Slit lamp Examination

DR.Prashant .P.PatelSenior resident,

Aravind Eye Hospital, Tirunelveli.

Slit lamp Biomicroscopy

• It is a dynamic examination in which eye and ocular adnexa are scanned anteroposteriorly and horizontally.

• Slit Lamp: It is a misnomer since slit is only one of the various other diaphragmatic opening present in the instrument.

• Slit lamp biomicroscopy:1) Term introduced by Mawas in 1925.2) Examination of living eye by means of microscope

and slit lamp.

• The slit-lamp is one of the important examination tools of ophthalmologists.• One of the most important advantages of slit-lamp examination is that one

can examine the eye structure in three dimensions (3D).

• There are three basic requirements for appreciation of depth with a slit-lamp.

• The first depends upon the clinician possessing a third grade of binocular vision called stereopsis.

• The second involves the direction of the incoming light source, and is

dependent upon the fact that the light beam can be moved so it comes in from one side or the other.

• The third involves the shape of the slit.

History• Purkinje: One of the first individuals to apply

microscopy to the living eye , who studied the iris with an adjustable microscope by illuminating the field of view.

• Louis de Wecker : He made the uniocular slit-lamp

combined an eyepiece, objective and adjustable condensing lens within a tube.

• It was improved by Siegfried Czapski who added binocularity to the microscope.

• However, none of the units had sufficient and adjustable illumination.

• 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.

• Henker and Vogt improved upon Gullstrand’s device in 1911 by creating an adjustable slit-lamp by combining Czapski’s microscope with Gullstrand’s slit-lamp illumination.

Basic design of slit lamp

• The three main components of the modern slit-lamp are:

1) Illumination system

2)Observation system

3) Mechanical system

Mechanical system

• It is mainly concerned with:• Positioning of patient.• Adjustment for observer and patient.• Adjustment of illumination and observations system.• It generally contains following hard ware.

Fixation light

Head rest

Canthal alignment mark

Chin rest

Lock for slit lamp base Joy stick

Power unit

Chinrest adjust-ment knob

Height adjustment switch

• Mechanical coupling: Mechanical system provides coupling of microscope and illumination system along a common axis of rotation that coincides their focal planes.

• This arrangement ensures that light falls on the point where microscope is focused.

Illumination SystemIt is based on Kohler illumination

•The light source L is imaged in the objective O by the collector system K. The objective in turn produces an image at S in the mechanical slit located next to the collector system. The image of the light source at O is the exit pupil of the system.

•The filament is imaged on to the objective lens but the mechanical slit is imaged on to the patient’s eye.

• Köhler illumination provides a very homogeneous slit image.

Illumination System

• The illumination system of most slit-lamps consists of two different designs.

• The first design: the Haag-Streit type illumination, allows de-coupling in the vertical meridian.

• Such vertical de-coupling is particularly useful when performing gonioscopy to minimize reflections and for indirect fundus examination to gain increased peripheral views.

In the Zeiss type the illumination comes from below.

The second design: the Zeiss type illumination system, does not allow decoupling in the vertical meridian.

Illumination system control

• Angle• Width• Type1.Neutral density2.Red free 3.Cobalt blue

• Height• Intensity

Observation system

• Should have following Characteristics :• Optimum stereoscopic observation.• Selectable magnification.• Large field of view.• Large depth of field.• Enough space in front of the microscope for manipulations on the eye.

Observation system is composed of

• An objective lens: Two planoconvex lenses with their convexities put together.

• Magnifying lenses• Telescope• Pair of prisms: To reinvert the image.• Eyepiece• Converging tubes: They are converged at an

angle of 10-15 degree, to provide good stereopsis.

• The object is located at the object side focal

point of the magnifying lens that magnifies the object image projecting it virtually to infinity.

• The image is than viewed with respective magnification through telescope.

Change in magnification

• Grenough type:• Galilean changer type:• Czapskiscope with rotating objective:• Zoom system:

The Grenough type(Classical HaagStreit)

Flip lever to changemagnification

• The Galilean Magnification changer

The Galilean Magnification changer

• It utilises the Galilean telescopes to alter the magnification.

• It has two optical components:• 1)Positive lens 2)Negative lens• Lenses are arranged in turret arrangement.• It provides large range of magnifications,

typically five.

The Galilean Magnification changer

The Galilean Magnification changer

Knob to changemagnification (3or 5step)

Czapskiscope with rotating objectives

• The different objectives are usually placed on a turret type of arrangement that allows them to be fairly rapidly changed during examination.

