Ординатура / Офтальмология / Английские материалы / Eye Essentials Assessment and Investigative Techniques_Doshi, Harvey_2005

Advanced instrumentation

164 Introduction

Advances in technology have led to improvements in methods of measuring and assessing ocular structures.The increasing use of the laser has developed alongside microprocessor improvements allowing miniaturization of hardware and improved software capability. Increasingly, much of the optometric assessment may be automated and digital encoding of clinical data allows accurate storage, measurement and analysis.While this chapter is necessarily a brief overview of such technology (both because

of the limited space in a book such as this as well as the rapid advances making in-depth discussion rapidly outdated), it is important to recognize the increasing role of technology in optometry.

Scanning laser ophthalmoscopy

The confocal scanning technique was introduced around 1980 by Petran and Boyde and has been adopted in many clinical and biological fields.The basic principle involves the production of a collimated polarized laser beam which is deflected stepwise across the two x and y-directions by a scanning unit.The

continually deflecting beam is then reflected by a dichroic mirror (the beamsplitter) to pass through the objective lens which focuses the light to the point at the level to be viewed. Light emitted from the target reaches the beam-splitter again which remains transparent to the selected wavelength from the target point, while eliminating all out-of-focus light (from points above or below the target point).The x-y deflection allows this to be done for all points in a plane at the established level and, by doing this for a variety of planes, a sequence of ‘slices’ through the object viewed may be imaged.The wavelength of the beam will be important in terms of its penetration of whatever is being viewed.

The main advantage of this system is that it can be used to carry out retinal investigations in ways that enable the

Scanning laser ophthalmoscopy

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Figure 10.1 The Optos Panoramic 200

ophthalmologist or optometrist to gain more information about the retina than can be obtained from conventional imaging techniques.

The Optos Panoramic 200 (Figure 10.1) is designed to enable an area of up to 200° of the fundus to be produced as one image and without the use of collaging or pupil dilation.The SLO tends to be superior to standard imaging methods when cataract is present and can be used for both fluorescein and indocyanine green angiography.

The large field image produced by the Panoramic combined with the unfamiliar coloring often takes some experience to interpret. Indeed the instrument initially led to an increase in referrals of eyes subsequently found to be within healthy bounds. However, the improved view of the peripheral fundus (Figure 10.2), remarkably in an undilated eye, has improved the clinician’s ability to detect and manage peripheral retinal disease.

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Figure 10.2 Improved view of the peripheral fundus by the Panoramic 200 allows greater accuracy in detecting peripheral lesions

Scanning laser polarimetry

By its simplest definition, a polarimeter is an instrument capable of measuring either the polarity (unidirectional property of light) or rotations to that polarity. As light passes through a structure, the nature of the change to the light gives information about the properties of that structure.

Layered structures may exhibit properties not found when the individual materials are assessed. An example of this is when

waves traversing a medium, with their electric fields perpendicular to the medium, experience a different index of refraction than the waves parallel to the plane of the medium.This difference is due to the varying boundary conditions imposed on the electric fields parallel and perpendicular to the interfaces between the different layers.This property is known as birefringence and forms the basis of the operation of the instrument known as

the GDx (the GDx Access is the model used in modern practices).

Scanning laser polarimetry

The retinal nerve fiber layer shows birefringence of an incident

laser and the influence that it has upon the incident confocal laser 167 beam from the instrument is interpreted to infer the thickness of

the layer.This results in the nerve fiber layer thickness profiles shown in the Figure 10.3.

Figure 10.3 Nerve fiber layer thickness profiles produced by the GDx (reproduced with permission from Zeiss)

Advanced instrumentation

Because the assessment is made with measurement compared to 168 a reference plane, the technique is not affected by refractive error.

As might be expected, one problem in the development of such a technique has been that birefringence occurs as light passes through the cornea and this must be compensated for if an accurate assessment of the nerve fiber layer is to be achieved.The GDx and GDx Access include such a compensator and this is usually set at a default appropriate for a standard cornea (an 80% setting is typical). If the compensation is not accurate, this may be predicted by unusual data that are not borne out by other assessment.

Much has been published in recent years concerning the accuracy of the GDx unit and nerve fiber analysis.The fact that changes to the NFL may be a useful early indicator of glaucomatous change is well-established and many ophthalmologists have for years used bright or red-free light assessment of the peripapillary area to look for deviations in the way the nerve fibers reflect light. A wedge-shaped area of less reflection would, for example, be treated as suspicious.

The use of NFL analysis as a tool in the effective screening of glaucoma has been found to have a high sensitivity and specificity. More recently, the use of data about the superior to nasal and the inferior to nasal NFL data ratios has been confirmed as effective and sensitive in differentiating between glaucomatous and nonglaucomatous eyes.

There is also evidence to confirm NFL differences may be demonstrated in the eyes of ocular hypertensives.When the technique was assessed to determine its diagnostic accuracy, one study has found that it was effective at differentiating between normal patients and those with glaucomatous damage. However, even the best algorithm tested failed to detect a substantial number of subjects with severe damage.

