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 Table of Contents    
ORIGINAL ARTICLE
Year : 2017  |  Volume : 10  |  Issue : 2  |  Page : 91-95  

Effect of phacoemulsification on measurement of retinal nerve fiber layer and optic nerve head parameters using spectral-domain-optical coherence tomography


Dr. Rajendra Prasad Center for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India

Date of Web Publication29-Jun-2017

Correspondence Address:
Tanuj Dada
Room No. S1, 1st Floor, Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi . 110 029
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ojo.OJO_93_2016

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   Abstract 


PURPOSE: The purpose of this study is to determine the effect of phacoemulsification cataract extraction on measurement of retinal nerve fiber layer and optic nerve head parameters using spectral domain optical coherence tomography.
MATERIAL AND METHODS: A prospective, hospital-based study of 100 patients of 40 years of age and above, with no other ocular morbidity except cataract and planned for phacoemulsification with IOL implantation (SN60WF) at a tertiary centre at AIIMS, New Delhi, India. All patients underwent imaging with Cirrus SD-OCT model 400 and the optic disc cube 200x200 protocol at baseline and at 1 month follow up. Paired sample t-test was used to compare the RNFL parameters and ONH parameters.
RESULTS: The mean age of subjects was 56.6 ± 12.3 years (70 males, 30 females). The average RNFL increased from 92.6 ± 5.4 μm to 101.3 ± 5.6 μm after phacoemulsification, an increase of 9% (P = 0.003) and the signal strength increased from 5.6 ± 0.5 to 7.6 ± 0.7, increasing by 35.7% (P = 0.004). There was a significant increase in the disc area (P = 0.004) and rim area (P = 0.004) but no significant change in vertical cup-disc ratio (P = 0.45) or average cup-disc ratio (P = 0.075). The quadrant-wise RNFL thickness increase in inferior, superior, nasal, and temporal quadrants was 12.6% (P = 0.001), 10% (P = 0.001), 5.6% (P = 0.001), and 3.2% (P = 0.001), respectively. The change in RNFL thickness was maximum in posterior subcapsular cataract (P = 0.001) followed by cortical (P = 0.001) and nuclear (P = 0.001) subtypes.
CONCLUSIONS: A significant increase in RNFL thickness and signal strength was observed after cataract surgery using SD-OCT. The maximum change in RNFL thickness was in the inferior quadrant, where RNFL thinning is a significant predictor of glaucoma progression. The posterior subcapsular cataract interfered with RNFL measurement maximally due to its density and proximity to nodal point. After the cataract surgery, a new baseline needs to be established by obtaining fresh OCT images for assessing the longitudinal follow-up of a glaucoma patient.

Keywords: Cataract, optic nerve head parameters, phacoemulsification, retinal nerve fiber layer, spectral-domain optical coherence tomography


How to cite this article:
Jha B, Sharma R, Vanathi M, Agarwal T, Sidhu T, Tomar A, Dada T. Effect of phacoemulsification on measurement of retinal nerve fiber layer and optic nerve head parameters using spectral-domain-optical coherence tomography. Oman J Ophthalmol 2017;10:91-5

How to cite this URL:
Jha B, Sharma R, Vanathi M, Agarwal T, Sidhu T, Tomar A, Dada T. Effect of phacoemulsification on measurement of retinal nerve fiber layer and optic nerve head parameters using spectral-domain-optical coherence tomography. Oman J Ophthalmol [serial online] 2017 [cited 2019 Dec 7];10:91-5. Available from: http://www.ojoonline.org/text.asp?2017/10/2/91/209121




   Introduction Top


Glaucoma is a group of eye diseases characterized by progressive optic neuropathy and recognizable patterns of optic disc and visual field damage. It is associated with the death of retinal ganglion cells whose axons form the retinal nerve fibers. This chronic, progressive loss of retinal nerve fiber layer (RNFL) causes thinning [1] which can be objectively detected with imaging. Recent advances in ocular imaging technology provide objective and quantitative information about the health and thickness of RNFL and optic nerve head (ONH). Confocal scanning laser ophthalmoscopy, scanning laser polarimetry, and optical coherence tomography (OCT) have been used to quantitatively and objectively measure optic disc topography and RNFL thickness.[2] These are used to predict glaucoma development and have the potential to detect and measure structural progression.[3],[4],[5],[6],[7],[8],[9],[10]

