Menoufia Medical Journal

ORIGINAL ARTICLE
Year
: 2015  |  Volume : 28  |  Issue : 4  |  Page : 891--896

Evaluating the role of OCT in optic disc analysis in glaucoma patients


Saber H Elsied, Sarhan A El Sebaey, Faried M Wagdy, Hanan E Hegazy 
 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia Governorate, Egypt

Correspondence Address:
Hanan E Hegazy
Talaat Harb Street, Shebin Elkom, Menoufia Governorate, 32512
Egypt

Abstract

Objectives The aim of the study was to evaluate the role of optical coherence tomography (OCT) in optic disc analysis in glaucoma patients. Background Forty patients underwent full ophthalmic examination, including a review of medical history, best-corrected visual acuity, slit-lamp biomicroscopy, intraocular pressure measurement, gonioscopy, dilated fundoscopic examination, automated perimetry, and OCT at 1 month and after 6 months. Patients and methods Stratus OCT was used for ocular imaging in patients with dilated pupils. All patients had optic nerve head and retinal nerve fiber layer thickness (RNFL) measured during the two visits. Quality assessment of stratus OCT scans was evaluated by an experienced examiner masked to the patient«SQ»s other test results. Good-quality scans had to have focused images from the ocular fundus, adequate signal strength (>6 for RNFL and macula scans), and the presence of a centered circular ring around the optic disc (for RNFL scans). Results There was a statistically significant difference between patients at 1 month and after 6 months as regards OCT (RNFL), especially with regard to the inferior quadrant, with the P value of the RNFL inferior quadrant, which was the most affected after 6 months, at 0.03 and the P value of the rim area being highly significant at 0.001. Conclusion An OCT would be significantly informative early in the disease course to diagnose a preperimetric glaucoma, to confirm on a visual field, and to follow up longitudinal glaucoma progression.



How to cite this article:
Elsied SH, El Sebaey SA, Wagdy FM, Hegazy HE. Evaluating the role of OCT in optic disc analysis in glaucoma patients.Menoufia Med J 2015;28:891-896


How to cite this URL:
Elsied SH, El Sebaey SA, Wagdy FM, Hegazy HE. Evaluating the role of OCT in optic disc analysis in glaucoma patients. Menoufia Med J [serial online] 2015 [cited 2020 Apr 5 ];28:891-896
Available from: http://www.mmj.eg.net/text.asp?2015/28/4/891/173609


Full Text

 Introduction



Glaucoma is currently defined as a disturbance of the structural or functional integrity of the optic nerve that causes characteristic pathological changes, which may also lead to visual field defects over time. This disturbance can usually be minimized by adequate lowering of intraocular pressure (IOP) and enhancement of optic nerve head (ONH) circulation. Primary open-angle glaucoma (POAG) is described distinctly as a multifactorial form of chronic and progressive optic neuropathy with a characteristically acquired loss of optic nerve fibers. Such loss develops in the presence of an open anterior chamber angle, characteristic visual field abnormalities, and elevated IOP, which affects the hemodynamic circulation of ONH and axoplasmic transport. POAG is a major worldwide health concern, because it has an insidious onset and leads to irreversible blindness [1] .

In glaucoma there is a relatively slow loss of retinal ganglion cell axons. Early loss is usually in the midperipheral visual field. The disease becomes symptomatic at a relatively late stage when central vision is affected and the visual acuity declines or extensive loss of peripheral vision occurs. Blindness due to glaucoma can largely be minimized by early glaucoma discovery. Although the visual damage is irreversible, it can usually be arrested, and to achieve this early diagnosis has to be made [2] .

