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)

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.

Postoperative visualization

In the postoperative period, after vitrectomy and gas tamponade, the view of the fundus may be obscured by vitreous hemorrhage. It is usually possible to glean a view of the upper fundus by looking through the gas bubble. The observer however, needs to be positioned lower than the patient. Specifically, the aim is to look through the lower flat bottom surface of the gas bubble. Asking the patient to stand, while the physician remains seated, can work well. Equally, if other parts of the fundus are to be viewed, the patient can be asked to lie on their side. Again, if the vantage point is lower than the fluid level, the ophthalmologist can easily see the lower nasal or temporal half of the fundus. If the bubble is small, then fundoscopy is difficult. It would be better to rely on B-mode ultrasonography. One additional point should be made, i.e., looking through the bubble, the retina often appears to be attached. This may simply be due to the buoyancy of the bubble displacing subretinal fluid laterally and posteriorly.

Replace globe volume

Air is used in conventional scleral buckling surgery after the drainage of subretinal fluid. The air prevents subretinal fluid being recruited again. The air also restores the intraocular pressure. At this stage, effectively a flat retinal tear would be treated. Without the presence of subretinal fluid, cryotherapy would be limited and break localization would be more accurate. The fact that the intraocular pressure is normal also means that suture can be applied to the sclera more safely. In this way, air would be serving a role as an intraoperative tool.

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.

Fig. 104.1 Preparation for gas injection. (A) Pure gas is stored in a cylinder with a regulatory valve. Two sterile filters should be connected between the cylinder and the syringe in use. (B) As physiological dead space exists within the system, accuracy maybe affected by the air contained within these spaces. Pure gas should then be drawn from the cylinder and the syringe flushed a few times to ensure complete evacuation of air from the dead space. Appropriate amount of pure gas is then drawn into the syringe. (C) The syringe with one filter is then disconnected. The three-way tap is then turned to the other unused filter, and air is drawn in to achieve the appropriate concentration of air–gas mixture.

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.

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.

Fig. 104.3 Technique of air–gas exchange in a vitrectomized eye. (A) Injection of gas through syringe connected to a needle. Air inside eye is flushed out through infusion, which is disconnected externally. (B) Injection of gas is done through a syringe connected to the infusion, and gas is flushed into the eye. Air is allowed to exit the eye through the needle inserted with plunger of the syringe removed.

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.

Fig. 104.4 Injecting gas in pneumatic retinopexy or post-scleral buckle cases. (A) Detachment is located superiorly. Injection of gas is done via a needle, with the patient lying supine. (B) When the patient assumes an upright position, bubble tamponades the break and displaces subretinal fluid (SRF). Detached retina is opposed to the original position.

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 macular hole surgery

When first described, macular hole surgery was not complete without the injection of intraocular gas tamponade followed by facedown posture for 1 week. This provides a mechanical effect by the buoyancy force of the bubble, over the macular hole, in hope to assist closure. The injecting technique is identical to that in retinal detachment surgery with vitrectomy approach. The duration of postoperative posturing has been a topic of debate in recent years. Similar closure rate was found between air and 20% SF₆, and that between 20% SF₆ and 12% C₃F₈.³⁷ The choice of gas is generally based on the surgeon’s preference and experience. The authors’ choice of gas is 12% C₃F₈ followed by facedown posturing until dissolution of the bubble.

In displacement of subretinal blood

Pneumatic displacement of subretinal blood clot has been found to be therapeutic in treating polypoidal choroidal vasculopathy, macroaneurysm, choroidal neovascularization, and trauma. It has been shown to allow speedier recovery of vision, and may potentially reduce the harmful effect of blood on the photoreceptors. The original procedure was to treat within 1 week from onset of hemorrhage, coupled with tissue plasminogen activator (TPA) injection. There are recent reports showing similar effects without the use of TPA, and also of variable duration from onset of up to 30 days.

Prior to injection, careful patient selection should be done. Distinction between subretinal blood and intraretinal blood should be made. Injecting gas for intraretinal blood will not displace the clot, but rather increases the chances of blood dispersing into the vitreous. When there is retinal thinning or lattice formation particularly inferiorly, or when there is no PVD, the risk of inducing retinal breaks cannot be omitted, and close monitoring should be performed.

The injection technique is identical to that in pneumatic retinopexy, where 0.3 mL 100% C₃F₈ should be used, with or without the injection of TPA. This is followed by a strict facedown posture for a few days.

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.

Fig. 104.5 Technique of postoperative gas exchange 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.

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.

