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

Examination techniques

Due to the positioning of the illumination system the images are

only visible through one of the eyepieces of the observation 39 system.

Specular reflection is the preferred choice of illumination for observing the corneal endothelium. In order to achieve a good view the slit beam is normally shortened and widened.

To achieve the best view of the endothelium high magnification should be used, typically in the order of 40× (Figure 3.3). At such a high magnification slight adjustments to the focusing

of the instrument are required. Additionally slight adjustment of the illuminating system may also be required to optimize the image.

Surfaces such as the corneal endothelium are visible by specular reflection as they are not perfectly smooth. Light is reflected at various angles, which allows the ‘roughness’ of the surface to be assessed. Light that is not reflected back towards the microscope appears dark whereas that which is reflected back appears light.

Retro-illumination

Viewing certain ocular structures with direct illumination sometimes poses difficulty, either as a result of their proximity or as a result of their structure.Take for example the limbal vasculature, its close proximity to the corneo–scleral junction often results in excessive glare from the sclera when viewed in direct illumination (Figure 3.4). Indirect illumination is often more beneficial, however the relative lack of illumination often makes these fine vessels difficult to see. Another example is corneal nerves, which are notoriously difficult to see as they enter the cornea beyond the limbus. In both examples these structures are best viewed by illuminating them from behind rather than directly in front.

As the name indicates retro-illumination involves illuminating the object of interest from behind.The reflective nature of some of the ocular structures is utilized to create a secondary diffuse source of illumination from within the eye. A good example of diffuse illumination that most practitioners have experienced is the

Principles of the slit-lamp biomicroscope

Figure 3.3 Method for obtaining specular reflection of the endothelium (after Chauhan, K. Reproduced with permission from Doshi & Harvey

Investigative Techniques and Ocular Examination, Butterworth-Heinemann 2003)

dark appearance of a cataract seen against the red-reflex generated by a retinoscope. In a slit-lamp examination the diffuse reflector is often the iris. It is a popular misconception that when

Summary

41

DOF

Figure 3.4 Retro-illumination to view corneal vasculature near the limbus. In this area, uncoupling of the limbus is not necessary (after Chauhan, K. Reproduced with permission from Doshi & Harvey Investigative Techniques and Ocular Examination, Butterworth-Heinemann 2003)

performing retro-illumination the slit-lamp has to be decoupled. It is only necessary when the corneal apex is being viewed (Figure 3.5b), the separation angle is too large, high magnification is used and the beam is narrow. For small separation angles and areas close to the limbus the cornea (or associated structures) can be viewed without uncoupling (see Figure 3.5a).

Summary

The methods described within this chapter are only to be used as an indicator. It is essential for each clinician to develop their own

Principles of the slit-lamp biomicroscope

Figure 3.5 Retro-illumination to view an object away from the limbal area of the cornea (after Chauhan, K. Reproduced with permission from Doshi & Harvey Investigative Techniques and Ocular Examination, Butterworth-Heinemann 2003)

routine, which should be methodical and effective. Slit-lamp examination is a dynamic technique, with more than one form of illumination technique being utilized at any point in time while the practitioner is making his/her assessment. In the following chapter the application of a slit-lamp routine is discussed to more specific clinical conditions.

Further reading

Chauhan K (2003) Principles of the slit lamp biomicroscope. In Doshi S and Harvey W (eds) Investigative Techniques and Ocular Examination, pp 27–32. Eds Butterworth-Heinemann, Optician: Oxford, UK.

Morris J and Stone J (1997) The Slit Lamp Biomicroscope in Optometric Practice. AOP/Optometry Today: Fleet, UK.

Slit-lamp examination of the anterior segment

46 Introduction

No slit-lamp routine can be prescriptive and moreover it must be flexible enough to address a patient’s specific symptoms or history. Indeed each examination will generally differ from the last.Thankfully the slit-lamp is such a versatile instrument that it is able in most instances to meet such high demands. In Chapter 3 specific illumination techniques were described to aid and optimize examination.While this is a common way to describe the slit-lamp technique the reader should remember that this could lead to a rigid, nonflexible approach to examination. Moreover, it is likely that in a methodical, structured examination more than one illumination technique will be utilized in unison.

