Egyptian Retina Journal

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 7  |  Issue : 2  |  Page : 36--40

Changes in retinal nerve fiber layer thickness after panretinal photocoagulation in diabetic retinopathy


Ehab Ismail Wasfi1, Kamel Abd El-Naser Soliman1, Rania Mohammed Mohammed2, Ali Natag Ryad1,  
1 Department of Ophthalmology, Assiut University Hospital, Assiut, Egypt
2 Department of Ophthalmology, Assiut Ophthalmology Hospital, Assiut, Egypt

Correspondence Address:
Dr. Rania Mohammed Mohammed
31- Kolta - Mohammed Ali Makarem Street
Egypt

Abstract

Context: Panretinal photocoagulation (PRP) is the gold standard treatment for high risk proliferative diabetic retinopathy. Aim: The evaluation of peripapillary retinal nerve fiber layer (RNFL) thickness changes in eyes undergoing PRP by optical coherence tomography. Materials and Methods: RNFL thickness was measured before PRP, then after PRP by 1, 3, and 6 months, respectively. Results: Mean peripapillary RNFL thickness increased significantly 1 month after P (P = 0.001), then showed insignificant decrease at 3 months' post-PRP (P = 0.1), then it showed significant decrease at 6 months (P = 0.0001) compared to baseline. Conclusions: PRP should be used with a great caution with least number of shots to avoid excessive damage to inner retinal layers.



How to cite this article:
Wasfi EI, Soliman KA, Mohammed RM, Ryad AN. Changes in retinal nerve fiber layer thickness after panretinal photocoagulation in diabetic retinopathy.Egypt Retina J 2020;7:36-40


How to cite this URL:
Wasfi EI, Soliman KA, Mohammed RM, Ryad AN. Changes in retinal nerve fiber layer thickness after panretinal photocoagulation in diabetic retinopathy. Egypt Retina J [serial online] 2020 [cited 2021 Dec 2 ];7:36-40
Available from: https://www.egyptretinaj.com/text.asp?2020/7/2/36/308382


Full Text



 Introduction



Diabetic retinopathy significantly impacts the vision and accounts for about 20% of visual impairment in working-aged adults.[1] Without treatment, nearly 50% of patients with high-risk proliferative diabetic retinopathy (PDR) experience severe vision loss within 5 years.

Panretinal photocoagulation (PRP) is considered the gold standard treatment for PDR. The Early Treatment Diabetic Retinopathy Study recommends the application of up to 2000 visible end-point burns on the retina.[2] PRP is a procedure that involves applying laser burns to the peripheral retina, from which the thermal energy induces tissue coagulation. In this procedure, laser light is absorbed primarily by melanosomes within the retinal pigment epithelium, where light energy is used to coagulate tissue by increasing tissue temperature, liquid vaporization, and protein denaturation. The coagulative effect mostly affects the outer retina, but several studies have demonstrated a detrimental effect from PRP on adjacent tissues, including the retinal nerve fiber layer (RNFL).[3]

PRP is theorized to stop the progression of ischemic disease by improving retinal oxygenation and decreasing the drive for vascular endothelial growth factor production by the retina.[3] RNFL thickness has been shown to be affected by PRP, and the appropriate titration of intensity should aim to achieve photocoagulation of the outer retina while limiting damage to the inner retina. Over time, peripheral retinal damage can lead to a decrease in dark adaptation and a constricted visual field.

Diabetes mellitus (DM) is considered to be a neurovascular disease, so PRP should be used with great caution to avoid excessive damage to the retina, which can add to the pathological process of DM itself and the associated decrease in RNFL thickness. The use of subthreshold pulsed laser is recommended by many researchers to avoid annual scar expansion growth rate.[4],[5] Peripapillary RNFL can be measured by optical coherence tomography (OCT), which is a high-resolution imaging technology that provides a cross-sectional (histological) view of the retina. OCT is characterized by being safe, repeatable, and noninvasive, which is important for the diagnosis, follow-up, and treatment of diabetic retinopathy.[5]

The aim of this study is evaluation of peripapillary RNFL thickness changes in eyes undergoing PRP by using spectral domain OCT (SD-OCT).

 Materials and Methods



This is a prospective, noncontrolled observational study, conducted between January 2019 and December 2019. This study was approved by our institute's committee of medical ethics and written informed consent was obtained from all patients. This study involved 34 eyes of 30 diabetic patients aged 35–70 years' old with PDR confirmed by fluorescein angiography who were scheduled for PRP. Patients with densely opaque media (as dense corneal opacity, cataract, or vitreous hemorrhage), glaucomatous patients, or patients with any other associated retinopathies were excluded.

