Special Adjuncts to Treatment

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Chapter 104 Special Adjuncts to Treatment

Special adjunct to treatment

Introduction

The first use of intraocular gas in treating retinal detachment dates back a century.1 At that time, the causal relationship between retinal break and detachment was not fully appreciated. It was only later, when the importance of localization and sealing of retinal breaks was recognized, that the concept of air injection was introduced.2 Rosengren described his technique of internal tamponade with air after subretinal fluid drainage, coupled with external diathermy to create adhesion, and demonstrated an increase in success rate in retina detachment repair.2 The technique of scleral buckling was introduced in the 1950s and in the 1960s, complicated retinal detachments were treated with a combination of scleral buckling and intraocular gas injection.3 As air is absorbed quickly, other longer lasting gases were sought.4 Sulfur hexafluoride and the perfluorocarbons have proved themselves to be the most popular intraocular gases. In the 1980s, pneumatic retinopexy was first introduced by Lincoff, and later popularized by Hilton and Grizzard.5 It was made possible with the use of expansile gases and this procedure obviated the need for scleral buckling for some patients. More significantly, pneumatic retinopexy transformed retinal detachment surgery from an inpatient operation, to an office-based procedure that has a reasonably high reattachment rate in selected patients. For many reasons, pneumatic retinopexy never gained popularity among surgeons in European countries. With the advent of vitrectomy,6 the use of intraocular gas became indispensable. The combination of closed three-port pars plana microsurgical approach with long-acting gases improved the success rates especially for the more complicated situations such as proliferating vitreoretinopathy and giant tears. Indication for intraocular gas extended to macular hole repair and pneumatic displacement of submacular hemorrhage. There are those surgeons who argued that we are using vitrectomy and gas in cases that might equally be treated with scleral buckling. The findings of randomized trials do little to change this trend.7

Physical properties of intraocular gases

The properties of an ideal intraocular gas are listed in Box 104.1. In reality, no single gaseous product has all the desired properties. A variety of gaseous products have been investigated for intraocular use.812 It is good to understand the different characteristics of the available products, so that we as surgeons can make rational choices. Of interests to the clinician are the longevity inside the eye, expansion ratio in pure form, and the nonexpansile concentration. Gases could be used in their pure forms, or as a mixture with air. The expansile property could be adjusted by mixing the pure form with air in different proportions (Table 104.1). In daily practice, air, sulfur hexafluoride (SF6), perfluoroethane (C2F6), and perfluoropropane (C3F8) are most commonly used. Table 104.2 highlights the physical properties of these gases. Historically, xenon was used for its shortest intraocular longevity.

Table 104.1 Gases investigated for intraocular use

Nonexpansile Expansile
Air Sulfur hexafluoride (SF6)
Xenon (Xe) Perfluoromethane (CF4)
Nitrogen (N2) Perfluoroethane (C2F6)
Helium (He) Perfluoropropane (C3F8)
Oxygen (O2) Perfluorobutane (C4F10)
Argon (Ar) Perfluoropentane (C5F12)
Krypton (Kr) Octafluorocyclobutane (C4F8)
Carbon dioxide (CO2)  

Air was the initial gas to be tried in retinal surgeries. If the vitreous cavity is entirely filled with air, then the bubble does not dissolve or disappear for 5–7 days. It is nonexpansile for all intent and purposes. This is not a drawback but rather an advantage. In Europe, air is often used in conventional scleral buckling surgery. Subretinal fluid is drained first, followed by the injection of air. This effectively restores the anatomy, in that the retina becomes re-apposed to the underlying retinal pigment epithelium and choroid. Cryotherapy can then be applied. The application can be precise and limited as there is no need for a large ice-ball or to freeze through a depth of subretinal fluid. Equally, localization of the retina break can be precise. The scleral buckle need only be as low profile as the retinal break/breaks, as the retina is already attached.13 The air injection is useful in three specific ways. First, the intraocular pressure is restored after the air injection. Second, the surface tension of the air bubble means that the retina can be kept opposed and attached (had saline been injected instead, the liquid might go through the retinal breaks and the retina might redetach again). Third, air is nonexpansile. There is no concern about causing traction to the inferior retina and causing new retinal breaks. This last point is not often appreciated. The use of air combined with scleral buckling is highly popular in Europe. There are many complications, including the break up of the air bubble into “fish eggs” by poor injection techniques. Inferior breaks, however, is not one of the complications noted, despite a large number of publications on this technique.14 This is in contrast to pneumatic retinopexy. The fact that the bubble expands may cause further collapse of the gel with trans-gel traction. Because the gas bubble floats, this trans-gel traction is transmitted to the vitreous base inferiorly to give rise to inferior retinal breaks.15 The fact that air does not expand makes it uniquely safe when combined with scleral buckling and nonvitrectomized eyes. Air has also enjoyed a resurgence of popularity when combined with vitrectomy. The success of retinal detachment relies on identification of all offending retinal breaks and sealing them. With the increasing use of endolaser, it is generally acknowledged that adhesion can develop much more quickly. Logically, there is therefore no need for prolonged tamponade. The duration of tamponade from a gaseous bubble only needs to be long enough for chorioretinal adhesion to develop. In the absence of any risk factors for developing proliferative vitreoretinopathy and when the causative retinal breaks are confined to 1 or 2 clock-hours, air is a perfectly acceptable tamponade. The rapid absorption, if anything, is an advantage. It simply means that the patients can be rehabilitated quicker and that they can travel by air sooner.

