Pars plana vitrectomy (PPV) with silicone oil tamponade (SOT) has been adopted for complex rhegmatogenous retinal detachment (RRD) and proliferative vitreoretinopathy for decades.1–4 The standard protocol dictates that the silicone oil should be removed from the treated eyes as soon as a stable retinal situation is achieved due to the complications of long-term silicone oil tamponade.5–7 These complications include cataract, glaucoma, band keratopathy, retinal toxicity, silicone oil migration, recurrent retinal detachment, transient hypotony, and vitreous hemorrhage.7–11 Notably, transient hypotony after silicone oil removal (SOR) is considered one of the most prevalent complications; however, its etiology has not been well-studied since the post-SOR hypotony spontaneously relieved within a few days in most cases.12–14
Despite its transience, post-SOR hypotony may lead to potentially harmful consequences, especially in the era of minimally invasive and sutureless PPV. For example, it may induce wound reopening and vessel exudation, both of which are well-recognized to be the causes of postoperative complications such as choroidal detachment, vitreous hemorrhage, and endophthalmitis.15–17 Therefore, investigating its etiology would facilitate the procedures of SOR with minimal complications.
After careful examination of the SOR procedure, we noted that, although the scleral incision is closed, the inner incision is not sutured after SOR. Furthermore, it has been reported that the incidence of post-SOR hypotony covaries with the sizes of scleral incision.18–22 Thus, we reasoned that the unsutured inner incision could be a possible cause of post-SOR hypotony. Additionally, imaging with by ultrasound biomicroscopy (UBM) and optical coherence tomography (OCT) has revealed that most cases of post-operative hypotony are accompanied by ciliary body detachment, which can also be a consequence of the connecting of supraciliary cavity and vitreous cavity through the unsutured inner incision.23–25 Thus, we hypothesized that the unsutured ciliary incision after SOR results in ciliary body detachment and hypotony, and that alternative procedures without damaging the ciliary body will reduce the incidence of postoperative complications. To examine this hypothesis, we designed and performed a prospective study and provided an alternative procedure that leads to fewer complications than the current procedure.
Patients and Methods
In this prospective study, patients were recruited from the Department of Ophthalmology, Tongji Hospital of Huazhong University of Science and Technology (HUST), Wuhan, China. This study complied with the Declaration of Helsinki and was approved by the institutional review boards of HUST. All recruited volunteers were provided informed consent before participation after explanation of the nature and possible consequences of the study.
To better investigate the relationship between the scleral incision and hypotony after SOR, we specially selected patients with an aphakic eye and high degree myopia requiring for SOR and no need for intraocular lens (IOL) implantation. All the patients diagnosed with a RRD and managed with PPV combined with phacoemulsification in previous first surgeries were eligible for the study. For all patients, the retina was completely attached with SOT for more than 3 months. Patients with unsuccessful retina reattachment, high/low intraocular ocular pressure (IOP) (< 8 mm Hg or > 25 mm Hg), history of corneal refractive surgery, silicone oil entering the anterior chamber prior to SOR, or implanted IOL were excluded from the study. The patients were randomly scheduled into two groups for SOR. According to the recruited sequence, patients with odd numbers were separated into the scleral group and those with even numbers were separated into the corneal group. The pars plana group was treated with pars plana incision and the corneal group was managed by two transparent corneal incisions. In this study, IOP less than 8 mm Hg was defined as hypotony, whereas IOP less than 5 mm Hg was considered as extreme hypotony.
