Management of Subretinal Fluid and Surface Tension Agents During Vitreoretinal Surgery
Management of Subretinal Fluid and Surface Tension Agents During Vitreoretinal Surgery
Some basic principles apply.
Steve Charles, MD, FACS, FICS
A significant number of pars plana vitrectomy procedures are performed for retinal detachment, as well as lamellar or full thickness macular hole and foveoschisis, all of which require surface tension management agents. Interfacial surface tension management agents are usually inappropriately called tamponade agents; many surgeons believe that the term “tamponade” means “to press,” but it is actually derived from a French word meaning “to plug.” It promotes understanding and more effective use of these agents when physics-based terminology is used.
The properties of these agents vary widely. Density, interfacial surface tension, viscosity and average molecular weight are unrelated but drive the behavior and, therefore, the clinical application of these agents. The agents can be divided into intraoperative, short-to-medium term (gas or perfluorocarbon liquids) and longer-term (silicone oil).
Low-density agents, such as silicone oil, air and air-gas mixtures, float; density (the Archimedes principle) determines whether the agent floats or sinks in aqueous, while interfacial surface tension determines its primary effect. For example, viscoelastic agents, such as hyaluronan, are non-Newtonian, pseudoplastic materials; translated into surgeon's language, this means that they have minimal interfacial surface tension in aqueous and flow through retinal breaks. Thus, they cannot be used for retinal detachment or macular hole repair.
Although silicone oil is used for complex retinal detachments, typically in proliferative vitreoretinopathy, the interfacial tension of silicone oil in aqueous is approximately 50% less than a gas/aqueous interface. This fact explains why some patients are attached after fluid-air exchange and internal drainage and air-silicone exchange are performed, and partial inferior retinal detachment is noted the day after surgery. In such situations, the residual traction exceeds the force due to surface tension.
Viscosity does not determine the rate of emulsification, just the fluidic resistance to injection and removal; 5,000-cSt oil requires five times more time to inject or extract than 1,000-cSt oil, assuming equal injection pressures and extraction vacuums. There is no evidence that 5,000-cSt oil produces less emulsification than 1,000-cSt oil; the interfacial surface tension is equal.
Although n-perfluorooctane (PFO) has nearly doubled the density of aqueous and therefore sinks, it has only two-thirds the viscosity of water and therefore is easy to inject and remove through 25- or 27-gauge cannulas. PFO causes subretinal fluid, aqueous humor, liquid vitreous (hyaluronan), infusion fluid and the flap of giant breaks to float anteriorly in the supine surgical patient. Although air and air-gas mixtures float, they produce minimal buoyancy-based pressure and provide no benefit in epimacular membrane or macular edema patients.
For surface tension agents to be used after the vitreous has been removed, there must be an exchange of infusion fluid for the agent, often with an intermediate step of exchange for air. Use of logical terminology is crucial in this context as well; removal of fluid while infusing air is fluid-air exchange (FAX), not gas-fluid exchange. Word order matters. Exchanging air for silicone oil is air-silicone exchange (ASX); similarly, fluid-silicone exchange is FSX, which is less commonly performed.
Gary Abrams introduced the critical concept of isoexpansive gas mixtures (25% SF6, 18% C3F8) over three decades ago, yet some surgeons still use potentially unsafe methods of gas injection. The volume of the vitreous cavity cannot be reliably estimated; injection of pure gas into an eye filled with air can produce a very small, ineffective bubble on the first postoperative day or a blind eye from a central retinal artery occlusion.
Often, pain from high intraocular pressure is masked by PRN postop pain meds, especially in vit-buckle patients; buckles produce significant pain while sutureless vitrectomy is painless. The author has not performed a vit-buckle in two de cades. Partial fluid-air exchange and use of higher gas concentrations have no advantages and not infrequen tly produce high IOP or small, ineffective bubbles.
Another source of high IOP in gas cases is incorrect mixing of the gas with air, often caused by confusing cubic centimeters of gas with percentage; using cc notation on surgeon preference cards only works if the syringe size has not changed.
Air-silicone exchange is usually preferred over fluid-silicone exchange because it is easier to keep oil out of the anterior chamber, and drainage of subretinal fluid, as well as endolaser retinopexy, can be performed under air. This approach also enables the surgeon to determine whether residual traction is present, requiring additional traction relief; this issue will be discussed below, in the section on subretinal fluid drainage.
There is no advantage to visualizing the fundus during air-silicone oil exchange and therefore no need for the slower method of injecting oil through the infusion cannula so that an endoilluminator can remain in the eye. Some surgeons close one sclerotomy, ie, removal of one cannula and inject oil, while forcing air back into the infusion system and console air pump; this method offers no advantages and has many disadvantages. Although a filter is used on the console air source, there is the potential for the contamination of subsequent patients with this approach.
The Constellation (Alcon) autostopcock appropriately prevents air from moving back into the infusion system and console. A far better approach is to inject oil with a short, thin-walled cannula into the superotemporal port, with the superonasal port open to allow air to escape. This method is a normotensive process, and by positioning the superonasal cannula at the highest point, it ensures a full oil fill. The air infusion line should be clamped with a small hemostat when the oil fill is almost complete, and oil injection, preferably with the Constellation (or Accurus) VFC, should be slowed to allow the final air bubbles to escape from the open, superonasal cannula.
Perfluorocarbon liquids, such as PFO, should be injected under direct visualization with a MedOne dualbore cannula to allow infusion fluid egress, not a 25-g short needle or single-bore cannula without fundus viewing. The cannula tip should be positioned near the retina, nasal to the disk, and a small bubble injected, preferably using the Constellation (or Accurus) VFC with the pressure set at 10 psi, not the 80 psi used for oil.