When the retinal breaks are multiple and their locations widely separated (in terms of clock-hours), then it would be desirable to have a large postoperative bubble. Long duration of tamponade is not particularly needed, if proliferative vitreoretinopathy (PVR) is not anticipated. Sulphur hexafluoride (SF6) in these cases might be suitable. The nonexpansile concentration of SF6 is 20%. As a rule of thumb, if the vitreous cavity were totally filled, a bubble of 20% SF6/air would last for about 2 weeks. The bubble would be relatively large in the first few days to give a sufficiently large area of tamponade to widely separated retinal breaks. SF6 is seldom used neat. It is inert, nontoxic, colorless, and is five times heavier than air. Hydrolysis only occurs at high temperatures (>500°C). We will discuss the choice of tamponade agents later in this chapter.

The perfluorocarbon gases have the generic chemical formula (CxF2x+2, where x can be 1–4). These are inert gases with no odor or color. Water solubility varies according to the carbon chain length. The longer the carbon chain, the lower the solubility in water, hence the longer is the intraocular longevity. For instance as a rough guide, 1 mL of pure C2F6 expands 3.3 times when injected into the eye, and stays in the eye for 4–5 weeks; but for 1 mL of C3F8, the same volume expands four times, and stays for 6–8 weeks.

When a gas bubble is injected into the eye, two forces act on the gas bubble. There is a downward force caused by gravity and there is an upward force generated by buoyancy. Gravity equals the weight of the intraocular gas. Archimedes’ Principle states that any floating object displaces its own weight of fluid. For instance, 1 mL of C3F8 weighs 0.001 g. Hence 1 mL of C3F8 displaces 1 mL of fluid, which weighs 1 g (specific gravity of water is 1.0). Therefore buoyancy is 1 g upwards (1 g equals 0.0098 Newton; gram is used here for ease of understanding). Net weight acting on the C3F8 bubble is therefore 0.999 g (i.e., 1 g buoyancy minus 0.001 g gravity). This force is pushing the bubble upwards. In terms of magnitude, this upward force is large compared with that of a silicone oil (SO) bubble. This upward force is the same order of magnitude as the downward force associated with perfluorocarbon liquids, with a specific gravity close to 2 g/mL. The orthodoxy is that perfluorocarbon liquid (PFCL) is too heavy to be left in the eye, as this may cause retinal damage.16 It is interesting that no-one speaks of gas bubbles pressing too hard and causing toxic changes to the upper retina.17

The classical concept of tamponade is that the bubble makes contact with the retina and prevents water from gaining access to the subretinal space via the retinal breaks. The interfacial tension between water and gas is high. The bubble therefore wants to stay as one bubble and it would not go through a small aperture such as a retinal break. To do so, it would need to deform. The bubble would need to assume a smaller radius and this would require higher surface energy.

In practice, an air or gas bubble would seldom have a chance to go through retinal breaks, except when they were associated with fixed retinal folds. If a retinal detachment were mobile, as soon as a bubble was injected, the bubble would float to the uppermost position of the vitreous cavity. Any subretinal fluid would be displaced inferior to the bubble. The upper retina would be opposed to the underlying retinal pigment epithelium, including any retinal breaks that might be situated in that upper part of the retina. Furthermore, there are those who believed that direct contact between the retina break or bubble might not be necessary. Clinical studies have shown that inferior retinal breaks can be successfully treated with vitrectomy, gas tamponade and no scleral buckling.18 One school of thought is that gas bubbles (for that matter oil bubbles) act inside the eye as splints, reducing intraocular currents. In the absence of traction, the lack of intraocular currents would allow the retina to settle back. There may or may not be a need for direct contact between the bubble and the retinal breaks.