Zoom System

• Zoom system allows continuously various degree of magnification.

• E.g, Nikon photo slit lamp &Zeiss-75 Sl

Magnification can also be changedby changing the eyepiece power

Clinical Procedure

• Before using the slit-lamp, it is important to ensure that the instrument is correctly set up.

• The eyepieces should be focused for the observer for his/her own refractive error.

• Often a little more minus correction is required than the observer’s actual refractive error due to accommodation and proximal convergence.

• The Pupillary distance (pd) is adjusted for the observer (perhaps the pd should be slightly less than that usually measured to account for proximal convergence).

• Check that the observation and illumination systems are coupled, and the slit-beam is of even illumination and has sharply demarcated edge (otherwise irregularity of the beam may be falsely interpreted as irregularity of tissues).

• The slit-lamp examination is conducted in a semi dark room.

• Patient is seated in front of slit-lamp on an adjustable stool and his/her head is steadied by placing chin on chin-rest and his forehead rests on the bar of head-rest.

• Adjust the chin-rest so that the patient’s eyes are approximately level with the black marker on the side of the head rest.

• Focus the slit-beam on the eye by moving the joystick either towards or away from the patient.

• The examination should be commenced using the X10 eyepieces and the lower powered objective to locate the pathology and higher magnification should then be used to examine it.

• Use the lowest voltage setting on the transformer.• Select the longest slit-length by means of the

appropriate lever.• The angulation between the observation arm and

the illumination arm is adjusted.

Examination methods –Types of Illumination.

• Slit lamp Provides three basic types of Illuminations.• 1)Focal Illumination: Achieved by narrowing the slit horizontally or

vertically, provides isolation of the specific areas of eye /cornea for observation.

• 2) Oblique illumination: It is essential for detecting and examining findings in different layers of the cornea.

• 3)The Optical Section: The narrow slit beam slices through the eye revealing the internal details of the tissue at all layers.

Types of Illumination

• Dffuse Illumination: Terminology :It is the type encountered in everyday life.

For example light from sun or a light bulb that diffusely illuminate one’s surroundings.

Principle :It is a Initial survey examination of the face, eyelids and ocular surface.

If one directly proceeds with the magnified examination one is likely to miss skin disorders( such as acne rosacea), eyelid lesions ( such as molluscum contagiosum, small chalasion, mild ptosis).

Technique: It can be done with torch light ,

Diffuse illumination with slit lamp

1)Swing the microscope aside or keep it at 30-40’ of angle.

2)Opening the slit beam to full height and width.3)Dialing in the neutral density filter.• Beam is only 8-14 mm diameter and therefore

must be moved over the eyelids and ocular surface.

• It can reveal location and general pattern of eyelid, conjunctival, corneal lesions.

Sclerotic Scatter

• Terminology :In this technique for illuminating cornea, the slit beam is directed at the scleral limbus and illumination is transmitted into cornea by total internal reflection.

• Opacities within the cornea scatter the light back to the observer.

• Principle: Opaque sclera scatters the light at the point of illumination, some of the light is directed in to corneal stroma, where it travels through the entire cornea by repeatedly reflecting from its anterior and posterior surfaces.

• The light emerges around circumference of the cornea, where it encounters the opaque sclera and create a glowing halo.

• Should be used early in the examination because,

1) The patient acclimates to bright light of the slit lamp before it is directed in to the pupil.

2) It accurately reveals the presence and pattern of corneal opacities.

3) It helps to identify faint opacities that are difficult to see in direct illumination.

• Technique: • SLIT BEAM : Moderate width, Directed at the

3- or 9-o’clock scleral limbus• MICROSCOPE: Independently focused onto

the cornea.

Direct focal slit illumination

• Terminology : Projection , of a narrow slit beam at an angle, to the corneal surface, producing an optical section that slices through the cornea and eye.

• Principle: A direct narrow slit beam optically cuts through the cornea , providing a cross sectional view that reveals its contour and its internal structure.

• It forms two parallel curved surface, one that follows anterior corneal surface and one that posterior corneal surface.

• Two surfaces are joined by a block of light scattered in the stroma to create a geometric figure that resembles an elongated ice cube.

• This is known as parallelepiped/ optical block/optical section.