The cannula tip should be positioned at the highest point of the initial, small bubble and injection of PFO continued while retracting the dual-bore cannula, keeping the tip of the cannula at the top of the expanding bubble. This method ensures that a single bubble is created and no PFO is lost through the proximal infusion fluid egress port, which is used to prevent increasing IOP as the PFO is injected.
PFO removal or exchange for gas or oil is the inverse of the injection process; the soft-tip cannula should be positioned anteriorly, above the PFO in the aqueous layer, and moved posteriorly, removing aqueous humor, residual liquid vitreous, subretinal fluid, and infusion fluid before the PFO is removed. PFO–silicone oil exchange using a chandelier for illumination results in less giant break slippage than an intermediate PFOair exchange step, as we have learned from David Wong.
Intraocular lens fogging occurs because the IOL is cooled by room temperature infusion fluid, and the air in the vitreous cavity is saturated with water vapor. It occurs with all IOL materials if the vitreous cavity air is exposed to the surface of the IOL. YAG capsulotomy or a capsular defect is necessary for fogging to occur, in addition to a defect in the anterior vitreous cortex. Silicone IOLs produce more posterior capsular opacification than acrylic IOLs and therefore have higher YAG rates.
In addition, silicone IOLs are thicker and have a higher thermal mass, thereby retaining cold longer, explaining the higher fogging risk. Applying viscoelastic to the posterior surface of the IOL can be effective for fogging but is more difficult with 25-g trocar cannula systems. Viscoelastics increase silicone oil emulsification and should be used sparingly.
A better way to manage IOL fogging is to return to fluid infusion, remove the air, attach the retina with PFO, apply endolaser to the retinal breaks and perform PFO–isoexpansive gas exchange if gas was planned. If oil is required, simply inject oil while removing fluid and air; when the oil reaches the IOL surface, the fogging will disappear, and further traction removal, if required, and endolaser can be applied “under” oil.
REMOVAL OF SUBRETINAL FLUID
One of the many advantages of vitrectomy over scleral buckles for retinal detachment repair is the ability to drain subretinal fluid through retinal breaks instead of the sclera/choroid/RPE. This technique is referred to as internal drainage of subretinal fluid and was developed by the author over three decades ago.
Once again, proper terminology is important; this is not gas-fluid exchange, which actually means removal of gas and replacement with fluid. Correct words matter. Surgical step temporal sequence matters as well; internal drainage of subretinal fluid should be initiated before fluidair exchange, which was also developed by the author. If a fluid-air exchange is performed first, subretinal fluid migrates posteriorly, making drainage of subretinal fluid more difficult and necessitating a drainage retinotomy. If internal drainage of SRF is initiated first, pre-existing retinal breaks with the flaps removed by the vitreous cutter can be used for internal drainage of SRF, thereby preventing or reducing posterior migration of SRF.
The soft-tip cannula is safer than any rigid cannula for internal drainage, although the vitreous cutter can be used for larger breaks. The soft-tip cannula is less likely to damage the RPE and choroid, especially if the patient moves under local anesthesia, and it can be extended under the retina posteriorly to drain more SRF. Because the soft-tip cannula is endopening, unlike the side port on the vitreous cutter, more complete SRF removal can be accomplished, waiting until highly viscous SRF migrates pos teriorly and repeated drainage can remove most of the subretinal fluid. Diathermy is used to mark the drainage retinotomy by some surgeons, although the author finds this to be an unnecessary step.
Combining drainage of SRF with fluid-air exchange has been termed the reattachment experiment by the author. This step should be done slowly while watching the retina; residual traction is definitely present if air becomes subretinal, indicating the need for vitreous removal, end-opening forceps membrane peeling, scissors segmentation/delamination of epiretinal membrane and/or retinectomy (not outdated relaxing retinotomy) “under” air.
Stanley Chang introduced the extremely valuable concept of using perfluorocarbon liquids to remove SRF, reposition giant break flaps, and stabilizestabilize the retina during PVR membrane peeling. This technique is essential for giant break repair and is highly useful for the IOL fogging scenario described above. It is not necessary in most retinal detachment cases, but in contrast to internal drainage of SRF, all SRF can be removed with PFO.
Removal of all SRF facilitates endolaser retinopexy and may justify using air instead of SF6 or, according to the so-called Spanish method, avoiding using either air or gas. If SRF does not initially egress from retina breaks, a soft-tip cannula can be placed through the break and be used to remove SRF without removing PFO. If SRF migrates anterior to the most anterior break, the soft-tip cannula can be pushed anteriorly through the break to remove the SRF, the break can be extended anteriorly, or a small drainage re tinotomy can be performed just inside the ora.
Silicone oil (Figure 1) is used for what the author terms rhegmatogenous confinement, ie, for breaks that were not visualized during surgery, breaks that may occur in the future, or for what the author calls “retinopexy avoidance.” The rationale for retinopexy avoidance is the notion that PVR is, to some extent, iatrogenic due to ex cessive retinopexy, with respect to number of spots, lesion intensity or both.
Figure 1. Depiction of a two-port 25-gauge reoperation for epimacular membrane under silicone oil.
Because inflamed eyes are more likely to have PVR recurrences, it is better to postpone the office laser retinopexy until all subretinal fluid and intraretinal edema absorbs and inflammation subsides. This process can be performed one month before oil removal.
Better understanding of the physical properties of surface tension agents results in more rational application of these essential agents, as well as better methodology. RP
|Steve Charles, MD, FACS, FICS, is clinical professor of ophthalmology at the University of Tennessee College of Medicine in Memphis. Dr. Charles reports significant financial interest in Alcon. He can be reached via e-mail at email@example.com.
Retinal Physician, Issue: November 2011