Fig. 6.4  The stereo photograph of the patient imaged in Fig. 6.1 showing a wedge retinal nerve fiber layer defect in the inferior quadrant of the left eye

6.1.3  Scanning Laser Polarimetry (SLP)

Scanning laser polarimetry is a technique used to evaluate the peripapillary RNFL thickness based on the birefringent properties of the retinal nerve fibers, which induce a change in retardation of polarized light in proportion to their thickness [20]. Birefringence is described in terms of polarization axis and polarization magnitude. In the anterior segment, the cornea and lens are also birefringent and may affect the measurements.

The commercial version of SLP is the GDX. It is currently in its fifth generation and is equipped with a variable cornea compensator (VCC) that allows eye-specific compensation of anterior segment birefringence. Studies have demonstrated good correlation between SLP measurements and RNFL thickness [21–24]. Higher correlations were described for the inferior and superior quadrants. GDX tends to underestimate the RNFL thickness in the nasal and temporal sectors [24]. Despite not measuring the RNFL thickness accurately in all sectors, studies have demonstrated the utility of SLP in clinical practice to discriminate between healthy and glaucomatous eyes [23–25]. The most highly discriminating parameter on the GDX device is the nerve fiber indicator

(NFI) – a support vector machine-derived parameter trained to discriminate between healthy and glaucomatous eyes. Recently, several advances have been made in the GDX. A new measurement algorithm that improves the signal-to-noise ratio of measurements by reducing the presence of atypical retardation patterns has been developed. Atypical retardation patterns may be present in 15–51% of glaucomatous eyes, [23, 24] and are more frequently observed in older subjects and in high myopia. In order to minimize the atypical retardation patterns, a new algorithm known as enhanced corneal compensation (ECC) that improves the signal-to-noise ratio by extracting the retinal retardation mathematically from the total retardation image has been developed [21, 24]. The new ECC algorithm increases the ability of SLP to discriminate between healthy and glaucomatous patients, especially in those cases with high atypical retardation patterns and moderate-to-high myopia [20, 21, 24]. Studies show that the SLP-ECC is at least as reproducible as the SLP–VCC [20, 21, 23]. ECC improves the correlation between visual function and RNFL measures in SLP.

Summary for the clinician

››Evaluation of the optic nerve is of the highest importance in making a diagnosis of early glaucoma, and thus a quantitative evaluation of the optic nerve is highly desirable.

››The newest generation confocal scanning laser ophthalmoscope has improved image scaling and alignment, a new classification system, and an expanded normative database that includes different ethnicities. Its greatest strength is that the newest software is compatible with previous versions, expanding its ability to longitudinally study an individual nerve.

››Optical coherence tomography RNFL thickness measurements provide the strongest structure-function correlation of all imaging devices. The normative database does not include ethnicity at this point in time and measurements may be affected by high refractive error, media opacity, and severe glaucoma.

››Scanning laser polarimetry estimates the RNFL, and new measurement algorithms have improved the signal-to-noise ratio and anterior segment birefringence over previous versions.

6.2  Is One Optic Nerve Imaging Technique Better or More Promising than the Others for Helping to Detect Glaucoma and Its Progression?

There is enough data in the literature to support the use of imaging technology as a complementary tool to clinical evaluation in glaucoma diagnosis and monitoring [6, 8, 10, 24–27]. All imaging technologies have their inherent advantages and limitations. Limita­tions include low reproducibility in high refractive errors, and with large or small optic disc sizes, inaccurate reference planes or presence of atypical retardation patterns in some imaging technologies, and an inability to detect some optic disc abnormalities such as disc hemorrhages. The data suggest that objective imaging technologies and subjective assessment of stereo optic disc photographs by experts are similar in their ability to identify early glaucoma [25, 26]. However, imaging technologies have the following advantages: (1) they provide objective and quantitative measurements of the optic disc and RNFL and (2) there is less variability between observers when examining printouts.

All available technologies have proven ability to discriminate well between healthy and glaucomatous nerves. Many studies that evaluate and compare the different technologies have concluded that they all perform similarly in making the diagnosis of early glaucoma, while other studies have demonstrated that one technology may outperform the others in detecting early optic disc and RNFL abnormalities. Medeiros et al. showed that RNFL imaging with GDX VCC showed superior performance compared to HRT for detecting early damage in patients suspected of having glaucoma [20]. In another study, Caprioli et al. concluded that OCT might detect glaucomatous damage earlier than other imaging techniques or clinical evaluation of optic disc photographs in eyes with normal visual fields [19].

All technologies have high reproducibility, good correlation with structural (stereoscopic disc photography) and functional (visual fields) tests, and might be useful in early glaucoma diagnosis and monitoring. The OCT is believed to have the strongest correlation with visual field findings and is able to detect early changes in the RNFL before it is recognized by standard visual field exams [19]. But the OCT technology has the drawback of not having a normative morphometric database and statistical analysis for optic disc evaluation. It does have a normative database for RNFL

thickness evaluation, which is its best performing para­ meter and the most reproducible of all OCT measurements. Another limitation of the OCT is that newer versions are not compatible with older versions of the instrument, making it more difficult to carry on a longitudinal evaluation for detection of progression. A study performed by Wollstein et al. showed that the OCT performed better than visual field tests to detect progression, although the specificity has yet to be established [15]. It is a promising technology, especially with the new versions (ultra-high speed and ultrahigh resolution) yet to come [17].

The new HRT-3 is another promising technology with its improvement in diagnostic accuracy and enhan­ cement of image scale and alignment [8, 9]. It has the strength of having a normative, ethnic-specific database and statistical analysis program available to help in glaucoma diagnostics and monitoring. The HRT-3 has a new diagnostic classification system independent of the contour line tracing (i.e.,it is operator independent) and reference plane [5, 6, 8]. It also has different statistical analysis algorithms to detect glaucoma progression. It is a promising technology in detecting and quantifying glaucoma progression.

The SLP with the new ECC algorithm increases its ability to discriminate between healthy and glaucomatous nerves, increases its correlation with functional tests, and reduces the effect of atypical retardation patterns,­ making it a promising technology to help in early glaucoma diagnosis [20–24]. However, it does not have a statistical analysis program to detect glaucoma progression.

In summary, all imaging technologies, especially the OCT and SLP, are promising to help the ophthalmologist in early glaucoma diagnosis, while HRT is promising to help detect and quantify glaucoma progression.

Summary for the Clinician

››At the present time, OCT and SLP are best for detection of RNFL defects and most promising for the early detection of glaucoma.

››In its current state, HRT is most promising to detect and quantify progression.

››All imaging technologies are highly reproducible in most eyes and show correlation with disc photographs and functional tests.

››All imaging technologies are excellent complementary tools in the diagnosis and monitoring of glaucoma.