If, however, contact were important, then the shape of the bubble would determine the effectiveness of tamponade. When buoyancy was large, the bubble would take on the shape of the vitreous cavity and assume a flat bottom. This way, most of the volume would contribute to making contact with the retina. Thinking about it in another way, very little volume would be wasted to forming the meniscus. If the buoyancy were small, as would be the case with SO, the shape of the bubble would be rounded. In other words, very little of the volume would contribute to making contact with the retina; much of the bubble would go to form the meniscus, which would make no contact with the retina. The extreme case would be a spherical bubble inside a spherical cavity. Unless the fill is 100%, the contact would go from nil to total. In practice, total tamponade is probably unachievable. Most surgeons use the so-called nonexpansile concentration of gases. By definition, the volume of the bubble decreases in the postoperative phase. Those who use “slightly expansile” concentration are risking high intraocular pressures. In a normal eye, any increase in intraocular pressure would result in an increase in outflow that in turn would lower the intraocular pressure. If the vitreous cavity was totally filled with gas, the only aqueous available to increase the outflow would be that in the anterior chamber (AC). The AC would then be shallow. If the iris lens diaphragm was lax, then secondary angle closure might ensue. In practice, we could not know, for a given individual patient, precisely what the nonexpansile concentration of each gas might be. Using expansile concentration would therefore be unnecessarily risky.

Functions of gas

The main functions of a gas bubble inside the eye are to: (1) provide internal tamponade; (2) flatten folded retina; (3) enable visualization; (4) replace globe volume; (5) reduce intraocular currents.

Internal tamponade

Providing internal tamponade for retinal detachments has been the main indication of intraocular gas use.19 The purpose is to oppose the break by utilizing the surface tension of the bubble. The surface tension of gas is high compared with liquid tamponade agents such as SO. The interaction between buoyancy, weight, shapes of intraocular gas bubbles and contact have been eluded to previously.

It is worthwhile mentioning that the shape of a gas bubble varies with its volume. When a small gas bubble is injected, it takes on a rounded shape. This is observed daily when pneumatic retinopexy is performed. For example, where 0.3 mL of C3F8 was injected, this bubble stays relatively rounded until it expands in size over the next 24–48 hours. It then clearly adopts a flattened shape (this shape is referred to as a spherical cap). When a gas bubble is small, its shape is mainly determined by its surface tension. Because the surface tension is high, the bubble is rounded. When the bubble expands, buoyancy becomes important. Every molecule of the bubble wants to float upwards, which is why the bottom of the bubble has a flattened shape. Lincoff made this observation many years ago and went on to suggest a means of assessing the size of intraocular gas bubble by observing this flattened bottom surface of the gas bubble.20 In terms of upward force, it is greatest at the apex of the bubble, whereas it is near zero at the bottom. Table 104.3 gives an estimation of the volume of the gas injected and the effective arc of tamponade. A study was made in the past, using a model eye constructed of surface modified polymethylmethacrylate to mimic the hydrophilic retinal surface. The efficiency curve plots the arc of contact against the percentage fill. It was shown that the curve was sigmoidal. Initially, the plot was exponential. It showed that a relatively small bubble would provide a large arc of contact. The plot was linear; the fill and contact was proportional and towards the end, the plot was exponential again. This time, a slight underfill would leave a large arc of retina not in contact with the bubble.21

Table 104.3 Changes in arc of contact with gas bubble volume (assume a vitreous cavity diameter of 21 mm)

Arc of contact (degrees) Gas bubble volume
90 0.28 mL
120 0.75 mL
150 1.49 mL
180 2.40 mL

The tamponade bubble also acts to seal the break, such that cellular elements can no longer escape from under the retina into the vitreous cavity. This was considered important in preventing proliferative vitreoretinopathy. However, cellular elements that have already gone into the vitreous cavity tend to concentrate in the thin film of fluid just beneath the bubble. This accounts for why postoperative proliferative vitreoretinopathy is more commonly found inferiorly.22

Unfolding and folding of the retina

The surface tension and buoyancy force of the bubble can help to unfold the retina. Circumferential folds sometimes occur with high radial buckles. If subretinal fluid was drained and air was injected, these folds would be less prominent. The so-called retinal redundancy would be minimized, as the retina would be made to follow closely the contour of the indent. Equally, if subretinal fluid drainage was incomplete and a large bubble was injected, retinal fold could occur. When these folds involve the macula, the patients would be very symptomatic, complaining of distortion and poor vision.23 This complication could be prevented by achieving a more complete drainage of subretinal fluid before injection and judicious posturing of the patients immediately postoperatively. This posturing might involve “steam-rolling” with the patient lying first with the retina break lowest most, then turning slowly to position the bubble to the posterior pole, followed by posturing on the correct side.24 This type of maneuver aims to use the bubble to express the subretinal fluid out through the retinal break and to protect the macula being affected by retinal folds.

