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OPERATIVE TECHNIQUE
The technique detailed here is essentially the same as that originally described by Hilton and Grizzard in 1986, with a few modifications. The operation is usually performed in an outpatient department or in the surgeon’s office.
ANESTHESIA
Pneumatic retinopexy can be accomplished with topical, subconjunctival, or retrobulbar anesthesia, depending on the surgeon’s and patient’s preferences. Sensitive patients may do better with a retrobulbar injection.
An injection into the anterior muscle cone will usually produce anesthesia without akinesia initially. This has the advantage of allowing the patient to position his eye to cooperate with cryopexy. After the retrobulbar anesthetic is given, cryopexy is administered promptly before the eye becomes akinetic.
Rarely, general anesthesia (avoiding the use of nitrous oxide) may be indicated if the patient is very young and/or apprehensive.
RETINOPEXY
Pneumatic retinopexy is generally performed in one session with cryopexy applied to the retinal breaks prior to gas injection (Figure 8–1B). An alternative technique involves a two-part procedure, utilizing laser instead of cryotherapy. The first part of the procedure consists of injection of a gas bubble into the vitreous cavity. The patient maintains appropriate head positioning at home, with follow-up in the surgeon’s office. Once the break is reattached, usually the next day, laser treatment is applied.
The photocoagulation is greatly facilitated by use of the laser indirect ophthalmoscope (LIO), which makes it easy to position the patient’s head to move the gas bubble away from the break(s). Treatment can also be applied with a slit lamp laser delivery system by tilting the patient’s head as needed. It is generally best not to attempt to laser until the retina is completely reattached in the treatment area.
Although one can treat the breaks through a moderately large gas bubble, one must be careful not to overtreat. Gas has an insulating effect, conducting heat away from the laser spot at a slower rate than vitreous, which may result in excessive thermal burns with retinal necrosis and hole formation.
Cryopexy versus laser
A one-part procedure with cryopexy is usually preferred over a two-part procedure with laser. Retinal breaks are easier to find when they are detached, and a one-part procedure is usually more convenient.
Certain circumstances might indicate the use of laser instead of cryotherapy. Very posterior breaks are easier to treat with laser than with cryo. The chorioretinal adhesion with laser may be obtained more quickly and firmly than that with cryo. If multiple large breaks are present, laser may be better than extensive cryopexy. When there has been a recent surgical incision in the eye, laser may be safer than cryo because it may be easier to avoid applying pressure to the eye.
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Transcleral photocoagulation may provide some advantages over both cryopexy and transpupillary laser. Like transpupillary laser photocoagulation, one can see exactly where the treatment will be applied and where it has been applied. As with cryopexy, it allows treatment in detached retina. Treatment can even be applied through a preexisting buckle.
STERILIZING THE EYE
A sterile lid speculum is utilized. About six drops of undiluted povidone-iodine solution are instilled directly onto the cornea and conjunctiva, and left in contact with the eye for a few minutes. During this time the gas can be prepared. The injection site is then dried with a sterile cotton-tipped applicator, and the eye is ready for paracentesis and injection of gas.
Be certain to use a povidone-iodine solution which does not contain alcohol or detergent. Preoperative topical antibiotics add nothing to the sterility of a careful sterile prep. Meticulous sterility is mandatory. No cases of endophthalmitis have been reported following pneumatic retinopexy when povidone-iodine solution was used as described above.
PREPARING THE GAS
A pressure-reducing system is attached to the gas cylinder to allow drawing of the gas from a low pressure reservoir. High pressure can blow out the millipore filter and render it useless in sterilizing the gas. A urinary external catheter can be attached to the cylinder, or a step-down valve system can be used. Alternatively, the gas may be drawn into a large syringe and then transferred to a small syringe (Figure 8–3).
The selected gas is drawn through a millipore filter into a 3 ml syringe in a sterile fashion. The tube connecting the gas cylinder with the syringe, including the filter, is flushed through with gas to ensure no dilution with room air. A few milliliters of gas are drawn into the syringe, discarded, and the syringe is filled again. A disposable 30-gauge, one-half inch (12 mm) needle is then placed tightly on the syringe, and excess gas is expelled to leave the exact amount intended for injection. The gas should not be stored in the syringe for more than a few minutes prior to injection because room air will infiltrate the syringe and dilute the gas sample.
MAKING ROOM FOR THE GAS
The intraocular volume must be decreased before and/or after the gas injection to make room for the gas. We recommend that a paracentesis be performed before the gas injection, especially if the globe has been weakened by surgery or trauma within the past six weeks, or if there is significant glaucomatous optic nerve damage. Laser may be preferred over cryopexy in these cases, since cryopexy may also cause dehiscence of the unhealed wound or raise pressure excessively.
Paracentesis before or after the injection is usually necessary, but prolonged ocular massage, including compression from cryopexy, may be sufficient in some cases.
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C
Figure 8–3. Methods for drawing low-pressure gas into syringe. (A) Balloon system. (B) Valve system. (C) Syringe system.
Paracentesis
The intended paracentesis site should be sterilized with Betadine solution as described above. A 30-gauge, 1/2-inch needle on a 1 ml syringe with the plunger removed is passed obliquely into the anterior chamber through the limbus, staying over the iris with the bevel up. Fluid will flow passively into the syringe. As the flow of fluid slows, gentle pressure on the sclera with a cotton-tipped applicator will facilitate fluid egress. Remove as much fluid as can be obtained, up to about 0.45 ml. Fluid flow can be quite slow at the end, and it may take a few minutes with the needle in the eye to remove sufficient fluid.
