Incidence of Ocular Hypertension Following Silicone Oil Use after Vitrectomy Surgery

C l i n M e d International Library Citation: Nariani A, Khanna N, Chawla A, Momi R, Gabrielian A, et al. (2015) Incidence of Ocular Hypertension Following Silicone Oil Use after Vitrectomy Surgery. Int J Ophthalmol Clin Res 2:023 Received: February 09, 2015: Accepted: May 11, 2015: Published: May 14, 2015 Copyright: © 2015 Nariani A. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Nariani et al. Int J Ophthalmol Clin Res 2015, 2:3 ISSN: 2378-346X


Introduction
Silicone oil (SO), also known as polydimethylsiloxane, is a synthetic polymer made of consecutive Si-O units [1].It was first introduced by Cibis in 1962 for use in retinal detachment surgery [2].Today, SO is an important adjunct for internal tamponade in a wide variety of vitreoretinal surgeries, especially in eyes with proliferative vitreoretinopathy and diabetic tractional retinal detachments.Silicone oil has unique chemical characteristics that make it a useful tool in intraocular tamponade.These include an effective buoyant force and high surface tension [3].There are currently two types of SO available to use in vitreoretinal surgery.The two different SO's are differentiated based on viscosity.The 1000cs SO is less viscous and has a lower molecular weight, and the 5000cs SO is more viscous and has a higher molecular weight [4].
Unfortunately, SO has been associated with complications, such as cataract, keratopathy, and glaucoma [5][6][7][8][9].Secondary glaucoma has been reported to occur at any time in the post-operative period.This secondary glaucoma can manifest with a wide range of IOP's and vision loss [4,5,10].The true incidence of secondary glaucoma after pathogenesis responsible for early onset SOIOH.These include: 1) pupillary block 2) inflammation 3) pre-existing glaucoma, and 4) migration of silicone oil into the anterior chamber with subsequent mechanical obstruction to filtration [4].This contrasts with the possible mechanisms for intermediate and late-onset SOIOH, which are: 1) infiltration of trabecular meshwork by SO bubbles 2) chronic inflammation 3) synechial angle closure 4) rubeosis iridis 5) migration of SO (emulsified and non) into anterior chamber [4].It is difficult to ascertain the relative contribution of these individual factors to the development of SOIOH.This could be because the exact timing of early, intermediate and late onset of SOIOH are poorly defined in the literature.
In our study, SOT with 1000cs SO after PPV seemed to be associated with the development of SOIOH in 33.3% of patients.Literature review shows that the true incidence of glaucoma after SOT is difficult to establish, with averages from previous studies (1979 to 2007) ranging from 0-56% [4].
We excluded patients with acutely elevated IOP in order to investigate a more homogenous group of patients.These included patients who had non-sustained IOP spikes in postoperative days 1-21 and patients with SO overfill.SO overfill can lead to acute glaucoma with shallowing of the anterior chamber and oil displacement into anterior chamber [3].As a result, all patients had a small meniscus of silicone oil to ensure the absence of overfill.Additionally, as is customary in our practice, all underwent three consecutive air-fluid exchanges upon SO removal to ensure that all oil had been removed.
Age and family history of glaucoma had a positive correlation with the development of SOIOH.This is consistent with the tendency of glaucoma to be an age-related disease.One third of patients with SOIOH had a positive history of glaucoma, which supports our knowledge that genetic predisposition is an important part of the multi-factorial nature of glaucoma, and has important implications for patients undergoing SOT.
The length of SOT was associated with increased incidence of SOIOH.This suggests that SO should be removed as soon as there is no further need for it.Unfortunately, the majority of patients with SOIOH had persistence of the disease, even after SO removal.This could imply that the pathogenesis of SOIOH includes irreversible damage to the angle structures.Additionally, small silicone bubbles could have persisted in the trabecular meshwork, even after oil removal.Patients with SOIOH were 1.7 times more likely to have retained SO in the anterior segment.Retained SO in the anterior segment could increase the risk of developing SOIOH.Patients in the SOIOH group had a 10 fold higher increase in average post-operative IOP.
In our study the incidence of retained silicone oil was 20% and 33%, in the disease free and SOIOH groups respectively.Even though data on incidence of retained silicone oil is limited in literature, a recent study found that 17% of patients retain silicone oil in the anterior chamber after repair of complex retinal detachments with silicone oil tamponade [11].Patients with SOIOH were 1.7 times more likely to have retained SO in the anterior segment.Retained SO in the anterior segment could increase the risk of developing SOIOH.Notably, there is potentially a correlation between lens status and retained SO. 5 of 6 (83.3%) in the SOIOH free group with retained SO where phakic. 2 of 5 (40%) in the SOIOH group with retained SO where phakic.Another 2 of 5 (40%) in the SOIOH group with retained SO were aphakic.All patients undergoing SOT had increased IOP post-operative, with SOIOH group having a nearly 10 fold higher increase in IOP.