Dynamics of the gas bubble inside the eye

Different phases of gas resorption

After injection, the gas bubble inside the eye undergoes three phases before complete resorption. The three phases are expansion, equilibration, and dissolution. This occurs when pure expansile gases (i.e., SF6, C2F6, and C3F8) are injected. Air does not expand and this will be discussed later. Due to lower water-solubility than nitrogen, pure SF6, C2F6, and C3F8 will expand when injected into the eye. This is because nitrogen diffusion rate into the bubble is higher than the rate of gas dissolving into surrounding tissue fluid compartment. Expansion is most rapid in the initial 6–8 hours, and is similar for all gases. This is because the rate is mostly affected by the convection currents in the surrounding vitreous fluid.25 The bubble reaches its maximum size when the gaseous diffusion in and out of the bubble equilibrates. For SF6, this occurs around 1–2 days after injection; for C3F8, it takes 3–4 days to reach maximum expansion.26 This has practical implications, as intraocular pressure (IOP) may rise if the outflow facility cannot cope with the rapid increase in intraocular volume. It has been found that the eye can accommodate up to 1.2 mL of pure expansile gas injection without significant IOP change.12,26 This equals 20–25% of the vitreous cavity volume. In eyes with occludable angles, pure expansile gas should therefore be avoided, or prophylactic IOP lowering agents should be used.

Equilibration phase begins when the partial pressure of nitrogen in the bubble equals that in the surrounding fluid compartment. During this phase, there is a small net diffusion of expansile gas into the fluid compartment. This can be explained by the higher solubility of nitrogen, such that nitrogen equilibration is reached at a faster rate than other gases. Hence, the bubble diminishes slightly in volume during this phase. Duration of this phase differs for different expansile gas, and is dependant on solubility. For C3F8, this phase lasts 2–3 days.26

When partial pressure of all gases within the bubble equals that in the fluid compartment, the dissolution phase begins. The gas compartment gradually decreases in size as gases dissolve into the fluid compartment. The decrease in volume follows first-order exponential decay.27 This phase is the longest among all three phases. Despite the fact that it may take up to 6–8 weeks for a bubble to completely resorb, internal tamponade is often only effective during the initial 25% of the bubble’s lifespan. This is because it requires at least 50% of the initial size to provide an effective tamponade. If the bubble is smaller than 50% or it breaks into a few smaller bubbles (i.e., fish eggs), internal tamponade is ineffective and no therapeutic effect can be achieved, even though it may still remain in the eye for a long time. Figure 104.1 illustrates gaseous transfers in and out of the bubble during the three phases.

Air, which is already a mixture of gases, does not expand and enters the dissolution phase immediately after injection. This is because the partial pressure of nitrogen, oxygen, and carbon dioxide roughly equal that in the blood. Since equilibrium has already been reached during gaseous exchange in the lung, dissolution phase begins immediately after injection.

In clinical practice, expansile gas is often mixed with air to give a “nonexpansile” concentration. This can be interpreted as injecting two separate gas compartments into the eye, one being pure expansile gas, the other being pure air. The reduction in volume of the air compartment compensates for the increase in volume of the expansile gas compartment. When the appropriate ratio of these two compartments is met, the overall gas compartment volume remains constant. The percentages of gas/air mixtures to produce a nonexpansile volume are outlined in Table 104.2.

The time taken for complete resorption of the bubble also depends on other factors such as lens status, aqueous turnover, presence of vitreous, presence of periretinal membranes, ocular blood flow, and ocular elasticity.27 The lifespan of SF6 and C3F8 may be more than twice as long in phakic nonvitrecomized eyes than in aphakic vitrectomized eyes.28

Special considerations when under general anesthesia

During general anesthesia, the anesthetic gases inhaled may interfere with intraocular gas volume. Nitrous oxide (N2O) is, respectively, 34 times and 117 times more water-soluble than nitrogen and SF6.28 Therefore when there is a gas bubble in the eye, nitrous oxide quickly diffuses from the fluid compartment into the bubble, and increases the bubble volume. If SF6 is used, the bubble may increase up to three times its original size during anesthesia with nitrous oxide. Because of its high solubility, maximum IOP rise may occur after 15–20 minutes of nitrous oxide use; and IOP decreases once it is discontinued, as it diffuses out of the body through ventilation. It has been found that the concentration of nitrous oxide in the lung alveolars is reduced by 90% after it has been stopped for 10 minutes. Therefore in practice, nitrous oxide should be discontinued for at least 15 minutes prior to intraocular gas injection, to avoid interference in the desired bubble volume. If it has been continued during gas injection, the resultant bubble will be smaller than expected.

