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Evaluation of Changes in Peripapillary Nerve Fiber Layer Thickness after Deep Sclerectomy with Optical Coherence Tomography
Evaluation of Changes in Peripapillary Nerve Fiber Layer Thickness after Deep Sclerectomy with Optical Coherence Tomography Gema Rebolleda, MD, PhD, Francisco J. Muñoz-Negrete, MD, PhD, Susana Noval, MD Purpose: To detect and quantify changes, using optical coherence tomography (OCT), in the peripapillary retinal nerve fiber layer (RNFL) thickness in patients with glaucoma who underwent deep sclerectomy. Design: Prospective, controlled, interventional case series. Participants: Thirty-four consecutive patients who underwent monocular deep sclerectomy (surgery group) and medically treated fellow eyes (control group). Methods: Quantitative analysis of the peripapillary RNFL by OCT and automated perimetry were performed before surgery and 6 months after surgery in both eyes. Main Outcome Measures: The changes in RNFL thickness overall and by quadrant were evaluated and studied with respect to age, preoperative visual field test global indices, postoperative changes in intraocular pressure (IOP), and postoperative changes in visual field global indices. Changes observed in RNFL thickness were compared between eyes after surgery and in fellow eyes. Results: The IOP decreased from a baseline mean of 23.6⫾5.1 mmHg to 11.7⫾2.9 mmHg (P⬍0.001) 6 months after surgery at the time of OCT testing. The mean percent IOP change was 48.4⫾15.7%. No significant changes in the mean RNFL thickness overall or by quadrant were observed after surgery or in the mean deviation (MD) and pattern standard deviation after surgery. There was no significant difference in the RNFL thickness between eyes in the surgery group and those in the control group. The mean preoperative visual field MD was significantly (P ⫽ 0.006) worse in eyes with a postoperative decrease in the overall RNFL thickness compared with those with an increase in the RNFL thickness. No correlation was found between RNFL thickness changes and age or changes in the visual field global indices. There was no significant difference between eyes with an IOP reduction of more than 50% and those with a reduction in IOP less than 30% (P ⫽ 0.514). Conclusions: The authors found no significant changes in the peripapillary RNFL thickness measured 6 months after deep sclerectomy. The only significant factor related to RNFL thickness changes after surgery was the preoperative visual field MD (P ⫽ 0.038). Ophthalmology 2007;114:488 – 493 © 2007 by the American Academy of Ophthalmology.
The optic disc sometimes is seen to be less excavated when the intraocular pressure (IOP) falls. This anatomic change has been documented previously after trabeculectomy by stereoscopic disc photographs, computer-assisted planimetry, optic nerve head analysis, and confocal scanning laser ophthalmoscopy.1–7 In contrast, there is controversy about the effect of IOP reduction on the peripapillary retinal nerve fiber layer Originally received: March 18, 2006. Accepted: June 13, 2006. Manuscript no. 2006-341. From the Glaucoma Unit, Department of Ophthalmology, Hospital Ramón y Cajal, Universidad Alcalá, Madrid, Spain. The authors have no financial interest in the materials mentioned in the article. Correspondence and reprint requests to Gema Rebolleda, MD, PhD, Glaucoma Unit, Ophthalmology Department, Hospital Ramón y Cajal, Carretera Colmenar Viejo km 9, 1, E28034 Madrid, Spain. E-mail: grebolleda@ telefonica.net.
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© 2007 by the American Academy of Ophthalmology Published by Elsevier Inc.
(RNFL), which also is considered a marker of structural optic nerve damage. Some studies have found no significant changes in the retinal cross-sectional area using confocal scanning laser ophthalmoscopy and an optic nerve analyzer (Rodenstock Instrumente GmbH, Munich, Germany).6,7 Optical coherence tomography (OCT) evaluates and quantifies the peripapillary RNFL thickness in vivo. Aydin et al8 reported a significant increase in the mean peripapillary RNFL thickness assessed by OCT in 38 eyes after filtering surgery that was not correlated with changes in the visual field parameters. Recently, Leung et al9 reported structural and functional recovery in a patient with juvenile open-angle glaucoma, which was documented quantitatively by OCT after trabeculectomy. Deep sclerectomy is a nonpenetrating filtering procedure that facilitates IOP control with fewer complications than trabeculectomy.10 –13 To the best of our knowledge, no study has been published regarding the peripapillary RNFL ISSN 0161-6420/07/$–see front matter doi:10.1016/j.ophtha.2006.06.051
Rebolleda et al 䡠 RNFL Changes after Deep Sclerectomy thickness changes after this filtering surgery. Using OCT, we conducted a prospective clinical study to assess and quantify changes in the RNFL after deep sclerectomy.
Patients and Methods Consecutive patients with bilateral open-angle glaucoma scheduled for unilateral deep sclerectomy at the Ramón y Cajal Hospital from January 2004 through July 2005 were enrolled. The study protocol was reviewed and approved by the Institutional Review Board, and informed consent was provided by all patients. Eyes were scheduled for deep sclerectomy when the IOP exceeded the target pressure, when a visual field defect and glaucomatous optic nerve damage progressed despite maximum tolerated medical therapy, or both. Each patient underwent a complete ophthalmologic examination, including visual acuity testing, IOP measurement (Goldmann applanation tonometry), biomicroscopy, gonioscopy, indirect ophthalmoscopy, and visual field testing on the same day as the OCT imaging. Optical coherence tomography was performed during the month before surgery and 6 months after surgery in both eyes. Changes in the RNFL thickness and visual field indices between eyes undergoing deep sclerectomy (surgery group) were compared with those of the contralateral eyes in which the IOP was controlled medically (control group). The RNFL thickness postoperative change was analyzed for several potential related factors (age, preoperative overall RNFL thickness, postoperative IOP change, and visual field global indices). Patients were selected based on their ability to perform perimetry reliably and on the clarity of the ocular media. Only patients with primary open-angle glaucoma and pigmentary glaucoma were included in this study. Patients were excluded if they had ocular pathologic features other than glaucoma or were unwilling to participate. The amount of IOP postoperative change was not considered as an eligibility criterion for inclusion in the study.
Visual Field Testing All patients underwent Humphrey visual field testing using Swedish Interactive Threshold Algorithm standard 24-2 perimetry (Carl Zeiss Meditec, Dublin, CA). A reliable visual field test was defined as one with less than 30% fixation loss and false-positive or false-negative responses. The preoperative and postoperative mean deviation (MD) and pattern standard deviation (PSD) were used for the analysis.