All the surgery procedures were performed by Drs. Sun and Zhang. The following examinations and measurements were performed 1 day before SOR and on days 1, 3, and 7 and 1 month after SOR: IOP with applanation tonometry; anterior structure with high-frequency ultrasound biomicroscopy (hf-UBM) (80MHz UBM IUI-traSound system; iScience Interventional, Menlo Park, CA), and fundus examination by indirect ophthalmoscopy. We also assessed age, gender, axial length, IOP at retinal detachment, and duration of SOT. All the surgical procedures were performed under local anesthesia with the retrobulbar injection of 1.5 ml 2% lidocaine + 1.5 mL 0.75% bupivacaine, and the pupils were dilated with a mixed eyedrop (0.5% tropicamide + 0.5% adrenaline). In patients undergoing corneal SOR, two transparent corneal incisions were made at 4-o'clock position with 1 mm width for insertion of infusion cannula and 12-o'clock position with 3 mm width for passive SOR, respectively. The infusion pressure was maintained at 25 mm Hg. When the silicone oil was completely removed, the incisions were hydrated with balanced salt solution (BSS), and both incisions were sutured with Nylon 10.0 without any corneal leakage. In patients undergoing pars plana SOR, conjunctival peritomy was performed followed by two scleral incisions 3.5 mm from the corneoscleral limbus. The inferior incision was used for a 23-gauge insertion of infusion cannula connected to BSS maintaining the ocular pressure at 30 mm Hg, whereas the superior incision was used for a 20-gauge trocar connected to a homemade active vacuum aspiration composed by a 60-mL injection syringe and the tube of blood transfusion apparatus. The slightly higher infusion pressure in the pars plana group was because the vacuum aspiration need a higher pressure to maintain a stable and normal intraoperative IOP. A tiny corneal incision was also made to remove the residual silicone oil droplets in the anterior chamber. The 23- and 20-gauge scleral incisions and conjunctival peritomy were carefully sutured with 8-0 Vicryl sutures without any scleral leakage. To avoid the incision leakage, which could also induce hypotony, after completely suturing all the incisions, all eyes were carefully checked with the negative fluorescein stain and to confirm they could maintain stable IOP for 10 minutes. At the end of surgery, after closing all the incisions, IOP of all eyes in both groups was adjusted to normal level by filling or evacuating BSS from the corneal incision. To avoid the potential infection risk caused by tonometer, the IOP measurement during surgery was evaluated with finger palpation by the same experienced surgeon.
Hf-UBM was performed on the patients in both groups 1 day before and on the first, third day, and seventh day, as well as at 1 month after SOR. The examination was performed by the same doctors, and patients' information was blinded to the investigators.
Data relating to participant demographics and clinical characteristics were analyzed using GraphPad Prism (version 5.01; GraphPad Software, San Diego, CA) and SPSS (version 22.0 SPSS Statistics; IBM, Armonk, NY). Single-factor variance analysis with Chi-square test or Mann-Whitney U test was performed as appropriate. Descriptive statistics were presented as mean ± standard deviation. A P value less than .05 was considered to be statistically significant; “ns” represented not significant.
At screening, 30 patients with aphakic and high-degree myopia eyes were found eligible for the study. One patient due to redetachment of the retina on the fourth postoperative day, four patients with preoperative glaucoma, and three patients with strong demand for IOL implantation were excluded from the list. In the final analysis, 22 eyes out of 22 patients were enrolled, and each group consisted of 11 patients. The confounding variables (age, gender, axial length, and duration of SOT) between two groups were not significantly different (P > .05; Table 1). IOP measurements from retinal detachment to 1 month after SOR are shown in the Table and in Figure 1. According to the data, both groups had reduced IOP at retinal detachment. On the first postoperative day, all patients in the corneal group had normal IOP, whereas in the pars plana group, nine out of 11 patients experienced hypotony, including three patients with IOP lower than 5 mm Hg. The IOP in the pars plana group was significantly lower than in the corneal group (7.1 mm Hg ± 3.3 mm Hg vs. 14.1 mm Hg ± 1.9 mm Hg; P < .001). Hypotony was partially relieved on the third day and completely relieved on the seventh postoperative day as well as 1 month after SOR in all of the hypotonic patients (average IOP: 9.1 mm Hg ± 2.6 mm Hg, 14.2 mm Hg ± 3.0 mm Hg, and 16.6 mm Hg ± 1.7 mm Hg, respectively). The forest plot of risk factors associated with hypotony after SOR demonstrated the surgical pathway as the only risk factor of hypotony (Figure 2).
Comparison of Baseline Characteristics and IOP Variation BetweenThe Corneal and Pars Plana Groups
Intraocular pressure (IOP) variation curve of corneal and pars plana groups from retinal detachment to 1 month after silicone oil removal (SOR). Both groups had similarly reduced IOP at retinal detachment point and normalized IOP before SOR. On the first and third postoperative day, IOPs in the pars plana group were significantly lower than the corneal group (P < .001) (7.1 mm Hg ± 3.3 mm Hg vs. 14.1 mm Hg ± 1.9 mm Hg and 9.1 mm Hg ± 2.6 mm Hg vs. 13.9 mm Hg ± 1.5 mm Hg, respectively.) The IOP returned to normal level on the seventh postoperative day, as well as at 1 month after SOR in all of the hypotonic patients (average IOP: 14.2 mm Hg ± 3.0 mm Hg and 16.6 mm Hg ± 1.7 mm Hg, respectively).
Forest plot of risk factors associated with hypotony after silicone oil removal. Single-factor variance analysis was used to evaluate the risks factors associated with hypotony. OR = odds ratio; CI = confidence interval; SOT = silicone oil tamponade; M = months;IOP = intraocular pressure; RD = retinal detachment; SOR = silicone oil removal.
