Optic Nerve: Comparison of Technologies

Core Messages

››Each optic nerve imaging technology has its unique strengths and weaknesses.

››Each technology is in evolution to minimize it’s limitations.

››Frequency of imaging will be dictated by variables of the individual patient, such as stage of glaucoma or suspicion of progression.

6.1  Why Image the Optic Nerve?

Optic disc evaluation is of utmost importance in early glaucoma diagnosis and in monitoring progressive nerve damage. Optic disc and retinal nerve fiber layer (RNFL) abnormalities will often precede corresponding functional loss in glaucoma [1–3]. Therefore, it is imperative to have an objective, quantitative, and reproducible imaging technique capable of making an early diagnosis and monitoring the disease.

There are many imaging techniques available for optic disc and RNFL evaluation in glaucoma. Confocal scanning laser ophthalmoscopy (HRT; Heidelberg Retina

Tomography; Heidelberg Engineering, Heidelberg, Germany), scanning laser polarimetry (GDX; Carl Zeiss Meditec, Dublin, California, USA), and optical coherence tomography (OCT; Carl Zeiss Meditec, and others) are widely used among glaucoma specialists, but the

C. Souza (*)

Ophthalmology Department, Federal University of Sao Paulo, Sao Paulo, Brazil

e-mail: ce.bsouza@uol.com.br

mainstay of clinical practice remains subjective optic disc evaluation with stereo optic disc photography.

6.1.1  Confocal Scanning Laser

Ophthalmoscope

The HRT is a confocal scanning laser ophthalmoscope with high axial optical resolution that uses a diode laser (670 nm wavelength) to sequentially scan the retinal surface at multiple focal planes. Studies have demonstrated good reproducibility between different examiners; however, clinically significant variations do exist between examiners because of the differences in how individuals draw the contour line around the optic nerve [4, 5]. Good correlation among stereo photographs (Figs. 6.1 and 6.2), planimetry, and functional tests such as automated perimetry has also been found [6, 7].

The confocal scanning laser ophthalmoscope is currently in its third generation. The new HRT-3 software (Heidelberg) features improvements in image scaling and alignment, a new diagnostic classification system, and an expanded normative database. The new shapebased analysis (the Glaucoma Probability Score) does not require an examiner to draw a contour line around the optic disc, which decreases the inter-operator variability, and it is independent of a reference plane position­

[5, 6]. The HRT-3 software has a new, larger normative and ethnic-specific database that adjusts to age-related changes in the optic disc and optic disc size, with perhaps a higher accuracy in the analyses provided by the instrument [7]. The new scaling and alignment algorithm improve the ability to measure stereometric parameters such as area and volume based measurements, height variation contour, and RNFL cross-sectional area. They also improve the progression analyses [8, 9]. The two

Fig. 6.2  The stereo photograph of the patient in Fig. 6.3 showing inferior, superior, and temporal rim thinning in the left eye

different diagnostic classification systems available, Moorfield’s regression analysis (MRA) and Glaucoma Probability Score (GPS), have a similar capability to discriminate between normal and glaucomatous patients [6–­8]. In addition to MRA and GPS, two linear discriminant functions (LDFs) [proposed by F.S. Mikelberg (FSM) and R. Bathija (RB)] were included in the new HRT-3 software [8, 9]. There are currently two progression algorithms on the HRT-3 software: trend comparison analysis (TCA) and ‘trend analysis’ (see Chap. 3 for fuller discussion). The HRT-3 has an advantage over other imaging technologies in that it is compatible with earlier software versions of itself (HRT-2 and HRT-1), and therefore it is possible to analyze HRT-3 images with prior versions of the instrument. This allows glaucoma progression to be detected over a much longer period of time, which is a real advantage in longitudinal studies [8].

6.1.2  Optical Coherence Tomography

(OCT)

The OCT is a high-resolution imaging technique based on the optical principle of low-coherence interferometry. It is capable of providing cross-sectional images of ocular structures [10–12]. OCT works by measuring the time delay difference between laser light reflected at various retinal layers and a reflected reference beam. The Stratus OCT (Carl Zeiss Meditec, Inc.) has an axial resolution of approximately 8–10 mm [13]. The OCT can scan the peripapillary retina (RNFL scan), the optic nerve head (ONH), and the

macular region. Peripapillary RNFL scans are obtained using a fixed circle of 3.4 mm diameter centered on the optic nerve. To measure the RNFL thickness, the OCT first determines the retinal boundaries (vitreoretinal interface and retinal pigment epithelium [RPE]). The algorithm to detect the boundaries remains imperfect, especially in cases in which the RNFL reflectance is low, such as in severe glaucoma. A good quality image is essential for correct interpretation of the exam; a focused fundus image with the circle centered on the optic disc is necessary for correct RNFL thickness measurements. A minimal acceptable signal-to-noise ratio is six. A normative database is available with Stratus OCT; however, ethnicity and age under 18 years were not included as variables in constructing the database.

There are many studies demonstrating the reproducibility of OCT RNFL thickness measurements [11–13]. Several studies confirmed that RNFL thickness is useful to distinguish mild-to-moderate glaucoma patients from healthy subjects (Figs. 6.3 and 6.4) [14–16]. Optic nerve head parameters appear to be less useful and reproducible than RNFL thickness in detecting glaucoma, and they are limited by the lack of morphometric data. Age, ethnicity, axial length, and refractive error (myopia) are known to affect the RNFL thickness measurements [17]. As seen in some studies, optic disc size also appears to affect the measurements, although not confirmed in a study performed by Hougaard and colleagues [18]. Movement artifacts, media opacity, high myopia, and severe glaucoma can limit OCT scan measurements and reproducibility [16-18]. Good correlation between other imaging techniques and functional tests, such as automated perimetry, has been described [19]. Compared to other imaging techniques, OCT has the strongest structurefunction correlation and can possibly detect structural abnormalities before visual field defects. The new software for the Stratus OCT provides a statistical analysis program (GPA) to facilitate the detection of progression by comparing the RNFL thickness over time. The next generation – ultra-high speed, ultra-high resolution OCT (so called Fourier or Spectral Domain OCT, which is currently being marketed by a number of companies) – with three-dimensional views will attempt to improve the limitations of the Stratus OCT and may enhance the early glaucoma diagnosis and progression detection [17]. Studies on this newest generation of OCT are in progress.