Optical Coherence Tomography Scanning All OCT scans were obtained using the StratusOCT (Carl Zeiss Meditec) software version 3.0 after pupillary dilation with 1% tropicamide to a minimum diameter of 5 mm. Circular 360° OCT scans were obtained using the fast RNFL thickness scan, with a diameter of 3.4 mm on the peripapillary RNFL. Quality assessment of the scans was carried out by an experienced examiner (SN). Good scans were defined as focused images from the ocular fundus, with an adequate signal-to-noise ratio and a centered, circular ring around the optic disc. Images with less than 90% satisfactory A scan or a signal-to-noise ratio of less than 25 dB were excluded.14
Surgical Technique All surgical procedures were performed by 2 of the authors (GR, FJMN), who used a standard surgical procedure. First, a fornix-
based conjunctival flap was made, the sclera was exposed, and hemostasis by wet-field cautery was performed. A one-third scleral thickness superficial parabolic flap (5.0⫻5.0 mm) was dissected at the 12-o’clock position at least 1.0 mm into the clear cornea. A second flap of deep sclera was dissected, Schlemm’s canal was deroofed, and a trabeculo-Descemet membrane window was created. The deep scleral flap was excised, the juxtacanalicular trabeculum and Schlemm’s endothelium were removed using small blunt forceps (ab externo trabeculectomy; Huco Vision S.A., Saint-Blaise, Switzerland), and a reticulated hyaluronic acid implant (SKGel 3.5; Corneal Laboratories, Paris, France) was placed on the scleral bed. The superficial scleral flap was sutured with 2 to 4 interrupted nylon 10-0 buried sutures. Postoperative treatment included a combination of dexamethasone and tobramycin 6 times daily for 2 weeks. The dosage was tapered by 1 drop weekly until discontinuation after 12 weeks. When the vessel density increased or flattening occurred, we intensified the postoperative antiinflammatory treatment (prednisolone acetate every 1 to 2 hours during waking hours). When filtration through the trabeculo-Descemet membrane was insufficient because of an elevated IOP, a goniopuncture was performed with the neodymium:yttrium–aluminum– garnet laser in the thinnest anterior portion of the trabeculo-Descemet membrane.
Statistical Analysis We calculated the required sample size to obtain a statistical power of 0.80. For a mean RNFL postoperative change of 10 m, a standard deviation of 20 m,8 and an ␣ value of 0.05, the sample size was approximately 35 eyes. A paired t test was used to analyze RNFL thickness differences in individual eyes and to compare parameters before and after surgery. A comparison between the 2 study groups was carried out, and the t test Pearson correlation was used to analyze the association between parameters. The number of glaucoma medications and the visual acuity were compared using the Wilcoxon signedrank test. A statistics program (SPSS version 10.0 for Windows; SPSS Inc., Chicago, IL) was used for all analyses. A P value of 0.05 or less was considered significant.
Results Thirty-four eyes of 34 patients underwent deep sclerectomy; the fellow eyes served as the control eyes. The patient demographics and baseline characteristics for both groups are shown in Table 1. The mean IOP and the mean number of medications used before surgery were significantly lower in the control group (P ⫽ 0.001). The mean preoperative IOP in the surgery group was 23.6⫾5.1 mmHg; 6 months after surgery, the mean IOP decreased to 11.7⫾2.9 mmHg (P⬍0.001). The mean IOP change (preoperative IOP⫺postoperative IOP) was 11.9⫾5.4 mmHg (range, 4 –20 mmHg; median, 12 mmHg). The mean percent IOP change was 48.4⫾15.7% (range, 17%–72%; median, 53%). In 26 eyes (76.4%), the IOP reduction exceeded 30%. At 6 months, the complete success rate, defined as an IOP of 21 mmHg or less without treatment, was 100%. Thirty eyes (88.2%) achieved an IOP of 15 mmHg or less without medication. Neodymium:yttrium–aluminum– garnet goniopuncture was performed in 6 eyes (17.6%). The mean number of medications used decreased significantly after deep sclerectomy from 2.3⫾0.7 to 0 after surgery (P⬍0.001). No significant change was found between the visual acuity before and 6 months after surgery (P ⫽ 0.458). Postoperative complications included one choroidal detachment that resolved spontaneously. No other complication was observed.
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Ophthalmology Volume 114, Number 3, March 2007 Table 1. Demographic Data and Clinical Characteristics of the Study Population No. of Patients
%
16 18
47.1 52.9
30 4
88.2 11.8
Gender Women Men Glaucoma type Primary open angle Pigmentary
Mean ⴞ Standard Deviation
Range
Median
56.8⫾16.7
25–85
60.5
23.6⫾5.1 17.5⫾3.4 ⬍0.001
17–35 12–24
22 18
2.3⫾0.7 1.4⫾1.2 ⬍0.001
1–4 0–3
2 2
⫺7.0⫾6.8 ⫺5.5⫾5.1 0.220
⫺24.9 to 0.3 ⫺17.1 to 1.0
⫺4.1 ⫺3.5
4.9⫾3.3 4.7⫾3.4 0.752
1.2–12.5 1.0–13.9
3.7 3.5
0.82⫾0.2 0.84⫾0.3 0.625
0.1–1.0 0.3–1.0
0.9 0.9
Age (yrs) Preoperative IOP (mmHg) Surgery group Control group P value Preoperative medication Surgery group Control group P value Preoperative VF MD (dB) Surgery group Control group P value Preoperative VF PSD (dB) Surgery group Control group P value Preoperative VA Surgery group Control group P value
IOP ⫽ intraocular pressure; VF MD ⫽ visual field mean deviation; VF PSD ⫽ visual field pattern standard deviation; VA ⫽ visual acuity.