Special attention is required for patients undergoing general anesthesia for nonocular purposes while they still have an intraocular gas in situ. Severe visual loss resulting from central retinal artery occlusion and choroidal ischemia have been reported.29,30 This was thought to be due to the uncompensated rapid rise in IOP during surgery as a result of nitrous oxide diffusion into the bubble. For this reason, every patient with an intraocular gas bubble should be given a wristband to wear, indicating clearly the type and time of intraocular gas injection. It should be worn throughout the lifespan of the bubble.

Response to changes in altitude

Assuming most patients remain at a similar altitude after intraocular gas injection, the bubble size would not change significantly. However, when there is a change in altitude, significant changes in bubble size may occur. This is especially important for patients undergoing air travel shortly after surgery, because airplane cabin pressure is only equal to atmosphere pressure at an altitude of 8000 feet. Climb rate occurs at roughly 2000–3000 feet per minute during airplane ascent, and the rapid expansion in bubble size may be translated into IOP rise.31 Central retinal arterial occlusion may result. There have been reports of severe ocular pain as a result of air travel with an SF6 bubble in situ.32 It has been reported in animal studies that a bubble equivalent to 10% of the vitreous cavity or 0.6 mL may be safe for air travel. Up to 1.0 mL of gas was reported to be tolerable without significant IOP change.31 However, this is entirely dependent on outflow facility, and some surgeons feel that no volume is safe for air travel.

For the same reason, air bubble size may change during scuba diving.33 During scuba diving, gaseous equilibrium under atmospheric conditions may be interfered from inhalation of oxygen from compressed air tanks. On returning to surface, the bubble expands inside the eye and gives rise to an increase in IOP.

Preparation for injection

Gases of highest purity from either a disposable or reusable cylinder should be used. Prior to obtaining gas from the cylinder, gas pressure within the cylinder should be checked to ensure no gas leakage has occurred, which may affect the concentration of the gas inside. Silicone tubing is first connected to the cylinder at one end, and to two 0.22 µm Millipore filters (Millex-GS) at the external end. A 50 mL syringe is then connected to the filters. The syringe is then flushed two to three times to remove air trapped within the tubing and filters. Pure gas is then drawn into the syringe to the desired volume. For pure gas injection, the syringe could then be connected to either a needle or the infusion for use. For air–gas mixtures, the syringe should be disconnected from the cylinder at the junction between the two filters, having one filter still connected on the syringe. Sterile air is then drawn into the syringe to achieve the desired concentration of air-gas mixture. The filter is disconnected and syringe connected to a needle of the infusion for use. The gas or gas mixture should be used immediately to avoid inaccuracy in the concentration as a result of air influx from the surroundings. Figure 104.2 illustrates how gas is prepared for injection.

It must be reminded that human error can incur in the process of gas preparation. It is therefore imperative that all operative theatre staffs are acquainted with the techniques of gas preparation. It is also vital that they do not confuse the different gases for use, and proper labeling is done to ensure the correct gas is injected into the correct eye. When mixing pure gas with air, it also especially important to ensure that the correct concentration is produced. This is because an expansile bubble in an eye intended for a nonexpansile bubble could cause devastating consequences.

Clinical applications and surgical techniques

The most common indications for intraocular gas injection is to assist: (1) retinal detachment surgery with vitrectomy; (2) pneumatic retinopexy; (3) retinal detachment surgery with scleral buckle; (4) macular hole surgery; (5) displacement of subretinal hemorrhage; and (6) postvitrectomy gas exchange in vitrectomized eyes. The techniques discussed below are based on the authors’ personal surgical experiences and preferences, and may not necessarily be applicable in all cases.

In vitrectomy for retinal detachments

This is the one of the most common indications for intraocular gas injection. After full vitrectomy and relieving of tractions, fluid–air exchange is used to flatten the retina. Residual traction is indicated by persistence of subretinal fluid and inability of retina to flatten out. Fluid has to be reinstilled and all traction must be relieved prior to proceeding. When the retina is flat, and air is in situ, endolaser can be performed. The advantage of performing laser under air is that the peripheral retina is easier to visualize. In cases where perfluorocarbon liquid (PFCL) is required to flatten the retina, or to assist drainage of subretinal fluid, endolaser can be performed under PFCL, and PFCL–air exchange should be performed afterwards. Direct PFCL–gas exchange is generally not advised as (1) residual traction could be seen on extraction of PFCL, fluid has to be reinstilled for further intervention, and gas injected would be wasted, and (2) closure of the wound after gas injection could potentially cause gas leakage and postoperative hypotony.

