Second Year 99/00
Clinical Optometry 3
Introduction
The slit lamp is essentially a simple and generally under-used piece of equipment. It consists of an illumination system and a binocular observation system, which when correctly aligned will result in a coincidental focus of the slit and microscope.
Illumination system
Basically a short focus projector projecting an image of the illuminated slit aperture on to the eye. This part of the system should be flexible to allow various sizes and shape of slit beam. Usually a rheostat is incorporated and the lamp house can be rotated. Neutral density, cobalt blue and red free filters are usually available, and occasionally a diffuser and polariser.
Observation system
Consists of a binocular microscope with parallel or convergent eyepieces (some practitioners may get diplopia with parallel eyepieces). Generally magnification from 6x to 40x is allowed by varying the eyepiece and objective lens of the microscope.
Note
The illuminating system and observation system are normally focused at the same point - coupled. Although there are some occasions when it is better to focus the slit at a different point to where the observation system is set-de-coupled.
Before commencing any work with the slit lamp it is important to ensure that your instrument is correctly set up:
Using the slit lamp
When using the slit lamp it is important that you are aware of the various techniques you are employing and when to use them. As with any technique, a general routine should be established, in most cases when examining the eye and adnexa a general rule of thumb might be-use a large field of view initially and then focus in on detail when required with larger amounts of magnification.
Commence the examination using the 10x eyepieces and the lower powered objective i.e. 1x. Use the lowest voltage setting on the transformer. Select the longest slit length by means of the appropriate lever (17). Adjust the chin rest by means of control 27 so that the patient's eyes are approximately level with the black marker on the side of the head rest. Adjust the height of the slit lamp until the slit beam is centred vertically on the patient's eye. Focus the slit beam on the eye by moving the joystick (1) either towards or away from the patient. Coarse positioning can be effected without using the microscope but critical focussing should be carried out whilst viewing through the microscope.
The slit width is varied by rotating either the left hand or right hand knurled control 10. To vary the angle between illumination and microscope use one or other of these same controls as handles.
The slit should be set primarily in the vertical position, but any desired inclination can be achieved by means of the ball handle 15 (notches at 45°, 90°, 135°; stops at 0° and 180°). By tripping the latch 11 and tilting the slit lamp column, the beam can be introduced from as much as 20° below the horizontal. This is mainly used for carrying out gonioscopy.
For observation by sclerotic scatter or other dissociated forms of examination the centring screw 13 is loosened, so that the slit image can be moved away from the centre of the field of observation. The image is centred again by tightening the screw.
Carry out the techniques described below.
Description of Haag Streit 900
1) Control lever for horizontal coarse and fine adjustment.
2) Fixing screw for the horizontal movements.
3) Gliding plate
4) Accessory drawer
5) Pilot light
6) Fuse
7) Rotary switch
8) Rail covers
9) Headrest
10) Fixing screw for the microscope arm.
11) 2 knobs for coupling the microscope arm to the lamp arm (invisible).
12) Roller for setting the angle between microscope and illumination.
13) Index for reading angles.
14) Scale for reading the angle between microscope and illumination unit.
15) Guide plate for pre-set lenses and applanation tonometer.
16) Level adjustment control for the chin rest.
17) Chin rest.
18) Fixing screw for the microscope.
19) Lever for changing the objectives.
20) Interchangeable eyepieces.
21) Knurled rings for setting the eyepieces.
22) Interchangeable illumination mirror.
23) Knurled knob for lateral adjustment of the fixation light.
24) Clamping nuts for the lamp housing.
25) Cover for lamp housing.
26) Scale for slit diaphragms.
27) Lever for three filters.
28) Control for rotation of slit, for varying the slit length and interposing the blue filter.
29) Lever marker.
30) Fixation lamp.
31) Handle for focusing the fixation target.
32) Slide for pre set lens.
33) Centring screw.
35) 5° stop.
34) Latch for the angle of inclination.
35)Controls (2) for setting the slit width.
Methods of observation
The slit lamp offers a variety of illuminating and observing methods which do not all have to be used in routine examination, but the ability to perform them should be practised. Not all slit lamps will allow all methods to be performed.
The following is a suggestion as to the way a routine technique should be performed-it is by no means the only way to perform a slit lamp routine and indeed not all practitioners will do so. A good routine is one that you feel comfortable with and that allows you to detect and investigate any irregularities and note the normal condition (base-line data) of the eye.
The examiner uses one hand to operate the joystick and the other to control the magnification, fixation light position and the type and angle of illumination, merging from one method to another until the examination is complete. The setting-up and use of each kind of illumination must be practised until the proceedings become automatic. NOTE: regardless of the type of illumination used, to examine the entire cornea/sclera/lens you must ask the patient to look up, down, right, left (holding the upper or lower lid when necessary).
