Anterior confocal microscopy

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Figure 10.6 Zeiss Stratus OCT

Figure 10.7 Macular hole image produced by ocular coherence tomography

Anterior confocal microscopy

Confocal microscopy has been a familiar technique in university and hospital research departments, allowing a practitioner to carry out cell counts, and monitor minor structural changes. Such work on the endothelium is the basis of much of our knowledge about how this structure changes with age or with

Advanced instrumentation

external insult (such as hypoxic stress) and much significant

174knowledge has been gained. Other research, such as looking at the effects of topical therapeutic agents, contact lenses and developmental variations may all be assessed in vivo with a significant advantage over in vitro techniques.

Like many other areas of optometric technology, the advances in refractive surgery have led to an increased demand for good resolution yet easy to achieve confocal corneal images. One of the best indications of postoperative corneal recovery (either for a photorefractive treatment such as Lasik or phototherapeutic intervention such as graft surgery) is to look for changes to cellular structure within the cornea, or to visualize debris build-up. For example, a high-resolution confocal view of the stroma post-Lasik may allow changes to the keratocytes to be detected, as well as increases in cell debris or corneal nerve changes. Doing this at intervals after surgery gives an excellent indication of corneal recovery and is an integral part of most refractive surgeons’ follow-up routine.

The technique also has a more noniatrogenic use, however, in the detection and monitoring of corneal disease. For example, the Langerhans cell population in the plane of the basal cell layer near the limbus is useful in assessing the keratoconic, particularly after a full-thickness graft. Cellular changes and guttata may be detected at an early stage in Fuch’s dystrophy. Microcystic response, either related to hypoxic insult or keratitis, may be monitored with much greater accuracy than a slit-lamp would afford. It is even possible to analyze erythrocyte flow in a neovascular response. Furthermore, the increasing use of mitomycin drugs in corneal recovery may be monitored. In summary, the confocal microscope adds an extra dimension

to corneal assessment over and beyond the slit-lamp and, as such, is an increasingly important tool in the armory of any corneal or contact lens specialist, either optometric or ophthalmologic. An extra benefit to being able to focus on any plane in the cornea is the ability to accurately assess corneal thickness.The distance between the confocal planes imaged

at the front and back surfaces of the cornea provides this information.

Topographers

Topographers

In 1847 the English physician Henry Goode used the reflection of a square target from the cornea to decide upon corneal shape in what is claimed to be the first keratoscope. Independently, in 1880 the Portuguese oculist Antonio Placido introduced the now famous black and white concentric ring target with a viewing hole to analyze the distortion of the reflected rings from the cornea; the first photokeratoscope. It is a variation of the Placido’s disk which is the basis of most modern computerized topographers. At this stage, assessment of corneal contour was primarily qualitative, with subjective assessment being made of distortion of the reflected image.The introduction of computer analysis of the reflected targets, which began in earnest in the late 1980s, allowed the development of the quantification of reflected image changes. By sampling changes to the image at many points, a very accurate profile of the corneal contour may be constructed, and then represented in a variety of representations or ‘maps’.

The various maps are usually color coded; red areas show where the cornea is steepest and blue areas where it is flatter (Figure 10.8). Difference maps may allow a change in profile, for example after contact lens wear, to be shown.

More recent topographers, such as the Orbscan, use a slit-beam scanning device which allows not only the anterior corneal surface profile to be measured, but also the posterior surface. Furthermore, topographers may also represent the tear profile underneath a contact lens, so providing a useful alternative to the far more subjective fluorescein assessment.

The increasing use of topographers in contact lens practice has led to some software to generate the ideal contact lens parameter for corneal fitting. It is also the basis of the resurgence in popularity of orthokeratology, where the deliberate remolding of the cornea

Advanced instrumentation

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Figure 10.8 Topography – color-coded map. Note cooler colors indicate flatter areas, warmer colors are steeper

by selective lens fitting to overcome refractive error, where vastly improved accuracy and predictability has been made possible.

The increased use of topographers as part of the preoperative assessment before refractive surgery has also resulted in increasing numbers of early corneal dystrophies, such as keratoconus, to be detected.This shows that the machines

have a diagnostic as well as a biometric role.

Aberrometry

When describing the very great amount of aberration that affects light passing through the human eye, Helmholtz once remarked ‘If an optician wanted to sell me an instrument which had all these defects . . . I should give him back his instrument’. Refractive correction may compensate for the so-called lowerorder aberrations, but the final image is reduced by an array of

Aberrometry

higher-order aberrations.The impact of these may be measured

using an aberrometer.This relies upon an analysis of how a 177 wavefront passing through the eye is altered. If the eye is

aberration free, then an incident plane wave will be refracted to form a regular wave.The eye, however, is not aberration free and hence the resultant wave is distorted (Figure 10.9a,b). It is

Figure 10.9a Parallel rays from a distant object are focused at a point in an eye without aberrations

Figure 10.9b Parallel rays from a distant object are focussed at various retinal positions in an eye with aberrations

Advanced instrumentation

possible to measure the change from the regular wave at many

178points so recording the wavefront aberration. Most aberrometers use a system called the Hartmann–Shack.This employs an array of tiny lenses (lenslets) through which the aberrant wavefront passes. Each lenslet forms a point image which would be a perfect point for each were the wavefront to be aberration free. However, the point spread at each point focus reveals the overall aberration.The accurate analysis of this gives useful information which may be used, for example, in programing the laser used in refractive surgery such that the postoperative corneal profile has less aberrations, or in specifying contact lens design with minimal aberration for the cornea to which it is to be fitted.

Further reading

American Academy of Ophthalmology (1996) Preferred Practice Pattern: Primary Open Angle Glaucoma American Academy of Ophthalmology: San Francisco.

Index

Index

Index

Index