When the eye is filled with air, air–gas exchange can be performed. The infusion line should be kept in place, and IOP controlled by the air-insufflation pump of the vitrectomy machine. The other two sclerostomy wounds should then be closed. This should be done with suturing in a 20-gauge system or the trocars be removed in a 23-gauge system, and air-tightness ensured. The syringe holding the desired gas or gas/air mixture should then be connected to the infusion line, at a site as closest to the eye as possible. This is to minimize dead space in the tubings that may interfere with the desired concentration of the gas. A 27-gauge needle connected to an empty syringe, with plunger removed, is then inserted through the sclerostomies, or through the sclera at the same plane as the sclerotomies, to allow exit passage for the air inside. The gas or gas/air mixture is then flushed into the eye through the infusion line. Flushing the eye with a minimum of 25 mL of gas or gas/air mixture is required to achieve an identical concentration to that in the original syringe. The infusion line is then pulled and the last sclerostomy closed. Another method is to inject the gas or gas/air mixture directly into the eye through the sclera or sclerostomy, and let air inside to exit via the infusion line, which is opened to atmosphere on the other end. In both techniques, the needle tip, be it for exit passageway, or for injection, has to be clearly visualized through the cornea before any air–gas exchange is performed. This is to avoid the inadvertent insertion of the needle in the suprachoroidal space. If gas has leaked during the sclerostomy closure, additional gas could be injected directly to maintain a normal IOP at the end of surgery. Conversely, if IOP is high, gas could be released by either depressing the sclerostomy wound or by inserting a syringe into the eye to relieve part of the gas. Figure 104.3 shows how this is performed.

The choice of gas is sometimes based on the availability of gases, and the surgeon’s experience and preferences. In general, the choice of gas is dependent on the intended duration of tamponade. For simple cases where duration required is short, air could be used. In more complicated cases where longer tamponade is desired, nonexpansile concentration of gas/air mixture (18% SF6 or 14% C3F8) should be used.28,34 When a larger bubble is needed, a gas/air mixture with an expansile concentration should be used. This is especially important for inferior breaks where a larger bubble could provide better tamponade. A larger bubble also has the advantage of being able to unroll folded retina. In the Silicone Study, C3F8 has been found to be more effective than SF6 in cases with PVR.14,35,36

In pneumatic retinopexy

As mentioned earlier, careful patient selection is important, as failure of opposing the retina may subject the patient to further operations. Before gas is injected, there are several prerequisites: (1) the retinal detachment has to be in the superior half of the retina; (2) the break or hole is ideally solitary or grouped within 1 to 2 clock-hours; (3) there are no inferior breaks or retinal thinning; and (4) preferably the presence of posterior vitreous detachment (PVD). This is an office procedure, and local anesthesia is usually sufficient in experienced hands. In contrast to buckled cases, internal traction is not counteracted therefore opposition of the retina to choroid has to be provided by the bubble initially, and later by the chorioretinal adhesion induced by cryotherapy.

Gas is injected only after adequate cryotherapy. Pure expansile gas should be used. In practice, 0.3 mL of 100% C3F8 is used most commonly. First, the injection should be on the side of the break. If the break is located at 12 o’clock, then the injection is at midline. Gas is then injected through a 27-gauge needle, 3.5–4 mm behind limbus. Normally 0.3 mL 100% C3F8 is used. To avoid fish-egg formation (small bubbles instead of one large bubble), the injection site should be rotated such that it is in the uppermost part. The needle should be inserted just deep enough to penetrate all layers and the injection force should be swift and constant, aiming at creating a single bubble. After injection, the injection site should be rotated laterally before pulling the needle out of the eye. This is to ensure the bubble moves away from the opening before the needle is retrieved, to prevent leakage. If fish egg has formed, the sclera can be gently tapped a few times to promote fusion of the small bubbles. Figure 104.4 illustrates how this is performed. After injecting gas, AC paracentesis can be performed to counter the increase in intraocular volume. The patient’s head is then rolled 180°, to the facedown position. This serves to unroll any folded retina associated with the break. The patient is then instructed to assume this position as much as possible, until complete dissolution of the bubble has occurred. Careful monitoring is required during the postoperative period for proper opposition of the retina, resolution of subretinal fluid (SRF), and any new breaks formation inferiorly. In cases where opposition is doubtful, SRF persists, or new breaks were found, a reoperation with either scleral buckling or vitrectomy approach has to be performed.

