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Optic
disc imaging forms an essential part of the management of glaucoma
suspects and patients with established glaucomatous visual field
loss. The widest application of optic disc imaging is in glaucoma
management and is the perspective of this review.
 Technological
advances have brought imaging devices into clinical practice, and
these offer considerable advantages over previous methods of recording
the appearance of the optic disc, such as drawings and monoscopic
photographs. In this review, the various forms of imaging are outlined
and their clinical application in diagnosis and management is condiered.
The
various forms of imaging permit quantitative measurement of optic
disc and retinal nerve fibre layer (RNFL) structure. There are potential
advantage of quantiative imaging over perimetry, particularly in
early disease process [1].
A number
of different instruments, each making use of different optical principles,
has been introduced over the last 15 years. The technologies are
continually evolving and each is at a different stage of development
and clinical evaluation.
Stereoscopic
photography
The
only CE marked, dedicated stereoscopic optic disc camera available
in the UK is the Discam (Marcher Enterprises Ltd). Stereoscopic
image pairs are taken in succession at video frame rates. Newer
instruments are full colour and this is an advantage over all forms
of scanning imaging devices (eblow). The field of view is 12 and
pupil dilatation is required for imaging. The images provide a high
magnificaiton, stable picture that can be easier to evaluate than
the image obtained with indirect ophthalmoscopy. New software enables
an observer to make magnification-corrected measurements of optic
disc features. The measurements are, however, subjective, and have
greater between-observer variability than the semi-automated scanning
devices.
Scanning
laser tomography
This
technology, in the form of the Heidelberg retina tomograph (HRT,
Heidelberg Engeineering GmbH), has been available for around 10
years. A compact version (the HRT II) has been released more recently
for clinical use. The field of view is 15 and imaging can be performed
through an undilated pupil. Images are monochromatic and the confocal
optics enable the determination of a surface height map (topography).
The margin of the optic disc is outlined by an observer and a reference
plane is positioned parallel to the surface and set below the surface
[2]. Structures that lie within the disc margin (contour) and above
the reference plane are denoted as neuroretinal rim. Space below
the reference plane is denoted as optic cup (Figure 1).
Scanning
laser polarimetry
This
first prototype of this instrument was developed about 10 years
ago, and was first released commercially as the GDx Nerve fiber
analyzer (Laser Diagnostic Technologies Inc). The second generation
product is called the GDx Access. The field of view is 15 and imaging
should be performed through an undilated pupil. The polarised laser
scans the fundus, building a monochromatic image. The state of polarisation
of the light is changed (retardation) as it passes through birefringent
tissue (cornea and RNFL). Corneal birefringence is eliminated (in
part) by a proprietary 'corneal compensator'. The amount of retardation
of light reflected from the fundus is converted to RFNL thickness.
Sub-optimal compensation of corneal birefringence is currently being
addressed by the manufacturer with hardware and software modifications.
Low-coherence
interferometry
The
first commercial application of this technology was released by
Humphrey Instruments (now Zeiss Humphrey Systems) in 1995, as the
Optical coherence tomography scanner. Second and third generations
have been produced, giving faster scanning and greater depth resolution.
The principle is analogous to B scan ultrasonography, using a light
source instead of sound. Imaging is performed through a dilated
pupil. The OCT 3 performs a linear scan on the retina with a near
infrared (low coherence) light beam. The depth resolution is 10
µm. OCT software locates borders (changes in reflectivity)
such as the vitreoretinal interface, the interface between RNFL
and inner retinal layers, and the outer retina/choroid interface.
Laser
optical cross-sectioning
The
commercial instrument utilising this principle is the Retinal thickness
analyzer (RTA, Talia Technology Ltd). The RTA projects a narrow
slit of green laser light at an angle on the retina and acquires
an image from a different angle on a digital camera. An optical
cross-section of the retina is seen, with reflectance peaks that
correspond to the RNFL/inner limiting membrane and the retinal pigment
epithelium. The software measures the distance betweenthe peaks
to obtain retinal thickness. The macula, peripapillary area and
optic disc may be scanned. Software to derive an optic disc topography
has also been developed.
