As described before, the present follow-up study had a relatively rigorous design. Evaluation indicators included retinal perfusion system, choroidal capillary plexus, retinal and choroidal layers thicknesses. Before us, study designs of previous researches on fundus observation after RRD surgery divergent widely. Combined operation with cataract surgery [20, 21], either or both scleral buckling and PPV [12, 13, 17–21], various intraocular tamponade choice[12, 15, 17, 21], could all have impact on fundus structure and vasculature. Moreover, few study focused on the observation of different tamponade effects of silicone oil with other intravitreal materials on fundus structure and vasculature changes after retinal reattachment. We performed only single PPV for retinal repair to eliminate potential bias due to surgical option. It is unethical to randomly choose intravitreal SO or sterilized air as tamponade materials during procedure. After adequate communication with each patient, considering of patients’ will of undertaking a second operation (SO removal), the decision was made by both the patients and the surgeon. It is understandable that the SO group had relatively larger extent of detachment than the Gas group (Table 1). However, the symptom duration, number of retinal tears had no significant difference between Gas group and SO group. Thus, we believe that the intraocular pathological conditions between two groups were similar and our work could be reliable in observation of different tamponade effects of silicone oil and sterilized air on postoperative fundus changes. The recommended time for SO removal is yet to reach a consensus. Theoretically, the removal of SO is to avoid SO-related complications such as cataract, glaucoma, and keratopathy. As routine practice in our ophthalmic clinical center, RRD patients underwent retinal repair with anatomically well-reattached retina according to OCT follow-up and no sign of obviously progressive PVR for around 3 months are suggested for SO removal. Thus, the duration of observation in the present study was 12 weeks.
Retinal vessel density, choroidal thickness and vessel density change
The retinal and choroidal vasculature changes after PPV for retinal repair have not reached a commitment, which is due to different study designs of previous researches as described above. For retinal perfusion, Wang et al, found significant increase in macular perfusion during observation (12 weeks) after retinal reattachment[21]. However, in their study, the PPV operation was combined with cataract surgery. Phacoemulsification has been demonstrated to increase retinal VD after surgery [23, 24]. Wu et al. found significantly lower SCPVD and DCPVD in macular-off RD following Gas tamponade eyes than the fellow eyes[18]. Although such finding is consistent to our results, we have to realize that the length of their cross sectional observation time was relatively longer than us (3.6 ± 2.4 months, range 2–9 months). In the present study, we found decreased retinal perfusion in both SO tamponade and Gas tamponade eyes from 6 weeks to 12 weeks (Table 3). Of which, only SCPVD was found significant decreased in Gas tamponade eyes, and SO tamponade led to pronounced decrease in both SCPVD and DCPVD (Table 3). Although the macular perfusion system and structure could be influenced after macular detachment. We still postulate that intraocular SO tamponade might lead to poorer postoperative macular perfusion than Gas tamponade, especially in deep capillary plexus vessel density.
For choroidal capillary plexus status, previous studies used choroidal thickness as an evaluation indicator. Akkoyun et al. reported increased subfoveal choroidal thickness 1 week after retinal repair, which they thought represent postoperative inflammatory reaction[25]. Karimi et al reported time-related reduce of subfoveal choroidal thickness in SO tamponade eyes after PPV which may possibly due to tamponade effect of silicone oil[16]. However, choroidal thickness change is not necessarily associated with choroidal vessel change[26, 27]. According to our observation, we failed to find any significant result of choroidal thickness change during observation. We only found significant increase of parafoveal CCPVD in SO tamponade eyes from 2 weeks to 6 weeks (p = 0.013, Table 2). We thought that it may be attributed to the relatively greater amount cryopexy and laser photocoagulation during procedure following SO use, which lead to more severe choroidal inflammation after surgery. Compared to choroidal thickness, choroidal vessel density might be a more sensitive method to monitor postoperative choroid status. Future study should verify the choroidal vasculature change in long-term follow-up.