Results
Forty-five patients that met the criteria were evaluated.The demographics of the SOIOH free group and the SOIOH group are given in Table 1.The average age of all the patients was 43.5 years (range 2-76).The mean follow-up was 21 months (range 6-60).The average length of SOT was 8.2 months (range 4-38).Average pre-and post-operative IOP for the combined group was 13.6 ± 4.6mmHg and 17.5 ± 7.9mmHg respectively.IOP increased by 3.6mmHg (±1.1mmHg; p=0.0009; 95% CI: 1.74-6.17)overall post-operatively for the combined group.
Six of thirty (20%) patients in the SOIOH free group and 5 of 15 (33%) in the SOIOH group had retained silicone oil in anterior chamber after SO removal (Figure 1).Of the 6 patients with retained SO in the SOIOH free group 5 were phakic and one had a posterior chamber intraocular lens (PCIOL).In the SOIOH group 2 out of 5 patient with retained SO were phakic, 1 had a PCIOL and 2 were aphakic.All 11 patients with retained silicone oil in anterior chamber had oil visible in the anterior chamber.Two of the eleven patients (1 from each group) had silicone bubbles in the anterior chamber and angle.
The average time to develop SOIOH was 9.7 months (range 1-38).Thirteen of fifteen eyes (87%) had persistent SOIOH even after SO removal.SOIOH resolved after SO removal in 2 of the 15 patients with SOIOH.In one patient SOIOH resolved after having an additional trip to the operating room to remove persistent SO droplets from the anterior chamber by washout.The rest of the SOIOH patients were managed medically.

Discussion
The development of SOIOH can be categorized according to the postoperative time of onset.SOIOH that develops early in the postoperative period has a different pathogenesis than intermediateand late onset SOIOH.There are many proposed mechanisms of  Anterior chamber washout was done in one patient with retained SO in the anterior chamber after SO removal due to persistently elevated IOP despite maximum medical management.This resulted in successful resolution of SOIOH.The remaining patients with SOIOH were managed medically.No patient required surgical management of SOIOH.Given the success of anterior chamber washout in resolving SOIOH, it should be attempted in all patients with persistently elevated IOP and visible SO in the anterior segment.
There is conflicting data regarding the development of SOIOH after the use of the 1000cs or 5000cs SO.For example, one study suggested the 1000cs is more likely to cause an elevated IOP [12], but another study did not find such a difference [13].Therefore, it would be plausible to investigate 1000cs compared to 5000cs SOT in a prospective, randomized controlled study, in order to determine if there is a significant difference between the two oils in causing SOIOH.This could potentially lead us to modify our use of silicone oil by selecting the "safer" oil for SOT in the near future.Additionally, the issue of ocular hypertension with the current management of SOT, may be overcome through the development of a non-SO vitreous substitute in the future.
Our study was limited by its retrospective nature and a relatively small number of patients.We defined SOIOH to be a persistent elevation of IOP after post-operative week #3.Were this a prospective study, we would have included visual field and pachymetry testing to define the SOIOH group with more accuracy.Despite these limitations, however, our follow-up suggested that over one third of patients with SOT develop SOIOH, with length of SOT being associated with an increased incidence of SOIOH.Since the majority of patients continue to have persistent SOIOH even after SO removal, SOT should be minimized as much as possible, without compromising the success of the surgery.

Figure 1 :
Figure 1: Visible silicone oil in the anterior chamber of a patient lying supine.

Table 2 : Results Average preoperative IOP (mmHg) Average postoperative IOP (mmHg) Length of SOT (months) Average increase in IOP (mmHg) Retained SO in anterior chamber after SO removal
Silicone Oil Induced Glaucoma, SOT: Silicone Oil Tamponade, SO: Silicone Oil, IOP: Intraocular Pressure