• Technique:• Best used after the lesion is located by

sclerotic scatter, diffuse illumination.• Examiner than focuses narrow slit of light

over this lesion.• SLIT BEAM: Narrow• Approximately 30’ angle between slit beam

and microscope, can be increased up to 90’.

Broad tangential illumination

• Terminology: A wide beam is oriented at an extremely oblique illumination angle, causing it to project tangentially across the corneal surface.

• Principle : Extreme angle of incidence of the slit beam results in decrease of light reflected and scattered by the cornea, this in turn reduces background glare causes surface abnormalities to stand out.

• It is most useful for examining corneal surface.

• Technique:• SLIT BEAM: wide• ANGLE between slit beam and microscope:

70-80’.• This highlights irregularities of corneal

surface such as epithelial defect, PEES etc.

Proximal( indirect) illumination

• Terminology: It requires that the slit beam is directed adjacent (proximal) to the area of interest to illuminate it indirectly.

• Principle: It combines the Principle of Sclerotic Scatter and Retro illumination.

• When directed adjacent to opaque area of the cornea, the illumination of the slit beam is internally reflected within the cornea causing light to spread throughout the stroma, light striking the opacity is scattered and some of the scattered light is reflected back to the observer.

• It is used to define the an opaque area of the cornea and to identify details within the opacity.

• Technique:• SLIT Beam:• Short: 2-3 mm• Slightly broad: 0.2 mm• Directed adjacent to area of interest.• Angle between microscope and slit lamp is 15

degree.

Retroillumination from Iris

• Terminology: It is a technique of illuminating an area of cornea using light reflected from structure posterior to cornea such as iris.

• Direct type: Cornea illuminated by light is viewed directly.

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

Direct type of Retro illumination

Indirect type of Retro illumination from Iris

• Principle: When light of direct illumination strikes a corneal opacity, it scatters and some of the light is reflected back towards the examiner. This form of illumination often washes out and obscures details of the lesion and provide little information about optical qualities and internal structure of small lesion.

• When retroillumination is used these details stands out more prominently because the lesion is less likely to scatter and more likely to obstruct and refract the reflected light.

• Observer looks for three optical phenomena in retroillumination.

• 1) Obstruction of light by densely opaque abnormalities appear black against the light beam. e.g, pigment deposit.

• 2)Substructure of the droplets or refractile material in retroillumination.

• 3)Distortion of light especially near the edge of the abnormality by refractile lesions that have different refractive index than the media in which they are contained.

Technique:Slit beam: narrow to medium, slit height is

reduced, area of corneal pathology is positioned directly over the slit beam light reflected from the iris, either by moving the instrument or by altering the patient’s gaze.

Direct retroillumination

Indirect retroillumination.

Retroillumination from the fundus:

• Terminology : Light entering the pupil is reflected from RPE and choroid and emerges from the pupil with orange red glow, commonly called as red reflex.

• When examiner views the cornea against this reflex he/she is able to detect lesions that are to subtle for visualization by other techniques.

• Principle: Same that of the Retroillumination from iris.

• Dense scars : Obstructs the reflected and they appear as dark silhouettes.

• Translucent/transparent objects: Corneal guttae, DM Folds, Lattice corneal dystrophy, epithelial oedema stands out as brightly refractile contours.

• Technique: Slit lamp is aligned coaxial with microscope. Then decentered to the edge of the pupil.

• Slit width: Medium and curved at one edge to fit in pupil.

• Slit height: Reduced to 1/3 .

Specular reflection

• Terminology: The smooth surfaces of cornea reflect incident light like a plain mirror following SNELL’S LAW.

• When angle of incidence is equal to angle of reflection as measured from line drawn perpendicular to the surface. THIS IS KNOWN AS SPECULAR REFLECTION.

• Principle: Surface light reflex from the tear air interface is brightest specular reflection emanating from the cornea, but this does not permit examination of individual epithelial cells, because the light is reflected from tear film.

• Intensity of anterior reflection is great because the difference in refractive index between air and tears is large.

• Intensity of posterior reflection from endothelial surface is much less because the difference in index of refraction between aqueous humor and endothelium is much less.

• Light Reflected from the endothelium is 0.22% of total incident light.

• Because cornea is curved , only a small part of the incident light beam is reflected in a specular manner which forces the observer to narrow the slit beam in to eliminate the surrounding glare.

• Mirror smooth posterior surface of endothelium is broken by the intercellular spaces, which do not reflect the light, thus appears dark boundaries outlining a regular mosaic.

• The regular endothelial mosaic pattern can be disrupted by various pathologic entities.

