Abstract
purpose. To evaluate whether erroneous compensation for anterior segment retardation can be estimated and used to correct peripapillary (PP) retinal nerve fiber layer (RNFL) retardation measurements.
methods. Retardation measurements (for the 780-nm wavelength), given as RNFL thickness by the scanning laser polarimeter, were obtained at the macula and PP retina in 45 eyes of 45 normal subjects and 53 eyes of 53 patients with early glaucoma. The correlation of macula and PP retardation was assessed. The normal range for RNFL retardation was defined as 97.5th minus 2.5th percentile (normal subjects). This was calculated for uncorrected PP RNFL retardation and for PP RNFL retardation corrected by retardation measurements taken in the macula (analysis 1) and in the temporal aspect of the PP measurement annulus (analysis 2). Further ranges were defined at different percentile cutoffs, and normal and glaucomatous eyes were classified as abnormal if retardation measurements were below each cutoff. The accuracy of classification by uncorrected and corrected measurements was assessed by receiver operating characteristic curve analysis. Uncorrected and corrected RNFL retardation was correlated with visual field mean deviation (MD).
results. PP retardation correlated significantly with of macular retardation in normal (r 2 = 0.71, P < 0.000) and glaucomatous (r 2 = 0.41, P < 0.000) eyes. The normal range for uncorrected PP retardation was 25.4° and for corrected retardation, 18.0° (analysis 1) and 14.6° (analysis 2), a reduction of 29% and 43%, respectively. For a specificity of 85%, the sensitivity to identify glaucomatous eyes of uncorrected and corrected (analyses 1 and 2) retardation was 26%, 55%, and 66%, respectively. Corrected PP retardation measurements correlated better with visual field MD (analysis 1: r 2 = 0.21; analysis 2: r 2 = 0.18) than did uncorrected measurements (r 2 = 0.05).
conclusions. Erroneously corrected anterior segment birefringence significantly affects PP RNFL retardation measurements. Retardation arising from the cornea–corneal compensator interaction can be partially estimated from the macula and temporal aspect of the PP measurement annulus, allowing correction of PP RNFL retardation. This provides a narrower normal range and greater sensitivity for glaucoma diagnosis.
Glaucoma causes the death of retinal ganglion cells and their axons.
1 2 3 4 It is assumed that axonal loss results in a reduction in the thickness of the retinal nerve fiber layer (RNFL). Measurements of RNFL thickness should therefore be a reliable means of predicting the loss of visual function in glaucomatous eyes. Scanning laser polarimetry is a relatively recent innovation designed to allow in vivo measurement of the thickness of the RNFL.
5 6
The scanning laser polarimeter (SLP) is a confocal scanning laser ophthalmoscope with polarizing and polarization-analyzing optics. A laser beam is scanned across the fundus, and reflected light returns through the pupil to a detector. The theoretical basis of scanning laser polarimetry is that the state of polarized light is changed as it passes through a linearly birefringent material. Linear birefringence refers to the property of a material in which light polarized in one direction travels faster through the material than light polarized in the perpendicular direction (it has a “fast” and a “slow” axis). This difference in speed causes a phase shift between the perpendicular light beams (retardation). Principal ocular structures exhibiting birefringence are the cornea,
7 8 RNFL,
6 9 and Henle fibers,
10 with the cornea contributing the largest component to measurements of retardation in whole eyes.
10 The cornea, in fact, behaves as a biaxial crystal with three different indices of refraction (in the
x,
y, and
z directions).
7 11 However, the fastest principle axis is normal to the corneal surface so that, when light passes through the central cornea perpendicular to the surface, only the difference in refractive index between the
x and
y directions give rise to retardation.
7 11 The cornea, therefore, acts as a fixed retarder, with its slow axis nasally downward,
7 11 when light passes perpendicularly through the central corneal. In the parafovea, the symmetrical arrangement of Henle fibers acts as a uniaxial birefringent crystal with the slow axis arranged radially around the fovea.