Fundal exam in the postoperative period

Examining the fundus with an in situ bubble can be difficult in inexperienced hands. When there is a near complete fill, examination through the bubble is relatively easy. As the bubble gets smaller, the retina is easier to see through the gas compartment. This is due to reflections from the gas–fluid interface. To visualize the whole fundus, the head has to be tilted sideways, to allow the bubble to cover different areas for inspection. When the gas compartment is smaller than the fluid compartment, examining the patient when lying supine helps to bring the bubble behind the pupil and reduces glare.

Intraocular pressure measurements

Maximum expansion of the bubble occurs within the first postoperative day. During this period, monitoring of intraocular pressure (IOP) is important, as an overfilled expansile bubble may predispose to central retinal arterial occlusion. Measurement with applanation tonometry has been found to be more accurate than Tonopen or Schiotz tonometer.³⁹ Risk of having an IOP rise is lower with air injection or nonexpansile gases. For high-risk cases, prophylaxis with oral Diamox and topical timolol should be given, especially in cases having pre-existing glaucoma.

Laser photocoagulation

When more photocoagulation is deemed necessary, it can be done through the bubble. Hypotony may cause corneal striaes when contact lenses are applied on the eye for photocoagulation. This can be overcome by temporarily injecting air into the eye to increase the IOP, which could then be released afterwards with a needle and syringe. The peripheral fundus may not be easily visualized with a wide-angle contact lens, and photocoagulation via laser indirect ophthalmoscopy (LIO) may be necessary. In cases where LIO is not possible, cryotherapy should be used. It has been reported that up to 70% of redetachment can be flattened with the use of fluid–gas exchange coupled with supplementary photocoagulation.⁴⁰

Vision change after surgery

It should be emphasized preoperatively to the patient that vision will drop after injecting gas. This is vital in cases of macular-on retinal detachments, where preoperative vision may be normal. Vision is usually poor during the lifespan of the bubble, mainly due to diffraction and glare. After dissolution of the bubble, vision may slowly return, provided the patient has good visual prognosis.

Changes in altitude

As mentioned above, the gas bubble changes in size at different altitudes. It is therefore important to advise the patient to refrain from changing altitudes. If the change in altitude is gradual and could be compensated for by outflow facility, IOP change may not be apparent and would not cause significant problems. However, rapid changes in altitude may cause a sudden expansion in bubble volume and IOP, which may not be compensated for in time, and central arterial occlusion may occur. Air travel and scuba diving should therefore only be permitted after complete dissolution of the bubble. Loss of light perception on sudden ascent from sea level to 4300 feet in 20 minutes by car has been reported.⁴¹

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.

Hypotony

The bubble can leak from sclerotomies either at the end of operation during wound maneuvers, or postoperatively, through a leaky wound. This results in hypotony. This should not be overlooked, as choroidal effusion or hemorrhage may occur with prolonged hypotony. Observation is enough for mild cases, but when hypotony is prolonged or if the risk for choroidal hemorrhage is high, reinjecting the eye with more gas is indicated. This could be performed using the pars plana approach at the slit lamp.

Subretinal gas

Migration of gas into the subretinal space can occur both intraoperatively, or in the postoperative period. It can occur either because the bubble is smaller than the tear, or when persistent traction elevates the retina and allows passage for gas into the subretinal space. If noted during surgery, the bubble can be displaced with the help of scleral depression externally. During the postoperative period, if the bubble has gone into the subretinal space, it can affect proper attachment of the break and may lead to redetachment. If away from the break and it does not affect break attachment, it can be left alone and will usually be absorbed within a few days. If break adherence is affected, reoperation may be required, and presence of residual traction should be inspected and released prior to reinjection of gas.

Gas in the anterior chamber and corneal decompensation

This may occur in aphakic eyes or in pseudophakic eyes with a nonintact posterior capsule. View of the fundus is often compromised if this happens. If noted intraoperatively, AC could be filled with viscoelastics prior to proceeding. If found postoperatively, it can usually be left alone and will be absorbed within a few days. Reoperation is seldom required. However, prolonged contact of the bubble in aphakic eyes with the use of expansile gases may predispose the corneal endothelium to hypoxia and decompensation.⁴⁴ This is mainly due to the interruption of aqueous flow to the endothelium, which in turn reduces the oxygen supply. Avoiding lying supine may reduce bubble–endothelium contact and potentially reduce the risk of corneal decompensation in such cases. In patients with cervical spine problems that prevent them from posturing, a large bubble should be avoided.

Intraocular lens capture

With combined phacovitrectomy and intraocular gas injection, the IOL may be pushed forward into the AC, causing optic capture. This could be prevented by limiting the anterior capsulorrhexis to a size smaller than the optic of the IOL. The patient should also be advised to avoid a faceup posture. The condition may be left alone if there is no tilting of the IOL and disturbance to vision is minimal. However, repositioning of the IOL may be needed if dislocation has occurred or pigment dispersion is significant.