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BLOG: After vitrectomy, what's in the vitreous chamber?
This is a simple question, but one that can take some time to explore. There are two types of vitrectomies: anterior and pars planar. An anterior vitrectomy is typically performed during or after a cataract surgery in which the posterior capsule has torn and lens fragments have fallen into the anterior vitreous. If the posterior capsule is torn, then the anterior vitreous could prolapse through into the posterior chamber and maybe the anterior chamber.
Removal of this vitreous and the lens fragments is necessary to lower the risk of complications from the surgery, especially endophthalmitis.
A pars planar, or primary, vitrectomy is typically performed to treat retinal detachment or non-clearing vitreous hemorrhage. Vitrectomy will alleviate any vitreoretinal traction and allows for a complete tamponade of the vitreous cavity. This is especially helpful in large detachments or posterior retinal breaks, both of which are not easily managed with a scleral buckle.
Let’s start with the steps of a typical surgical technique for a vitrectomy for a macula-on retinal detachment, with a focus on what is filling the vitreous chamber.
1. Three trocars with cannulae (basically a small tube with a point for insertion, see Figure 1) are inserted into the pars plana through the conjunctiva and sclera to gain access to the vitreous cavity. Through the ports are inserted a: light pipe, a vitrector and an infusion cannula (using balanced salt solution [BSS]) to maintain positive pressure.
Trocars with cannulae are inserted into the pars plana through the conjunctiva and sclera.
2. The vitreous is removed via the vitrector, starting with the core vitreous and then the posterior vitreous. As vitreous is removed, it’s replaced with the BSS. This must be done carefully, because if vitreous is removed at too fast a rate, it will outrun the infuser and cause the eye to collapse.
3. Any subretinal fluid is drained via the retinal break.
4. At this point, all the vitreous should be removed (besides the vitreous base), and the vitreous chamber is filled with BSS. The surgeon will now perform a fluid-air exchange (FAX) and then perform any retinopexy with endolaser. The FAX is done because a non-liquid medium is needed for the laser.
5. Next, in order to achieve retinal tamponade, there is an exchange of air to gas (AGX) or silicone oil (ASX). Gas is the most common tamponade, especially in the patient’s first procedure. Sulfur hexafluoride (SF6) and octafluoropropane (C3F8) are the most common gases used.
6. Finally, the instruments are removed, IOP is adjusted, and the cannulae are removed (sutures are typically not required for small-gauge vitrectomies).
At this point, gas fills the vitreous chamber, the retina is pressed against the retinal pigment epithelium (RPE) by the gas, and the RPE pumps are starting to re-adhere the retina. But eventually the gas will diffuse out into the ocular tissues. An SF6 bubble will double in size within 24 to 48 hours and will last 1 to 2 weeks. A C3F8 bubble expands to almost quadruple the original size and will last 6 to 8 weeks. The surgeon will determine if SF6 is good enough, or if C3F8 is needed. If gas has failed to fix the retinal detachment in past vitrectomies, then the surgeon might use silicone oil, which would require another surgery to remove it.
But what of our original question: What eventually fills the vitreous chamber after the gas diffuses out? The answer is aqueous humor. In a typical patient, the ciliary body makes enough aqueous humor to replace the entire anterior chamber volume every few hours – plenty fast enough to fill the vitreous chamber. As the gas seeps out and the bubble rises, aqueous humor fills the empty space. Of course, this happens even if the surgeon doesn’t use gas.
In cases of vitreous hemorrhage, there is (often) no retinal break, so no need to tamponade the retina. The surgeon will remove the vitreous and fill the vitreous chamber with BSS. The BSS will slowly filter out of the eye, being quickly replaced with aqueous. For this reason, all else being equal, patients immediately following vitrectomy for vitreous hemorrhage have the potential to see much better than those immediately following surgery for retinal detachment, because the gas causes visual distortion until the bubble shrinks enough to see over it (or under it, depending on your point of view).
I think it’s important to remember that in a non-vitrectomized eye each chamber has its own pressure, and this pressure affects the others. The anterior chamber should have the lowest pressure, and the vitreous chamber the highest. Because gases and liquids in nature will move from a higher pressure container to a lower one, this helps aqueous humor leave the posterior chamber in which it’s produced and flow into the anterior chamber, where it’s filtered out of the eye. Aqueous misdirection is a rare complication that can occur after some (typically incisional angle-closure) surgeries, where the aqueous flows posteriorly, flattening the anterior chamber and closing the angle.
And the lesson about keeping each chamber with its own pressure is felt very strongly during cataract surgery. If the posterior capsule is ruptured, then great care must be taken to keep the anterior chamber pressure higher than that of the vitreous chamber, lest the vitreous prolapse forward. This higher pressure can be achieved with viscoelastic, which not only protects against surgical trauma to the cornea and absorbs ultrasound energy but also can be injected behind the torn capsule and help tamponade the vitreous.