The mean preoperative and postoperative MD was ⫺7.0⫾6.8 dB (range, ⫺24.9 to ⫺0.3 dB; median, ⫺4.1 dB) and ⫺6.4⫾6.1 dB (range, ⫺24 – 0.17 dB; median, ⫺5.8 dB), respectively (P ⫽ 0.264). A significant correlation was found between the preoper-
ative overall RNFL thickness and the preoperative MD (r ⫽ 0.553; P ⫽ 0.002). The mean preoperative and postoperative PSDs were 4.9⫾3.3 and 4.3⫾3.2, respectively (P ⫽ 0.431). Table 2 summarizes the mean peripapillary RNFL thickness measured by OCT in both groups. No significant changes were found in the mean overall RNFL thickness in either group. The mean overall RNFL thickness change in the surgery group was ⫺2.6⫾10.6 m (range, ⫺26.5 to 17.4 m; median, ⫺1.45 m; P ⫽ 0.166). In this group, the RNFL thickness decreased in 20 eyes (58.8%) and increased in 14 eyes (41.2%) after surgery. In the control group, the mean overall RNFL thickness change was ⫺2.6⫾8.9 m (range, ⫺22.8 to 15.5 m; P ⫽ 0.114). Analyses of the changes in the RNFL thickness by quadrant in the surgery group showed a nonsignificant increase in the RNFL thickness in the superior and temporal quadrants after surgery. There was an increase in the RNFL thickness in 17 eyes (50%) in the superior and nasal quadrants and in 16 eyes (47%) in the inferior and temporal quadrants. No significant differences were found in the analyses of the quadrants between the groups. To evaluate the relationship between the preoperative glaucomatous damage and the postoperative changes, we divided the surgery group into 2 subgroups of 17 eyes each on the basis of the preoperative median RNFL thickness (62 m). There was no significant difference between the groups in overall changes (P ⫽ 0.732) or quadrant changes (Table 3). However, the mean preoperative visual field MD was significantly worse in eyes with a mean postoperative decrease in overall RNFL thickness (⫺9.9⫾7.9 dB) compared with those with a RNFL thickness increase (⫺3.7⫾3.2 dB; P ⫽ 0.006). The RNFL thickness change was unrelated to the postoperative IOP reduction. Nineteen eyes with an IOP reduction of more than 50% did not have a significant change in RNFL thickness compared with 8 eyes with a reduction in IOP of less than 30% (P ⫽ 0.514). We did not find a significant correlation between the IOP reduction and the changes in the visual field MD (P ⫽ 0.567) or the visual field PSD after surgery (P ⫽ 0.472). The correlations between the RNFL changes after surgery and age, IOP change (mmHg), percent IOP change, preoperative MD, PSD, and the change in the visual field MD and PSD are shown in Table 4. The only significant correlation was found between the RNFL thick-
Table 2. Mean Peripapillary Retinal Nerve Fiber Layer Thickness Changes in Eyes That Underwent Deep Sclerectomy Compared with the Contralateral Eye of the Same Patient
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Variable
Preoperative (m)
Postoperative (m)
Mean Difference (m)
P Value
Overall Surgery group Control group
66.6⫾20.2 74.1⫾21.8
64.0⫾21.8 71.7⫾20.3
⫺2.6⫾10.6 ⫺2.6⫾8.99 P ⫽ 0.978
0.166 0.114
Superior Surgery group Control group
78.1⫾23.7 91.5⫾29.4
82.2⫾28.6 86.7⫾24.9
2.1⫾18.8 ⫺4.79⫾21.8 P ⫽ 0.380
0.755 0.209
Nasal Surgery group Control group
58.1⫾21.5 60.6⫾18.8
56.3⫾25.2 62.5⫾18.2
⫺1.82⫾14 1.88⫾11.8 P ⫽ 0.242
0.453 0.359
Inferior Surgery group Control group
72.3⫾29.2 85.1⫾35.1
69.7⫾26.4 85.5⫾32.1
⫺2.6⫾18.9 0.38⫾17.2 P ⫽ 0.497
0.427 0.898
Temporal Surgery group Control group
54.5⫾22.3 57.7⫾19.4
56.2⫾20.4 57.3⫾20.7
1.6⫾18.4 ⫺0.44⫾15.3 P ⫽ 0.612
0.605 0.868
Rebolleda et al 䡠 RNFL Changes after Deep Sclerectomy Table 3. Mean Retinal Nerve Fiber Layer Thickness Changes According to Mean Preoperative Overall Retinal Nerve Fiber Layer Thickness Postoperative Preoperative Overall ⱕ62 m ⬎62 m P value
Overall
Superior
Inferior
Nasal
Temporal
⫺3.2⫾10.95 ⫺1.9⫾10.47 0.732
⫺0.82⫾18.5 5.29⫾17.9 0.335
⫺3.8⫾17.9 ⫺0.17⫾8.5 0.413
⫺1.29⫾19 ⫺3.9⫾19.3 0.691
6.7⫾19.7 ⫺3.4⫾15.8 0.109
ness changes after surgery and the preoperative MD (r ⫽ 0.354; P ⫽ 0.038). There was no significant change in the control group in the MD (P ⫽ 0.099), PSD (P ⫽ 0.064), or IOP (P ⫽ 0.097) 6 months after surgery.
Discussion Several studies reported less cupping of the optic disc after IOP reduction in some patients after glaucoma surgery.1–7 This observation is more likely to be due to a simple shift in anatomic structures rather than recovery or reversal of damage. When IOP is lowered, there is less stretch on the lamina cribrosa, and the disc is able to return to its normal position. However, there is no consensus regarding whether the changes associated with IOP reduction occur only in the optic nerve head or also in the peripapillary RNFL.7–9 Until recently, the assessment of the RNFL has been largely subjective. Optical coherence tomography, developed to assess tissue thickness in vivo, is a noninvasive imaging technique, allows high-resolution cross-sectional ocular imaging, and evaluates and quantifies the peripapillary RNFL thickness. Optical coherence tomography provides real-time, immediate, objective, and quantitative assessments of the RNFL within a short time during 1 visit and offers a reproducible technique with a standard deviation of measurements of 10 to 20 m for the mean overall RNFL thickness and 15 to 30 m for measurements of each clock hour.15,16 In the current study, we prospectively evaluated the peripapillary RNFL thickness changes by OCT in 34 patients who underwent monocular deep sclerectomy. We elected deep sclerectomy because it may offer success rates comparable with those of trabeculectomy and may miniTable 4. Pearson’s Correlation between Changes in Mean Retinal Nerve Fiber Layer Thickness and Age, Intraocular Pressure, and Visual Field Global Indices in the Surgery Group Parameter
r
P Value
Age IOP change (mmHg) IOP change (%) Preoperative VF MD Preoperative VF PSD Change in VF MD Change in VF PSD
⫺0.202 0.01 ⫺0.007 0.354 ⫺0.153 0.387 0.391
0.243 0.954 0.970 0.038 0.427 0.068 0.072
IOP ⫽ intraocular pressure; VF MD ⫽ visual field mean deviation; VF PSD ⫽ visual field pattern standard deviation.