10 The retardation arising when polarized light passes through the birefringent cornea and Henle fibers is explained by klein Brink and van Blokland.
10 Retardation arising from two linear retarders is added or subtracted, depending on the angle the slow axes of the retarders make with each other. Thus, as the beam passes around a circle in the perifoveal area, corneal and retinal retardations are alternately summed and subtracted, with troughs 180° apart and peaks 180° apart.
10
To quantify the component arising from the RNFL, it is necessary to neutralize the corneal component. A commercial SLP (GDx Nerve Fiber Analyzer; Laser Diagnostic Technologies, Inc., San Diego, CA), incorporates a proprietary “cornea polarization compensator” designed to cancel the polarization effects of the cornea.
12 When the fast axis of the compensator is aligned to the slow axis of the cornea, and the magnitude of birefringence is equal to the population mode, the effect of corneal birefringence is neutralized. If the fast axes of the two are aligned, the birefringent effects are additive. Thus, if the slow axis of the cornea is rotated (in either direction) and out of alignment with the fast axis of the corneal compensator, measured retardation from the combined cornea and corneal compensator is greater than zero. The corneal compensator has a fixed optic axis and a fixed magnitude of retardation, so that the corneal component of retardation is eliminated only if the position of the slow axis and magnitude of retardation of the cornea in the eye being imaged are identical with the mode of the population. However, if either the axis or magnitude of retardation of the cornea lies outside the normal range, retardation resulting from the interaction of the cornea and the compensator manifests in measurements made from the fundus. Because the retardation pattern arising from the Henle fibers around a fovea-centered annulus in the perifoveal retina is radially symmetrical,
10 any modulation of the measured retardation around an annulus in the perifoveal retina must result from uncompensated, or erroneously compensated, retardation elsewhere in the laser pathway. The slow axis of corneal birefringence is typically 15° nasally downward.
7 11 Greenfield et al.
11 established that there is considerable interindividual variation in the axis of corneal birefringence and demonstrated that the corneal axis is significantly associated with RNFL and macular summary retardation parameters. Furthermore, they suggested that the macular measurements might be helpful in estimating the corneal polarization axis. Knighton et al.
13 have shown that there is also a wide variation in the magnitude of corneal retardation. Recently, Zhou et al.
14 have demonstrated that the axis and magnitude of anterior segment retardation estimated from the macular retardation pattern agrees well with direct measurement of the same variables, using a corneal polarimeter (although the magnitude of retardance was generally lower when measured from macular polarimetry). The corneal polarimeter enables a view of the fourth Perkinje image through crossed polarizers and a variable waveplate.
13
Retardation measured from the macula represents the combined effects of the birefringence of the corneal compensator, the cornea, and the radially arranged Henle fibers. Because the axons of the RNFL are approximately radially arranged in the peripapillary retina, the interaction of the corneal compensator–corneal birefringence with peripapillary RNFL birefringence may be similar to that occurring in the macula. It may therefore be possible to use the macular retardation pattern to correct the peripapillary measurements for erroneously compensated corneal birefringence.
The purpose of this study was to apply this empiric approach to determine whether the ability of peripapillary RNFL measurements to discriminate between normal and glaucomatous eyes can be improved by correcting the peripapillary retardation measurements with retardation measurements made in the macula.
Forty-five eyes of 45 normal subjects and 53 eyes of 53 subjects with glaucoma were imaged with the SLP. Normal subjects were members of staff, friends or spouses of patients, or volunteers, attending the Glaucoma Division of the Jules Stein Eye Institute, Los Angeles (37 subjects), and the Glaucoma Research Unit of Moorfields Eye Hospital, London (8 subjects). Inclusion criteria were ametropia less than 5 D, visual acuity of 20/30 or better, normal visual fields, intraocular pressure of less than 21 mm Hg, no previous history of ocular disease, and no family history of glaucoma involving a first-degree relative. All subjects performing a normal field test were included, regardless of optic disc appearance. One eye was included in the study, chosen at random if both were eligible.