In scleral buckling for retinal detachments

Intraocular gas injection is generally not required, provided adequate drainage of SRF and relieving of traction with buckle has been achieved. However, its use is still invaluable in certain cases, for example when fishmouthing of the break on a circumferential buckle is seen and is insufficiently opposed by the buckle, or as a “salvage” procedure to save the patient from a reoperation.

For intraoperative use, we prefer injecting the gas towards the end of operation. This is mainly because the view of the fundus will be obscured by the bubble after injection. The injection technique is identical to that in pneumatic retinopexy. It should be stressed that the major therapeutic component under these circumstances is still the sclera buckle, whereas gas bubbles only act as an adjunct, and should not be relied on totally to treat the detachment.

The “salvage” procedure for post-scleral buckled cases is an invaluable tool to save the patient from a reoperation. When after sclera buckling surgery, a persistent layer of SRF remains underneath the break, or meridional folds are present, or there is fishmouthing of the break, injecting gas could be of help. The technique is identical to pneumatic retinopexy; 0.3 mL gas should be injected followed by AC paracentesis. Depending on the location of the break and likelihood of redetachment, different gases could be used. For instance, for superior breaks with minimal SRF, air could be used. For inferior breaks with fishmouthing breaks, an expansile bubble of 0.3 mL 100% C3F8 would be desired, although a reoperation would likely be required.

In postvitrectomy gas exchange

This technique is invaluable for recurrent detachment, and can avoid the need for reoperation.38 Success rate is highest when there is no evidence of PVR. If PVR has already set in, gas injection may be complicated by formation of new retinal breaks or extension of existing breaks, which usually occur at the edge of laser marks. A fluid–gas exchange could be performed at the slit lamp, via a 30-gauge needle connected to a syringe filled with the desired gas of injection. The needle is inserted at 3.5–4 mm posterior to the limbus, at the inferotemporal quadrant, from a dependent angle, aiming towards the center of the globe. Fluid–gas exchange is then performed via a push–pull technique. When the plunger is pushed, gas is injected into the eye. This is followed by aspiration of intraocular fluid by pulling the plunger. This cycle is repeated until the bubble has reached the desired size. Special note has to be taken to visualize the needle tip prior to any movement of the plunger. This is to avoid any inadvertent entry of the needle into the suprachoroidal space. If the patient is aphakic, the procedure could be performed by inserting the needle into the AC through the cornea, instead of the pars plana approach. The choice of gas is dependent on the condition of the redetachment. If a larger bubble is desired, expansile gas should be injected; whereas if a smaller bubble is needed, air or nonexpansile concentration of gas/air mixture could be used. Figure 104.5 shows how this could be done at the slit lamp.

Gas–gas exchange is sometimes required. For instance, when a patient is required to undergo air travel before the expected dissolution of the bubble, another gas of shorter longevity (i.e., air or xenon) could be exchanged. This could be done using the same technique described above, with the needle connected to two syringes via a three-way stopper. The bubble in the eye is aspirated into one syringe, which is followed by injection of the desired gas into the eye from the second syringe. It should be noted that it is difficult to ascertain an accurate concentration of the second bubble with this method.

Postoperative care

Head posture after intraocular gas injection

Proper opposition of the break is only ensured by proper posturing of the head. This is done such that the break is located at the uppermost part of the eye, and be in direct contact with the bubble. Facedown posturing with the usage of expansile gas has another advantage of preventing pupil block glaucoma or optic capture. Another potential advantage is that this reduces the contact between the posterior surface of the lens in a phakic patient with the gas bubble, and reduces the risk of cataract development. This should be done by assuming a facedown or prone posture immediately after surgery. If facedown or prone posture is difficult, or the patient needs to take rest from prolonged facedown position, lying laterally on the opposite side of the break is also accepted (i.e., lying on the left for a right side break). This could be facilitated with the use of pillows designed for posturing purposes. As chorioretinal adhesion from laser or cryotherapy takes 2–3 days to become effective, the initial tamponade by bubble is the main force to keeping the retina attached. This should be conveyed to the patient and compliance should be checked every visit or over the phone. If the patient has good compliance to facedown posturing, precipitates could be noted on the central corneal endothelium, which are sometimes referred as “positioning spots”. This is due to gravitational deposition of intraocular inflammatory elements, which is usually more obvious in combined phacovitrectomy cases. In nonvitrectomized cases, it is especially important for the patient to assume proper posturing. Movement of the bubble may cause traction in the vitreous gel and may induce new breaks. This is problematic if inferior breaks are induced, since the bubble cannot provide tamponade for these breaks and reoperation is likely. Posturing is needed until complete dissolution of the bubble has occurred. As the bubble gets smaller, patients with good visual prognosis may report seeing the bubble inferiorly. Fundal exam and sometimes history taking can tell when the bubble has completely dissolved. Wristband indicating an in situ bubble can only be cut at this time.

