What pressure is exerted on the retina by heavy tamponade agents?

Biomechanical considerations for optimising subretinal injections

Subretinal injection is the preferred delivery technique for various novel ocular therapies and is widely used because of its precision and efficient delivery of gene and cell therapies; however, choosing an injection point and defining delivery parameters to target a specified retinal location and area is an inexact science. We provide an overview of the key factors that play important roles during subretinal injections to refine the technique, enhance patient outcomes, and minimise risks. We describe the role of anatomical and physical variables that affect subretinal bleb propagation and shape and their impact on retinal integrity. We highlight the risks associated with subretinal injections and consider strategies to mitigate reflux and retinal trauma. Finally, we explore the emerging field of robotic assistance in improving intraocular manouvrability and precision to facilitate the injection procedure.

Subretinal Injection Under Perfluorocarbon Liquids to Avoid Foveal Dehiscence

SUMMARY

We describe a novel technique modification of subretinal injection to reduce the risk of foveal dehiscence during subretinal tissue plasminogen activator delivery for submacular haemorrhage, using a perfluorocarbon liquid filled vitreous cavity and an eccentric injection point, with a viscous fluid injector system controlled injection.

PURPOSE

To describe a novel method of subretinal injection to reduce the risk of foveal dehiscence during subretinal tissue plasminogen activator delivery for submacular hemorrhage.

METHOD

Description of technique with illustrative case description and details of four cases treated. The subretinal injection is delivered under a perfluorocarbon liquid bubble filling 80% of the vitreous cavity. An eccentric injection point is used and the tissue plasminogen activator injected under a pneumatically controlled viscous fluid injection system with a 38-g polyimide cannula and low injection pressure.

RESULTS

Four cases have been treated with controlled subretinal injection extending under the fovea without dehiscence, including one case with apparent preexisting foveal dehiscence and a preretinal clot.

CONCLUSION

The technique allows the creation of a low more diffuse subretinal bleb compared with without perfluorocarbon liquid, minimizing hydraulic stress on the fovea during injection and could be applied to other subretinal injection scenarios where the fovea is at risk of hydraulic blowout. Further experience of the technique is needed to validate this initial report.

What Is the Cause of Toxicity of Silicone Oil?

Purpose: To investigate the toxicity of the low-molecular-weight components (LMWCs) in ophthalmic silicone oils (SilOils) on retinal cell lines. Methods: The toxicity of six types of LMWCs were studied and compared with conventional SilOil 1000 cSt. In vitro cytotoxic tests of LMWCs, in both liquid and emulsified forms, on three retinal cell lines (Müller cells (rMC-1), photoreceptor cells (661W) and retinal pigment epithelial cells (ARPE-19)) were conducted using a transwell cell culturing system. The morphology and viability of cells were assessed by light microscopy and Cell Counting Kit-8 (CCK-8) assay at different time points (6, 24 and 72 h). The ARPE-19 apoptotic pathway was investigated by Mitochondrial Membrane Potential/Annexin V Apoptosis Kit at different time points (6, 24 and 72 h). Results: Apart from dodecamethylpentasiloxane (L5), all liquid LMWCs showed varying degrees of acute cytotoxicity on retinal cell lines within 72 h. Emulsified LMWCs showed comparable cytotoxicity with liquid LMWCs on retinal cell lines. Cyclic LMWCs, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) had significantly higher cytotoxicity when compared with their linear counterparts decamethyltetrasiloxane (L4) and L5 with similar molecular formula. Using ARPE-19 cells as an example, we showed that LMWCs induce the apoptosis of retinal cells. Conclusions: Most LMWCs, in both liquid and emulsified forms, can induce acute cytotoxicity. In addition, cyclic LMWCs are suspected to have higher cytotoxicity than their linear counterparts. Therefore, LMWCs are suspected to be the main cause of the long-term toxicity of ophthalmic SilOil, due to their toxicity and propensity to cause ophthalmic SilOil to emulsify. The amount of LMWCs should be considered as the paramount parameter when referring to the quality of SilOil.

Silicone oil in vitreoretinal surgery: indications, complications, new developments and alternative long-term tamponade agents

Silicone oil (SO) has been used as a long-term tamponade agent in the treatment of complicated vitreoretinal diseases for about half a century, during which time many advances in surgical techniques and technologies have been made. This review summarizes the chemical and physical properties of SO, its indications and complications, including particularly emulsification. The mechanisms and risk factors for emulsification are discussed, as well as novel strategies for its effective removal. Finally, the review focuses on new improved formulations of SO, including research into slow-release pharmacological agents within SO and provides an overview of alternatives to SO for the purpose of long-term tamponade that are being developed.

