Patients and Methods
This is a retrospective study analyzing data collected from patients diagnosed with glaucoma or glaucoma suspect. The research protocol followed the tenets of the Declaration of Helsinki, and was approved by the Duke University institutional review board before to data collection.
From the Duke Imaging Center's database, adult patients with a diagnosis of glaucoma or glaucoma suspect were identified who underwent bilateral SD-OCT (Spectralis, Heidelberg Engineering, Carlsbad, CA) using the Duke glaucoma scan protocol between January 1, 2010, and June 30, 2010. The Duke glaucoma scan protocol, described in detail previously, includes measurement of the RNFL circumferential peripapillary scan, as well as a macular thickness analysis protocol. Briefly, the macular thickness analysis measures retinal thickness along 61 lines in the central 20 degrees of each eye. The area is divided into an 8×8 mm grid, centered on the foveal pit and aligned with the optic disc, and consisting of small 3 degree by 3 degree squares, with the mean retinal thickness of each small square displayed. In addition, the average total retinal thickness, as well as the average retinal thickness for each macular superior-half and inferior-half, are calculated and displayed for each eye. The RNFL measurement is displayed as an average thickness for the quadrants as well as with the superior and inferior quadrants divided into nasal and temporal sections each. Figure 1 shows an example of the SD-OCT data collected from each patient. OCT scan results with artifacts were excluded.
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Figure 1.
Example of spectral domain optical coherence tomography (SD-OCT) with asymmetry analysis. This example illustrates how the inferior defect in the left eye may have been easily overlooked without the macular thickness analysis. A, Visual field and SD-OCT analysis of the retinal nerve fiber layer (RNFL). The pattern standard deviation plots show a dense superior altitudinal defect and an inferior nasal defect in the right eye, with corresponding RNFL thinning inferiorly more than superiorly. In the left eye, the visual field shows some nonspecific central and nasal areas of decreased sensitivity. The RNFL average thickness by quadrant is normal; however, the RNFL tracing shows a dip below normal infero-temporally. B, Asymmetry analysis of the posterior pole. The retinal thickness map overlays the infrared photo of the posterior pole for each eye. Inferior thinning is noted in both eyes, seen quite clearly in the color map; in addition, the RNFL defect noted in the left eye in (A) is seen to extend well into the macula. The asymmetry analyses are displayed as gray-scale maps, with respective thinning shown progressively darker (30 μm difference appears black, 20 μm difference gray, and no difference or thickening white). The asymmetry analysis comparing the right and left eyes (located between the photo-maps) shows significant global thinning of the right eye compared with the left. The asymmetry analysis comparing the superior and inferior hemifields as well as the average retinal thickness measurements (total, as well as superior and inferior averages) are shown below the photo-maps. In the right eye, there is marked inferior thinning, consistent with the RNFL and visual field findings. In the left eye, inferior thinning is also notable.
From the patients who had undergone imaging as described above, those with automated Humphrey (Carl Zeiss Meditec Inc., Dublin, CA) visual field (HVF) examination done within 1 year of the OCT, were chosen. Those patients with visual fields flagged on the printout as with low reliability in either eye were excluded. Global visual field parameters such as mean deviation (MD) and pattern standard deviation (PSD) were recorded. On chart review, patients with any significant coexisting ocular pathology that would confound the retinal thickness measurement, including diabetes with retinopathy, age-related macular degeneration, epiretinal membrane, or media opacity (≥3+ nuclear sclerosis cataract or greater than trace posterior subcapsular cataract) were excluded.
Macular thickness parameters as measured by the SD-OCT were compared with parameters of the HVF tests for each patient; both eyes were included and were analyzed separately. SD-OCT parameters included total average macular thickness, average macular thickness in the superior and inferior macular halves, as well as differences in total and macular half-thicknesses between the right and left eyes. The average thickness of the circumpapillary RNFL in the superotemporal and inferotemporal quadrants for each eye were also recorded. The HVF parameters used included the MD and PSD. Asymmetry of the visual fields was calculated by the difference in MD scores between the 2 eyes. The cup-to-disc ratio of each eye, as noted by the treating glaucoma specialist in the patient's medical chart, was also recorded.
Correlations between the macular thickness values and the visual field parameters, as well as the RNFL parameters, were determined using linear regression analysis, using SAS 9.2 software (SAS Institute Inc., Cary, NC), with correlations expressed as a Pearson coefficient of correlation. The bivariate fit analyses, along with the linear fit, are expressed as graphical plots and models.