Types of illumination are:
1) Diffuse illumination
2) Direct (focal) illumination
(a) narrow beam (optic section)
(b) broad beam (parallelepiped)
(c) conical beam
3) Indirect illumination
4) Retro illumination
(a) direct
(b) indirect
5) Specular reflection
6) Sclerotic scatter
7) Oscillatory illumination
8) Tangential illumination
Diffuse illumination
This is a good method of observing the eye and adnexa in general. Diffusers are sometimes supplied with slit lamp and can be 'flipped-over' the illuminating system. Diffusers are generally ground glass plates that cover the light source, if these are not available a piece of tissue might suffice to diffuse the light although this can sometimes be precarious as the tissue often falls off! The slit should be opened wide and the magnification should be set as low as possible to enable a large field of view.
Direct/focal illumination
This is the most common method of viewing all tissues of the anterior eye, the focused slit is viewed directly by the observer through the microscope. The magnification can be increased quite markedly (10x to 40x or more) to view any areas of interest in greater detail.
Any size or shape of beam can be used, and it is important to be aware of the main options.
(a) Broad beam (parallelepiped)
Generally a very wide beam is used for surface study, whilst a very narrow one is used for sections. A useful combination of the two is the parallelepiped section of the cornea, which uses a 2mm slit width enabling corneal surface as well as stroma to be studied. This allows us to ascertain the depth of any interesting feature e.g. foreign body, corneal abrasion. Direct illumination on the front surface of the crystalline lens reveals the 'orange peel' effect (produced by the cell bodies of the lens fibres just below the front surface of the lens), and on the iris allows observation of iris pattern.
(b) Narrow beam (optic section)
This technique is only used if you wish to investigate something. It should not be used to search for abnormalities. However, once an abnormality has been found it is easier to determine the precise depth using an optical section. The resolution of the section can be improved by reducing the slit width to a minimum, and be viewed more clearly by increasing the magnification. Generally the angle between the illuminating and observation systems should be set around 45 to 60 degrees, however to increase the amount of cross section, this angle can be further increased to 90 degrees. A good corneal section will allow at least 4 layers to be seen - tears (outer), epithelium (and Bowman’s membrane), stroma seen as the central grey granular area and the fainter back line which is the endothelium (and Descemet’s membrane).
When looking at the lens, variations in refractive index and transparency can be seen allowing observation of the Y sutures and any lens opacities. Opacities scatter and reflect more light and therefore appear white against the grey background. Sometimes due to pigmentation, opacities may appear blue (congenital) or brown.
van Herick’s Technique
A further use of an optical section is to assess the degree of openness of the anterior chamber angle. The method is as follows:
Use a low magnification (6x or 10x) to give an adequate field of view. Set the beam 60 degrees to the side of the microscope and place a narrow slit as close to the limbus as possible and normal to the cornea. To assess anterior chamber depth you compare the width of the cornea seen by the optical section with the dark section seen between the front surface of the iris and the back of the cornea. The findings are classified into four grades:
GRADE 4: The ratio of aqueous to cornea is 1:1 - open angle
GRADE 3: The ratio of aqueous to cornea is 1:2 - open angle
GRADE 2: The ratio of aqueous to cornea is 1:4 - indicates narrow angle, which should be viewed by gonioscopy.
GRADE 1: The ratio is smaller than 1:4 - indicates dangerously narrow angle, which is likely to close.
(c) conical beam
An important modification of direct illumination is used when inflammatory cells within the anterior chamber are suspected (as in acute anterior uveitis). This will show up as aqueous flare, and can only be accurately seen using a conical beam of light, set at an angle of between 45 and 60 degrees and focused onto the front surface of the cornea. Using the pupil as a dark background, aqueous flare may be seen in the space between the focused beam of light on the cornea and the out of focus beam on the lens. In the normal eye this space will be perfectly clear. To use this technique the room illumination must be completely dark.
Indirect illumination
This simply means looking at tissue outside the area which is directly illuminated and can be used in conjunction with most of the above techniques. Structures are often easier to see under indirect illumination as glare is reduced e.g. opacities, corneal nerves and limbal vessels. When using the slit lamp direct and indirect illumination are viewed simultaneously, structures viewed in the illuminated field are seen under direct illumination, but as this does not fill the whole of the field of view, anything which reflects or scatters light from outside the illuminated area is being viewed by indirect. It is therefore important to look at the entire field of view and not just the illuminated patch.
If you wish to view a certain feature by indirect illumination first locate it by direct illumination and keeping your viewing system unchanged swing the lamp to one side. Occasionally it is necessary to de-couple the beam to enable you to move the beam to one side.
Retro-illumination
This is another form of indirect viewing. The light is reflected off the deeper structures, such as the iris or retina, while the microscope is focused to study the more anterior structures in the reflected light. The most typical use is to study the cornea in light reflected from the iris, or the lens in light reflected from the retina. Features that are opaque to light appear dark against a light background (e.g. scars, pigment, and vessels containing blood). Features that scatter light appear lighter than the background (e.g. oedema of the epithelium, corneal precipitates). Note that this method is useful for examining the size and density of opacities, but not their location.
1) Use a parallelepiped focused (for example) on the iris.
