Slit lamp biomicroscopy
OP1201 – 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
 Routine examination of
anterior segment
 Adnexa through to anterior
vitreous
 Problem-based examination
of anterior segment
 Contact lens examination
 Assessment of anterior
chamber depth and angle
With accessories
 Gonioscopy
 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 5th
option
Zoom systems
 This tends to be found on high-end Nikon, Topcon
and Zeiss instruments
 Magnification can vary between 7x to ~ 40X
 I find that the image quality is not as good with zoom
magnification
Change eyepieces or objective
Eyepieces
Objective
 Often two sets provided with
 Flip arrangement for rapid
slit lamp
 Typical values 10x, 12.5x, 15x or
20x
 Inconvenient so rarely used
 Generally unnecessary on
change
 Usually only two options due
to space confinements
 Typical values are 1x and 2x
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 12-
14mm)
 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
Microscope
Lamp
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 6-10x
 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 (10-25x)
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.
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.
Microscope
Lamp
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.
Microscope
Lamp
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 narrow-
moderate 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