mize the risk of postoperative complications.10 –13 We used the fast RNFL thickness scan, which reduces the examination time and improves the accuracy and centration of the scans.15 We did not find significant changes in the overall peripapillary RNFL thickness (P ⫽ 0.166) or in the quadrant analysis (Table 2) in the surgery group. Similarly, there was no difference between the preoperative and postoperative MD and PSD of the visual field results. Similar results have been reported using several devices. Sogano et al7 used a Rodenstock optic nerve head analyzer and reported that although the cup volume decreased and the rim area increased significantly after trabeculectomy, the RNFL height did not change 2 to 6 months after trabeculectomy. Irak et al6 used confocal scanning laser ophthalmoscopy to evaluate 49 eyes 3 months after filtration surgery and did not find a significant change in the RNFL cross-sectional area. Lesk et al,4 using the Heidelberg Retina Tomograph (Heidelberg Instruments, Heidelberg, Germany), reported that pressure-dependent changes in RNFL thickness were not statistically significant. Although the optic disc sometimes is seen to be less excavated when IOP is lowered, there is not a corresponding increase in the RNFL thickness, so the explanation of this finding is not likely a restoration or increase in the number of axons, but rather relaxation of the stretch of tissues that support the optic disc structures. Contrary to these results, Aydin et al8 reported a significant increase in the overall peripapillary RNFL from 72.8 to 81.7 m after filtration surgery measured by OCT in 18 eyes that underwent trabeculectomy and in 20 eyes that underwent combined trabeculectomy and cataract extraction. The overall measurement and the measurement of the quadrants (except the inferior quadrant) to determine the RNFL thickening were significant for the entire study group. In the current study, a nonsignificant mean overall RNFL decrease from 66.6 to 64.0 m was found after surgery. Moreover, this change was similar to that in eyes in the control group in which the RNFL decreased from 74.1 to 71.7 m (Table 1). The differences between our findings and those of Aydin et al8 could be attributed to several factors. First, the absence of a control group in the study of Aydin et al precludes achieving definite conclusions, taking into account the normal test–retest variability of the device. The report of Aydin et al was retrospective and the results obtained should be interpreted cautiously; a bias in the selection of the population cannot be disregarded. Moreover, those au-
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Ophthalmology Volume 114, Number 3, March 2007 thors assumed that the RNFL thickness does not change after cataract surgery, and they combined the data obtained from trabeculectomy or combined cataract extraction and trabeculectomy. However, changes in the ocular media, such as posterior subcapsular and cortical cataracts, could impair the ability to perform OCT, with more low-quality and degraded image artifacts than in patients with a clear lens.17–19 When cataracts are removed along with glaucoma surgery, the subsequent changes in the eye’s optic nerve along with the reliability index of measurements could change the RNFL values. To avoid these artifacts, we included only eyes with clear ocular media. The mean preoperative RNFL thickness was 6.2 m lower in our study than in the study of Aydin et al.8 Curiously, despite a higher mean thickness before surgery than in our patients, the mean preoperative MD was worse, a finding that can be explained partially by an increase in the diffuse visual field defects as the result of cataract artifacts. In fact, to avoid the effect of cataract removal on the visual field test, they analyzed the data in only 15 eyes that had undergone only trabeculectomy. We did not find a significant correlation between the RNFL postoperative changes and age, preoperative overall RNFL thickness, visual field global indices, changes in IOP, or visual field global indices (Tables 3, 4). In our study, based on the absence of significant RNFL thickness changes, no significant changes occurred in the visual field global indices. However, in the study of Aydin et al,8 significant thickening of the peripapillary RNFL was not associated with a clinical improvement in visual field parameters. It has been reported that there is a greater likelihood of glaucomatous progression by OCT compared with automated perimetry. This may reflect OCT hypersensitivity or true damage identified by OCT before detection by conventional methods.20 In addition, the differences may be the result of different degrees of preexisting glaucomatous damage. Some experimental and clinical studies have shown that restoration of anatomic position is more likely to occur in the early stages of glaucoma.21,22 Although the difference was not significant, eyes with an RNFL thickness less than 62 m before surgery had a mean decrease that was 1.4 m more than those with an RNFL thickness of 62 m or more. Taking in consideration that some changes may occur because of higher fluid content of tissues at lower ambient hydrostatic pressure than at high IOP, thin RNFL probably had less opportunity to expand. We found a significant correlation between the postoperative RNFL thickness changes and the preoperative MD. With higher degrees of glaucomatous visual field damage, the chances were less of an increase in the RNFL thickness after surgery. Moreover, the mean preoperative visual field MD was 6.2 dB worse in eyes showing a decrease in the overall RNFL thickness after surgery than in those with an increase in the RNFL thickness (P ⫽ 0.006). Several studies have shown a high correlation between the degree of improvement in the optic nerve head morphologic features and the percent of IOP reduction.2,4,6,7 Aydin et al8 reported that after filtration surgery, a 0.5-m increase
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in the mean RNFL could be expected for each 1-mmHg decrease in IOP.10 Although a difference in IOP reduction could explain different results, the mean IOP change after surgery obtained in our study was 1.3 mmHg more than that obtained by Aydin et al.8 Moreover, in our study, the mean percent of IOP change was 48.4⫾15.7% (median, 53%). Twenty-six eyes (76.4%) had an IOP reduction of more than 30%, similar to data (73.7%) reported by Aydin et al.8 The minimal and nonsignificant changes in RNFL thickness observed in our study may represent normal test–retest variability, taking into account that no significant differences were observed between the 2 study groups (Table 2). The mean IOP and visual field global indices also remained stable in the control group. Mechanisms that explain an improvement in RNFL thickness with IOP reduction are unclear. After the retinal nerve fiber is damaged, it cannot regenerate. One plausible explanation is recovery of compressed RNFL; however, there is no direct evidence showing RNFL compression at the peripapillary RNFL with high IOP. It is possible that some axons are able to function marginally while the IOP is high and can recover some physiologic functions when the IOP is lowered, but this is a biomechanical or physiologic restoration, not an anatomic one. Changes observed in the optic nerve head probably have a mechanical source because, unfortunately, regeneration of axons is not expected. In our study, we did not find a significant change in peripapillary RNFL thickness after successful deep sclerectomy, despite obtaining a significant IOP reduction. The only factor significantly correlated with changes in the RNFL thickness was the preoperative MD—that is, with a poor preoperative MD there was less chance for RNFL thickening after IOP reduction. In conclusion, our results indicate that OCT measurements are not affected by IOP reduction after deep sclerectomy. Additional studies are necessary to support or reject our findings and to provide supplementary data.
References 1. Greenidge KC, Spaeth GL, Traverso CE. Change in appearance of the optic disc associated with lowering of intraocular pressure. Ophthalmology 1985;92:897–903. 2. Katz LJ, Spaeth GL, Cantor LB, et al. Reversible optic disk cupping and visual field improvement in adults with glaucoma. Am J Ophthalmol 1989;107:485–92. 3. Funk J. Increase of neuroretinal rim area after surgical intraocular pressure reduction. Ophthalmic Surg 1990;21:585– 8. 4. Lesk MR, Spaeth GL, Azuara-Blanco A, et al. Reversal of optic disc cupping after glaucoma surgery analyzed with a scanning laser tomograph. Ophthalmology 1999;106:1013– 8. 5. Kotecha A, Siriwardena D, Fitzke FW, et al. Optic disc changes following trabeculectomy: longitudinal and localisation of change. Br J Ophthalmol 2001;85:956 – 61. 6. Irak I, Zangwill L, Garden V, et al. Change in optic disk topography after trabeculectomy. Am J Ophthalmol 1996;122: 690 –5. 7. Sogano S, Tomita G, Kitazawa Y. Changes in retinal nerve fiber layer thickness after reduction of intraocular pressure in chronic open-angle glaucoma. Ophthalmology 1993;100:1253– 8.