Complications and management

Cataract formation

Gas-induced cataract is usually in the form of feathery posterior subcapsular cataracts. It can also appear as vacuoles at the superior portion of the lens. Incidence is higher if the eye is two-thirds or more filled with gas. It is also more likely to occur if the gas of choice is of higher purity and longer longevity.11,42 Assuming a prone position, as well as leaving a thin layer of anterior hyaloid help prevent this from occurring. These help to isolate the bubble from the lens. If in mild form, gas-cataracts tend to resolve without treatment. For persistent opacities, which are more likely to occur with gases of longer longevity, surgical removal may sometimes be required, especially when view of the fundus is compromised. If cataract extraction has to be performed when the bubble is still in situ, aspirating the gas before cataract extraction is needed. Otherwise, the bubble will push the posterior capsule upwards and increase the risks of complications.

Raised intraocular pressure

Expansile gases or gas/air mixtures of high purity tend to cause IOP rise more frequently. A 26–59% incidence was reported.43 It is usually due to overfill or expansion of the bubble, which cannot be compensated for by the outflow facility. This is usually short-lived and can be managed without difficulty using antiglaucoma medications. Refractory cases may be due to outflow compromisation. For cases with peripheral anterior synechiae (PAS), pre-existing angle closure glaucoma, or neovascular glaucoma, care must be taken when choosing the gas for injection. In general, air or a nonexpansile gas/air mixture should be used in these cases, to reduce the risk of postoperative IOP rise. Other than medical treatment, excess gas could be partially aspirated to reduce the volume and hence IOP.

Secondary glaucoma with intraocular gas use is infrequently seen. It can occur with angle disruption from forwardly displaced lens–iris diaphragm, by a large bubble. This is more frequently seen in patients who cannot adopt a prone posture after surgery. Therefore in patients with cervical spine problems or who cannot assume a facedown posture, nonexpansile gas or air should be used, and a complete fill should be avoided.

Perfluorocarbon liquid in vitreoretinal surgery

Introduction

Perfluorocarbon liquid (PFCL) was initially designed for use as a blood substitute.45 Clark and Gollan first used it as an oxygen transporter in a mouse model.45 In humans, its use was involved in coronary angioplasty to deliver oxygen to ischemic myocardial tissue. PFCL has a high oxygen carrying capacity, and is also chemically inert. In 1982, Haidt and associates first examined its use as a vitreous substitute.46 Clark later examined its possibility as an intraoperative tool, as well as postoperative vitreous substitute.47

In 1987, Chang pioneered its use in humans.48 He investigated the possibility of PFCL as an intraoperative tool to assist the manipulation of the retina in complicated retinal detachments (RD). This was acknowledged by many as a major advancement. The use of PFCL has greatly improved retinal attachment rates, especially in complicated RD. This chapter will cover the physical and chemical properties of PFCL, surgical techniques, and the potential complications that may arise with its use.

Types and properties of perfluorocarbon liquid

PFCL is a synthetic fluorinated hydrocarbon containing carbon–fluorine bonds. Some also contain other elements such as hydrogen, bromide, and nitrogen. Their chemical structures can be either straight chains or cyclical. Straight chain compounds contain carbon chains from C5 to C9, whereas cyclic compounds are made up of carbon chains from C5 to C17. For compounds with a carbon chain shorter than C5, e.g., perfluoropropane (C3F8) and perfluoroethane (C2F6), they exist in gaseous form at room temperature. In general, all PFCL are odorless, colorless, low viscosity, and have higher specific gravity and density than water. They are stable under high temperatures, and do not absorb wavelengths of commonly used lasers. A few low-density PFCLs have been investigated for potential use in ophthalmology. This includes perfluoro-n-octane (C8F18),49 perfluoroethylcyclohexane (C8F16),50 perfluorodecaline (C10F18),51 perfluoro-octylbromide (C8F17Br),52 perfluorophenanthrene (C14F24),16,52 perfluorotributylamine (C12F27N),53 and perfluorotri-n-propylamine (C9F21N).54 Details are listed in Table 104.4. Chemical and physical properties vary according to chemical structures. Of these, C8F18 was found to possess higher efficacy and was approved by the US Food and Drug Administration, for intraocular use.

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