OCT uses light reflectance signal from the retina to measure RNFL thickness. Several software programs are there to perform segmentation of RNFL automatically by detecting the internal limiting layer and outer RNFL border in each A-scan of OCT frame.[11] For ONH analysis, an automated software algorithm defines the margin of the optic disc as the termination of Bruch's membrane. Once this boundary has been identified by spectral-domain OCT (SD-OCT), ONH parameters are automatically generated by a Carl Zeiss Meditec analysis algorithm developed for Carl-Zeiss Meditec Cirrus SD-OCT model 400 (Zeiss Instruments, Dublin. CA, USA, software version 3.0), which does not involve user interaction. Specific features of the ONH are measured, calculated, and displayed in a data table format for use by the clinician.[12]

Measurement of RNFL progression and ONH changes in glaucoma patients has clinical relevance in prognostication of the disease and formulation of a treatment plan. As cataract is one of the most commonly encountered pathological conditions in ophthalmology, it is important to understand the effect of cataract on RNFL thickness measurements and ONH changes by OCT. This is of practical significance as cataracts and glaucoma are frequently coexistent conditions both occurring in the elderly.[13] Cataract formation, cataract progression, and cataract surgery are inevitable events noted during long-term follow-up of glaucoma patients. There are a few studies evaluating the effect of cataract surgery on the measurement of RNFL thickness using SD-OCT, but the effect of cataract surgery on ONH parameters has not yet been documented. The purpose of this study was to determine the effect of phacoemulsification cataract extraction on the measurements of RNFL thickness and ONH parameters using SD-OCT.


   Materials and Methods Top


This was a prospective, hospital-based study, approved by the Institutional Review Board. The study conformed to the Declaration of Helsinki. One hundred consecutive patients with age ≥40 years who had no ocular disease other than cataract and planned for phacoemulsification with intraocular lens (IOLs) implantation were recruited to participate in the study. Patients with intraocular pressure >21 mmHg, tilted disc or any other disc anomaly, retinal or optic nerve pathology or media opacity other than cataract that might affect RNFL measurements, family history of glaucoma, history of diabetes, history of ocular surgery, and patients not willing for follow-up were excluded from the study. All patients underwent a comprehensive ophthalmological examination at baseline. The examinations included best-corrected visual acuity, refractive error, slit lamp biomicroscopy, posterior segment examination, intraocular pressure measurement, SD–OCT, and grading of cataract. Any patient with refractive error ≥ ±5 diopter was excluded from the study. Cataract was graded into nuclear, cortical and posterior subcapsular depending on the predominant type present based on the lens opacity classification system III.[14]

All patients underwent imaging with Carl-Zeiss Meditec Cirrus SD-OCT model 400 (Zeiss Instruments, Dublin. CA, USA, software version 3.0), and the optic disc cube protocol was used after pupillary dilation with 1% tropicamide. One eye of each patient was scanned three times using the optic disc cube 200 × 200 protocol. If both the eyes of a patient fulfilled the inclusion criteria, the one with better scan quality was included in the study. All scans were acquired in the same session by same examiner (BJ) within 5–10 s of each other. OCT scans of signal strength <5 were excluded from the study. The scan with the best signal strength was selected from the three scans for each eye.

Cataract surgery was performed under topical anesthesia by a single surgeon (TD) in all cases. The surgery involved a standard phacoemulsification with an in-the-bag monofocal hydrophobic foldable IOLs (SN60WF, Alcon, TX, USA) implantation through a clear corneal incision.

Patients were followed up after 1 month of phacoemulsification cataract surgery and best corrected visual acuity, anterior segment, and posterior segment examination were repeated. SD-OCT was repeated by the same examiner using the same protocol. The parameters evaluated at baseline and 1 month after phacoemulsification cataract surgery were RNFL thickness (360°, 4 quadrants), signal strength, and ONH parameters (optic rim area, optic disc area, average cup–disc ratio, and vertical cup–disc ratio). Statistical analysis was performed using SPSS v 15 (SPSS Inc, Chicago, IL, USA). Paired sample t-test was used to compare the RNFL parameters and ONH parameters. The statistical significance level was accepted as P ≤ 0.05.