Rebolleda et al 䡠 RNFL Changes after Deep Sclerectomy 8. Aydin A, Wollstein G, Price LL, et al. Optical coherence tomography assessment of retinal nerve fiber layer thickness changes after glaucoma surgery. Ophthalmology 2003;110:1506 –11. 9. Leung CK, Woo J, Tsang MK, Tse KK. Structural and functional recovery in juvenile open angle glaucoma after trabeculectomy [letter]. Eye 2006;20:132– 4. 10. Shaarawy T, Mansouri K, Schnyder C, et al. Long-term results of deep sclerectomy with collagen implant. J Cataract Refract Surg 2004;30:1225–31. 11. El Sayyad F, Helal M, El-Kholify H, et al. Nonpenetrating deep sclerectomy versus trabeculectomy in bilateral primary open-angle glaucoma. Ophthalmology 2000;107:1671– 4. 12. Ambresin A, Shaarawy T, Mermoud A. Deep sclerectomy with collagen implant in one eye compared with trabeculectomy in the other eye of the same patient. J Glaucoma 2002; 11:214 –20. 13. Rebolleda G, Muñoz-Negrete FJ. Deep sclerectomy with mitomycin C in failed trabeculectomy. Eye. In press. 14. StratusOCT Model 3000 User Manual. Dublin, CA: Carl Zeiss Meditec; 2003:4 – 6. 15. Paunescu LA, Schuman JS, Price LL, et al. Reproducibility of nerve fiber thickness, macular thickness, and optic nerve head measurements using StratusOCT. Invest Ophthalmol Vis Sci 2004;45:1716 –24.
16. Schuman JS, Pedut-Kloizman T, Hertzmark E, et al. Reproducibility of nerve fiber layer thickness measurements using optical coherence tomography. Ophthalmology 1996;103: 1889 –98. 17. Kanamori A, Escano MF, Eno A, et al. Evaluation of the effect of aging on retinal nerve fiber layer thickness measured by optical coherence tomography. Ophthalmologica 2003;217: 273– 8. 18. Ray R, Stinnett SS, Jaffe GJ. Evaluation of image artifact produced by optical coherence tomography of retinal pathology. Am J Ophthalmol 2005;139:18 –29. 19. Degering RF, Aschmoneit I, Kamppeter B, et al. Optical coherence tomography and confocal scanning laser tomography for assessment of macular edema. Am J Ophthalmol 2004;138:354 – 61. 20. Wollstein G, Schuman JS, Price LL, et al. Optical coherence tomography longitudinal evaluation of retinal nerve fiber layer thickness in glaucoma. Arch Ophthalmol 2005;123:464 –70. 21. Shirakashi M, Nanba K, Iwata K. Reversal of cupping in experimental glaucoma. Ophthalmologica 1991;202:194 –201. 22. Topouzis F, Peng F, Kotas-Neumann R, et al. Longitudinal changes in optic disc topography of adult patients after trabeculectomy. Ophthalmology 1999;106:1147–51.
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The optic disc sometimes is seen to be less excavated when the intraocular pressure (IOP) falls. This anatomic change has been documented previously after trabeculectomy by stereoscopic disc photographs, computer-assisted planimetry, optic nerve head analysis, and confocal scanning laser ophthalmoscopy.1–7 In contrast, there is controversy about the effect of IOP reduction on the peripapillary retinal nerve fiber layer Originally received: March 18, 2006. Accepted: June 13, 2006. Manuscript no. 2006-341. From the Glaucoma Unit, Department of Ophthalmology, Hospital Ramón y Cajal, Universidad Alcalá, Madrid, Spain. The authors have no financial interest in the materials mentioned in the article. Correspondence and reprint requests to Gema Rebolleda, MD, PhD, Glaucoma Unit, Ophthalmology Department, Hospital Ramón y Cajal, Carretera Colmenar Viejo km 9, 1, E28034 Madrid, Spain. E-mail: grebolleda@ telefonica.net.
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© 2007 by the American Academy of Ophthalmology Published by Elsevier Inc.
(RNFL), which also is considered a marker of structural optic nerve damage. Some studies have found no significant changes in the retinal cross-sectional area using confocal scanning laser ophthalmoscopy and an optic nerve analyzer (Rodenstock Instrumente GmbH, Munich, Germany).6,7 Optical coherence tomography (OCT) evaluates and quantifies the peripapillary RNFL thickness in vivo. Aydin et al8 reported a significant increase in the mean peripapillary RNFL thickness assessed by OCT in 38 eyes after filtering surgery that was not correlated with changes in the visual field parameters. Recently, Leung et al9 reported structural and functional recovery in a patient with juvenile open-angle glaucoma, which was documented quantitatively by OCT after trabeculectomy. Deep sclerectomy is a nonpenetrating filtering procedure that facilitates IOP control with fewer complications than trabeculectomy.10 –13 To the best of our knowledge, no study has been published regarding the peripapillary RNFL ISSN 0161-6420/07/$–see front matter doi:10.1016/j.ophtha.2006.06.051
Rebolleda et al 䡠 RNFL Changes after Deep Sclerectomy thickness changes after this filtering surgery. Using OCT, we conducted a prospective clinical study to assess and quantify changes in the RNFL after deep sclerectomy.
Patients and Methods Consecutive patients with bilateral open-angle glaucoma scheduled for unilateral deep sclerectomy at the Ramón y Cajal Hospital from January 2004 through July 2005 were enrolled. The study protocol was reviewed and approved by the Institutional Review Board, and informed consent was provided by all patients. Eyes were scheduled for deep sclerectomy when the IOP exceeded the target pressure, when a visual field defect and glaucomatous optic nerve damage progressed despite maximum tolerated medical therapy, or both. Each patient underwent a complete ophthalmologic examination, including visual acuity testing, IOP measurement (Goldmann applanation tonometry), biomicroscopy, gonioscopy, indirect ophthalmoscopy, and visual field testing on the same day as the OCT imaging. Optical coherence tomography was performed during the month before surgery and 6 months after surgery in both eyes. Changes in the RNFL thickness and visual field indices between eyes undergoing deep sclerectomy (surgery group) were compared with those of the contralateral eyes in which the IOP was controlled medically (control group). The RNFL thickness postoperative change was analyzed for several potential related factors (age, preoperative overall RNFL thickness, postoperative IOP change, and visual field global indices). Patients were selected based on their ability to perform perimetry reliably and on the clarity of the ocular media. Only patients with primary open-angle glaucoma and pigmentary glaucoma were included in this study. Patients were excluded if they had ocular pathologic features other than glaucoma or were unwilling to participate. The amount of IOP postoperative change was not considered as an eligibility criterion for inclusion in the study.
