Ординатура / Офтальмология / Английские материалы / Retinal and Vitreoretinal Diseases and Surgery_Boyd, Cortez, Sabates_2010

Introduction

Intraocular gas is a useful adjunct in the management of a wide variety of disorders of the posterior segment. Gases have been used advantageouslyfortheoutpatientmanagement of retinal detachment (pneumatic retinopexy) as well as an invaluable intraoperative tool during vitreous surgery, macular hole repair,1-3 and the management of complicated retinal detachment. Intraoperatively, gases have been used to manipulate the retina mechanically, to flatten retinal detachments and displace subretinal fluid, to maintain a clear view of the retina by displacing vitreous hemorrhage, and to maintain ocular tone. The function of the gas bubble postoperatively is to provide temporary internal tamponade of retinal breaks until a permanent chorioretinal adhesion occurs.

Innovations in the development and application of longer-acting gases in the last 25 years have provided a greater understanding of the clinical behavior of intraocular

gases in vitreous surgery. In this chapter we will summarize the current state of the art and review the indications, technique, and complications of intraocular gases in ophthalmic surgery. Emphasis is given to the characteristics of the gases most commonly used in vitreoretinal surgery.

History

Ohm is credited with the first injection of an intraocular gas (air) in 1911 to repair a retinal detachment,4 although the significance of closing retinal breaks in achieving reattachment was not known until Gonin's later discovery.5,6 It was Rosengren, however, who synthesized these concepts and advocated the use of intraocular air in conjunction with diathermy and drainage of subretinal fluid to create permanent retinal apposition to the retinalpigmentepithelium(RPE).7-9 Rosengreńs success was soon overshadowed by the innovation and success of scleral buckling in the mid-20th century.10

Work by Norton11,12 and others13-18 popularized the intraocular use of sulfur hexafluoride (SF6) gas for internal tamponade of retinal breaks. Gases with longer duration and greater expansile properties—the perfluorocarbon family of gases—were first investigated by Vygantas19 et al in 1973 and came into greater clinical use in the early 1980’s.20-28

Pneumatic retinopexy was introduced by Domínguez29 in Spain and popularized by Hilton and Grizzard.30 This technique, unlike the conventional repair of retinal detachment, could be performed in the outpatient setting without surgical buckling techniques (Figure 1). The advantages and limitations of this alternative treatment modality are beyond the scope of this chapter but are reviewed in several excellent works.29-32

Bubble Dynamics

An expansile concentration of gas injected intraocularly undergoes three phases: bubble expansion, equilibration, and bubble dissolution. Certain environmental factors and specific gas characteristics govern the behavior of the gas bubble through these stages.

The first stage of gas transfer occurs after a pure gas is injected into the vitreous cavity. Rapid bubble expansion commences as host tissue gases(oxygen,carbondioxide,andwater vapor) diffuse into the bubble along partial pressure gradients.10,33-35 Nitrogen enters the bubble more slowly than oxygen and carbon dioxide. Continued bubble expansion is halted (maximal expansion is achieved) when the

rate of pure gas diffusion out of the bubble is equal to the rate of nitrogen diffusion into the bubble. Principal factors contributing to a slow rate of diffusion include a low coefficient of diffusion, poor water solubility, and a high molecular weight.10,17 Although the time required to achieve maximal expansion varies among specific gases, the most rapid rate of initial bubble expansion is fairly constant despite the type of expansile gas used and typically occurs in the first 6 to 8 hours.10 Vitreous convection currents are thought to play a major role during this time period, although this theory is based on in vitro studies.34

The second stage—nitrogen equilibration— occurs as the partial pressures of nitrogen within the gas bubble and in the surrounding tissue equilibrate.17,35 At the beginning of this phase, the bubble has achieved maximal expansion, but soon loses volume as pure gas leaves the bubble (because of a high pressure gradient) at a rate greater than that at which nitrogen enters it (as a result of a decreasing partial pressure gradient).17,34,35

The third stage is slow, predictable reabsorption. The concentration of various gases within the bubble remains constant throughout bubble dissolution that follows first-order exponential decay.28 Although gas bubbles may remain in the eye for periods of several days to several weeks (depending on the specific gas injected), the actual therapeutic effect of such bubbles is of much shorter duration. Thompson reported that the maximal effective duration of tamponade is equal to the passage of three half-lives.36

Chang noted that a 50% gas bubble volume is desirable to achieve effective tamponade of most retinal breaks; however, this therapeutic volume of gas is only present in the eye for approximately 25% of the actual duration of gas in the eye.10

