OP1201 – Basic Clinical Techniques

Part 1

Dr Kirsten Hamilton-Maxwell

Today’s goalsBy 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

Why slit lamps are so greatSlit 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 qualityStereoscopic imageFlexible illuminationFlexible magnification

Therefore there are many different usesEven more when attachments are added

What can we use them for?On their own With accessories

Routine examination of anterior segmentAdnexa through to anterior

vitreous

Problem-based examination of anterior segment

Contact lens examination

Assessment of anterior chamber depth and angle

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

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 practiceAdaptable to suit different practitionersGood resolutionGood depth of focusA wide range of magnifications

MagnificationSlit lamps provide variable magnificationLower magnifications are used for general

assessment and orientationHigher magnifications are used for detailed

inspections of areas of interestThere 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 methodA 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 separation

It 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

A Galilean telescopeParallel 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 magnificationsMag highest when the

convex lens is near objective

Reversal of these two telescopes produces two further minifications

No telescope provides 5th option

Zoom 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

Change eyepieces or objectiveEyepieces ObjectiveOften two sets provided with

slit lamp Typical values 10x, 12.5x, 15x or

20xInconvenient so rarely usedGenerally unnecessary on

modern slit lamps

Flip arrangement for rapid change

Usually only two options due to space confinements

Typical values are 1x and 2x

Lever

The slit lamp

What makes a good slit?A good slit needs to be

BrightEvenly illuminatedFinely focusedHave well defined, straight edgesFlexible in terms of size, shape, colour and intensity

The illumination also needs toProvide good colour rendering to detect subtle colour

changes

Slit widthContinuously variable (0 to

12-14mm)May be graduated to allow

measurementNarrow slits are used to “slice”

through the cornea to determine depth or thickness

Wide slits are used to inspect surfaces

Slit heightMay be continuous or set to fixed

heightsUsually a combination of the twoMay 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

FiltersSlit lamps may have some/all of the following filters

DiffuserHeat reductionNeutral densityPolarisingRed freeCobalt blueWratten (in observation system)

Methods of illuminationDirectIndirectRetro-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

Direct illumination

LampMicroscope

The light and the microscope are both pointed at the object of interest

Direct illuminationThere are several different forms, named simply by how

wide the slit isDiffuse (usually not a slit at all)Wide beamParallelepipedOptical section

The 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 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 importantThe 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 simultaneously

You’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

Effect of angle (cornea)

45º: balance of surface and depth

5º: surface only 85º: depth only

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 beam

This eye has iris naevi (freckles)

ParallelepipedDefault method for

corneal inspectionShows 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 sectionAllows judgement of

thickness or depthUse 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.

LampMicroscope

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.

Indirect illuminationGood 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 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

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

Directly illuminated

Indirectly illuminated

LampMicroscope

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.

Retro-illuminationLight may be reflected from 2 main structures:

Iris: this back-lights the corneaFundus: this back-lights the lens

Opacities 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 high

For fundus retro-illumination, use a short slit with narrow-moderate width, narrow angle of illumination (0-10º), and moderate magnification

Directly 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).

Retro-illuminated

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-illuminationAt 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 readingElliott, 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