All procedures conformed to the ARVO Statement on Use of Animals
in Ophthalmic and Vision Research. Four mixed-breed dogs of 15- to
20-kg body weight were used in this study. The dogs were anesthetized
with halothane gas using a fluotec MK 3 vaporizer (Orchard Park, NY).
Pupils were dilated using 10% phenylephrine hydrochloride and 1%
tropicamide eye drops. A conjunctival peritomy was made, followed by
three sclerotomies each 3.5-mm posterior to the limbus. The
inferotemporal port was used for an infusion, and then via the superior
two ports a vitrectomy was performed using techniques that are commonly
used to strip the posterior hyaloid away from the retina in patients
with macular holes. Specifically, at the beginning of the vitrectomy
procedure, the aspiration pressure of the vitreous cutter was increased
to 200 mm Hg without activating the guillotine cutter. The vitreous
cutter was positioned close to the surface of the optic nerve head to
engage the posterior hyaloid and yet not aspirate retinal tissue. Under
the high-power magnification of the surgical microscope, the posterior
hyaloid was observed as it was aspirated into the port of the vitreous
cutter, and then the engaged vitreous was detached slowly from the
macular region. The vitreous was then cut and removed. To ensure that
no vitreous remained attached to the retinal surface, a silicone
soft-tip cannula was swept across the surface of the macula while
continuously aspirating through the cannula. If vitreous was present,
the cannula would engage it and the silicone rubber tip would bend.
This bending of the tip is akin to the bending of a fishing rod when a
fish is hooked onto the line. No such “fish-strike” sign was seen
in any of the dogs, indicating a thorough and complete stripping of the
posterior hyaloid. After the vitrectomy, the superotemporal port was
enlarged circumferentially to 3.5 mm to allow introduction of the
electrode array into the eye. The electrode array was introduced
through the superotemporal port and held in the mid-vitreous cavity
with intravitreal end-gripping forceps (Grieshaber Co, Kennesaw, GA).
Next, a retinal tack loaded in a retinal tack holder (Grieshaber Co)
was introduced through the superonasal port. The electrode array was
then tacked onto the area centralis. Usually, two tacks were used to
affix the electrode array to the retina. During the procedure, to
prevent any choroidal bleeding at the site of retinal tacks, the
intraocular pressure was raised by adjusting the height of the infusion
fluid bottle. The cable from the array exited the eyeball and was
passed underneath the lateral rectus muscle and sutured at several
points to the sclera. The cable was left subconjunctivally.
Superotemporal and superonasal sclerotomies were closed with 7-0 vicryl
(Ethicon, Somerville, NJ) mattress sutures, and the conjunctiva was
closed with interrupted 6-0 vicryl sutures (Ethicon). Antibiotic and
steroid ointment was applied at the end of the surgery. The dogs were
then extubated and when fully alert returned to their kennels.
Postoperatively, prednisolone tablets (1 mg/kg body weight once daily)
were given for 1 month and then tapered over the next 2 months. Topical
antibiotic and steroid ointment were also applied for the first 1
month.
Color fundus photography, fluorescein angiography, electroretinography
(ERG), and visual evoked potentials (VEP) were obtained preoperatively
and at postoperative weeks 1 and 2. These tests were then repeated
every 2 weeks up to 2 months and then at every 1-month interval.