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

SF6 is commonly used in vitreoretinal surgery, such as retinal detachment and proliferative diabetic retinopathy. The effect of the gas bubble on smoothing retinal folds and flattening fish mouth tears is only required for a short period. Air is preferable in situations in which the volume of gas is adequate to tamponade the break. SF6 should be considered when the expansion properties of a gas are desirable to achieve a larger volume, which will tamponade a larger break or multiple breaks. A frequent use of SF6 is in the management of macular holes, in which vitrectomy is done to remove the posterior hyaloid from the surface of the retina in the macular area, and a mixture of 20% gas and 80% air is introduced to create

atamponade effect and re-attach the edges of the hole (Figure 26). This procedure, if successful, leads to improvement of vision in

asignificant number of patients.

Use of C3F8

C3F8 is also inert, colorless, and odorless, is inflammable, and expands four times its volume. Depending of the volume injected inside the eye, it lasts about fifty five days.

The most common situation in which C3F8 is used is in rhematogenous retinal detachment with severe PVR, and severe diabetic retinopathy, where the surgeon peels vitreoretinal membranes, and re-attaches the retina after making a fluid-gas exchange. C3F8 is the gas mostly used in PVR surgery. Its use allows the neuroretina to be in contact with the retina pigment epithelium for about 10-15 days with a complete tamponade effect. This

Figure 26: Mechanism of Fluid-Gas Exchange to Reattach the Retina Following Vitrectomy, Relief of Traction and Tamponade Effect in Macular Holes. Fluid-gas exchange is performed following vitreoretinal surgery to maintain the retina attached to the pigment epithelium. First, the intraocular fluid is exchanged with air, until air (A) fills 100% of the cavity as shown. The air is introduced into the vitreoretinal cavity by an air pump via an infusion cannula (C) placed in the inferior temporal quadrant. At the same time, fluid (F) is aspirated via an aspiration cannula (B) placed in the superior nasal quadrant. Next, the air is exchanged with a mixture of gas and air (20% gas with 80% air if SF6 is used and 15% gas with 85% air if C3F8 is used). A light probe (E) for endoillumination is present in the superior temporal quadrant during this exchange. (Art from Jaypee Highlights Medical Publisher).

makes possible a stable chorioretinal adhesion from either laser or cryotherapy. C3F8 may also be used in cases of iatrogenic retinal breaks while removing or delaminating the membranes. When this complication arises, the use of gases makes the final results much more satisfactory.

These gases, SF6 and C3F8 are used industrially. Consequently, they are available in most countries. It is important to request medical grade, and to filter them before use.

Fluid Gas Exchange for Gas Injection

After performing vitreoretinal surgery to flatten the retina, a fluid-gas exchange is frequently done. Air is introduced into the vitreoretinal cavity through the infusion cannula, by an air pump usually integrated in the vitrectomy machine. Fluid is drained from the same cavity, and from the subretinal space, through an extrusion cannula. At the same time that the fluid is being extruded through the cannula, air is being introduced y the air pump, filling the vitreoretinal cavity until it occupies 100% of its space.

The air-gas exchange is done by insufflating an air-gas mixture with a concentra-

cannula (Figure 27). To obtain the 15% con-

Figure 27: Mechanism of Air-Gas Exchange to Leave an Intraocular Mixture of Gas and Air. The gas mixture is introduced (blue arrows) via the infusion cannula (C) as the air (A) exits (red arrows) the eye via the open sclerotomy (S). (Art from Jaypee Highlights Medical Publisher).

centration of C3F8, in the air-gas mixture, one should aspirate 3 cc of pure C3F8 into a 20 cc syringe; then air is aspirated until the 20 cc are completed. A concentration of 20% SF6 and 80% air is prepared by aspirating 4 cc of SF6, followed by 16 cc of air.

Another alternative, after the eye has been left insufflated with air (instead of fluid), is to wash the air from inside the eye by injecting the prepared concentrations of gas through one of the already closed sclerotomies, connecting a millipore filter and a 30-gauge needle to the syringe, and allowing

the excess gas to be extruded through another needle connected to a small syringe, with the plunger removed, and introduced through the opposite closed sclerotomy.

The Use of Intraocular Gases vs Silicone Oil in Rhegmatogenous Retinal Detachment

Comparative Effectiveness

In the United States, a multicenter control trial was made to evaluate the effectiveness of silicone oil in comparison to the C3F8 or SF6 gases, in cases of PVR. The study concluded that there was no difference in results in attaining a permanent reattachment of the retina.

In general, the tamponade elected in a particular case depends on the experience and preference of the surgeon. In a retinal detachment due to a large giant tear, we now prefer the use silicone oil as the tamponade.

Problems With Silicone Oil

The problem with using silicone oil is that we have to perform a second surgical procedure to withdraw this substance. This needs to be done about three to six months after the initial surgery, when the retina is re-attached and stable from the tamponade

effect. Futhermore, silicone oil left in the vitreous cavity for long periods of time (over six months) may lead to sight-threatening complications such as keratopathy, glaucoma and cataracts. In cases of inferior retinal detachment due to inferior breaks, silicon oil may not be the best option; due to its low density that may result in limited or reduced inferior tamponade. Another issue is that silicone oil may stimulate a peri-silicone proliferation of scar tissue. These are the main reasons why many retinal surgeons do not like to use silicone in most of their cases.