   Results Top


Hundred eyes of 100 patients were included in the study. There were fifty patients of posterior subcapsular cataract, 24 of cortical cataract, and 26 of nuclear type. The mean age of the patients was 56.6 ± 12.3 years with a maximum number of patients falling in the group 51–60 years. One eye of each patient was included. [Table 1] shows the participants' characteristics. There was a significant increase in best-corrected visual acuity after cataract surgery [Table 2]. The average RNFL increased from 92.6 ± 5.4 μm to 101.3 ± 5.6 μm after phacoemulsification, an increase of 9% (P = 0.003) and the signal strength increased from 5.6 ± 0.5 to 7.6 ± 0.7, increasing by 35.7% (P = 0.004). Quadrant-wise RNFL thickness increased significantly after cataract surgery for all the quadrants as shown in [Table 2] (increase in inferior, superior, nasal, and temporal quadrants being 12.6%, 10%, 5.6%, and 3.2%, respectively). There was a significant increase in disc area and rim area post cataract surgery. However, the average cup-disc ratio and vertical cup-disc ratio showed no significant change. When analyzing the change in RNFL thickness among cataract subtypes, there was a significant increase in each type, the increase being maximum in posterior subcapsular cataract followed by cortical and nuclear subtypes. A similar trend was seen in the change in signal strength, maximum change seen in posterior subcapsular variety.
Table 1: Patient characteristics in the study

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Table 2: Change in parameters postcataract surgery

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   Discussion Top


OCT has become an indispensable technique for high-resolution cross-sectional imaging of the retina.[15] Measurement of RNFL thickness using OCT has clinical relevance in the prognosis of disease and formulation of the treatment plan.[16],[17] Cataract and glaucoma are frequently seen to co-exist in the elderly.[13] Glaucoma therapy itself leads to acceleration of cataractogenesis.[18] We studied the effect of phacoemulsification on the measurement of RNFL thickness and ONH parameters using SD-OCT. This is the first study in literature to study the effect of cataract removal on ONH parameters using SD-OCT.

In the present study, a significant increase in RNFL thickness and signal strength was observed after cataract surgery using SD-OCT. We found an increase of 9% in 360° average RNFL thickness and an increase in signal strength by 35.7%. The mean change in average RNFL in our study was 8.6 μ. This is comparable to the results by Cheng et al.[19] and Kim et al.[20] who used SD-OCT to study the effect of cataract removal on RNFL measurements. We found a significant increase in RNFL thickness in all the four quadrants. This was similar to study by Cheng et al.[19] Kim et al. however found a significant increase in average thickness, all four quadrant thickness except temporal and all clock hour thickness except 4–6 h.[20] The difference in our result may be explained by the difference in sample size in the two studies. We found the maximum change in RNFL thickness after cataract surgery in the inferior quadrant which has important clinical implication as thinning of average and inferior quadrant RNFL are found to be significant predictors in glaucoma progression.[16] The change in RNFL and signal strength can be explained by the fact that cataract impedes signal transmission to and reflection from the retina. The delayed time-of-flight information, in turn, affects the spatial delineation of RNFL layer leading to a falsely low measurement.

We found all the subtypes of cataract to significantly affect RNFL measurement, posterior subcapsular cataract interfering maximally. The high density and typical morphology of posterior subcapsular cataract leading to obscuration of nodal point explain the strong effect of this subtype on imaging. This is similar to the results by Savini et al.[21] but they had used time domain OCT. The type of cataract causing a maximum decrease in the studies using SD-OCT has been found to be nuclear by Cheng et al. and cortical by Kim et al. This may be due to the difference in classification of cataract subtypes, difference in exclusion and inclusion criteria and our bigger sample size.

The difference in RNFL measurement after different types of IOL implantation is another concern. We have used a single type of IOLs in all patients in our study. Kim et al.[22] studied the influence of blue light-filtering IOL on RNFL measurements by SD-OCT and found no significant difference in perioperative changes of RNFL measurements between yellow and clear IOL groups.

We also found a significant increase in disc area and rim area after phacoemulsification which has not been reported in the literature yet. The significant increase may be explained by the improvement in signal strength leading to better scanning. We, however, could not get any significant change in vertical cup-disc ratio or average cup-disc ratio. This may be because of change in disc area as well as rim area which probably counterbalance each other and prevent any change in cup-disc ratio.

There are, however, some limitations in our study. First, the study was carried out on normal patients, and hence, the results cannot be applied to glaucoma patients. The smaller number of patients in each cataract subtype is another limitation. We also did not study the relationship of cataract severity to change in RNFL thickness and ONH parameters. Other limitation is that we found a small, but statistically significant increase in disc area and rim area which can be due to the variability among OCT measurements. We also did not find the effect of other kind of IOLs in the measurement of RNFL thickness.

In conclusion, our data suggest that the presence of lenticular opacity significantly affects RNFL thickness measurements and ONH parameter study using Cirrus OCT. In clinical practice, an increase in RNFL thickness and ONH measurements can be expected after cataract surgery and we recommend obtaining fresh OCT images after cataract surgery to define a new baseline for patients who need glaucoma follow-up.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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