2Today’s goals By the end of today’s lecture, you should be able to… List the uses of the slit lamp biomicroscopeIdentify the main components of the slit lamp; be able to operate these componentsDiscuss and perform a series of basic illumination and magnification techniquesA second lecture will follow where we talk about combining these techniques into a routine and look at some examples
3Why slit lamps are so great Slit lamp assessment is considered to be the gold standard device for the assessment of the anterior segment of the eye in clinical practiceThis is because they provide…Excellent image qualityStereoscopic imageFlexible illuminationFlexible magnificationTherefore there are many different usesEven more when attachments are added
4What can we use them for? On their own With accessories Routine examination of anterior segmentAdnexa through to anterior vitreousProblem-based examination of anterior segmentContact lens examinationAssessment of anterior chamber depth and angleGonioscopyFundoscopyOcular photographyContact tonometry (Goldmann)PachymetryCorneal sensitivity measurements (aesthiometry)Laser photocoagulation
5Basic Design Viewing arm Illumination arm BiomicroscopeAdjustable focus eyepiecesMagnification dialIllumination armThe “slit lamp”Slit size, shape and filter controlsVariable size, shape, colour and brightnessBiomicroscope and illumination are mechanically coupled around central pivot point (copivotal)Both focus at the same point (parfocal)Both arms can swing independently 180º along horizontal – there is a scale in degreesBoth always central regardless of angle (isocentric)Moveable base plate and joystick control
7A good biomicroscope has… Adequate working distance between the microscope and the eye to allow the practitioner to access the eyeConvenient size for use in practiceAdaptable to suit different practitionersGood resolutionGood depth of focusA wide range of magnifications
9Magnification Slit lamps provide variable magnification Lower magnifications are used for general assessment and orientationHigher magnifications are used for detailed inspections of areas of interestThere are several ways to do thisCommon methods: Littmann-Galilean telescope and zoom systemsLess common methods: Change the eyepieces and/or change the objective lens
10Littmann-Galilean telescope method A separate optical system is placed in between the eyepiece and the objectiveIt consists of a rotating drum that house 2 Galilean telescopes plus a pair of empty slotsOptics refresher: Galilean telescopes consist of a positive and negative lens that provide magnification based on the lens powers and their separationIt is easy to identify whether the slit lamp you are using has this insideThe magnification dial will click into place as you turn it, and there will be numbers on the dial that correspond to the magnification in each position
11A Galilean telescope Parallel light enters and exits. Magnification is typically the intended outcome.However, if you look from the other side, the image will be minified.
12Two telescopes produce two magnifications Mag highest when the convex lens is near objectiveReversal of these two telescopes produces two further minificationsNo telescope provides 5th option
13Zoom systemsThis tends to be found on high-end Nikon, Topcon and Zeiss instrumentsMagnification can vary between 7x to ~ 40XI find that the image quality is not as good with zoom magnification
14Change eyepieces or objective Often two sets provided with slit lampTypical values 10x, 12.5x, 15x or 20xInconvenient so rarely usedGenerally unnecessary on modern slit lampsFlip arrangement for rapid changeUsually only two options due to space confinementsTypical values are 1x and 2xLever
16What makes a good slit? A good slit needs to be BrightEvenly illuminatedFinely focusedHave well defined, straight edgesFlexible in terms of size, shape, colour and intensityThe illumination also needs toProvide good colour rendering to detect subtle colour changes
17Slit width Continuously variable (0 to 12-14mm) May be graduated to allow measurementNarrow slits are used to “slice” through the cornea to determine depth or thicknessWide slits are used to inspect surfacesGrading allows measurements of features of interest
18Slit height May be continuous or set to fixed heights Usually a combination of the twoMay be graduated to allow for measurementLong slits are used to view most structures in front of the pupil, while short slits pass through the pupil much betterShort slit also used to assess the clarity of the anterior chamberGrading allows measurements of features of interest
19Slit orientation Achieved by rotating lamp housing Grading allows measurements of features of interest
20Filters Slit lamps may have some/all of the following filters Diffuser Heat reductionNeutral densityPolarisingRed freeCobalt blueWratten (in observation system)
22Methods of illumination DirectIndirectRetro-illuminationSclerotic scatter (next year)Specular reflection (next year)Conical section (next year)A combination of these methods is used to view the anterior eye structures