The aim of glaucoma management is to prevent visual loss either by medical or by surgical treatment. Medical treatment of open-angle glaucoma can induce undesirable changes within the conjunctiva and Tenon's capsule, in addition to the transient conjunctival changes related to allergy and toxicity that most topically applied eyedrops can induce. These clinical observations suggest that a possibility for enhanced postoperative scarring of surgically created filtering blebs might be more likely in patients receiving long-term antiglaucoma medications. Also glaucoma surgery can arrest the progression of the disease through a reduction in IOP and is successful in maintaining normal IOP. The outcome of surgery is improved if the operation is performed early before the increased IOP causes serious damage to the optic nerve fibers. The prognosis is poor if the surgery is performed in the late stages of this disease [3] .

 Patients and methods



Inclusion criteria

Patients with POAG and fulfilling the following criteria were eligible for inclusion in the study: a high IOP greater than 21 mmHg, followed up by phasing; open angle of the anterior chamber on gonioscopy; laucomatous visual field defects; a trophy of retinal nerve fiber layer (RNFL) detected by optical coherence tomography (OCT); and glaucomatous optic disc changes.

Exclusion criteria

The following patients were excluded:

Patients with opaque media.

Patients with visual acuity (V.A) less than 1/60.

Patients with other ocular pathologies that can cause secondary glaucoma.

Patients with other major retinal or neural pathologies.

Patients unsuitable for visual field testing or OCT imaging.

Stratus OCT

The commercially available optical coherence tomograph Stratus OCT (Carl Zeiss Meditec, Dublin, California, USA) was used for ocular imaging of patients with dilated pupils. All patients had ONH and RNFL thickness measured during the same visit. Quality assessment of Stratus OCT scans was evaluated by an experienced examiner masked to the subject's other test results. Good-quality scans had to have focused images from the ocular fundus, adequate signal strength (>6 for RNFL and macula scans), and the presence of a centered circular ring around the optic disc (for RNFL scans). For macula and ONH scans, the radial scans had to be centered on the fovea and optic disc, respectively. RNFL scans were also evaluated as to the adequacy of the algorithm for detection of the RNFL. Only scans without overt algorithm failure in detecting the retinal borders were included in the study.

RNFL thickness measurements

The fast RNFL algorithm was used to obtain RNFL thickness measurements with Stratus OCT. Three images were acquired from each subject, with each image consisting of 256 A-scans along a 3.4-mm-diameter circular ring around the optic disc. The parapapillary RNFL thickness parameters automatically calculated with existing Stratus OCT (software version 4.0, Carl Zeiss Meditec, Dublin, CA, USA) Fast and evaluated in this study were average thickness (360° measure), temporal quadrant thickness (316°-45°), superior quadrant thickness (46°-135°), nasal quadrant thickness (136°-225°), and inferior quadrant thickness (226°-315°).

Optic nerve head measurements

The Fast Optic Disc scanning protocol (software ver. 4.0) was used to obtain ONH measurements with Stratus OCT, as described elsewhere. In ONH scans, the device automatically determines the disc margin as the end of the retinal pigment epithelium/choriocapillaris layer. One can manually adjust the demarcation of the edge of the retinal pigment epithelium to improve the outlining of the disc margin. However, the automatically determined default disc margin was used in this study to minimize subjectivity.

Statistical analysis

The data collected were tabulated and analyzed by statistical package for the social science, version 16 (SPSS Inc., Chicago, Illinois, USA) on an IBM compatible computer.

Quantitative data were expressed as mean and SD (X + SD) and analyzed by means of the Student t-test for comparison of two groups.Qualitative data were expressed as number and percentage [N (%)] and analyzed by means of the χ2 -test.All of these tests were used as tests of significance at P values less than 0.05.

 Results



In [Table 1] and [Figure 1], P values greater than 0.05 are considered nonsignificant, values less than 0.05 are considered significant, and values less than 0.001 are considered highly significant.{Figure 1}{Table 1}

[Table 1] shows that the P value of OCT (disc area) was nonsignificant at 0.06, P value of cup/disc (C/D) ratio by OCT was significant at 0.02, P value of C/D horizontal was significant at 0.03, P value of C/D vertical was nonsignificant at 0.17, and P value of the rim area was highly significant at 0.001.