In vitro experiment to elucidate the mechanism of the 'soft shell technique' for preventing subretinal migration of perfluoro-octane

AIM

Perfluorocarbon liquid (PFCL) can migrate into subretinal space in detached and stiffened retina with open holes during vitreoretinal surgery. An innovative 'soft shell' technique was introduced to reduce the complication using hyaluronate (HA) to 'cover' the retinal hole. This study aims to study the effectiveness of this technique in vitro.

METHODS

Ex vivo porcine retina was mounted on a transwell insert. Beneath the retina was an aqueous solution. Two retinal holes were made using needle punctures. One of the two retinal holes was covered with HA. Perfluoro-n-octane (PFO) was added above the retina incrementally using a syringe pump. The height of PFO required to cause the migration of PFO through the retinal holes was measured. The 'pendant drop' method was carried out to measure the interfacial tensions between the PFO and aqueous, and between PFO and four different concentrations of HA solution.

RESULTS

A statistically higher PFO level was required to cause the migration of PFO through the retinal hole with HA coating than without HA coating (Tobit regression with p<0.05). The use of HA was associated with 2.39-fold increase in hydrostatic pressure before the collapse of the PFO interface at the retinal holes. The interfacial tension between PFO and HA solution with concentrations of 0.05%, 0.25%, 0.5% and 1% were 54.2±0.6, 55.3±0.6, 59.5±1.5 and 68.3±1.3 mN/m, respectively (mean±SD). The interfacial tension between PFO and aqueous with 1% HA coating (68.3±1.3 mN/m) was significantly higher than that without (37.4±3.4 mN/m) (p<0.05).

CONCLUSIONS

The interfacial tension between HA and PFO is higher than that between aqueous and PFO. This is a plausible physical explanation of how the 'soft shell' technique might work to prevent subretinal migration of PFCL.

Tamponade or filling effect: changes of forces in myopic eyes

Myopia is the most common ocular abnormality. Its high and growing prevalence has contributed to a recent surge in surgical interest in the disorder, since retinal detachment in eyes with high myopia differs from that in emmetropic eyes or eyes with low myopia. The myopic eye, because of its specific anatomy, poses special challenges that need to be overcome to ensure the appropriate use of vitreous substitutes. However, intraocular tamponades have shown great potential for revolutionizing retinal detachment surgery and vitreomacular surgery in general in myopic eyes. We provide an updated review of the clinical use of vitreous substitutes in the myopic eye, paying particular attention to analyzing the ideal function of endotamponade agents and comparing the effects of these agents on the physical and biological properties of the eye.

The impact of axial length on retinal tamponade for gas, silicone oil, and heavy silicone oil, using an in vitro model

Background

To investigate whether ocular axial length influences the tamponade efficacy of three commonly used agents: gas, silicone oil, and heavy silicone oil.

Methods

A series of filling experiments was conducted using 19-mm and 25-mm surface-modified spherical model eye chambers to mimic the vitreous cavity. For each agent, tamponade efficacy was assessed across a range of percentage fills, and comparison was made between the two model eye chambers. The behavior of each tamponade agent was quantified by measuring (1) the maximum height of the tamponade bubble, and calculating (2) the arc of retinal contact subtended by the tamponade bubble.

Results

Polynomial regression analysis found no statistically significant difference between the regression models for the different-sized model eye chambers for bubble height or arc of retinal contact subtended. This applied to all of the tamponade agents under investigation.

Conclusions

Across the range of cavity sizes under investigation, no significant difference in tamponade efficacy (as reflected by the measured dimensions of the tamponade bubble) was identified.

High-density silicone oil (Densiron) as a primary intraocular tamponade: 12-month follow up

BACKGROUND

The high-density silicone oil (Densiron 68), a mixture of F6H8 with silicone oil, seems to be a therapeutic option, at least in selected patients with complex inferior retinal re-detachment, where standard procedures have already failed. In an interventional case series we used Densiron as a primary endotamponade.

METHODS

Twelve eyes of 12 patients aged 31 years to 85 years with inferior complex rhegmatogenous retinal detachment with secondary proliferative vitreoretinopathy (PVR) grades CP2 to CA8 were included. Surgical techniques (pars plana vitrectomy, membrane peeling, retinotomy, retinectomy, endophotocoagulation, cryocoagulation, endotamponade) did not include a scleral buckling procedure (except one eye). Mean duration of the Densiron endotamponade was 78.3 +/- 29.74 days, with a mean follow up after removal of 400.6 +/- 85.4 days.