23Direct illuminationThe light and the microscope are both pointed at the object of interestFocus light and microscope on same region of interest.MicroscopeLamp
24Direct illuminationThere are several different forms, named simply by how wide the slit isDiffuse (usually not a slit at all)Wide beamParallelepipedOptical sectionThe slit width will change what you can seeDiffuse/wide beam for an overall viewWide parallelepiped for broad views of one plane (e.g. Surface of a structure) and narrow parallelepiped for a balanced viewOptical section to “cut through” a tissue, for thickness and depth
25Effect of slit width (cornea) Wide beam: mostly surfaceParallelepiped: balance of surface and depthOptical section: mostly depth
26Why is the angle important The angle between the microscope and the illumination arms is important. Wider angles…Allow view of deeper layers without interference from reflections from upper layersThe wider the beam, the greater the angle needed to “see behind the surface layer”Allows estimation of depthAllows better perception of textureAllows direct/indirect/retro simultaneouslyYou’ll find a graduated scale located at the pivot point of the two slit lamp armsIt will give you the total separation between the two arms in degrees
27Effect of angle (cornea) 45º: balance of surface and depth5º: surface only85º: depth only
28This eye has iris naevi (freckles) Wide beam/DiffuseUsed for general inspection of eye and adnexaGood for colour assessmentContact lens fitWide slit, diffusing inserted, microscope in front, illumination angle 30–50°, magnification of 6-10xPatients are generally unable to tolerate the brightness of a wide beamThis eye has iris naevi (freckles)
29Parallelepiped Default method for corneal inspection Shows a block of tissue in 3-D, so good balance between surface and depth inspectionBeam about 2 mm, microscope/illumination, variable angle, medium to high mag (10-25x)This is a narrow parallelepiped being used to view iris and pupillary margin. The light first passes through the cornea but is out of focus there.
30Optical section Allows judgement of thickness or depth Use the narrowest slit possible (0.1 – 0.2 mm), angled beam (largest angle possible), high illumination, and a dark roomYou need very sharp focusA helpful tip: Even though this instrument is called a slit lamp, we hardly ever need to use a slit this narrow. Save it for when you need to work out the depth or thickness of a corneal lesion.
31Indirect illumination An object being viewed is illuminated indirectly when it lit by reflections/scatter of light that occur when the light is shone other than onto the object itself.MicroscopeLamp
32Indirect illumination Good for subtle detail, which would be obscured or washed out by large amounts of illuminationLight internally reflected within the cornea, or reflected by other surrounding tissueOpacities scatter light so they will appear light in colourThey are best viewed against the dark pupil (or dark iris, if your patient happens to have one)To achieve the effect, keep the slit width narrow to medium (2-4 mm), and view with a medium to wide angle. Magnification will vary depending on the size and extent of the object, but it’s typically medium to high for subtle defects
33Indirectly illuminated This picture shows a contact-lens related condition called neovascularisation. These are blood vessels in the cornea. In this example, we don’t move anything but our attention – the light and focus stays where it is. We can do this because the slit doesn’t light up our whole field of view
35Retro-illumination Microscope Lamp An object of interest is lit by retro-illumination when the light source is directed onto another structure so that the reflected light must pass through that object.MicroscopeLamp
36Retro-illumination Light may be reflected from 2 main structures: Iris: this back-lights the corneaFundus: this back-lights the lensOpacities will appear dark against a bright backgroundFor iris retro-illumination, use a narrow-moderate width slit, a wide angle of illumination, and magnification appropriate to the object size/extentDecoupling may be necessary when the magnification highFor fundus retro-illumination, use a short slit with narrow-moderate width, narrow angle of illumination (0-10º), and moderate magnification
37Indirectly illuminated Retro-illuminatedIndirectly illuminatedThis is the same example from earlier. The blood vessels in the indicated section are retro-illuminated because they are being lit from behind (the light has reflected off the iris).
38This is an example of retro-illumination of the lens (the light has reflected from the retina). This patient has cortical spokes, which are indicative of early cortical cataract.
39Marginal retro-illumination At the border of the zones illuminated by indirect and retro, therefore viewing technique is similar for retro with high mag, decoupling helpsObjects of higher refractive index show “reversed illumination”Useful to differentiate microcysts (high refractive index) from vacuoles (low refractive index)