Indications for Silicone Oil

Silicone oil is particularly indicated in patients who live at higher altitude than the surgical center where the surgery takes place and/or patient who need to travel by plane soonaftertheprocedure. Rapiddecompression of atmospheric pressure during air travel may cause an elevation of intraocular pressure in patients with a large intraocular gas bubble. This rise in pressure may compromise blood flow through the central retinal artery. Lincoff et al. support the indications from clinical experience, that volumes of intraocular gas up to 1.0 mL can be tolerated. Silicone oil allows the patient to travel by plane after a successful retinal surgery. When vitreoretinal traction has not been relieved or when it can be anticipated that it will recur, the use of silicone oil may be valuable. Clinical experience has prompted the restriction of silicone-oil tamponade to severe cases of retinal detachment, selected patients with complex diabetic retinopathy, viral retinitis,

and severe trauma. It seems that in eyes with severe anterior PVR and clinically significant posterior PVR changes, a better visual prognosis has been attained when silicone oil has been used. Some concerns about the long-term effect of silicone oil on the retina and other tissues, have been expressed.

PERFLUOROCARBON

LIQUIDS (PFCLs)

They represent a significant development in vitreoretinal surgery. Dr. Refojo in Boston, and Dr. Stanley Chang in New York, were the pioneers in their use. They observed that their weight, their superior tamponade-effect, and their very different refractive-index from water, also made them easily distinguishable from saline solution. Presently, the perfluorocarbon liquids (PFCLs) are used to re-attach the retina in PVR cases, when one have to see the retinal-traction and membranes intraoperatively, and in giant retinal tears, where the posterior edge of the tear is often rolled towards the vitreous cavity. When heavy liquids are introduced after vitrectomy has been performed, the retina is re-attached and the tear is again in contact with the pigment epithelium.

Another use of PFCLs is in the management of crystalline lenses and intraocular lenses traumatically luxated into the vitreous (Figures 28A and 28B), in the management of retinal detachments caused by ocular trauma.

Figure 28 A: Use of Perfluorocarbon Liquid for Dislocated Lens - Stage 1. In cases of removal of a hard dislocated lens, perfluorocarbon liquid (P) can be placed in the eye. Lens fragments float on top of the liquid at a safe distance from the retina. Here a large lens piece (L) is cracked and aspirated with the phaco tip (A) and manipulated with a tissue manipulator (cannula with endoilluminator (E)). Infusion cannula (I).

(Art from Jaypee Highlights Medical Publisher).

Figure 28 B: Use of Perfluorocarbon Liquid for Dislocated Lens - Stage 2. The last lens piece (L) is shown being aspirated from the eye with the phaco tip (A) as it floats on the perfluorocarbon liquid (P). Tissue manipulator (cannula with endoilluminator (E)). Infusion cannula (I). (Art from Jaypee Highlights Medical Publisher).

Some cases of uncomplicated rhegmatogenous retinal detachments arising from occult retinal break(s), may be treated by vitrectomy and PFCLs, rather than by a scleral buckle alone. PFCLs can help to identify the location of occult breaks by exerting a posterior flattening force that displaces the subretinal fluid through the peripheral break, exhibiting a schlieren sign, seen when two liquids of different retractive-index are mixed. The retinal break(s) can be located by observing the direction of schlieren flow. PFCLs may also produced a very effective tamponade in cases of inferior retinal breaks, when the use of silicon oil may be limited; however, some authors have found irreversible damage to the retina after prolonged periods during which PFCLs remains in the vitreous cavity.

In recent years, the development of second and third generations of heavy tamponade agents, which combine silicone oils and heavy liquids, such as Oxane HD (Bausch and Lomb, Toulouse, France), Densiron 68 (Fluoron Co, Neu-Ulm, Germany), and HWS 46-3000, have shown, at least in preliminarily results, better tolerance and tremendous potential in repairing complex retinal detachments.

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Giant retinal tears are not as prevalent as many other types of retinal detachments. However,theyareanimportantsubjectbecause they have traditionally been difficult to repair and have been associated with a lower success rate and more complications than other retinal detachments. While any patient can develop a giant retinal tear spontaneously or

after cataract surgery, several types of patients seem more prone than others to developing giant retinal tears. These include patients with hereditary vitreoretinal abnormalities such as Stickler syndrome.1-3 Giant retinal tears are also associated with trauma in approximately 20% of cases4 (Figures 1a and 1b).

Giant retinal tears are defined as retinal breaks greater than 90 degrees in extent or circumference. Giant tears are distinguished from other retinal breaks by their size and by the fact that the vitreous gel is attached only to the anterior flap. While a large retinal dialysis can also be considered a retinal tear, the pathogenesis of the two conditions is completely different (Figure 2).