In [Table 2] and [Figure 2], P values greater than 0.05 are considered nonsignificant, values less than 0.05 are considered significant, and values less than 0.001 are considered highly significant.{Figure 2}{Table 2}

[Table 2] shows that the P value of the RNFL superior quadrant was significant at 0.04, P value of the RNFL temporal quadrant was significant at 0.04, P value of the RNFL nasal quadrant was nonsignificant at 0.15, and the RNFL inferior quadrant was the most affected after 6 months with a P value of 0.03.

[Table 3] and [Figure 3] show that timilol was the most commonly used drug (10 patients, 25%). Other drugs used were Xlatan (four patients, 10%), Alphagan (eight patients, 20%), Cosopt (eight patients, 20%), Iprost (six patients, 15%), and Betoptic (four patients, 10%).{Figure 3}{Table 3}

 Discussion



Glaucomatous damage can be quantified using either structural or functional loss criteria, or a combination of both. Patients with glaucoma may present with the disease before damage is detectable with standard achromatic automated perimetry (preperimetric glaucoma) or with clear glaucomatous visual field defects (perimetric glaucoma) [4] .

OCT is a developed, noninvasive, noncontact technique for imaging of the layered structure of the retina [5] .

Attempts to measure RNFL thickness in normal and glaucomatous eyes using OCT have been made by several investigators [6] .

However, considerable interindividual variations in RNFL thickness have been demonstrated even in normal individuals [7] .

Some studies suggested that OCT may be superior to other imaging techniques, such as scanning laser polarimetry and Heidelberg retina tomography, for detecting a specific pattern of reduction in the average or focal (RNFL) thickness [6] .

The analysis of stratus (OCT) time domain software-provided parameters showed that RNFL measures and ONH topography parameters have the highest power to discriminate glaucomatous from healthy eyes [6] .

Our patients underwent a complete ophthalmological examination, visual field examination, and OCT at two visits with a 6-month time interval.

Among our glaucoma patients, there was reduction in the mean thickness of all quadrants of RNFL, with minimal change in the nasal quadrant and most reduction in the inferior quadrant.

Soliman et al. [8] and Kanamori A et al. [9] conducted a study with similar results. They noted that OCT has the ability to detect glaucomatous changes by measuring RNFL thickness, particularly the average and inferior quadrant. Further, average and quadrant thickness had good correlation with the mean deviation on the Humphery field analyzer. These results agree that glaucomatous visual field damage is likely to occur in a hemi-field area, with the superior field affected more than the inferior, and that the inferior segment of the disc is more susceptible to glaucomatous damage. Soliman et al. [8] had similar findings to Soltan-Sanjari et al. [10] , who compared average RNFL thickness and visual field loss among glaucoma patients using scanning laser polarimetry. They also studied the level of RNFL percentage loss in mild, moderate, and severe glaucoma compared with normal eyes.

It seems that using visual field loss as the only endpoint in clinical trials limits the interpretation of the behavior of the disease, because it is a subjective test with large variability and limited range and requires longer follow-up time, and there is no consensus on the best method of judging progression.

This is in agreement with the results of Manassakorn et al. [11] , as they found that there was a statistically significant difference in C/D between glaucoma patients at 1 month and after 6 months.

There was a statistically significant correlation between C/D and average RNFL thickness in glaucoma patients.

This is in agreement with the results of Anita et al. [11] . They reported that the fast optic disc algorithm performs as well as the fast RNFL thickness algorithm for discrimination of glaucoma performance at 1 month and after 6 months.

For the inferior quadrant and the superior quadrant these results were in agreement with those of Schuman et al. [12] .

However, our results regarding the temporal quadrant are not in agreement with those of Medeiros et al. [13] or with Nouri-Mahdavi et al. [14] in that the temporal quadrant comes after the inferior quadrant.

There was a statistically significant correlation between average RNFL thickness and the superior quadrant, as well as with the inferior quadrant and the nasal quadrant, but a nonsignificant correlation between average RNFL thickness and the temporal quadrant.