All patients were subjected to ophthalmic examination including best-corrected visual acuity (BCVA), intraocular pressure (IOP) measurement, slit-lamp biomicroscopy examination, fundus examination, and OCT scans of the peripapillary optic nerve head before starting the PRP protocol. These initial examinations were considered as the baseline measurements, then OCT scans were performed 1, 3, and 6 months following PRP.

The peripapillary RNFL thickness was measured using a Heidelberg OCT Spectralis, 2006 Germany. In the RNFL scan protocol, the peripapillary RNFL thickness was calculated using a 3.45 mm–radius ring centered on the optic disk. Pupillary dilatation was performed using Mydrapid 1% eye drops before the beginning of the scan. The 3.4 mm–diameter circular scan was positioned around the optic nerve head so that it was centralized in the center of the circle. The desired scanning position was then located while the examiner observed both the real-time display of the OCT in progress and the video image of the OCT probe beam location on the fundus. The RNFL thickness was reported individually for each scan as averages for each quadrant (temporal, superior, nasal, and inferior) in each average clock hour. OCT image registration was automatically confirmed with an OCT projection image (generated from three-dimensional-OCT data by summing different retinal depth levels) and localization of major retinal vessels. RNFL thickness in each cell was calculated and compared to the normative database of the device.

PRP was performed using doubled neodymium-doped yttrium aluminum garnet laser. Pupillary dilatation was performed using Mydrapid 1% eye drops before laser treatment, and local anesthetic eye drops were administered before starting the treatment. PRP treatment protocol involved laser burns administered over the entire retina, sparing the central macular area. The protocol was performed over two sessions, 1 week apart. The area of laser coverage extended two-disc diameters temporal to the fovea to the ora serrata in all four quadrants, sparing one-disc diameter in the peripapillary zone with the use of Goldmann three-mirror contact lens to provide a wide range of magnification. The parameters of the laser settings used were 200 μm laser spot size, 0.1–0.2 s, and 200–320 mw (sufficient to produce moderate intensity/gray–white burns). The burns were placed one spot size apart. The procedure was continued peripherally to achieve ∼800–1000 burns in each session, with a total of about 1600–2000 burns over two sessions, 1 week apart.

Patients were categorized into two groups according to the length of time they had been diagnosed with DM: Group A included patients who had been diabetic for >10 years and Group B included patients who had been diabetic for ≤10 years. Patients were also categorized according to their age into two groups: Group C included patients >50 years and Group D included patients ≤50 years.

All statistical analyses were performed using SPSS software (version 21.0; Armonk, NY, USA: IBM Corp). P < 0.05 was considered as statistically significant.

 Results



This study involved 34 eyes of 30 diabetic patients with a mean age of 56.4 years old. The demographic characteristics of patients are summarized in [Table 1].{Table 1}

The mean RNFL thickness increased significantly from 100.55 ± 15.11 μm at baseline to 103 ± 14.7 μm 1-month post-PRP (P = 0.001), then decreased to 99.18 ± 13.49 μm 3 months' post-PRP, but was insignificant (P = 0.1). Subsequently, the RNFL thickness decreased gradually, showing a statistically significant reduction (P = 0.0001) at 6 months' post-PRP of 97.55 ± 19.61 μm. At 1-month post-PRP, the most significant increase was seen at the inferior and temporal quadrants (superior and nasal quadrants were insignificant). At 3 months' post-PRP, the mean total RNFL decreased by 1.38 ± 5 μm from baseline, but it was an insignificant decrease. At 6-month post-PRP, a significant reduction in RNFL thickness was shown in all retinal quadrants, with a mean total RNFL decrease of 3 ± 1 μm when compared to baseline. The reduction in RNFL thickness varied from one retinal quadrant to another, with the most significant changes in the temporal, superior, and inferior quadrants [Table 2] and [Figure 1].{Table 2}{Figure 1}

The BCVA (logarithm of the minimum angle of resolution [LogMAR]) baseline mean was 0.53 ± 0.28. No change was detected at 1-month and 3 months' post-PRP. The mean BCVA (LogMAR) showed insignificant change at 6 months' post-PRP (0.55 ± 0.1, P = 0.09). There were no detectable changes between pre- and post-PRP refraction. There was no significant correlation between changes in RNFL thickness and changes in visual acuity 6-months after PRP (r = −0.2, P = 0.3).

The mean IOP measurement at baseline was 16.07 ± 1.25. At 1-month post-PRP, the mean was 16.089 ± 1.07 (14–18), which was an insignificant change (P = 0.09). At 3months' post-PRP, the mean was 16.189 ± 1.09 (14.3–18.4; P = 0.2), and at 6 months' post-PRP, the mean was 16.15 ± 1.1 (14–18; P = 0.4).