Visual Field Testing All patients underwent Humphrey visual field testing using Swedish Interactive Threshold Algorithm standard 24-2 perimetry (Carl Zeiss Meditec, Dublin, CA). A reliable visual field test was defined as one with less than 30% fixation loss and false-positive or false-negative responses. The preoperative and postoperative mean deviation (MD) and pattern standard deviation (PSD) were used for the analysis.
Optical Coherence Tomography Scanning All OCT scans were obtained using the StratusOCT (Carl Zeiss Meditec) software version 3.0 after pupillary dilation with 1% tropicamide to a minimum diameter of 5 mm. Circular 360° OCT scans were obtained using the fast RNFL thickness scan, with a diameter of 3.4 mm on the peripapillary RNFL. Quality assessment of the scans was carried out by an experienced examiner (SN). Good scans were defined as focused images from the ocular fundus, with an adequate signal-to-noise ratio and a centered, circular ring around the optic disc. Images with less than 90% satisfactory A scan or a signal-to-noise ratio of less than 25 dB were excluded.14
Surgical Technique All surgical procedures were performed by 2 of the authors (GR, FJMN), who used a standard surgical procedure. First, a fornix-
based conjunctival flap was made, the sclera was exposed, and hemostasis by wet-field cautery was performed. A one-third scleral thickness superficial parabolic flap (5.0⫻5.0 mm) was dissected at the 12-o’clock position at least 1.0 mm into the clear cornea. A second flap of deep sclera was dissected, Schlemm’s canal was deroofed, and a trabeculo-Descemet membrane window was created. The deep scleral flap was excised, the juxtacanalicular trabeculum and Schlemm’s endothelium were removed using small blunt forceps (ab externo trabeculectomy; Huco Vision S.A., Saint-Blaise, Switzerland), and a reticulated hyaluronic acid implant (SKGel 3.5; Corneal Laboratories, Paris, France) was placed on the scleral bed. The superficial scleral flap was sutured with 2 to 4 interrupted nylon 10-0 buried sutures. Postoperative treatment included a combination of dexamethasone and tobramycin 6 times daily for 2 weeks. The dosage was tapered by 1 drop weekly until discontinuation after 12 weeks. When the vessel density increased or flattening occurred, we intensified the postoperative antiinflammatory treatment (prednisolone acetate every 1 to 2 hours during waking hours). When filtration through the trabeculo-Descemet membrane was insufficient because of an elevated IOP, a goniopuncture was performed with the neodymium:yttrium–aluminum– garnet laser in the thinnest anterior portion of the trabeculo-Descemet membrane.
Statistical Analysis We calculated the required sample size to obtain a statistical power of 0.80. For a mean RNFL postoperative change of 10 m, a standard deviation of 20 m,8 and an ␣ value of 0.05, the sample size was approximately 35 eyes. A paired t test was used to analyze RNFL thickness differences in individual eyes and to compare parameters before and after surgery. A comparison between the 2 study groups was carried out, and the t test Pearson correlation was used to analyze the association between parameters. The number of glaucoma medications and the visual acuity were compared using the Wilcoxon signedrank test. A statistics program (SPSS version 10.0 for Windows; SPSS Inc., Chicago, IL) was used for all analyses. A P value of 0.05 or less was considered significant.
Results Thirty-four eyes of 34 patients underwent deep sclerectomy; the fellow eyes served as the control eyes. The patient demographics and baseline characteristics for both groups are shown in Table 1. The mean IOP and the mean number of medications used before surgery were significantly lower in the control group (P ⫽ 0.001). The mean preoperative IOP in the surgery group was 23.6⫾5.1 mmHg; 6 months after surgery, the mean IOP decreased to 11.7⫾2.9 mmHg (P⬍0.001). The mean IOP change (preoperative IOP⫺postoperative IOP) was 11.9⫾5.4 mmHg (range, 4 –20 mmHg; median, 12 mmHg). The mean percent IOP change was 48.4⫾15.7% (range, 17%–72%; median, 53%). In 26 eyes (76.4%), the IOP reduction exceeded 30%. At 6 months, the complete success rate, defined as an IOP of 21 mmHg or less without treatment, was 100%. Thirty eyes (88.2%) achieved an IOP of 15 mmHg or less without medication. Neodymium:yttrium–aluminum– garnet goniopuncture was performed in 6 eyes (17.6%). The mean number of medications used decreased significantly after deep sclerectomy from 2.3⫾0.7 to 0 after surgery (P⬍0.001). No significant change was found between the visual acuity before and 6 months after surgery (P ⫽ 0.458). Postoperative complications included one choroidal detachment that resolved spontaneously. No other complication was observed.
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Ophthalmology Volume 114, Number 3, March 2007 Table 1. Demographic Data and Clinical Characteristics of the Study Population No. of Patients
%
16 18
47.1 52.9
30 4
88.2 11.8
Gender Women Men Glaucoma type Primary open angle Pigmentary
Mean ⴞ Standard Deviation
Range
Median
56.8⫾16.7
25–85
60.5
23.6⫾5.1 17.5⫾3.4 ⬍0.001
17–35 12–24
22 18
2.3⫾0.7 1.4⫾1.2 ⬍0.001
1–4 0–3
2 2
⫺7.0⫾6.8 ⫺5.5⫾5.1 0.220
⫺24.9 to 0.3 ⫺17.1 to 1.0
⫺4.1 ⫺3.5
4.9⫾3.3 4.7⫾3.4 0.752
1.2–12.5 1.0–13.9
3.7 3.5
0.82⫾0.2 0.84⫾0.3 0.625
0.1–1.0 0.3–1.0
0.9 0.9
Age (yrs) Preoperative IOP (mmHg) Surgery group Control group P value Preoperative medication Surgery group Control group P value Preoperative VF MD (dB) Surgery group Control group P value Preoperative VF PSD (dB) Surgery group Control group P value Preoperative VA Surgery group Control group P value
IOP ⫽ intraocular pressure; VF MD ⫽ visual field mean deviation; VF PSD ⫽ visual field pattern standard deviation; VA ⫽ visual acuity.
