Slit lamp biomicroscopy OP 1201 Basic Clinical Techniques

Slit lamp biomicroscopy OP 1201 – Basic Clinical Techniques Part 1 Dr Kirsten Hamilton-Maxwell

Today’s goals By the end of today’s lecture, you should be able to… List the uses of the slit lamp biomicroscope Identify the main components of the slit lamp; be able to operate these components Discuss and perform a series of basic illumination and magnification techniques A second lecture will follow where we talk about combining these techniques into a routine and look at some examples

Why 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 practice This is because they provide… Excellent image quality Stereoscopic image Flexible illumination Flexible magnification Therefore there are many different uses Even more when attachments are added

What can we use them for? On their own With accessories Routine examination of anterior segment Gonioscopy Adnexa through to anterior vitreous Problem-based examination of anterior segment Contact lens examination Assessment of anterior chamber depth and angle Fundoscopy Ocular photography Contact tonometry (Goldmann) Pachymetry Corneal sensitivity measurements (aesthiometry) Laser photocoagulation

Basic Design Viewing arm Biomicroscope Adjustable focus eyepieces Magnification dial Illumination arm The “slit lamp” Slit size, shape and filter controls Variable size, shape, colour and brightness Biomicroscope 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 degrees Both always central regardless of angle (isocentric) Moveable base plate and joystick control

Biomicroscope

A good biomicroscope has… Adequate working distance between the microscope and the eye to allow the practitioner to access the eye Convenient size for use in practice Adaptable to suit different practitioners Good resolution Good depth of focus A wide range of magnifications

Magnification

Magnification Slit lamps provide variable magnification Lower magnifications are used for general assessment and orientation Higher magnifications are used for detailed inspections of areas of interest There are several ways to do this Common methods: Littmann-Galilean telescope and zoom systems Less common methods: Change the eyepieces and/or change the objective lens

Littmann-Galilean telescope method A separate optical system is placed in between the eyepiece and the objective It consists of a rotating drum that house 2 Galilean telescopes plus a pair of empty slots Optics refresher: Galilean telescopes consist of a positive and negative lens that provide magnification based on the lens powers and their separation It is easy to identify whether the slit lamp you are using has this inside The 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

A 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.

Two telescopes produce two magnifications Mag highest when the convex lens is near objective Reversal of these two telescopes produces two further minifications No telescope provides 5 th option

Zoom systems This tends to be found on high-end Nikon, Topcon and Zeiss instruments Magnification can vary between 7 x to ~ 40 X I find that the image quality is not as good with zoom magnification

Change eyepieces or objective Eyepieces Objective Often two sets provided with slit lamp Flip arrangement for rapid change Usually only two options due to space confinements Typical values are 1 x and 2 x Typical values 10 x, 12. 5 x, 15 x or 20 x Inconvenient so rarely used Generally unnecessary on modern slit lamps Lever

Illumination The slit lamp

What makes a good slit? A good slit needs to be Bright Evenly illuminated Finely focused Have well defined, straight edges Flexible in terms of size, shape, colour and intensity The illumination also needs to Provide good colour rendering to detect subtle colour changes

Slit width Continuously variable (0 to 1214 mm) May be graduated to allow measurement Narrow slits are used to “slice” through the cornea to determine depth or thickness Wide slits are used to inspect surfaces

Slit height May be continuous or set to fixed heights Usually a combination of the two May be graduated to allow for measurement Long slits are used to view most structures in front of the pupil, while short slits pass through the pupil much better Short slit also used to assess the clarity of the anterior chamber

Slit orientation Achieved by rotating lamp housing

Filters Slit lamps may have some/all of the following filters Diffuser Heat reduction Neutral density Polarising Red free Cobalt blue Wratten (in observation system)

Types of illumination

Methods of illumination Direct Indirect Retro-illumination Sclerotic scatter (next year) Specular reflection (next year) Conical section (next year) A combination of these methods is used to view the anterior eye structures

Direct illumination The light and the microscope are both pointed at the object of interest Lamp Microscope

Direct illumination There are several different forms, named simply by how wide the slit is Diffuse (usually not a slit at all) Wide beam Parallelepiped Optical section The slit width will change what you can see Diffuse/wide beam for an overall view Wide parallelepiped for broad views of one plane (e. g. Surface of a structure) and narrow parallelepiped for a balanced view Optical section to “cut through” a tissue, for thickness and depth

Effect of slit width (cornea) Wide beam: mostly surface Parallelepiped: balance of surface and depth Optical section: mostly depth

Why 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 layers The wider the beam, the greater the angle needed to “see behind the surface layer” Allows estimation of depth Allows better perception of texture Allows direct/indirect/retro simultaneously You’ll find a graduated scale located at the pivot point of the two slit lamp arms It will give you the total separation between the two arms in degrees

Effect of angle (cornea) 45º: balance of surface and depth 5º: surface only 85º: depth only

Wide beam/Diffuse Used for general inspection of eye and adnexa Good for colour assessment Contact lens fit Wide slit, diffusing inserted, microscope in front, illumination angle 30– 50°, magnification of 6 -10 x Patients are generally unable to tolerate the brightness of a wide beam This eye has iris naevi (freckles)

Parallelepiped Default method for corneal inspection Shows a block of tissue in 3 -D, so good balance between surface and depth inspection Beam about 2 mm, microscope/illumination, variable angle, medium to high mag (10 -25 x) 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.

Optical 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 room You need very sharp focus A 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.

Indirect 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. Lamp Microscope

Indirect illumination Good for subtle detail, which would be obscured or washed out by large amounts of illumination Light internally reflected within the cornea, or reflected by other surrounding tissue Opacities scatter light so they will appear light in colour They 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

Directly illuminated Indirectly 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

Retro-illumination 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. Lamp Microscope

Retro-illumination Light may be reflected from 2 main structures: Iris: this back-lights the cornea Fundus: this back-lights the lens Opacities will appear dark against a bright background For iris retro-illumination, use a narrow-moderate width slit, a wide angle of illumination, and magnification appropriate to the object size/extent Decoupling may be necessary when the magnification high For fundus retro-illumination, use a short slit with narrowmoderate width, narrow angle of illumination (0 -10º), and moderate magnification

Directly illuminated Retro-illuminated Indirectly illuminated This 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).

This 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.

Marginal retro-illumination At the border of the zones illuminated by indirect and retro, therefore viewing technique is similar for retro with high mag, decoupling helps Objects of higher refractive index show “reversed illumination” Useful to differentiate microcysts (high refractive index) from vacuoles (low refractive index)

Vacuole

Recommended reading Elliott, Section 6. 61 -6. 67. There are lots of examples on Elliott Online (pictures and video) For section 6. 64, parts 2, 3, 4, 5 and 6 only