The mean reduction in RNFL thickness 6-month post-PRP in Group A (2.9 ± 1.1) versus Group B (3.1 ± 0.9) did not have statistical significance [P = 0.7; [Figure 2]].{Figure 2}

The mean RNFL thickness in Group C was 101.36 ± 16.575 μm at baseline, then became 104.6 ± 1675 μm at 1-month post-PRP, 99.75 ± 14.9 μm 3-month post-PRP, and 98.5 ± 16.3 μm 6-month post-PRP. The mean RNFL thickness in Group D at baseline was 97.7 ± 9.9 μm, then became 101.00 ± 9.1 μm at 1-month post-PRP, 97.3 ± 7.4 μm 3-month post-PRP, and 94.56 ± 9.96 μm 6-month post-PRP. There was no statistically significant difference between both groups regarding RNFL thickness at baseline and at each follow-up interval (P = 0.6, 0.55, 0.66, and 0.5, respectively). The mean reduction in RNFL thickness 6-month post-PRP in Group C (2.9 ± 1 μm) versus Group D (3.4 ± 1 μm) did not have statistical significance [P = 0.2; [Figure 3]].{Figure 3}

 Discussion



The aim of this study was to evaluate the peripapillary RNFL thickness changes in eyes undergoing PRP for PDR using SD-OCT.

This study reported significant post-PRP changes, including increased thickness at 1-month post-PRP, followed by progressive thinning at 6-month post-PRP, as shown in [Table 2].

Thickening of the peripapillary RNFL early in the post-PRP period can be explained by PRP-induced edema and inflammation.[6] Retinal edema may occur soon after PRP, as laser treatment triggers retinal inflammation due to increased vascular permeability. However, in the long-term, a reduction in RNFL thickness is likely to occur due to damaged retinal cells, such as photoreceptors and ganglion cells. Significant RNFL thinning in the superior quadrant at 6-months post-PRP could be explained by the fact that microaneurysms and acellular capillaries are twice as common in the superior than inferior retina.

The results of our study are in-line with other researchers who reported an initial increase post-PRP followed by a decrease in RNFL thickness. Yazdani et al.[7] reported an increase of 3 μm (P = 0.04) at 1-month post-PRP, followed by a marginal significant decrease of 2.4 μm at 6-month post-PRP (P = 0.054) compared to baseline values.

Muqit et al.[8] reported a little differ from our results as a significant RNFL thickness increase (8 μm; P < 0.05) noted at 10 weeks' post-PRP. They assumed that concurrent poor glycemic control may contribute to the ongoing laser-induced inflammation at 10 weeks in their patients. They noted a significant reduction in RNFL thickness (4 μm; P < 0.05) at 6-month post-PRP as our result.

Our results varied from Eren et al.[9] as they mentioned persistent significant increase till 3-months post-PRP in all retinal quadrants, except the temporal quadrant, and attributed these results to axonal edema. However, a decreased thickness was reported at 6-month post-PRP in all quadrants compared to baseline values due to axonal loss secondary to laser burn.

Kim and Cho[10] compared RNFL thickness reduction after 6-month follow-up between a control group (mild-to-moderate non-PDR patients), and post-PRP treatment group (severe non-PDR patients). They reported a significant reduction in RNFL thickness (2.12 μm) at 6-month in the treatment group compared with baseline values; however, this reduction was not statistically significant compared to the reduction observed in control group suggesting that PRP does not necessarily damage RNFL if laser burn is only of a moderate degree. They also reported a greater significant reduction in RNFL thickness after PRP in patients with higher glycated hemoglobin level denoting that poor glycemic control makes RNFL more vulnerable to external insults such as laser burns.

In our study, no significant changes occurred in BCVA, as laser burns affect the peripheral field, and it is possible that the relatively short-term follow-up did not result in any BCVA changes as laser scar expansion had not yet developed.

Considering DM as a neurovascular disease and that tight glycemic control and other risk factors are much more important than the duration of DM could explain the greater decrease in RNFL thickness in the younger age group in our study.

Other risks of PRP include the fusion and expansion of laser burns over a long period of time. Laser scars expansion was reported to be more in the posterior pole with 12.7% mean annual expansion rate and 7.0% in the midperiphery.[11] Therefore, progressive destruction in the RNFL and visual field loss can be seen after PRP treatment. The extent of visual field loss has been attributed to higher laser power and greater retinal laser coverage. Hence, choosing sufficient parameters for optimum efficacy and safety is mandatory.

Limitation to this study includes relatively short-term follow-up, and lack of estimation of glycated hemoglobin level.

 Conclusion



PRP should be used with a great caution with least number of shots to avoid excessive damage to inner retinal layers.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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