromazine 0.03 mg/kg and hydromor- pigments lens is centered on the cornea and not on
phone 0.05 mg/kg, or • Error in labeling which eye is which on top of the nictitating membrane.
○ Dexmedetomidine 0.0025-0.005 mg/ recordings • Perform standardized protocol. Protocols are
kg (2.5-5 mcg/kg: dogs, cats) com- • Lack of standardized protocol for specific available in ERG unit’s software. To deter-
bined with hydromorphone 0.05 mg/ model of equipment on premises mine rod function, important in progressive
kg or butorphanol 0.2 mg/kg (dogs, • Lack of standardized anesthesia/sedation, rod-cone degeneration, a blue filter under
cats) leading to difficulty in interpretation of scotopic conditions must be used.
• Other anesthetic or sedation protocols may results • Use signal averaging. This produces an
be employed; be consistent to aid in correct • Excess noise due to failure to use signal averaged response after a predetermined
interpretation of the recordings. averaging and filters number of flashes (5-10) and enhances the
• General anesthesia and sedation depress the • Excessive muscle fasciculation in eyelids due signal-to-noise ratio.
ERG response. to inadequate sedation leads to excess noise in • Use bandpass filters to screen out unwanted
• Butorphanol is the sedative most likely to the readings. The use of an eyelid speculum frequencies.
depress ERG response. exacerbates muscle activity due to muscle • A normal ERG consists of a low-amplitude
• Topical anesthetic (i.e., proparacaine) stretching. negative a wave 10-12 msec after the light
Personnel (two people): • Dull electrode needles stimulus, followed by a large positive b wave.
• One to monitor anesthesia or sedation, • Gap in gold-foil electrode ring on contact In some tracings, the c wave (a third wave)
position the animal’s head, ensure that light lens electrode, leading to inaccurate reading is seen as a positive deflection.
stimulus is directed into the eye, cover the or artifact • The a wave is produced by the photoreceptors
fellow eye, and check to see that all electrodes • Expulsion of contact lens or reference elec- and the b wave by movement of potassium
are in position during test trodes during procedure, leading to artifact in and out of Müller cells and indirectly by
• One to run the computer program during or inaccurate reading bipolar cells.
the test • The 60-cycle interference caused by lights,
appliances in same room, noninsulated table Postprocedure
Anticipated Time • Stray light stimulus from computer monitor • Remove electrodes from eye and skin.
About 1-1 2 hours, including time for pupillary during procedure, leading to lower amplitude • Soak contact lens electrode in distilled water.
1
dilation readings Do not rub the inside surface of the lens to
Animal-derived errors: clean it because this action will remove the
Preparation: Important • Eyes are closed, leading to very-low-amplitude gold-foil circle.
Checkpoints readings. • Flush eye with sterile eye wash, and stain
• Knowledge of retinal physiology • Blinking during light stimulation, leading for corneal ulcer.
• Familiarity with ERG equipment to low-amplitude readings • Reverse sedation/recover from general
• Control of inflammation; especially impor- • Third eyelid is prolapsed, expelling the anesthesia.
tant in a pre–cataract surgery evaluation. contact lens or leading to very-low-amplitude • Measure amplitude of recordings, the
Lens-induced uveitis will decrease ERG readings due to the failure of light to reach amplitude from the nadir (lowest point) of
amplitudes. retina. the negative a wave to the peak of the positive
• Dilation of pupils • Patient too sedate, eye deviated ventrally, b wave.
• Bleaching of retinal pigments in rods light does not reach retina • Record findings on a separate disc or CD
by exposure to bright light before dark Interpretation errors: for a backup copy, and print/scan copy for
adaptation • Misinterpretation due to lack of knowledge animal’s record.
• Dark adaptation (4 minutes minimum, 20 of retinal physiology and test basis
minutes desirable) Alternatives and Their
• Lock room to avoid inadvertent exposure Procedure Relative Merits
to light and necessity of starting procedure • Measure intraocular pressure: if > 20 mm Genetic testing for inherited retinal disease:
again. Hg, do not dilate due to possible glaucoma. • Many tests are available for specific dog
• Use grounded outlet to decrease/eliminate Dilating pupil may cause iridocorneal angle breed–related inherited retinopathies but
60-cycle interference. closure. not for all breeds.
• Take off wristwatch to decrease 60-cycle • Dilate pupils with short-acting mydriatic (i.e., • Requires only blood test, so simpler than
interference. tropicamide) 30 minutes before sedation. ERG
• Set up computer, supplies, equipment before • Sedate or induce general anesthesia as Maze test under photopic and scotopic
dark adaptation. necessary. conditions:
• Place reference electrode in skin over ipsi- • May be gross screening test for retinal
Possible Complications and lateral zygomatic arch, 5 cm distal to lateral atrophy
Common Errors to Avoid canthus. • Unable to determine anatomic source of
Possible complications: • Place ground electrode in skin over top of defect in sudden blindness cases
• Corneal ulcers, especially in diabetics muzzle. If dog must be muzzled, place ground
(decreased corneal sensitivity and increased electrode in skin on forehead. If electrode SUGGESTED READING
risk of corneal ulceration) needles do not go into skin easily, discard Komáromy AM, et al: Technical issues in electro-
• Hypoxemia or death due to failure to monitor and use new electrodes to avoid discomfort diagnostic recording. Vet Ophthalmol 5:85-91,
animal during general anesthesia to animal and increase accuracy of reading. 2002.

www.ExpertConsult.com