• 1) large and small cells may form a heterogeneous population.

• 2)Irregularities in Descemet’s membrane e.g., cornea guttata , ridges, folds may displace the endothelial cells from the plane of reflected light so that they can not be clearly visualised and localised black area is viewed.

• 3)Pigment deposits/ keratic precipitates on the posterior surface of the endothelium may reflect light and may be seen as focal bright spots.

• Technique: It requires a more systematic, stepwise, careful approach.

• Patient is asked to look straight ahead.• Slit beam is projected on the central cornea from the

temporal side.• The height of beam : 3 to 4 mm• Width of beam: Moderately wide (0.5mm)• Angle of slit beam and microscope should be same

from the perpendicular to the corneal surface.• With this settings observer sees three lights:

• 1. Slit beam parallelepiped in the cornea.• 2) Beam reflected from the iris nasal to the

parallelepiped.• 3) The corneal light reflex( First Catoptric

image) temporal to parallelepiped.• Examiner focuses the slit beam at the level of

the endothelium and slowly moves it temporally towards the first catoptric image.

• As this being done entire corneal parallelepiped changes in appearance from grey black to lighter and brighter structure.

• When the corneal parallelepiped passes in front of the catoptric reflection, the bright surface reflex from the tear air interface dazzles the examiner and faint mosaic pattern of the endothelium becomes apparent, at this point magnification is changed to high power.

Aqueous flare and cells

• Conical beam: is a small circular beam used to examine the presence of cells and flare.

• Beam: Small circular pattern.• Light Source:45-60’ Temporally and directed in

to the pupil.• Biomicroscope: Directly in front of the eye.• Magnification: High• Focusing: Beam is focused between cornea and

anterior lens surface.

Filters• Sodium fluorescein is applied gently to the bulbar conjunctiva.• The patient should blink once or twice for the dye to be dispersed over

the eye.• If the epithelium of the conjunctiva or the cornea is damaged, the

fluorescein stains the underlying tissue. • The remaining dye fluoresces a yellow green colour when excited by

the blue light.• Healthy epithelium does not stain.• Uses: • Contact lens fitting, • Marginal tear film height measurement,• Tear film break up time,• Jone’s dye disappearance test

Red free filter

• The Red-free (green) filter to differentiate vascular from pigmented lesions.

• Blood vessels and small haemorrhages will take on a dark appearance with the use of the red-free filter, whereas pigmented lesions will remain dark.

• It makes the detail better by improving contrast.

Clinical Slit Lamp Microscopy• Examiner must integrate various types of illuminations in to flowing

examination of cornea that permits rapid and accurate observations of corneal disease.

• An example for sequence of illuminations in which the corneal lesion is to be seen:

• 1)Flash light/ diffuse illumination: To locate the pathology.

• 2)Sclerotic scatter: To see the pattern of abnormality.

• 3)Focal slit: To know the depth at which the lesion is located.

• 4)Proximal illumination : For internal details of the lesion.

• 5)Tangential illumination: For surface characteristics of the lesion.

• 6)Retroillumination from iris & fundus: For optical qualities of the lesion.

Clinical Drawings of Corneal Pathology

Colour coding:• Black: • Corneal limbus, • Scars ,• Degenerations, • Foreign Bodies, • Suture,• Contact lens.

• Blue: Designates oedema1)Shading: Diffuse stromal oedema.2)Small circles: Epithelial Oedema.3)Wavy Lines: DM Folds.

• Brown: Indicates Melanin or Iron Pigmentations

1)Pupil 2)Iris 3)Deposits of Melanocytes on Posterior

cornea. 4)Sundry Iron lines on the epithelium.

• Red1) Blood vessels: i) Wavy lines: Begin outside the limbus

indicates subepithelial vessels. ii) Straight lines : Begin inside the limbus

indicates stromal vessels.2) Rose bengal stain: RED DOTS indicates area

stained by rose bengal.3)Solid red shades: Indicates haemorrhage.

• Yellow: indicates presence of white blood cells.

• Hypopyon• Stromal infiltrate• Keratic precipitates

• Green: fluorescein staining of the cornea• Green coloured dots: PEES• Small Lines: Filaments• Shaded Outlines: Epithelial Defect.

Accessory Devices:

• Gonioscopy.• Pachymetry.• Applanation tonometry.• Slit lamp photography.• Slit lamp as a delivery system for argon,

diode,and YAG laser.

Thank you!!