Retinal layer thickness change
In most previous studies on retinal structure change after RRD surgery, the retinal layers were divided in a relatively general way. In the present study, the retinal layers were strictly segmented into seven layers as introduced in the method section. Due to different study design and observation length, the result of postoperative NFL thickness after retinal repair is a little bit controversial. The report of short term increase may be related to postoperative inflammation[15]. NFL thickness may have a decreasing trend in long term follow up[28]. In the present study, the NFL thickness kept steady from 2 weeks to 6 weeks in both SO and Gas tamponade eyes. From 6 weeks to 12 weeks, we found pronounced decrease of NFL thickness in SO tamponade eyes than in Gas tamponade eyes (p = 0.003, Table 5). NFL thickness decrease echoed the experimental results of inner retina thickness change and loss of neuronal connections after surgical repair of RRD[29]. Furthermore, previous study has also described the potential toxic effect of SO on ganglion cells[14]. Thus, according to the more significant decrease of NFL thickness in SO tamponade eyes, we may demonstrate that SO tamponade could have more negative impact on ganglion cells compared to Gas tamponade.
We found significant increase of foveal OPL thickness in SO tamponade eyes across the whole observation (p = 0.015,Table 4; p = 0.033, Table 5). Yasin et al. reported thicker INL and OPL in both SO tamponade and Gas tamponade eyes after successful retinal repair than in fellow eyes[15]. Theoretically, the inflammatory reaction after retinal detachment and reattachment repair may be observed as increase in the volume of OPL thickness. Intermediate filament protein like vimentin and glial fibrillary acidic protein extends throughout the cytoplasm of the Müller cells[30]. Marcel et al. also reported a time related increase of INL-OPL thickness after successful retinal reattachment[17]. In the light of these findings, we suspect the more pronounced increase of OPL thickness may indicate more severe and longer-lasting fundus inflammation due to SO tamponade.
The photoreceptor lied in the ONL + IS and the OS + RPE layer in the present study (Fig. 2). The integrity and thickness of IS/OS could be an important predictor of postoperative VA after RD surgery[31, 32]. We found significant increase of ONL + IS and OS + RPE thickness in both SO tamponade and Gas tamponade eyes across observation (Table 4&5), which might indicate photoreceptor recovery[33](One can even tell the increase from Fig. 1, A4, B4&C4). Furthermore, we found significant decrease of BRM thickness in Gas tamponade eyes from 2 weeks to 6 weeks (Table 4). Thickness change of Brunch’s membrane after retinal repair has not been previously reported.
Recent years, numerous researches has been done for the underneath mechanism of Silicone Oil-Related Visual Loss (SORVL). Some researchers considered such visual abnormalities to be related to SO tamponade effect[34], and several pathophysiologic hypotheses have been proposed. One theory speculated that photo-toxicity may have a role, as oil transmits light more in the blue spectrum than aqueous[35]. SO has also been reported to dissolve fat soluble elements from the retina[36]. The fat soluble macular pigments like lutein and zeaxanthin are thought to protect the macula from photo-oxidative damage[37]. In addition, lost buffering capacity of the vitreous cavity and presence of intraocular SO lead to impaired retinal homeostasis. Potassium, magnesium and cytokines levels were found to be influenced due to SO tamponade, which may cause metabolic exhaustion and degeneration of the Müller cells[38–40].
The present study has certain limitation. Due to clinical practical routine in our ophthalmic center, we only compared different tamponade effects between SO and gas, and the observation time was relatively short. In several previous studies, fellow eyes were used as control[13–15, 18, 21]. However, unaffected eyes with asymptomatic feature could have vascular abnormalities in fundus diseases like retinal vein occlusion and primary open-angle glaucoma[41, 42]. Retinal detachment could also be bilateral in quite a number of patients. Thus, we did not take fellow eyes data as baseline or control to avoid potential bias. The present study put emphasis on observation of various fundus changes due to different intraocular tamponades. We analyzed fundus changes in macular-off RRD patients, the impact of disease influence on postoperative fundus abnormalities should be taken into account.
In conclusion, we observed different intraocular tamponade impacts of silicone oil and gas on fundus vasculature and structure in macular-off rhegmatogenous retinal detachment patients after single pars plana vitrectomy. Silicone oil tamponade had relatively more negative effect on retinal vasculature and specific retinal layer thickness change during observation.