The mean preoperative and postoperative MD was ⫺7.0⫾6.8 dB (range, ⫺24.9 to ⫺0.3 dB; median, ⫺4.1 dB) and ⫺6.4⫾6.1 dB (range, ⫺24 – 0.17 dB; median, ⫺5.8 dB), respectively (P ⫽ 0.264). A significant correlation was found between the preoper-
ative overall RNFL thickness and the preoperative MD (r ⫽ 0.553; P ⫽ 0.002). The mean preoperative and postoperative PSDs were 4.9⫾3.3 and 4.3⫾3.2, respectively (P ⫽ 0.431). Table 2 summarizes the mean peripapillary RNFL thickness measured by OCT in both groups. No significant changes were found in the mean overall RNFL thickness in either group. The mean overall RNFL thickness change in the surgery group was ⫺2.6⫾10.6 m (range, ⫺26.5 to 17.4 m; median, ⫺1.45 m; P ⫽ 0.166). In this group, the RNFL thickness decreased in 20 eyes (58.8%) and increased in 14 eyes (41.2%) after surgery. In the control group, the mean overall RNFL thickness change was ⫺2.6⫾8.9 m (range, ⫺22.8 to 15.5 m; P ⫽ 0.114). Analyses of the changes in the RNFL thickness by quadrant in the surgery group showed a nonsignificant increase in the RNFL thickness in the superior and temporal quadrants after surgery. There was an increase in the RNFL thickness in 17 eyes (50%) in the superior and nasal quadrants and in 16 eyes (47%) in the inferior and temporal quadrants. No significant differences were found in the analyses of the quadrants between the groups. To evaluate the relationship between the preoperative glaucomatous damage and the postoperative changes, we divided the surgery group into 2 subgroups of 17 eyes each on the basis of the preoperative median RNFL thickness (62 m). There was no significant difference between the groups in overall changes (P ⫽ 0.732) or quadrant changes (Table 3). However, the mean preoperative visual field MD was significantly worse in eyes with a mean postoperative decrease in overall RNFL thickness (⫺9.9⫾7.9 dB) compared with those with a RNFL thickness increase (⫺3.7⫾3.2 dB; P ⫽ 0.006). The RNFL thickness change was unrelated to the postoperative IOP reduction. Nineteen eyes with an IOP reduction of more than 50% did not have a significant change in RNFL thickness compared with 8 eyes with a reduction in IOP of less than 30% (P ⫽ 0.514). We did not find a significant correlation between the IOP reduction and the changes in the visual field MD (P ⫽ 0.567) or the visual field PSD after surgery (P ⫽ 0.472). The correlations between the RNFL changes after surgery and age, IOP change (mmHg), percent IOP change, preoperative MD, PSD, and the change in the visual field MD and PSD are shown in Table 4. The only significant correlation was found between the RNFL thick-
Table 2. Mean Peripapillary Retinal Nerve Fiber Layer Thickness Changes in Eyes That Underwent Deep Sclerectomy Compared with the Contralateral Eye of the Same Patient
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Variable
Preoperative (m)
Postoperative (m)
Mean Difference (m)
P Value
Overall Surgery group Control group
66.6⫾20.2 74.1⫾21.8
64.0⫾21.8 71.7⫾20.3
⫺2.6⫾10.6 ⫺2.6⫾8.99 P ⫽ 0.978
0.166 0.114
Superior Surgery group Control group
78.1⫾23.7 91.5⫾29.4
82.2⫾28.6 86.7⫾24.9
2.1⫾18.8 ⫺4.79⫾21.8 P ⫽ 0.380
0.755 0.209
Nasal Surgery group Control group
58.1⫾21.5 60.6⫾18.8
56.3⫾25.2 62.5⫾18.2
⫺1.82⫾14 1.88⫾11.8 P ⫽ 0.242
0.453 0.359
Inferior Surgery group Control group
72.3⫾29.2 85.1⫾35.1
69.7⫾26.4 85.5⫾32.1
⫺2.6⫾18.9 0.38⫾17.2 P ⫽ 0.497
0.427 0.898
Temporal Surgery group Control group
54.5⫾22.3 57.7⫾19.4
56.2⫾20.4 57.3⫾20.7
1.6⫾18.4 ⫺0.44⫾15.3 P ⫽ 0.612
0.605 0.868
Rebolleda et al 䡠 RNFL Changes after Deep Sclerectomy Table 3. Mean Retinal Nerve Fiber Layer Thickness Changes According to Mean Preoperative Overall Retinal Nerve Fiber Layer Thickness Postoperative Preoperative Overall ⱕ62 m ⬎62 m P value
Overall
Superior
Inferior
Nasal
Temporal
⫺3.2⫾10.95 ⫺1.9⫾10.47 0.732
⫺0.82⫾18.5 5.29⫾17.9 0.335
⫺3.8⫾17.9 ⫺0.17⫾8.5 0.413
⫺1.29⫾19 ⫺3.9⫾19.3 0.691
6.7⫾19.7 ⫺3.4⫾15.8 0.109
ness changes after surgery and the preoperative MD (r ⫽ 0.354; P ⫽ 0.038). There was no significant change in the control group in the MD (P ⫽ 0.099), PSD (P ⫽ 0.064), or IOP (P ⫽ 0.097) 6 months after surgery.
Discussion Several studies reported less cupping of the optic disc after IOP reduction in some patients after glaucoma surgery.1–7 This observation is more likely to be due to a simple shift in anatomic structures rather than recovery or reversal of damage. When IOP is lowered, there is less stretch on the lamina cribrosa, and the disc is able to return to its normal position. However, there is no consensus regarding whether the changes associated with IOP reduction occur only in the optic nerve head or also in the peripapillary RNFL.7–9 Until recently, the assessment of the RNFL has been largely subjective. Optical coherence tomography, developed to assess tissue thickness in vivo, is a noninvasive imaging technique, allows high-resolution cross-sectional ocular imaging, and evaluates and quantifies the peripapillary RNFL thickness. Optical coherence tomography provides real-time, immediate, objective, and quantitative assessments of the RNFL within a short time during 1 visit and offers a reproducible technique with a standard deviation of measurements of 10 to 20 m for the mean overall RNFL thickness and 15 to 30 m for measurements of each clock hour.15,16 In the current study, we prospectively evaluated the peripapillary RNFL thickness changes by OCT in 34 patients who underwent monocular deep sclerectomy. We elected deep sclerectomy because it may offer success rates comparable with those of trabeculectomy and may miniTable 4. Pearson’s Correlation between Changes in Mean Retinal Nerve Fiber Layer Thickness and Age, Intraocular Pressure, and Visual Field Global Indices in the Surgery Group Parameter
r
P Value
Age IOP change (mmHg) IOP change (%) Preoperative VF MD Preoperative VF PSD Change in VF MD Change in VF PSD
⫺0.202 0.01 ⫺0.007 0.354 ⫺0.153 0.387 0.391
0.243 0.954 0.970 0.038 0.427 0.068 0.072
IOP ⫽ intraocular pressure; VF MD ⫽ visual field mean deviation; VF PSD ⫽ visual field pattern standard deviation.