2) For direct retro-illumination: the observed feature on the cornea is viewed in the direct pathway of reflected light. The angle between the microscope and the illuminating arm is about 60°.
3) For indirect retro-illumination: the angle between the microscope and slit-lamp arms is greatly reduced or increased so that the feature on the cornea is viewed against a dark background. It may be necessary to uncouple the beam.
Specular Reflection
This type of viewing is achieved by positioning the beam of light and microscope such that the angle of incidence is equal to the angle of refection. The light can be reflected from either the anterior (i.e. tears & epithelium) or posterior (i.e. endothelium) corneal surface. Note that the reflected light should pass through only one eyepiece, and therefore this method is monocular.
Method for viewing the anterior surface
1) The angle between the light and microscope arms should be about 45°
2) Use a magnification of about 25 times, and a moderate parallelepiped.
3) Gradually change the angle of the light source until a bright area of pre-corneal fluid is seen. Elevations and depressions of the anterior surface appear as dark spots in the brightly reflected area. Details of the pre-corneal film, mucous secretion, and corpuscular elements of tears may be seen.
Method for viewing the posterior surface
1) The angle between the light and microscope arms should be about 60°
2) A 2mm wide parallelepiped and magnification of 20-25x is used.
3) Find the image of the illuminating bulb, then move the light beam until the image of the bulb is just behind the posterior surface of the parallelepiped. Incidence = reflection when the dazzle from the pre-corneal fluid is seen.
4) Focus on the back of the parallelepiped. A mosaic of hexagonal endothelial cells will appear. The posterior endothelium and keratic precipitates may thus be studied.
Uses
1) To examine the epithelium and endothelium. Note that the quality of the reflection formed gives an indication of the quality of the surface from which the light is reflected.
2) Examination of the tear film.
3) Examination the crystalline lens.
Note
This technique is basically a method of evaluating the reflective qualities of the surfaces of the eye, and gives an indication of the surface quality. The four Purkinje image's are used as backgrounds - the first to view the tears / epithelium, the second to view the endothelium of the cornea, the third the anterior surface of the lens capsule and the fourth, the posterior surface of the lens capsule.
This method of illumination is particularly useful to examine the endothelium layer of the cornea (e.g. blebs, polymegathism), although very high magnification is necessary, at least 40x is required and to see individual cells 80x. As deeper layers are viewed with this method the angle between the two systems needs to be reduced (only 5 degrees between them is required when viewing the posterior lens capsule).
Sclerotic scatter
This method uses the principle of total internal reflection. A narrow vertical slit (1-1.5mm in width) is directed in line with the temporal (or nasal) limbus. A halo of light will be observed around the limbus as light is internally reflected within the cornea, but scattered by the sclera. Any corneal opacities, oedema or foreign bodies will be made visible by the scattering light, appearing as bright patches against the dark background of the iris and pupil. It is important that the room illumination is as dark as possible.
For this technique, the cornea is usually viewed by the observer's naked eye, from around the side of the microscope, however it is possible to observe the cornea using the microscope as well. To do this the slit must first be focused with direct/focal illumination (see below) on to the debris of the tear film. This ensures that the corneal surface is focused correctly. Then without moving the observation system or the entire slit lamp, the system is de-coupled and the illuminating system directed at the limbus. This will give sclerotic scatter and still allow the anterior surface to be viewed through the microscope, and allow the observer to use the most appropriate amount of magnification.
Oscillatory Illumination
A beam of light is rocked back and forth by moving the illuminating arm or rotating the prism or mirror. Occasional aqueous floaters are easier to observe. Can also be used to determine the extent of opacities in the crystalline lens.
Tangential Illumination
The iris is examined under very oblique illumination while the microscope is aligned directly in front of the eye. Useful for examining tumours and naevi of the iris.
Indications for slit lamp routine examination in private practice
It certainly can be argued that slit lamp examination is the very best method of examining the external eye and should therefore be used with every patient. Here is a list of ocular conditions and complaints that could warrant slit lamp examination:
1) Conjunctival injection
2) Lacrimation
3) Itchiness
4) Grittyness
5) Pain
6) Dry eyes
7) Photophobia
8) Haloes around lights
9) Narrow anterior angle
10) Recent foreign body sensation
11) All contact lens wearers
12) History of anterior uveitis
13) Ingrowing eyelashes
14) Pterygia
15) Blepharitis
16) Meibomium cysts
17) Styes (hordeolum)
18) Unexplained reduced visual acuity
19) Lens opacities (best seen through dilated pupil)
20) Iris pigmentation
21) Irregular pupil margins
22) Increased IOP over 40mm Hg (possible corneal oedema)
Attachments for the slit lamp
Goldmann tonometer - IOP measurement (applanation tonometry)
Pachometer - measures corneal thickness. Readings of 0.60mm and above suggest oedema may be present.
Gonioscope - used to assess anterior angle.
Binocular Indirect Ophthalmoscopy
- (Volk lens or Hruby)Used to view fundus.
Camera attachments - can be used to photograph and record the external eye of a patient
Aesthiometry - to measure corneal sensitivity.