8
Pneumatic Retinopexy
Pneumatic retinopexy (PR) is an office-based, sutureless, no-incision alternative to scleral buckling or vitrectomy for the surgical repair of selected retinal detachments. Cryotherapy is applied around the retinal break(s) to form a permanent seal. A gas bubble is injected into the vitreous cavity, and the
patient is positioned so that the bubble closes the retinal break(s), allowing resorption of the subretinal fluid (Figure 8–1A–F). As an alternative to cryotherapy, laser photocoagulation can be applied after the intraocular gas has caused the retina to reattach.
INTRAOCULAR GASES
CHOICE OF GASES
Sulfur hexafluoride (SF6) is the gas most frequently used with pneumatic retinopexy. Perfluorocarbon gases such as perfluoropropane (C3F8) are sometimes used, and success has also been reported with sterile room air.
In selecting a gas, it is important to understand the longevity and expansion characteristics of the gases. SF6 doubles in volume within the eye, reaching its maximum size at about 36 hours. It will generally disappear within about 10–14 days, depending on the amount injected. Perfluoropropane nearly quadruples in volume, reaching maximum size in about three days. The bubble will last 30–45 days in the eye. Room air does not expand, but immediately starts to reabsorb. The air bubble will be gone within just a few days (Table 8–1).
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A B
C D
E F
Figure 8–1. Pneumatic retinopexy procedure. (A) Volume of subretinal fluid is determined by inflow of fluid vitreous (green arrow) and outflow through retinal pigment epithelial pump into the choroid (red arrow). (B) Area of retinal break is treated with contiguous applications of transconjunctival cryotherapy. (C) With pars plana injection site uppermost, gas bubble is injected into vitreous with 0.5-inch, 30-gauge needle. (D) Head is positioned to place retinal break uppermost, thereby sealing break with intravitreal gas bubble. (E) With break closed, retina is usually reattached by first postoperative day. (F) Gas bubble is spontaneously absorbed. (Published with permission from Hilton GF, Grizzard WS: Pneumatic retinopexy: a two-step out-patient operation without conjunctival incision. Ophthalmology 1986;93:626–641.)
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Table 8–1. Intravitreal Gas Duration and Expansion |
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Gas |
Average Duration |
Largest Size By |
Average Expansion |
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Air |
3 days |
Immediately |
No expansion |
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SF6 |
12 days |
36 hours |
Doubles |
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C3F8 |
38 days |
3 days |
3X–4X |
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The initial expansion of SF6 and C3F8 is due to the law of partial pressures and the solubility coefficients of the gases involved. A 100% SF6 bubble injected into the eye contains no nitrogen or oxygen, but these gases are dissolved in the fluid around the bubble. Due to the law of partial pressures, nitrogen and oxygen will diffuse into the gas bubble. SF6 also starts to diffuse out of the gas bubble into the surrounding fluid which contains no SF6. However, nitrogen and oxygen diffuse across the gas–fluid interface much more quickly than SF6 because of the relative insolubility of SF6. The net result is an initial influx of gas molecules into the bubble, expanding its size until partial pressures equilibrate, net influx equals net egress, and maximum expansion is reached. Then the bubble gradually reabsorbs as the SF6 is slowly dissolved in the surrounding fluid. The diameter of the bubble shrinks at an approximately constant rate until the gas is gone. C3F8 expands more and reabsorbs more slowly because it is even less soluble than SF6.
The choice of type and amount of gas depends on the following considerations.
What size gas bubble is needed?
One must usually plan for a gas bubble more than large enough to cover all detached breaks simultaneously, and keep them covered for three to five days.
Computerized tomography studies on eyes with intravitreal gas bubbles showed that a 0.3 ml gas bubble covers over 45 degrees of arc of the retina (Figure 8–2), but it takes approximately a 1.2 ml bubble to cover 80–90 degrees. A highly myopic eye will require a larger volume of gas than an emmetropic eye to cover the same arc of the retina.
Usually, 0.4 to 0.6 ml of gas is injected into the eye. For room air PR, a larger bubble is generally needed, perhaps 0.8 ml, depending on the characteristics of the case. If it is desired to inject more than 0.6 ml, multiple paracenteses will likely be needed, one before the gas injection, and usually one or more after the gas injection. Alternatively, multiple gas injections may be performed, allowing the return of intraocular pressure toward normal between injections.
How long should the bubble remain in the eye?
It is optimal for the gas bubble to cover the break(s) for five days and then disappear as soon as possible. However, good results have been reported with only 3–4 days of tamponade, as with room air. The longevity of air is probably sufficient for most cases, but sometimes the chorioretinal adhesion may not be sufficiently mature when the air has been reabsorbed. Air also forfeits the advantage of postinjection expansion within the eye, necessitating an injection of a large volume.
184 II: Practice
Figure 8–2. Computerized tomography scan of gas bubble in eye. Surface tension results in rounding of bubble, decreasing arc of retina covered by bubble.
In most cases, the prolonged longevity of a perfluoropropane bubble is a disadvantage. A lingering gas bubble may induce tears, since movement of the head causes forcible movements of the vitreous when a gas bubble is in the eye. Also, air travel is contraindicated for a longer period of time with C3F8. However, it may also eliminate the need to reinject gas if a new break develops, and C3F8 allows the injection of a smaller amount of gas initially, thereby reducing the need for paracentesis.
Our gas of choice in most cases is SF6. We use C3F8 for the occasional case which requires an exceptionally large and long-acting gas bubble to tamponade large or widespread breaks. Most of the time we inject 0.4 to 0.6 ml of 100% SF6.
WHY GAS WORKS
The following characteristics of intraocular gases account for their efficacy in reattaching the retina:
1.Surface tension allows the gas bubble to occlude a retinal break instead of passing into the subretinal space. The surface tension of any gas is much higher than that of other substances in the eye. Once the break is occluded,