THE BIOMICROSCOPE (SLIT LAMP)-DR AKSHAY NAYAK

THE SLIT-LAMP BIOMICROSCOPE is a high-power binocular microscope with a slit-shaped illumination source, specially designed for viewing the different optically transparent or translucent tissues of the eye.

The science of examination with a slit lamp is called Biomicroscopy as it allows in vivo study of living tissues at high magnification.

Allvar Gullstrand: An ophthalmologist and 1911 Nobel laureate introduced the illumination system which had for the first time a slit diaphragm, therefore Gullstrand is credited with the invention of the slit lamp.

One of the most important advantages of slit-lamp examination is that one can examine the eye structure in three dimensions (3D).

Binocular vision- stereopsis is provided by slit lamp

TYPES

There are 2 types of slit lamp biomicroscope

1)Zeiss slit lamp biomicroscope 2)Haag streit slit lamp biomicroscope

In Zeiss type light source is at the base of the instrument while in Haag streit type it is at the top of the instrument.

Zeiss slit lamp biomicroscope Haag streit slit lamp biomicroscope

The three main components of the modern slit-lamp are:1) Illumination system

2)Observation system/Viewing arm

3) Mechanical system

Basic design of slit lamp

Illumination system-Kohler Principle

Telescope system

The telescope system provide considerable distance between the microscope and the patient’s eye, so that certain maneuver like foreign body removal from the cornea or using extra lenses for fundus examination can be done.

Parfocality Parfocality : The point at which the microscope is focused

corresponds to the point on which the light is focused, this coupling effect is called parfocality.

This is achieved by the microscope and the illumination system, having a common focal plane and their common axis of rotation also lies in that focal plane.

Illumination system The illumination arm and viewing arm

are parfocal. Light source is atop the illumination

arm in a lamp housing. The beam of light can be changed in

intensity,height,width,direction or angle and color during the examination with the flick of lever.

Observation system(microscope)

Observation system is essentially a compound microscope composed of two optical elements

1.an objective ,2.an eyepiece

It presents to the observer an enlarged image of a near object.

The objective lens consists of two planoconvex lenses with their convexities put together providing a composite power of +22D.

The eye piece has a lens of +10D.

Microscope is binocular i.e. it has two eyepieces giving binocular observer a stereoscopic view of eye.

The Grenough type(Classical HaagStreit)

Knob to changemagnification (3or 5step)

The Galilean Magnification changer

Mechanical system Joystick arrangement

Movement of microscope and illumination system towards and away from the eye and from side and side is achieved via joystick arrangement.

Patient support arrangementVertically movable chin rest and the provision to adjust height of table.

Fixation target:A movable fixation target greatly faciliates

the examination under some conditions.Mechanical coupling :

Provides a coupling of microscope and the illumination system along a common axis of rotation that coincides their focal planes. This ensures that light falls on the point

where the microscope is focused Has advantages when using the slit lamp

for routine examination of anterior segment of eye.

Prerequisites :

Switch on power & unlock base screw Cleaning the forehead band Changing paper strip from chinrest Comfortable sitting of pt. and the examiner Counseling the patient Proper positioning of the pt. Target fixation Adjust eyepieces to correct for examiner’s refractive error and

interpupillary distance Children may need to stand, or they can sit on parent’s lap or kneel

on a stable chair

Patient positioning

Alignment mark

Magnificationmay be changed byflipping a lever

Changing filters. biomicroscope

Microscope and light source rotate indepedently

Filters used in slit lamp biomicroscopy Cobalt blue filter

Used in conjunction with fluorescein stain

Dye pods in area where the corneal epithelium is broken or absent.

The dye absorbs blue light and emits green. Uses:

Ocular stainingRGP lenses fittingTear layerApplanation tonometry

Red free(green)filter:

Obscure any thing that is red hence the red free light , thus blood vessels or haemorrhages appears black.

This increases contrast ,revealing the path and pattern of inflammed blood vessels.

For Rose Bengal staining evaluation.

Illumination techniques

Includes Diffuse illumination Direct illumination

Parallelepiped Optic section Conical(pinpoint) Tangential Specular reflection

Indirect illumination Retro-illumination Sclerotic scatter Transillumination Proximal illumination

Diffuse illuminationAngle between microscope and

illumination system should be 30-45 degree.

Slit width should be widest.Filter to be used is diffusing filter.Magnification: low to medium

Applications:General view of anterior of eye:

lids,lashes,sclera,cornea ,iris, pupil,Gross pathology and media opacitiesContact lens fitting.Assessment of lachrymal reflex.

Optics of diffuse illumination Diffuse illumination with slit beam and background illumination

Direct Focal-Parallelepiped:

Constructed by narrowing the beam to 1-2mm in width to illuminate a rectangular area of cornea.

Microscope is placed directly in front of patients cornea.

Light source is approximately 45 degree from straight ahead position.

With narrow slit the depth and portion of different objects(penetration depth of foreign bodies, shape of lens etc) can be resolved more easily.

With wider slit their extension and shape are visible more clearly.Applications:

Used to detect and examine corneal structures and defects.

Used to detect corneal striae that develop when corneal edema occurs with hydrogel lens wear and in keratoconus.

Used to localize:Nerve fibers Blood vesselsInfiltratesCataractsAC depth.

Corneal epithelium-thin,optically empty(black)Rest of cornea , lens and vitreous are relucent-silvery appearance.

Optical section of lens

1.Corneal scar with wide beam illumination 2.optical section through scar indicating scar is with in superficial layer of cornea.