In this study, we found that eyes that showed progression by standard methods (i.e. SAP visual fields) had significantly higher rates of RNFL loss over time as measured by the Stratus OCT than did eyes that remained stable. Also, RNFL thickness parameters performed significantly better than ONH and macular thickness parameters for detection of change. To our knowledge, this is the first study to evaluate these OCT scanning areas for monitoring progression, and these findings may have significant implications for the use of this instrument to longitudinally evaluate glaucomatous eyes and those suspected of having disease.

Stratus OCT RNFL parameters performed well in discriminating eyes that progressed on the basis of visual fields from eyes that did not. The mean rate of change for the RNFL parameter average thickness in progressing eyes was −0.72 μm/year, which corresponds to ~0.8%/year, compared with 0.14 μm/year for nonprogressing eyes. However, there was wide variation in the rates of change among the eyes included in the study, as some of the eyes that were not detected as progressing on the basis of visual fields had significantly negative rates of change on Stratus OCT RNFL assessment. This could represent progressive glaucomatous damage that was not detected by conventional methods. In fact, there is some evidence that there can be observed changes in the RNFL before detectable damage to the optic nerve or visual fields [15] . In a longitudinal study of a prototype OCT, found that 22% of the eyes had significant change in OCT measurements without apparent deterioration of visual fields. Alternatively, the decline in Stratus OCT measurements over time in nonprogressing eyes could also represent age-related loss of RNFL thickness. Further follow-up of these subjects should clarify whether these cases represent age-related loss of nerve fibers, early detection of glaucoma progression, or simply instrument-related false-positive results.

The parameter inferior average showed the best performance in discriminating progressors from nonprogressors in our study. Changes in the inferior RNFL are in agreement with the expected pattern of damage in glaucomatous optic neuropathy. In fact, in several previous cross-sectional studies, this parameter has been found to be the best one to discriminate glaucomatous from healthy eyes. This is in agreement with the results of Parikh et al. [16] .

It should be noted, however, that changes may occur on different sectors of the optic disc and RNFL as glaucoma progresses, which could explain why a single parameter may not be able to detect all progressing eyes. Also, longitudinal evaluation of the RNFL with Stratus OCT may be affected by variations in the position of the circle of measurements around the optic nerve. These variations have been shown to result in decreased reproducibility [17] , and attempts have been made to develop tracking systems that could improve image registration and RNFL measurement reproducibility [17] . The use of SDOCTs is likely to overcome this limitation and result in improved reproducibility of RNFL measurements [18] . However, the ability of these instruments to detect progressive glaucomatous damage has not yet been evaluated.

Previous reports have found Stratus OCT ONH parameters to be reproducible [19] and to perform well in discriminating eyes with glaucomatous visual field loss from healthy eyes. However, in the present study, we found that these parameters had poor ability to differentiate progressing from nonprogressing eyes in a longitudinal setting. Although rates of change in the parameter cup area were significantly different between the two groups, Stratus OCT ONH parameters are obtained from only six radial scans centered on the optic disc. Therefore, interpolation is performed between the scans to obtain estimates of topographic parameters such as rim area and cup area.

 Conclusion



OCT can provide objective, quantitative, and reproducible images of the ONH and RNFL, both of which undergo structural changes in glaucoma before visual field loss.

Spectral domain OCT has been replacing time domain OCT in the diagnosis and follow-up of glaucoma, enabling three-dimensional video imaging of the ONH, three-dimensional RNFL thickness maps, and optical doppler tomography.

Minimum distance band RNFL deviation map, ganglion cell complex scan, and focal loss volume are new parameters allowing earlier detection and better evaluation of glaucoma change over time.

An OCT would be significantly informative early in the disease course to diagnose a preperimetric glaucoma, to confirm on a visual field, and to follow up longitudinal glaucoma progression.

 Acknowledgements



Conflicts of interest

There are no conflicts of interest.

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