Giant retinal tears occur at the posterior border of the vitreous base. The vitreous gel is not attached to the posterior flap, which moves independently of the anterior flap. The posterior flap can therefore fold backward and invert. In contrast, in a retinal dialysis the gel is attached to the retina posterior to the break; consequently, there is no inversion of the posterior flap (Figure 2). In retinal detachments associated with dialysis, the posterior hyaloid is usually attached, and epiretinal membrane proliferation and contraction are uncommon. Giant retinal tears, on the other hand, are almost always associated with posterior vitreous detachment and often associated with dispersion of pigment epithelial cells. Fibrocellular proliferation can easily occur on the retinal surfaces and the vitreous gel (Figure 3). Therefore, proliferative vitreoretinopathy (PVR) is a common sequel of giant retinal tear, which makes surgical management more challenging and the prognosis worse.5

Figure 3: Forces Involved in Cases of Giant Tear with PVR. Proliferative vitreoretinopathy (PVR) can occur as a result of a giant retinal tear. PVR has anterior and posterior components. In the anterior component, the types of traction forces are: 1.) Anteroposterior traction which displaces the posterior insertion of the vitreous base forward. It is recognized by posterior retraction of the iris and a circumferential trough anteriorly at the vitreous base. 2.) Anterior circumferential traction is caused by proliferative tissue on the posterior border of the vitreous, creating a circumferential ring at the posterior border of the vitreous base. It is recognized by radial retinal folds extending posteriorly from the posterior border of the vitreous base. 3.) Anterior perpendicular traction is caused by proliferation within the vitreous gel. This transvitreal traction pulls the anterior retina to the center of the vitreous cavity causing an anterior funnel shaped retinal detachment. Posterior PVR occurs posterior to the posterior border of the vitreous base. (Art from Jaypee Highlights Medical Publishers.) (Art from Jaypee Highlights Medical Publishers.)

Preoperative Assessment of the

Retinal Tear

The surgical approach should be guided by a preoperative study of the retina to determine the characteristics of the giant retinal tear (Figure 4). Using a slit lamp, the surgeon needs to determine the mobility of the vitreous and the degree of pigmentary dispersion.

Figure 4: Preoperative Assessment of Retinal Tear Characteristics. Preoperatively, the surgeon needs to determine the mobility of the vitreous (1-arrow) and degree of pigmentary dispersion (2). The relationship

(3) between the vitreous and the edge of the retinal flap (F) is noted. The mobility of the retinal flap is determined (4). Presence of additional small retinal breaks and tears (5) is assessed. The presence of periretinal proliferation is also determined (6). (Art from Jaypee Highlights Medical Publishers.)

The relationship between the cortical vitreous and the hyaloid posterior to the edge of the retinal flap should be noted (Figure 4). When the cortical vitreous has migrated under the inverted posterior flap of the tear, vitrectomy is necessary.6-8 In eyes in which the posterior hyaloid has not completely separated from the retinal surface opposite the tear, vitrectomy with a gas tamponade should be considered.

The mobility of the retinal flap must also be assessed. Vitrectomy is always necessary when the posterior flap is immobile. If the anterior edge of the tear is rolled, then the presence of epiretinal or subretinal proliferation should be suspected.

Although our main focus is on giant retinal tear, the surgeon should also look for the presence of additional small retinal breaks that should be treated with either laser or cryotherapy. A scleral buckle can be used alone without vitrectomy if the extent of the giant retinal tear is 120o or less and the posterior flap is not inverted.9 Scleral buckling alone can also be considered for a large retinal dialysis.

Surgical Principles for the Management of Uncomplicated Giant Retinal Tears

The surgical technique should be individualized for each case but follows the same

general outline and principles in most patients. The state-of-the-art treatment of giant retinal tears with or without PVR involves the use of perfluorocarbon (PFC) liquids. Using these liquids not only makes surgery simpler, but it also allows the surgeon greater confidence in planning the surgery and greater intraoperative control.10

THE IMPORTANCE OF

PERFLUOROCARBONS

Pioneered by Stanley Chang, M.D., PFC liquids have revolutionized vitreoretinal surgery. They are derived from hydrocarbons and are created by replacement of the hydrogen atoms by fluorine atoms. They are inert, colorless substances with very low water solubility. Most importantly, they are heavier than water and have a specific gravity approximately twice that of water.11 After intraocular injection of PFC liquid, the flattening forces are significantly greater than can be achieved with other materials used in vitreoretinal surgery, such as oil or gas (Figures 5, 6). Therefore, they facilitate dissection of membranes by flattening or stretching out the retina, particularly in the periphery.10 They are frequently used in the surgical management of giant retinal

Figure 5: Retinal Reattachment With Perfluorocarbon Liquid in Case of Giant Tear - Stage 1. In the case of retinal detachment with giant retinal tear, perfluorocarbon liquid (P) is injected into the vitreous cavity via the Chang cannula (N). Because the liquid has a specific gravity greater than water, it gravitates (blue arrow) to the posterior pole. This forces the subretinal fluid (S - red arrows) out through the giant retinal tear and out of the eye via the Chang cannula. The retina (R) is being forced to reattach (green arrow) in this manner. Infusion cannula (I). Endoilluminator (E). (Art from Jaypee Highlights Medical Publishers.)