The
clinical application of imaging is both for the diagnosis of glaucoma
and the detection of progressive disease. Illustrations will be
made with examples from one of the more mature technologies: HRT.
The other instruments may have a significant clinical role as they
are developed further.
Diagnosis
 None
of these instruments, used on its own, is diagnostic. They provide
measurement information that should be integrated with other clinical
information, such as intraocular pressure level and visual field
status.
The
instruments have a database of measurements from normal eyes. The
structural measurements are related to normative data in the same
way that visual field sensitivity is related to normative data in
perimetry. Classification is purely statistical and thresholds for
abnormality should be considered only as levels of probability.
Abnormalities other than glaucoma, such as tilted discs, may cause
measurements to fall outside the normal range. There is an overlap
of measurements between normal and glaucomatous eyes, so that classifications
such as 'within normal limits', 'borderline' and 'outside normal
limits', as seen in the HRT II (Figure 2) and GDx software, are
appropriate.
With
the Moorfields classification [3], approximately 80% of normal eyes
are identified as 'within normal limits' and 7% as 'outside normal
limits'. Approximately 67% of eyes with early glaucoma are 'outside
normal limits' and a further 20% are 'borderline'. Studies comparing
HRT, GDx and OCT have found that their ability to discriminate between
normal and glaucomatous eyes is generally similar [4, 5]. The GDx
performed slightly l ess
well [4]. However, it is anticipated that improved compensation
for corneal birefringence will result in an improved discriminating
ability.
Progression
The
greatest potential use of imaging instruments is in the detection
of glaucomatous progression. The good reproducibility of measurement
data increases the sensitivity of these instruments to detect progression.
Approaches to the statistical treatment of measurement data include
a 'change probability' analysis for surface height measurements
[6], similar to the 'change probability' in the Statpac software
for Humphrey perimetry.
It
is also possible to apply trend analysis to measurements, such as
neural rim area, made at different points in time (Figure 3). The
potential advantage of this form of analysis is that it gives an
estimate of the rate of change.
Mr
David Garway-Heath
Consultant Ophthalmologist, Moorfields Eye Hospital
References
1.
Chauhan BC, McCormick TA, Nicolela MT, LeBlanc RP. Optic disc and
visual field changes in a prospective longitudinal study of patients
with claucoma: comparison of scanning laser tomography with conventional
perimetry and optic disc photography. Arch Ophthalmol.
2001; 119: 1492-1499.
2. Burk RO, Vihanninjoki K, Bartke T, et al. Development
of the standard reference plane for the Heidelberg retina tomograph.
Graefes Arch Clin Exp Ophthalmol. 2000; 238: 375-384.
3. Wollstein G, Garway-Heath DF, Hitchings RA. Identification of
early glaucoma cases with the scanning laser ophthalmoscope. Ophthalmology.
1998; 105: 1557-1563.
4. Zangwill LM, Bowd C, Berry CC, et al. Discriminating between
normal and glaucomatous eyes using the Heidelberg Retina Tomograph,
GDx Nerve Fiber Analyzer, and Optical Coherence Tomograph. Arch
Ophthalmol. 2001; 119: 985-993.
5. Greaney MJ, Hoffman DC, Garway-Heath DF, et al. Comparison
of optic nerve imaging methods to distinguish normal eyes from those
with glaucoma. Invest Ophthalmol Vis Sci. 2002; 43:
140-145.
6. Chauhan BC, Blanchard JW, Hamilton DC, LeBlanc RP. Technique
for detecting serial topographic changes in the optic disc and peripapillary
retina using scanning laser tomography. Invest Ophthalmol Vis
Sci. 2000; 41: 775-782.
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