Irrespective of design of instrument the basic slit produced by the illumination system can be manipulated to facilitate examination in a number of ways.These include:

●Slit width.This can be varied between zero and 12–14 mm in most instruments.This can be particularly useful if it is necessary to measure the width of a lesion.

●Slit height.This is varied via a series of stops or in some instruments may be continuous, again this is useful when measuring and monitoring the size of a lesion.

●Slit orientation.The slit can be made to rotate through

180° in most modern instruments.This is usually achieved by rotating a turret within the illumination system.This is particularly useful when fitting toric soft contact lenses where the position of the axis maker can be determined

via a scale either on the slit-lamp illumination system or on one of the eyepieces.

●Slit decoupling. Normally the focal point of the slit produced by the illumination system and the focal point of the observation system are at the same point.The ability to decouple the two systems (move the illumination systems focal point) is essential to some of the illumination techniques previously described. In many of the Haag–Streit types of instrument the beam can be decoupled vertically as well as

Introduction

The normal white emission of the slit-lamp can be modified with the introduction of filters in the path of the beam.When used appropriately these can enhance the image under observation thus facilitating examinations.The normal filters incorporated in an illumination system are:

●Diffusing filter.These disperse light over a wide area allowing the observer to illuminate a wide width of the object under investigation.When used in conjunction with low magnification it is particularly useful in giving a good overview of the eye and adnexa (Figure 4.1). Diffusing filters come in many

types ranging from flip-over ground glass or plastic screens through to a ground flip side to the main slit mirror.

●Cobalt blue filter.This works by altering the emitted light so that the resultant beam is blue. It is most commonly used in

Figure 4.1 An overview of the eye and adnexa

Slit-lamp examination of the anterior segment

conjunction with a fluorescein stain and particularly in contact

48lens work, where a yellow Wrattan No. 12 filter may also be used to enhance areas of staining. Cobalt-blue filters are often useful in improving contrast to visualize intraor sub-epithelial corneal lesions such as whorl-like lesions seen in amiodrone keratopathy.

●Red-free filter.This filter results in the emitted beam being green.The result is that red objects are enhanced, appearing a much darker color (refer to example given in Figure 4.2). As a result these filters are frequently used to enhance blood vessels, particularly the limbal arcades (loops) or new vessels that are often very difficult to see particularly when these fine-caliber vessels are devoid of blood.

●Heat-reducing filters.These absorb light at the red end of the spectrum that are capable of causing thermal damage.They are particularly useful for long examinations as they enhance patient comfort.

Figure 4.2 The limbal vascular arcades are often difficult to see in white light and can be enhanced with a red-free filter

General examination of the eye

●Polarizing filters.These can be used to reduce the amount of glare and can be useful in techniques such as specular reflection.

General examination of the eye

In the absence of any specific symptoms the slit-lamp examination of the eye can be thought of, in the broadest terms, as a two-step procedure.The routine begins with a generalized examination of the eye and then continues using advanced examination techniques, for a more detailed examination of the ocular structures. Although the slit-lamp is the principal instrument for examination of the eye in a contact lens wearer, for the purpose of this chapter we will assume the eye under observation belongs to a non-lens wearer, although, where particularly relevant, reference will be made to differences between the two types of eye.

In most general examinations it is customary to begin at the front of the eye and move deeper in to examine the finer ocular structures. As such, it is normal to begin with an examination of the eyelids, tear-film and conjunctiva. Observing these structures is generally best performed using diffuse and direct illumination together with low magnification, but if an object of particular interest is seen, then higher magnification is frequently utilized.

Examination of the eyelids

A large angle between the illumination and observation systems and diffuse illumination together with low magnification offer the best set-up for a general examination of the surface of the eyelid. This allows the clinician to assess the eyelid position, examine for ptosis and check for any lid lumps or bumps.The external lid surface should be readily visible using this method and the practitioner should be investigating any localized or general