RESULTS

After Densiron removal, four patients (33.3%) showed a stable reattached retina without further interventions, while, in six patients (50%), recurrent retinal re-detachment appeared during endotamponade, generally within 2 months. One patient (8.3%) developed re-detachment 5 months after Densiron removal. One eye (8.3%) lost light perception due to severe intraretinal fibrosis with chronic hypotonia, despite complete retinal re-attachment. Visual acuity improved from mean logarithm of the minimum angle of resolution (logMAR) of 2.95 +/- 1.21 to 1.87 +/- 1.32 (statistically significant, P = 0.022). Side effects included temporary inflammatory reaction/fibrin accumulation (n = 2/2), moderate-to-severe intraretinal fibrosis (n = 3), elevated intraocular pressure (IOP) (n = 3), emulsification (n = 2), sterile hypopyon (n = 1), vitreous haemorrhage (n = 1) and chronic hypotony (n = 1).

CONCLUSION

Primary anatomical success rate of 33.3% was less encouraging than as expected. Especially, re-detachments within the posterior staphyloma in highly myopic patients were common during Densiron endotamponade. However, the surgical success increased to 75% after reintervention, even without the use of an additional encircling band. The observed adverse effects and the functional outcomes do not contraindicate the use of Densiron as an internal tamponade for a period of 3 months.

Effect of gravity in long-term vitreous tamponade: in vivo investigation using perfluorocarbon liquids and semi-fluorinated alkanes

PURPOSE

In order to investigate whether gravity is the reason for retinal degeneration in long-term vitreous tamponade, perfluorohexyloctane (F6H8), perfluorodecalin (PFD), and a mixture of F6H8/PFD were compared.

MATERIALS AND METHODS

Each group of 5 rabbits received a 3-month tamponade with either PFD (pure) (1.93 g/cm(3)), F6H8 (pure) (1.33 g/cm(3)), or a 1:1 mixture of F6H8/PFD (1.62 g/cm(3)). Electroretinograms (ERG) were performed pre- and postoperatively. Lower and upper retinal areas were compared using immunohistochemical methods. Transmission electron microscopy was performed to investigate alterations in the photoreceptors.

RESULTS

All three substances were tolerated well in rabbit eyes for up to 3 months. Dispersion was seen earliest with PFD and latest with pure F6H8. None of the substances demonstrated inflammatory reactions or vascular alterations. ERGs were not considerably altered with any of the substances. Histology of the retina showed alterations in the cell counts within the inner and outer nuclear layer that were not attributable to the gravity of the tamponading agent.

CONCLUSION

In contrast to previously published work, this study did not detect any tamponade-related structural damage of the retina after a 3-months tamponade in the rabbit model. Based upon this study, we conclude that gravity might not be causally linked to retinal damage.

Can the Sequential Use of Conventional Silicone Oil and Heavy Oil be a Strategy for the Management of Proliferative Vitreoretinopathy?

Introduction: Densiron is a novel long-term tamponade. Its specific gravity is 1.06 g/mL and as such it is heavier than water and provides support for the inferior retina. As proliferative vitreoretinopathy (PVR) has a propensity for the inferior retina, we used Densiron on a consecutive series of 97 cases with inferior pathology. We hypothesised that the sequential use of conventional and heavy silicone oil is a strategy for the management of PVR. Materials and Methods: A consecutive interventional case series involving the use of Densiron for PVR cases was studied. Patients were selected if conventional silicone oil and Densiron were used sequentially. Anatomical success was defined as total re-attachment in the absence of any tamponade agent for at least 3 months post oil removal. Results: Of the 97 patients, 10 patients fulfilled the criteria. Surgery involving Densiron was successful in re-attaching the retina in 7 of 10 cases, with one sequence of alternating light then heavy oil operation, and with one further surgery using silicone oil in the remaining 3 cases. The mean LogMAR preoperative vision was 1.57 and the postoperative vision was 0.82. In 8 of 10 patients, the final vision was 20/200 or better; in 5 of 10 patients, 20/80 or better. The mean follow-up was 19.5 months (range, 9 to 45). Conclusions: The sequential use of conventional silicone oil and Densiron may be a strategy in reducing the number of re-operations. Our case series shows that despite multiple surgical procedures, favourable visual outcome can be achieved. Key words: Densiron, Retinal detachment, Retinotomy, Silicone oil, Vitrectomy