Utilizing hf-UBM, we clearly observed the microstructure of anterior chamber and ciliary bodies of the eyes. One day prior to SOR, the ciliary bodies in all eyes of both groups were tightly attached to the sclera and emulsified silicone oil in the anterior chamber appeared as multiple, bright, tiny particles (Figure 3A). On the first postoperative day, the ciliary bodies remained tightly attached in the corneal group with normal IOP. On the contrary, in the pars plana group, nine of the 11 patients suffered ciliary detachment accompanied with hypotony (Figures 3B and 3C). In the eyes of most patients, the residual emulsified silicone oil particles after SOR were clearly identified in the anterior chamber and vitreous cavity. Meanwhile, we observed multiple silicone oil particles in the supraciliary cavity in six out of nine hypotonic eyes, which were supposed to only exist in the anterior chamber and vitreous cavity (Figure 4B). Three out of nine hypotonic eyes had noticeable gaps of ciliary body below the scleral incision site, which was a clear evidence of a passage between the vitreous cavity and supraciliary cavity (Figure 5). Additionally, two eyes with extremely low IOP suffered obvious vitreous hemorrhage, and the bleeding was spontaneously absorbed in 1 month.
A, B, and C are representative ultrasound biomicroscopy images from the 22 eyes before silicone oil removal (SOR): 13 eyes with normal intraocular pressure and nine eyes with hypotony on the first day after SOR, respectively. Before SOR, the ciliary body was tightly attached in all the eyes (arrowhead in A). On the first day after SOR, all the eyes in the corneal group and two normal pressure eyes in the pars plana group had the ciliary body still maintained tightly attached (arrowhead in B). However, in every hypotonic eye, we detected obvious ciliary detachment (triangle in C). The multiple tiny high echoes in both anterior chamber and vitreous cavity represent emulsified silicone oil particles (arrow in A, B) and the linear high echoes behind iris was standing for silicone oil interface (star in A).
A and B were from a normal intraocular pressure in the corneal incision group and a hypotonic eye in the pars plana incision group, respectively. Notice the detached ciliary body and multiple tiny high echoes in the supraciliary cavity (arrowheads in B) consistent with the emulsified silicone oil located in the vitreous cavity (arrows in A, B).
(A) Shows the shadows of the scleral incision and nearby sutures covering the pars plana in most of the images (star in A). In three out of the nine patients with hypotony, the shadows were smaller and further away from the scleral spur (star in B, C, and D), so we observed obvious ciliary gaps closed to the incision, which was a definite passage between the vitreous cavity and supraciliary cavity (arrows B, C, D).
Even though it is one of the most frequent postoperative complications, with the vary incidences from 3.5% to 55%,11,13,14,18–21,26–29 transient hypotony after SOR is still overlooked by many current research studies. It seems straightforward to consider scleral wound leakage or microleakage as the cause of postoperative hypotony. However, even with carefully suturing and a leakage test, which could ensure there was no leakage from the wound, the hypotony still occurred in some of the patients. Besides, the reported incidence with the sutureless 23-gauge to 25-gauge SOR did not increase compared to the sutured surgery.20,29,30 These clues remind us the leakage may not be the key factor of hypotony. In addition, according to the sporadic reports, the incidence of hypotony in 20-gauge era was much higher than the 23-gauge and 25-gauge surgery.18–22,26 Further studies conducted with UBM and OCT demonstrated that hypotony was strongly correlated with the size of scleral incision and ciliary body detachment, whereas the mechanism remained unclear.23–25 In our prospective study, we observed that the pars plana group had a much higher incidence of hypotony (81.8%) than the corneal group (0%), which revealed that the incision from pars plana is the main cause of transient hypotony.
To further study the mechanism of hypotony caused by scleral incision, we utilized hf-UBM to explore the changes of eyes before and after SOR. Unlike anterior OCT and regular UBM, it has three advantages. First, compared to OCT, hf-UBM can inspect the ciliary body with longer penetration depth, acquiring a better image of the ciliary body. Second, the regular UBM requires a water cup that restricts its application for postoperative eyes concerning about its risk of infection, whereas the tiny and sterile detector of hf-UBM is more comfortable and safer for postoperative examination. Finally, due to the frequency of 80MHz, it has much better resolution that can provide more details than regular UBM with a frequency of 30 MHz to 50 MHz. With this device, we observed that all nine of the hypotonic eyes were accompanied with ciliary detachment (Figure 3). This finding agreed very well with that of Roters' and Chen's, which were acquired with regular UBM and anterior OCT, respectively.24–25
Another crucial finding is that we clearly observed the gaps at the inner site of pars plana incision and multiple silicone oil particles in the supraciliary cavity (Figures 4 and 5), which normally only exist in anterior chamber and vitreous cavity. These observations are strong evidence of fluid flowing from the vitreous cavity into the supraciliary cavity. Additionally, the procedure of closing scleral incision does not include suturing the inner ciliary body incision. Thus, as illustrated in Figure 6, we concluded that, the unsutured ciliary incision acted as a passage between the vitreous cavity and supraciliary cavity, through which the fluid of vitreous cavity entered into the supraciliary cavity and led to ciliary detachment and secondary hypotony. Since the inner incision of pars plana created by the blade is regular and tiny, the incision could be self-closing. Once it closed, the fluid in the suprachoroidal space was immediately absorbed, hence, the ciliary detachment and hypotony spontaneously relieved after a few days from surgery.