If the posterior lens capsule is absent or open widely, paracentesis should not be performed through the limbus to avoid incarceration of vitreous in the limbal needle tract. With plunger in place, pass the needle through the pars plana, then angle it through the posterior capsular opening into the anterior chamber.
Ocular massage
If after paracentesis and gas injection the pressure is too high, the eye may be massaged to reduce intraocular volume. Retropulsion of the eye into the orbit dehydrates the orbital fat, but is less effective at reducing the intraocular volume.
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Instead, a scleral depressor is placed against the temporal equator, and the eye is pressed firmly against the bony nasal orbital wall. Firm pressure is applied for 45 seconds, then relaxed for 15 seconds to allow perfusion of the retinal vasculature. This cycle is repeated until the intraocular pressure is low enough. This maneuver causes egress of fluid from the eye, and also stretches the scleral fibers, allowing more ample intraocular volume. Preoperative medications for reducing the intraocular pressure do not help much.
INJECTING THE GAS
An injection site is selected 3–4 mm posterior to the limbus. This site should be away from large, open retinal breaks, highly detached retina, or detached pars plana epithelium. The head of the supine patient is turned 45 degrees to one side to make the injection site uppermost. The needle is then passed into the eye perpendicular to the sclera (Figure 8–4). The needle is pushed 6–8 mm into the eye to ensure that the tip is well into the vitreous, directing the tip away from areas of highly bullous detachment. The needle is then withdrawn until 3 mm of it remains in the eye (Figure 8–5). This will ensure that the tip remains in the vitreous but is shallow enough to prevent multiple small bubbles (“fish
Figure 8–4. Injecting gas into eye with injection site uppermost.
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Figure 8–5. Procedure of injecting gas into eye. (A) With injection site uppermost, needle is pushed 6–8 mm into eye to ensure tip is deep in vitreous. (B) Needle is withdrawn until 3 mm of needle remains in eye. Gas is then injected semi-briskly, creating a single bubble.
eggs”). Before performing the injection, a caliper can be set to the correct distance to help estimate when the needle is withdrawn to the proper point. We do not recommend trying to visualize the needle tip in the eye with an indirect ophthalmoscope.
With the needle in the correct position, a moderately brisk injection of the entire volume of gas is performed. This facilitates formation of a single bubble at the needle tip (Figure 8–1C). The injection should not be so brisk as to force bubbles of gas deep into the vitreous before their buoyancy can make them rise. Inject smoothly and fairly quickly, but not with excessive force. Hold the plunger down until the needle is withdrawn to prevent escape of gas back into the syringe.
Because some gas may escape instantly upon removal of the needle, a cottontipped applicator can be used to occlude the perforation site. The applicator must be pressed against the shaft of the needle and rolled immediately over the hole as the needle is withdrawn. The head is then rotated 90 degrees to move the gas away from the injection site, and the applicator is removed. Escape of gas into the subconjunctival space is not harmful, but may not leave sufficient gas in the vitreous cavity.
Alternatively, the patient’s head may be rotated 90 degrees to the opposite side before withdrawing the needle. This allows the bubble to float away from the injection site and prevents gas from escaping through the needle tract. This maneuver should be rehearsed with the patient prior to inserting the needle to ensure smooth coordination. With only 3 mm of the needle in the eye, this maneuver is unlikely to injure the lens. When the needle is then withdrawn, liquid vitreous will sometimes escape through the needle tract into the subconjunctival space, which may eliminate the need for additional paracentesis or massage. Vitreous incarceration in the pars plana injection site probably occurs, but is not known to cause clinically significant complications.
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AFTER GAS INJECTION
Following injection, the eye is examined with the indirect ophthalmoscope to make the following three determinations:
Is the central retinal artery occluded?
Examine the central retinal artery to ensure its patency. Paracentesis or massage may be performed to reestablish patency or strong pulsation of the central retinal artery. Occlusion of the central retinal artery can be safely observed for up to ten minutes. During this time, the intraocular pressure declines and the artery may reopen; if it does not, paracentesis or massage should be performed immediately.
As long as the central artery is open (widely patent or with strong pulsation), measurement of the intraocular pressure has little meaning. The pressure will soon return to normal and not increase, even though the gas is expanding.
Is a single gas bubble present or are there multiple small bubbles (“fish eggs”)?
Fish eggs are undesirable because a small gas bubble can pass through a retinal break into the subretinal space (Figure 8–6). See the section on “Injecting the Gas” above for techniques for avoiding the development of fish eggs, and see the section on “Special Procedures” below for suggestions on management of fish eggs.
Is the bubble mobile within the vitreous or is it trapped at the injection site?
If the bubble is beneath the pars plana epithelium, or trapped in the space bordered by the pars plana, the anterior hyaloid face, and the lens (the canal of Petit), it will not move when the head is turned and will take on a semicircular shape. This has been termed the “donut sign,” the “sausage sign,” or the “bagel sign.” Management of this occurrence is discussed below, under “Special Procedures.”
Figure 8–6. Multiple small intravitreal gas bubbles (“fish eggs”) with subretinal gas.