Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

232 Textbook of Visual Science and Clinical Optometry

be comfortably seated in a revolving stool with his chin and forehead pressed against chinrest and head rest respectively. Mastering the art of slit-lamp examination is a prerequisite for a good ophthalmologist and an efficient optometrist.

SALIENT TECHNICAL INFORMATIONS

•Nomenclature–Since the microscope has two eyepieces, it is binocular and offers the observer a binocular, stereoscopic and magnified view of the eye. Further, the light beam can be made very narrow producing a slit of light. Hence, it is called slit-lamp

biomicroscope (Fig. 16-1).

•Magnification–Magnification of image is obtained by multiplying the power of the oculars with the power of the objective lens. Thus, if the oculars are of 10X strength and the objective lens is of 2.5X, the resultant magnification will be 25X. Majority of slit-lamp biomicroscope are usually available with magnifications between 8X and 40X with intermediate magnifications of 12X, 16X and 25X. Magnification is changed by either changing the objective lens or by replacing the oculars, i.e. eyepieces. Objective lens is changed by either turning a dial on the side of the observation system or flipping a lever located below the observation system.

•Focussing–Focussing is controlled by a joystick. It is designed to move the slit-lamp biomicroscope laterally, axially and vertically. Vertical movement is controlled by rotating the joystick.

•Intensity of light–The illumination is controlled either by a rheostat or by rotating a knob. The height, diameter of the light beam can be altered by another knob. The width of the light beam is also controlled by another knob, located in the lower end of the illumination system.

•Click-stop position–A “click-stop” position indicates that the illumination system and the observation system are coaxial.

•The illumination system or light source can be moved away from the observation system, to change the angle of the incident light beam which is essential for various types of illumination techniques, discussed later in this chapter.

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•A frosted ground glass (if present) can be positioned in the path of the incident light by flipping a lever in the diffuse illumination technique.

•Halogen light is used in most of the examinations. Coloured filters (cobalt blue, red free, i.e. green) can be used by sliding a knob for procedures discussed later.

Fig. 16-1: Slit-lamp AIA–11 5 step, inset showing observation system of Slitlamp AIA–11 2 step (photo courtesy Appasamy Associates, India)

DIFFERENT TECHNIQUES OF ILLUMINATION

Diffuse Illumination (wide beam)

•The diffuse illumination is provided by placing a frosted or ground glass filter in front of the focussed light beam of slitlamp biomicroscope (Fig. 16-2).

•Usually, in diffuse illumination the incident light is projected intotheeyeobliquely(45°)andviewedunderlowmagnification.

•The light will be defocussed and will provide even illumination over the entire field of view of opaque tissues of the anterior segment (sclera, iris, eyelid margins, bulbar conjunctiva, etc).

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•First start with lower magnification and focus the corneal surface. Now move the oculars 20º to 30° away from the illumination source. Then, move the illumination source 20° to 30° on the opposite direction until a bright mirror-like reflection is seen through one ocular, i.e. eyepiece. Specular image is seen at every interface between structures of different refractive index. Hence, there should be three reflexes (Fig. 16-4);

Specular reflection

•Focus finely on the intermediate dim reflection to observe endothelial mosaic. Endothelium is best seen using only one ocular.

•Now the magnification may be increased to view the endothelial cells.

Fig. 16-4: Slit-lamp technique—Specular reflection, inset shows specular reflections withtheendotheliuminfocus(schematic). 1=Tearfilm–brightreflection,2=Endothelial mosaic–dim reflection and 3 = Light source–blurred image

iv. Tangential or oblique illumination

•In this technique, the angle between the observation system and illumination source is set at 90° (Fig. 16-5).

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•It produces long shadows for any elevation particularly on the iris surface. Even depressions are better appreciated.

Fig. 16-5: Slit-lamp technique—Tangential illumination

Indirect Illumination

In this technique the area of interest is viewed indirectly by an illumination from reflected light of the direct beam, i.e. structures

not within the focussed light beam are under observation.

i. Proximal illumination

•A moderately wide light beam is focussed on the areas adjacent to the lesion of interest (Fig. 16-6).

•The lesion is observed with scattered light against dark background. This results in a higher contrast of the lesion against a dark pupil.

ii. Sclerotic scatter

•A parallelepiped illumination beam is focussed at the temporal limbus.

•Illumination beam is set at an angle between 45° and 60° with the observation system, i.e. the microscope is focussed centrally on the cornea.

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Retroillumination

•In this technique, the incident light beam is reflected from the iris, anterior lens surface or retina to illuminate more anteriorly located structures of interest.

i. Direct retroillumination

•This refers to viewing the structure against an illuminated background.

•In direct retroillumination corneal opacities appear black against an illuminated background. Posterior capsular opacities or lental opacities appear dark against red glow of retina.

•The light beam should be directed from 45° angle and focussed behind the area of interest (Fig.16-8).

•In direct retroillumination objects that normally appear bright will appear dark.

Fig.16-8: Slit-lamp technique—Direct retroillumination

ii. Indirect retroillumination

•This refers to viewing the structures of interest against a dark background.

•The light beam is positioned in such way that the dark background is behind the structure of interest (Fig. 16-9).

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•The structure should not be within the pathway of the reflected light.

Fig.16-9: Slit-lamp technique—Indirect retroillumination

• = Corneal opacity

iii. Transillumination

•In transillumination the iris tissue is tested for passage of light through a defect within it.

•The slit-lamp light source and the microscope is positioned coaxially, i.e. click-stop position (Fig. 16-10) and focussed on the iris surface.

•Use a circular light beam equal to pupillary size and project through the pupil.

•Use lower magnification.

USE OF FILTERS IN SLIT-LAMP BIOMICROSCOPE

a.Cobalt blue filter: It is used in applanation tonometry, assessment of rigid contact lens fitting and fluorescein staining of cornea. It should be kept in mind that significant amount of light has been filtered out by the cobalt blue filter. Hence, brighter illumination should be employed while using cobalt blue filter.