The possible effects of the presence or absence of vitreous and/or the crystalline lens on the intraocular longevity of gas are still controversial. Wong and Thompson demonstrated that the phakic, non-vitrectomized eye maintains perfluoropropane (C3F8) and SF6 significantly longer than the aphakic, vitrectomized eye.37 Citing possible errors in methodology, Lincoff disputed the findings of Wong and Thompson which contradicted his prior study28 and he presented new data to support this claim.38

Clinical and Physical

Properties of Specific Gases

Variousgaseshavebeenusedforintraocular injection (Table 1), each with varying properties that make them desirable for a specific clinical situation. Gases may be classified by their chemical structure, behavior (expansile or non-expansile), and in vivo longevity (shortacting, intermediate-acting, and long-acting). The ideal gas is clear, colorless, nontoxic, and inert; maintains a high aqueous-gas interface surface tension; and remains in the eye sufficiently long to effect permanent chorioretinal adhesion.39 The three most commonly used gases in vitreoretinal surgery: air, SF6, and C3F8 (Table 2) are discussed.

Air

Air was the first gas used in vitreoretinal procedures.4,7-9 Air has a role as an intraoperative tool:

3)Perform a mechanical unrolling of large tears and flattening of fishmouth tears that result from retinal folds (although the perfluorocarbon liquids lend themselves more readily to this function).

4)Tamponade suspected but undetected retinal tears (in cases in which the retina fails to reattach) in the immediate postoperative period.

5)Provide retinal tamponade in cases of rhegmatogenous retinal detachment.10,36,40 Air remains in the eye for 1 to 2 days and does not expand under normal conditions.

Sulfur Hexafluoride (SF6)

SF6 was one of the earliest gases to be used for intraocular injection and is still widely used.41 Sulfur hexafluoride was chosen as an alternative to air when it was shown that this inert gas provided longer, more effective tamponade of retinal tears and detachments than air and was relatively free of harmful side effects.15,16,18 One milliliter of pure SF6 gas expands to approximately twice its original

size, attains maximal expansion two days after injection, and remains in the eye for 10-14 days. At a concentration of less than 20%, SF6 loses its expansile property.42

Perfluoropropane (C3F8)

The use of C3F8 has become routine in the management of complicated retinal detachment.41 Since its introduction by Lincoff in the early 1980s, C3F8 has been an attractive vitreous substitute in retinal detachment surgery because its prolonged intraocular duration (55-65 days) allows effective chorioretinal tamponade to occur.22,23 C3F8 remains in the eye four times longer than SF6 while exhibiting a side-effect profile similar to that of SF6.23 A C3F8 bubble expands to four times its original volume 72 hours after injection.22 C3F8 becomes non-expansile at a concentration of less than 12%42 and appears to be slightly more toxic than SF6.

Clinical Estimation of Gas Volume

Most clinicians describe gas fills as a percentage of the vitreous cavity based upon indirect ophthalmoscopy. For example, a 50% bubble describes a bubble whose inferior meniscus approaches the level of the fovea, whereas a 20% bubble would have as its inferior meniscus the level of the superior limbus. To avoid parallax error, it is imperative that the examineŕs eyes are at the same height as the patient́seyes and that the patient and examiner are sitting perfectly upright.37

Choice of Gas Based Upon

Clinical Situation

The surgeon must choose the proper gas, concentration, and volume to be injected. The primary role of the gas bubble is to provide internal tamponade of retinal breaks long enough for a permanent chorioretinal adhesion to form. The buoyant nature of gases not only provides internal tamponade, but this gas-fluid interface also serves to block the break and prevent fluid from entering the subretinal space.

To achieve these goals, the surgeon must choose a gas that will remain in the eye sufficiently long to perform these tasks. Superior breaks usually require smaller volumes of gas compared to inferiorly or posteriorly located lesions. Posterior and inferior breaks require larger bubbles and proper patient positioning for effective tamponade. A 50% bubble in the eye of a patient maintaining prone positioning is generally sufficient to treat posterior lesions effectively. Smaller bubbles (between 20-50%) are needed for treatment of macular holes with the patient prone.36 Lesser volumes can also be used for the treatment of superior breaks with the patient assuming upright positioning.36

Increased intraocular longevity—dictated by gas type and concentration—is required in the treatment of multiple or complicated breaks. Air is often injected intraoperatively during scleral buckling procedures after drainage of subretinal fluid to maintain ocular tone and is chosen because it will be rapidly absorbed into the blood stream after

serving its temporary support function. By contrast, perfluoropropane is more appropriate for treating multiple breaks complicated by residual vitreous traction in which prolonged tamponade is required. Several studies have attempted to outline decay rates of specific gases at varying concentrations in a variety of clinical conditions.23,28,38,42,43