The schematic of the mechanism of postoperative hypotony after silicone oil removal. The unsutured ciliary incision punctured by the blade acts as a passage between the vitreous cavity and supraciliary cavity. Through this gap the fluid and silicone oil particles (black dots) go into the supraciliary cavity leading ciliary detachment and secondary hypotony. After the inner incision of pars plana closing by itself, the fluid in suprachoroidal space will be absorbed and the ciliary detachment and hypotony will spontaneously relieve.
With the proposed mechanism, one can envision that any factor impeding the closure of pars plana incision may lead to higher incidence of hypotony, such as thinner pars plana and larger incision. Kim et al. reported long axial length as the only risk of hypotony.12 This is probably because the long axial length is frequently accompanied with thinner uvea including pars plana. Furthermore, according to the limited data, there is a rough tendency of increasing incidence of hypotony accompanied with larger incision (from 25-gauge to 20-gauge), although it requires a further validation by random prospective studies.11,13,18–19,22,27–28 In our study, the extremely high incidence of hypotony in pars plana group is probably due to the combination of long axial length and 20-gauge larger incision. On the contrary, scleral incision-free approach in the corneal indeed resulted in no incidence of hypotony in the 11 patients. Thus, we believe that this proposed alternative approach can significantly reduce the risk of complications after SOR.
In addition, two patients with significant hypotony suffered apparent vitreous cavity hemorrhage. We speculate that bleeding may originate from the blood vessels damaged by the pars plana incision or the suprachoroidal space due to extremely low IOP. It may further lead to potential complications such as choroidal detachment, suprachoroidal hemorrhage and even endophthalmitis if the incision is sutureless. Recently, with the widespread usage of sutureless techniques in current vitreoretinal surgery, the incidence of postoperative endophthalmitis became higher than 20-gauge era, with the postoperative hypotony was considered as an important cause.17,31 Thus, the risk of developing severe complications after transient hypotony further emphasizes the significance of our proposed alternative approaches that reduces the incidence of hypotony.
There are several limitations in this study, which include a relatively small case size, lack of comparison among different scleral incision size, and the limitation of design, which is unable to explain the causal relationship between hypotony and ciliary detachment. However, choosing the specific cases without lens can better illuminate the relationship between scleral incision and hypotony.
In conclusion, our study showed that pars plana incision is the crucial cause of ciliary detachment and consequent transient hypotony after SOR. Eyes with extremely low IOP may encounter vitreous hemorrhage. Therefore, using corneal limbus incision in patients with aphakic eyes to avoid the par plana incision is expected to reduce the incidence of hypotony after SOR with minimal complications.
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Comparison of Baseline Characteristics and IOP Variation BetweenThe Corneal and Pars Plana Groups
|Corneal||Pars Plana||P Value*|
|Eyes||n = 11||n = 11|
| Male||7||7||< .9†|
|Age (Years)||50.4 ± 8.1||51.5 ± 6.5||.74|
|Axial Length (mm)||30.7 ± 1.1||30.3 ± 1.6||.49|
|Duration of SOT (Days)||6.8 ± 2.3||5.9 ± 1.7||.38|
|IOP at RD (mm Hg)||10.0 ± 4.5||11.0 ± 3.7||.61|
|IOP Before SOR (mm Hg)||16.1 ± 2.5||16.3 ± 3.0||.94|
|IOP After SOR (mm Hg)|
| First day||14.1 ± 1.9||7.1 ± 3.3||< .005|
| Third day||13.9 ± 1.5||9.1 ± 2.6||< .005|
| Seventh day||15.2 ± 4.5||14.2 ± 3.0||.59|
| One month||15.9 ± 2.3||16.6 ± 1.7||.38|
|Ciliary Detachment||0||9||< .005†|