Subjects with glaucoma were patients attending the Glaucoma Division of the Jules Stein Eye Institute, Los Angeles (51 subjects), and the Glaucoma Research Unit of Moorfields Eye Hospital, London (2 subjects). Restriction criteria were ametropia less than 5 D, visual acuity of 20/30 or better, a reproducible visual field defect, normal open anterior chamber angle, and no other disorders that might cause visual field loss. One eye was included in the study, chosen at random if both were eligible.
All normal subjects and patients with glaucoma had consented to take part in prospective research into the early detection of glaucoma with new imaging technology at their respective institutions. Research protocols had been approved by the relevant institutional ethics committees and conformed to the tenets of the Declaration of Helsinki.
All visual field testing was performed with the Humphrey Field Analyzer (model 640 or 750; Allergan Humphrey, San Leandro, CA) and the 24-2 full-threshold or 24-2 Swedish interactive test algorithm (SITA) standard programs. Reliability criteria applied were fixation losses less than 30%, false-positive responses less than 15%, and false-negative responses less than 30%.
A glaucomatous visual field was defined as two or more contiguous points with a
P < 0.01 loss or greater, or three or more contiguous points with a
P < 0.05 loss or greater, in the superior or inferior arcuate areas, compared with perimeter defined age-matched control subjects, or a 10-dB difference across the nasal horizontal midline at two or more adjacent locations.
15 In addition, all patients had abnormal findings in a glaucoma hemifield test (Allergan Humphrey).
A normal visual field was taken to be one in which there were no sensitivity losses matching the criteria for glaucoma.
Measurements given by the SLP software are in micrometers of RNFL thickness. The RNFL thickness has been converted back to degrees of retardation, with the conversion factor of 3.0 μm per degree (Zhou Q, Laser Diagnostic Technologies, written communication, November 2001).
Data were analyzed by computer (Excel 97 SR-2; Microsoft Corp., Seattle, WA; and SPSS for Windows, ver.10.0.0; SPSS Science, Inc, Chicago, IL). Comparisons between data from the control and glaucoma groups were made by means of a two-tailed t-test, assuming unequal variance when the data were distributed normally. When the data were not normally distributed, the Mann-Whitney test was used. Measurement data before and after adjustment for corneal birefringence were compared by means of a paired t-test. P ≤ 0.05 was considered statistically significant.
The relationship between retardation measured in the macula and peripapillary region and between peripapillary retardation and visual field mean deviation (MD) was assessed by linear regression analysis. Significance was assumed at P < 0.05.
Analysis 1.
Comparisons were made between normal and glaucomatous eyes to determine differences in mean retardation around the perifoveal and peripapillary annuli and differences in the retardation modulation (maximum value minus minimum value for 10° sectors) around the perifoveal and peripapillary annuli and the modulation of the measure “peripapillary minus perifoveal retardation.”
The association of the mean magnitude of retardation around the perifoveal annulus and the mean magnitude of retardation around the peripapillary annulus was explored by linear regression analysis. Similarly, the association of the magnitude of retardation modulation around the perifoveal annulus and the magnitude of retardation modulation around the peripapillary annulus was explored by linear regression analysis. The analyses were performed separately in the normal and glaucomatous eyes.
A series of normal ranges for retardation measurement around the peripapillary annulus was defined from the normal eyes at cutoff levels ranging from the 1st to the 50th percentiles in each 30° segment. A normal or glaucomatous eye was categorized as abnormal if the retardation measurement fell below the cutoff level in any 30° segment.
The macular retardation measurement in each 30° segment was then subtracted from the retardation in the equivalent peripapillary 30° segment in all eyes, to give an adjusted retardation value. A second series of normal ranges was defined from cutoffs at the 1st to the 50th percentiles. Similarly, a normal or glaucomatous eye was categorized as abnormal if the retardation measurement declined below one of the percentile cutoffs in any 30° segment.
The categorization of eyes at the cutoffs from the 1st to 50th percentiles was used to generate receiver operating characteristic (ROC) curves.
Analysis 2.