Intraocular Pressure

As Thompson noted, scleral rigidity does not permit expansion of the eye after intraocular injection of gas.36 Therefore, an acute rise in intraocular pressure (IOP) is seen in approximately 36% of eyes after vitreous surgery;44 however, unlike some eyes injected with silicone oil, those injected with longacting gases rarely succumb to chronic IOP elevations.45 This phenomenon occurs for several reasons. Although the mechanical effect of the gas bubble against the iris-cili- ary body process may result in angle-closure glaucoma, this is actually one of the less common mechanisms. More frequently, an acute rise in IOP within the first 48 hours of surgery occurs as a result of the rapid expansion of the bubble, decreased aqueous outflow caused by intraocular inflammation, or decreased uveoscleral outflow in cases in which an encircling band has been placed.44,46 The rise in IOP as a result of intraocular gas injection can be difficult to differentiate from a similar rise caused by a steroid response. The distinction is typically based upon the fact that the former occurs in the early postoperative period while the latter becomes a problem later in the recovery period.

Because the Tono-Pen tonometer is known to underestimate IOP in gas-filled eyes,47,48 the surgeon should be mindful of established conversions48 or utilize more reliable methods such as Goldmann applanation tonometry.

The eye compensates for increased IOP by correspondingly increasing aqueous outflow; however, this mechanism often cannot overcome the large increases in IOP encountered in the early postoperative period. Management of the acute rise of IOP after vitreous surgery with long-acting gases typically involves topical or systemic carbonic anhydrase inhibitors. Anterior chamber paracentesis may provide

temporary relief of elevated pressure and, rarely, aspiration of previously injected gas may be required (Figure 2). The complication of central retinal artery occlusion must be considered after injection of an expansile concentration without removal of vitreous, as in pneumatic retinopexy. Frequent monitoring of the fundus in the first 6 to 8 hours after surgery is warranted in such cases.

Prior to intraocular injection of an expansile gas, gonioscopy should be performed to detect eyes with compromised angles, which may have areas of closure caused by peripheral anterior synechiae or neovascularization. Only

non-expansile gases should be used in these eyes, and all patients with a significant gas fill should be advised to assume a face-down position to avoid aphakic pupillary-block glaucoma, angle-closure glaucoma, and lenticular opacities.21

Effect of Gas on Ocular Structures

In addition to rises in IOP, intraocular gases can have a variety of adverse affects on the ocular structures. Chief among these are crystalline lens and capsular opacities, intraocular inflammation, vitreous changes, corneal endothelial damage, proliferative vitreoretinopathy (PVR), and macular pucker.49 Other complications include injection50 or subsequent migration of subretinal gas, creation of new retinal tears, anterior dislocation of an intraocular lens implant, and the rare possibility of endophthalmitis.

Lens opacities and corneal endothelial damage are thought to be caused by the mechanical effect of the bubble against these structures, not as a result of its chemical composition. The presence of the gas bubble is believed to interfere mechanically with the normal metabolism of these structures. Therefore, face-down positioning by the patient can alleviate the mechanical irritation of the gas bubble to the lenticular surface or the corneal endothelium in an aphakic eye. Most cases of keratopathy resolve over days to weeks,

but corneal decompensation is a real concern in an unfortunate subset of patients.

Recent studies on the rabbit retina suggest that retinal tamponade with long-acting gases results in histopathologic changes in the superior retina (thinning or disappearance of the outer plexiform layer and abnormally increased glutamate distribution).49 Further studies are needed to determine if these ill-effects are seen in the human retina as a result of prolonged tamponade by supposedly inert chemical gases.

Storage

Intraoculargasesaretypicallymaintainedin the manufacturer’s cylinder. Gas is dispensed from the cylinder via a two-filter system to guard against microbiologic or particle contamination. One syringe is used to collect the gas from the “dead-space” of the apparatus and is flushed several times before the second syringe is put in place and filled with pure gas. A 22-μm filter removes most particulate and microbiologic contaminants and should be used when filling syringes with either gas from the manufacturer’s cylinder or when collecting air from the operating room environment or outpatient setting.51 While temporary storage of gas in vacuum-sealed containers or in capped syringes for use in remote areas has been described, the practitioner should note that the concentration of the gas in these make-shift storage units varies considerably, especially with the passage of time.52,53