Conical beam(pinpoint)

Produced by narrowing the vertical height of a parallelepiped to produce a small circular or square spot of light.

Light source is 45-60 degree temporally and directed into pupil.

Biomicroscope: directly in front of eye.Magnification: high(16-25x)Intensity of light source to heighest

setting.

Focusing: Beam is focused between cornea and anterior

lens surface and dark zone between cornea and anterior lens observed.This occurance is called tyndall

phenomenon.

Tyndall phenomenon

Cells, pigment or proteins in the aqueous humour reflect the light like a faint fog.

The strongest reflection is possible at 90°.

Most useful when examining the transparency of anterior chamber for evidence of floating cells and flare seen in anterior uveitis.

Specular reflection

Established by separating the microscope and slit beam by equal angles from normal to cornea.

Based on snell’s law Angle of illuminator to microscope must be

equal and opposite. Angle of light should be moved until a very

bright reflex obtained from corneal surface which is called zone of specular reflection.

SPEC(tacular) lake REFLECTION

When such an area of reflection is established on corneal endothelium, its possible to see individual endothelial cells. Its because minute irregularities in the tissues cause some light in the zone of reflection not to be reflected to examiner.

Irregularities ,deposits ,or excavasation in these smooth surface will fail to reflect light and these appears darker than surrounding IN OTHERWISE BRIGHT ZONE.

Under specular reflection anterior corneal surface appears as white uniform surface and corneal endothelium takes on a mosaic pattern.

Used to observe: Evaluate general appearance of corneal endothelium Lens surfaces Corneal epithelium

Schematic of specular reflection.

Reflection from front surface endothelium

Indirect illumination The beam is focused in an area adjacent to ocular tissue to

be observed. Main application:

Examination of objects in direct vicinity of corneal areas of reduced transparency e,g, infiltrates,corneal scars,deposits,epithelial and stromal defects

Illumination: Narrow to medium slit beam Decentred beam

Retroillumination

Formed by reflecting light of slit beam from a structure more posterior than the structure under observation.

A vertical slit beam 1-4mm wide can be used.

Purpose:Place object of regard against a

bright background allowing object to appear dark or black.

Used most often in searching for keratic precipitates and other debris on corneal endothelium.

The crystalline lens can also be retroilluminated for viewing of water clefts and vacuoles of anterior lens and posterior subcapsular cataract.

Direct retroillumination from iris:

Used to view corneal pathology.A moderately wide slit beam is

aimed towards the iris directly behind the corneal anomaly.

Schematic ofdirect retroillumination fromthe iris.

direct retroillumination from the iris.

Indirect retroillumination from iris:Performed as with direct retroillumination

but the beam is directed to an area of the iris bordering the portion of iris behind pathology.

It provides dark background allowing corneal opacities to be viewed with more contrast.

Observe:Cornea, angles.

Direct type: Cornea illuminated by light is viewed directly.

Indirect type: Cornea viewed adjacent to area of illuminated by the reflected light.

Retroillumination from fundus(red reflex photography)

The slit illuminator is positioned in an almost coaxial position with the biomicroscope.

A wide slit beam is decentered and adjusted to a half circle by using the slit width.

The decentred slit beam is projected near the pupil margin through a dilated pupil.

Schematic ofretroillumination from the retina.

Example of retroillumination from the retina.

Uses of retro illumination

To see Lattice dystrophy Pseudoexfoliation Keratic precipitates Corneal scars Lens vacuoles Cataract

Sclerotic scatter It is formed by focusing a bright but narrow slit beam

on the limbus and using microscope on low magnification.

Such an illumination technique causes cornea to take on total internal reflection.

The slit beam should be placed approximately 40-60 degree from the microscope.

Corneal changes or abnormalities can be visualized by reflecting the scattered light- especially subtle opacities.

Schematic ofsclerotic scatter. Example of

sclerotic scatter.

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Transillumination

In transillumination, a structure (in the eye, the iris) is evaluated by how light passes through it.

Iris transillumination: This technique also takes advantage of the red

reflex. The pupil must be at mid mydriasis (3to 4 mm

when light stimulated). Place the light source coaxial (directly in line)

with the microscope..

Normally the iris pigment absorbs the light, but pigmentation defects let the red fundus light pass through..

Observe: iris defects (they will glow with the orange light reflected from the fundus)

Order of examination on slit lamp

Uses of slit lamp biomicroscopy

Diagnostic: Anterior segment and posterior segment diseases Dry eye

Procedures:ApplanationTear evaluationPachymetryGonioscopyContact lens fittingTherapeutic:

LaserFB removalEpilationYag capsulotomy

Anterior and posterior segment disease evaluation

Lids and lashes Conjunctiva and cornea Instillation of fluorescein and BUT

measurement Eversion of the lids Anterior chamber and angle

measurement Iris Crystalline lens Anterior vitreous

Injected conjunctivaMeibomian gland openings

pinquecula, INSTILLATION OF FLUORESCEIN PALPEBRAL CONJUNCTIVA EXAMINATION

Van Herrick Technique Used to evaluate angle of anterior chamber without gonioscopy Narrow slit angled at 60degrees at limbus Medium Magnification Depth of anterior chamber is evaluated to the thickness of

cornea.

Gonioscopy via slit lamp Single mirror gonioscope Angle of anterior chamber Three mirror gonioscope Fundus(central and peripheral) and angle of

anterior chamber

CENTRAL RETINA PHOTOGRAPHS WITH A 90-DIOPTER LENS

A moderate slit beam in the almost coaxial position gives the best results.

THANK YOU ….