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Retinal reattachment rates are much higher for simple RDs than for complicated RDs, and the visual results better. An outline is given for the reasons why retinal reattachment may not be achieved and why there may be functional (i.e. visual) failure after a technically successful operation.

1.8.1 Anatomical Failure:

A successful outcome will not occur if the wrong surgical approach is selected, if the techniques are incorrectly applied or if unavoidable or insurmountable technical problems arise. The surgical sequence is also important(67). The nature of the RD and attendant pathology has a large part to play in the final outcome and careful preoperative examination is imperative(62,145). Reattachment rates for simple RDs are good, varying from 84% - 96% after one or two operations(67,76,135,146- 8). Success is more likely in cases with round holes or dialyses and those with localised RDs, and less likely in aphakic eyes, those with PVR and those in whom a break cannot be identified preoperatively(147).

Retinal reattachment rates for complicated RDs are, however, not so good, varying from 55-80%, multiple operations often being required(104-5,149-153). Clarkson compared reattachment rates in 50 patients who had internal tamponade with

either air or SF6 gas. The gas injected eyes had better reattachment rates which was attributed to longer periods of break closure(154). The Silicone Oil Study Group (SSG) have undertaken two randomised, multicentre treatment trials of eyes with PVR > grade C3(104-5). Group I eyes had not had previous surgery, while Group II eyes had undergone vitreoretinal surgery. After vitrectomy and peroperative retinal reattachment eyes were randomised to internal tamponade with either SF6 gas or silicone oil in the first study, or C3F8 gas or silicone oil in the second study. The outcomes were macular reattachment rate, visual acuity and complications. In the first study macular reattachment was finally achieved in 80% of silicone oil filled eyes (Group I) compared to 60% of those treated with SF6 gas (p = <0.05), although repeat, surgery was required in 16 eyes in both groups, sometimes using different procedures (data were analysed by the original treatment modality). If the results of the first operation only are compared (Group I eyes only) retinal reattachment was achieved in 13/40 (33%) eyes treated with SF6 compared to 19/46 (41%) treated with silicone oil (Chi sq test, p = 0.4). The second study showed no significant difference in the final macular reattachment rates of Group I eyes (73% treated with C3F8 vs 64% treated with silicone oil) and Group II eyes (73% vs 61% respectively). If results of the first operation only are compared for Group I eyes macular reattachment was achieved in 29/67 (43%) C3F8 treated eyes compared to 27/63 (43%) eyes treated with silicone (Chi sq test, p = 0.96). In Group II eyes the rates were 30/71 (42%) and 21/63 (33%) respectively (chi sq test, p = 0.29). The authors conclude that results were better using C3F8 compared to SF6 gas, and that C3F8 gas compares favourably with silicone oil.

Retinal reattachment rates for eyes treated with silicone oil show that eyes with advanced PVR have the lowest reattachment rates, varying from 55 - 80% (104-5, 153,155), with anterior PVR carrying the worst prognosis( 125). Retinal reattach­ ment rates of 85-90% have been reported for eyes without PVR (97,155-157).

In these series a significant proportion of eyes treated with silicone oil redetached at a later date, often due to the development or progression of ERMs leading to traction RD, reopening of existing breaks, or the formation of fresh retinal tears. There are several theoretical reasons why retinal patching may prevent the development of ERMs.

ERMs have a cellular component (macrophages, glial cells, RPE cells and fibroblasts) within an extracellular matrix of collagens and FN(158). Viable RPE cells can be dispersed into the vitreous cavity by cryotherapy and proliferating RPE cells can reach the retinal surface via retinal breaks(88). Application of a retinal patch would not disturb the RPE and restoration of retinal continuity would prevent

RPE cells reaching the retinal surface. Subretinal and intravitreal fluids have been shown to contain FN and growth factors (possibly derived from RPE cells) which promote proliferation and migration of certain of the cell types implicated in the pathogenesis of ERMs(139) (from Gilbert 1988). Restoration of retinal continuity would confine these substances to the subretinal space. Cryotherapy, which would not be necessary with retinal patching, induces BRB breakdown and has been found to promote ERM formation in experimental models of PVR(136). All these factors offer theoretical reasons why retinal patching may result in a lower

incidence of postoperative ERM formation.

1.8.2 Visual Failure:

Visual function may not recover after a technically successful operation due to cystoid macular oedema (CMC), subretinal pigment, retinal folds and failure of macular recovery after macular detachment. Causes of late visual failure include macular pucker, cataract, glaucoma, keratopathy or retinal redetachment. Eyes with complicated RD carry the worse prognosis. Visual results from a series of publications are shown in Table 1.3(92,94-6,99-100,125). These patients had all undergone vitrectomy with silicone oil tamponade for different types of complicated RD. The findings are not, therefore, directly comparable. 26-63% of patients had early visual acuities of less than counting fingers despite successful anatomical results (defined as complete, or partial retinal reattachment including the posterior pole). The visual outcome of patients in the SSG trials are shown in Table 1.4 (104- 5) and early results are similar to the other studies. Early and late visual acuities are reported in the first SSG trial, and late visual acuities are reported in three of the other studies (Tables 1.4, 1.3). In the SSG trial vision deteriorated even in those patients with flat retinas; at 6 months 57% of patients with macular attachment had acuities of >5/200, compared to 46% at 24 months. In Leaver's study 33% of patients had worse vision at follow up, and in the other two studies acuities had dropped below CF in a further 24-40% of patients (Table 1.3). Causes of late visual failure included retinal redetachment and the development of complications. Chan's data are interesting(96) as the follow up in this group is long, including some of the first patients to have internal tamponade with silicone oil.

Retinal patching would probably not prevent CMC, nor would it influence macular function after macular detachment. Other causes of visual failure, such as retinal folding after scleral buckling, pigment fallout at the macula after cryotherapy, and macular pucker might, however, be prevented, as would some of the other sight- threatening complications of silicone oil.

Table 1.3 Visual acuity after internal tamponade with silicone oil

Date Author Ref Early VA Late VA > CF

% % 1979 Leaver 92 8 0 > C F 33 deterioration 1985 McCuen 94 41 > C F ND 1986 Cox 95 7 4 > C F ND 1986 Chan 96 90 "functional" 50 1987 Yeo 99 40 > 5/200 ND 1988 Federman 100 3 7 > C F 13 1988 Aaberg 125 40 > 5/200 ND VA = visual acuity ND = no data

Table 1.4 Visual acuity after internal tamponade with silicone oil - results of Silicone Study Group (SSG).

Date Study At 6 mn > 5/200 At 24 mn > 5/200

1992 SSG 1 Group 1 Macular attached 57% Macular detached 4% Overall 61% Macular attached 46% Macular detached 0% Overall 46% 1992 SSG II Group 1 Group 2 ND ND 46% 33%