Bearing this in mind, it seems as if the GDx provides a useful screening assessment of the NFL which makes it potentially valuable to an optometrist wanting a sensitive indicator of potential for glaucoma. It is also useful for monitoring glaucomatous NFL changes over a period of time. It does not, however, replace accurate disk analysis nor does it remove the need for pressures and fields to be monitored.

Scanning laser tomography

The accuracy of observation of an ocular structure, such as the optic disc, will be related to the skill and experience of the observer. Accurate photography of the disc allows a more accurate quantitative measurement of two-dimensional data, but the ability to accurately represent a three-dimensional model of the disk is yet more valuable. An instrument which might somehow store an accurate three-dimensional representation of the disk architecture could usefully detect any parameters which are outside an expected norm and would also be able to detect accurately any changes to the disk over a period of time.

When the Heidelberg Retinal Tomograph (HRT) was introduced in 1991, it was described as ‘the first scanning laser system for routine glaucoma exam’. Its cost and bulky size proved prohibitive to eye care professionals in general practice, but as a hospital-based research tool able to analyze and store accurate topographic information, it formed the basis of multiple research papers.Topographical analysis using a confocal technique, of course, meant the HRT was useful not only in glaucoma research but also in the investigation of other structural change diseases such as macular holes, macular edema, retinal detachments and neoplasms.There was also some application in the investigation of the atrophic changes in non-exudative macular degeneration.

In effect, the laser is focused and scanned rapidly across a common plane running through the optic nerve head. Elevation or depression of the optic disc relative to this flat plane is measured so allowing an accurate topographic representation of the head to be stored.The software may then predict whether the topography is normal or suspicious of glaucoma, and sequential assessment of a disk over a time period may very sensitively detect changes in disk topography as would occur with a chronic optic neuropathy.

In 1998 a team at Moorfields Eye Hospital completed a study which aimed to define the HRT parameters that best allow it to separate patients with early glaucoma from normal subjects. A total of 131 patients were examined, 51 of whom had glaucoma

Advanced instrumentation

leading to field loss (averaging a mean deviation of –3.6 dB).

170Data were taken from the normals regarding the relationship between the neuroretinal rim area and the optic disc area, and the cup/disc ratio and the optic disc area.The normal ranges

for these two parameters were defined mathematically (in terms of the 99% prediction intervals of the linear regression between the parameter and the optic disc area) for the whole disc and

for each of six predefined segments of disc. It was found that a high sensitivity (96.3%) and specificity (84.3%) in separating the normals from the glaucoma patients was achieved, and that the technique of linear regression provided a good separation method, the best figures being found when the log of the area of the neuroretinal rim was compared to the optic disc area.

A year later the same team published a study with the aim of trying to determine if an analysis of sequential optic disc images using the HRT would be able to demonstrate disk changes in ocular hypertensives before the development of reproducible field defects.They found that there was evidence of disc data recorded by the HRT showing changes over a 1 year period before any field loss was established.This suggested the machine might usefully play a role in glaucoma detection in an at-risk group prior to any field loss.

The HRT II was introduced in 1999 with the aim of providing the benefits of accurate laser scanning techniques in a more clinically friendly form and the instrument itself is similar to a small table-mounted slit-lamp unit with a separate computer and monitor (Figure 10.4).The basic unit itself can he dismantled easily and hence is readily portable.

It is marketed as an instrument for aiding the early detection and the monitoring of glaucoma. However, adapted software is available for analysis of macular edema, retinal analysis and corneal analysis (see below).The instrument contains a diode laser emitting at a wavelength of 670 nm (class 1 laser).The spread allows a field of view of 15° to be analyzed.

Figure 10.5 shows a reading taken from a diagnosed glaucoma patient.The six red crosses clearly demonstrate that the disk profile falls outside the expected norm in all six sectors.

Optical coherence topography

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Figure 10.4 The HRTII from Clement Clarke may come to be regarded as an essential instrument for screening for glaucoma, particularly in shared or managed care schemes (reproduced with permission from Doshi & Harvey Investigative Techniques and Ocular Examination, Butterworth-Heinemann 2003)

Optical coherence topography

As far back as 1878, Albert Michelson hypothesized that, if it were possible to split a beam of light into two parts and then transmit them along perpendicular paths, it might be possible upon receiving the returning beams to detect any differences in phase between them.This difference or interference would give valuable data about the surfaces from which the light has been reflected.This was the basis of the Michelson Interferometer, and is now the basis of the Zeiss Stratus OCT (Figure 10.6).

In this instrument, a light beam is sent simultaneously to the eye and a reference mirror.The light penetrates through retinal tissues ad is reflected back.The returning light is compared to the reference and this allows software to reconstruct a representation of the underlying tissues.This ability to, in effect, show a cross-section through retina makes the machine invaluable in monitoring lesions such as the macular hole shown in Figure 10.7.

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Figure 10.5 Reading taken from a diagnosed glaucoma patient. HRT II data