mize the risk of postoperative complications.10 –13 We used the fast RNFL thickness scan, which reduces the examination time and improves the accuracy and centration of the scans.15 We did not find significant changes in the overall peripapillary RNFL thickness (P ⫽ 0.166) or in the quadrant analysis (Table 2) in the surgery group. Similarly, there was no difference between the preoperative and postoperative MD and PSD of the visual field results. Similar results have been reported using several devices. Sogano et al7 used a Rodenstock optic nerve head analyzer and reported that although the cup volume decreased and the rim area increased significantly after trabeculectomy, the RNFL height did not change 2 to 6 months after trabeculectomy. Irak et al6 used confocal scanning laser ophthalmoscopy to evaluate 49 eyes 3 months after filtration surgery and did not find a significant change in the RNFL cross-sectional area. Lesk et al,4 using the Heidelberg Retina Tomograph (Heidelberg Instruments, Heidelberg, Germany), reported that pressure-dependent changes in RNFL thickness were not statistically significant. Although the optic disc sometimes is seen to be less excavated when IOP is lowered, there is not a corresponding increase in the RNFL thickness, so the explanation of this finding is not likely a restoration or increase in the number of axons, but rather relaxation of the stretch of tissues that support the optic disc structures. Contrary to these results, Aydin et al8 reported a significant increase in the overall peripapillary RNFL from 72.8 to 81.7 m after filtration surgery measured by OCT in 18 eyes that underwent trabeculectomy and in 20 eyes that underwent combined trabeculectomy and cataract extraction. The overall measurement and the measurement of the quadrants (except the inferior quadrant) to determine the RNFL thickening were significant for the entire study group. In the current study, a nonsignificant mean overall RNFL decrease from 66.6 to 64.0 m was found after surgery. Moreover, this change was similar to that in eyes in the control group in which the RNFL decreased from 74.1 to 71.7 m (Table 1). The differences between our findings and those of Aydin et al8 could be attributed to several factors. First, the absence of a control group in the study of Aydin et al precludes achieving definite conclusions, taking into account the normal test–retest variability of the device. The report of Aydin et al was retrospective and the results obtained should be interpreted cautiously; a bias in the selection of the population cannot be disregarded. Moreover, those au-
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Ophthalmology Volume 114, Number 3, March 2007 thors assumed that the RNFL thickness does not change after cataract surgery, and they combined the data obtained from trabeculectomy or combined cataract extraction and trabeculectomy. However, changes in the ocular media, such as posterior subcapsular and cortical cataracts, could impair the ability to perform OCT, with more low-quality and degraded image artifacts than in patients with a clear lens.17–19 When cataracts are removed along with glaucoma surgery, the subsequent changes in the eye’s optic nerve along with the reliability index of measurements could change the RNFL values. To avoid these artifacts, we included only eyes with clear ocular media. The mean preoperative RNFL thickness was 6.2 m lower in our study than in the study of Aydin et al.8 Curiously, despite a higher mean thickness before surgery than in our patients, the mean preoperative MD was worse, a finding that can be explained partially by an increase in the diffuse visual field defects as the result of cataract artifacts. In fact, to avoid the effect of cataract removal on the visual field test, they analyzed the data in only 15 eyes that had undergone only trabeculectomy. We did not find a significant correlation between the RNFL postoperative changes and age, preoperative overall RNFL thickness, visual field global indices, changes in IOP, or visual field global indices (Tables 3, 4). In our study, based on the absence of significant RNFL thickness changes, no significant changes occurred in the visual field global indices. However, in the study of Aydin et al,8 significant thickening of the peripapillary RNFL was not associated with a clinical improvement in visual field parameters. It has been reported that there is a greater likelihood of glaucomatous progression by OCT compared with automated perimetry. This may reflect OCT hypersensitivity or true damage identified by OCT before detection by conventional methods.20 In addition, the differences may be the result of different degrees of preexisting glaucomatous damage. Some experimental and clinical studies have shown that restoration of anatomic position is more likely to occur in the early stages of glaucoma.21,22 Although the difference was not significant, eyes with an RNFL thickness less than 62 m before surgery had a mean decrease that was 1.4 m more than those with an RNFL thickness of 62 m or more. Taking in consideration that some changes may occur because of higher fluid content of tissues at lower ambient hydrostatic pressure than at high IOP, thin RNFL probably had less opportunity to expand. We found a significant correlation between the postoperative RNFL thickness changes and the preoperative MD. With higher degrees of glaucomatous visual field damage, the chances were less of an increase in the RNFL thickness after surgery. Moreover, the mean preoperative visual field MD was 6.2 dB worse in eyes showing a decrease in the overall RNFL thickness after surgery than in those with an increase in the RNFL thickness (P ⫽ 0.006). Several studies have shown a high correlation between the degree of improvement in the optic nerve head morphologic features and the percent of IOP reduction.2,4,6,7 Aydin et al8 reported that after filtration surgery, a 0.5-m increase
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in the mean RNFL could be expected for each 1-mmHg decrease in IOP.10 Although a difference in IOP reduction could explain different results, the mean IOP change after surgery obtained in our study was 1.3 mmHg more than that obtained by Aydin et al.8 Moreover, in our study, the mean percent of IOP change was 48.4⫾15.7% (median, 53%). Twenty-six eyes (76.4%) had an IOP reduction of more than 30%, similar to data (73.7%) reported by Aydin et al.8 The minimal and nonsignificant changes in RNFL thickness observed in our study may represent normal test–retest variability, taking into account that no significant differences were observed between the 2 study groups (Table 2). The mean IOP and visual field global indices also remained stable in the control group. Mechanisms that explain an improvement in RNFL thickness with IOP reduction are unclear. After the retinal nerve fiber is damaged, it cannot regenerate. One plausible explanation is recovery of compressed RNFL; however, there is no direct evidence showing RNFL compression at the peripapillary RNFL with high IOP. It is possible that some axons are able to function marginally while the IOP is high and can recover some physiologic functions when the IOP is lowered, but this is a biomechanical or physiologic restoration, not an anatomic one. Changes observed in the optic nerve head probably have a mechanical source because, unfortunately, regeneration of axons is not expected. In our study, we did not find a significant change in peripapillary RNFL thickness after successful deep sclerectomy, despite obtaining a significant IOP reduction. The only factor significantly correlated with changes in the RNFL thickness was the preoperative MD—that is, with a poor preoperative MD there was less chance for RNFL thickening after IOP reduction. In conclusion, our results indicate that OCT measurements are not affected by IOP reduction after deep sclerectomy. Additional studies are necessary to support or reject our findings and to provide supplementary data.
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