SLIT LAMP EXAM
10 Steps to a Better Slit Lamp Examination
Brush up on this essential part of every eye examination.
By Etty Bitton, OD, MSc, FAAO
Are
you getting the most out of your slit lamp exam (SLE)? Like most, your slit lamp
skills have no doubt improved with time. To maximize the use of your slit lamp,
try adding the following 10 reminders to your routine.
1.
Clean
Up Your View
Whether or not you cover your slit lamp at the
end of the day, dust and oils from your eyelashes will soil the oculars. Make sure
you dust the mirror of the illumination column as well as the patient side of the
oculars. You can use a small brush, which usually comes as part of the slit lamp
accessories, to dust off the mirror. Adding a routine cleaning will keep your image
sharp.
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Figure 1. Turn the
knurled ring of the eyepiece to adjust the objectives.
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2.
Adjust Your Oculars
This
step may be more pertinent if you practice in a group or hospital setting whereby
several individuals share the equipment. Verifying the interpupillary distance of
the oculars prior to evaluation will maximize three dimensional (3D) viewing of
the ocular tissues. In addition, you should also adjust the objectives to your prescription
by turning the knurled ring (Figure 1) of the eyepiece. Daily handling can displace
the interpupillary distance of the oculars, hence a small adjustment could make
a big difference with respect to viewing comfort and rendering subtle 3D details
more evident.
3.
Follow
the Anatomy
A comprehensive SLE should
follow a logical sequence. It's best to begin with low magnification and observe
the external structures first, such as lids, lashes, inner and outer canthi and
tear meniscus. You can also evaluate the overall appearance of the bulbar conjunctiva,
cornea, iris and pupil.
Begin by sweeping the lid margin from
the outer canthus towards the inner canthus. Of particular interest for contact
lens (CL) wearers is a detailed examination of the lid margin, paying particular
attention to the lids, lashes and tear film. Examine the lashes for signs of blepharitis,
madarosis (loss of lashes) and trichiasis (misdirected lashes). Observing the tear
meniscus should reveal a uniform structure with little debris and an inferior meniscus
that's slightly larger than its superior counterpart. Note lid margin scars or deformities,
especially if they obstruct the flow of the tears towards the punctum. Meibomian
gland expression should require little force and reveal clear liquid secretions.
Meibomian gland dysfunction is often underdiagnosed, leading to an unstable tear
film and unexplained CL dropout. Make lid eversion a routine part of every anterior
segment examination, even more so in CL wearers. Check the lids for signs of papillary
response or hyperemia. Some silicone hydrogel lenses that have higher modulus have
resulted in a papillary response in some patients. Switching to a lens with lower
modulus may have beneficial effects.
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TABLE 1 |
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llumination Techniques |
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ILLUMINATION
|
ILLUMINATION
ANGLE (DEGREES) |
MAGNIFICATION* |
TISSUES
|
|
Diffuse |
30-45 |
Low |
External
overall view, lid, lashes, CL fit |
|
Direct: |
|
•
Broad beam |
30-45 |
Low-med |
Conjunctiva,
cornea |
|
• Parallelepiped |
30-45 |
Med |
Cornea,
meniscus, iris, lens |
|
• Optical Section |
30-60 |
Med |
Angle
estimation, corneal layers, lenticular layers |
|
• Conical Beam |
30-45 |
Med |
Anterior chamber (cells) |
|
Indirect |
45-60 |
Med |
Cornea |
|
Retroillumination |
0 |
Low-med |
Transillumination
of the iris, lenticular opacities
direct |
|
Specular
Reflection |
90 from microscope
|
Med-high |
Tear
Layer, endothelium |
|
Sclerotic scatter |
60 |
Low |
Corneal
scars, central edema |
|
Tangential |
70-80 |
Med |
Iris
(freckles, suspicious nevi) |
|
* Low: 6X to 10X, Med: 10X to 16X, High: 25X
to 40X |
A
detailed examination of the corneal structures (epithelium, stroma and endothelium)
is next, varying magnification and illumination as needed. Examine the iris, pupil
and crystalline lens last because the direct illumination is more bothersome for
the patient. A systematic SLE will improve efficiency, uncover reasons for CL intolerance,
reduce chair time and limit omissions of tissue anomalies.
4.
Know
Your Illuminations
Can you differentiate between specular reflection
and sclerotic scatter? Most practitioners vaguely remember learning about illuminations,
but the specifics have developed cobwebs over time. Most of us perform SLE under
a somewhat automated mode. We constantly switch from one type of illumination to
another without realizing it.
A review of illuminations may be helpful in
reminding us of those used less frequently but with inherent advantages. Table 1
offers a quick overview of different illuminations and which structures are best
viewed under these illumination techniques. Some slit lamp models have the ability
to tilt the illumination column (towards the patient) up to 20 degrees (in 5 degree
increments). This oblique illumination is useful in reducing reflections during
gonioscopy. In modern slit lamps, manufacturers have replaced most tungsten type
bulbs, which have a redder light, with more efficient halogen illumination, which
provides a bluer light. Fluorescein is also useful in performing the Seidel test,
which identifies aqueous leaking.
5.
Enhance
Your View
Adding ophthalmic dyes to your SLE will improve
your diagnostic evaluation. Sodium fluorescein highlights areas of corneal stress
while lissamine green is best at highlighting conjunctival problems. Better tolerated
than rose bengal, lissamine green is observed in white light and provides additional
insight for patients presenting with dry eye symptomology or CL intolerance.
Document staining, noting the form, depth
and extent. The shape of staining can be micropunctate (resembling small dots),
macropunctate (larger dots) or coalescent staining (a patch). Its depth can be limited
to the epithelium or it may enter the stromal layers. You can approximate the extent
in percentage of the surface area affected by the staining. Improving your documentation
of staining will facilitate your observation for any progression or regression at
follow-up visits.
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Table
2 |
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Effect of Magnification
on Field of View* |
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MAGNIFICATION |
FIELD OF VIEW |
|
6X |
35.1mm |
|
10X |
22.5mm |
|
16X |
14.1mm |
|
25X |
8.8mm |
|
40X |
5.6mm |
|
* Using a 12.5X eyepiece
(Topcon SL-7E) |
6.
Filters
We perform most of a SLE using white light. When
using fluorescein, use a yellow (Wratten #12) barrier filter in conjunction with
a cobalt (blue) filter to maximize viewing of the fluorescence. Positioning the
yellow barrier filter in the path of the returning light and not in the path of
the incident light will enhance the contrast significantly. Newer slit lamps have
integrated the yellow barrier filter in the ocular housing, as its use is becoming
more commonplace.
You can use the red-free (green) filter to
differentiate vascular from pigmented lesions. Blood vessels and small hemorrhages
will take on a dark appearance with the use of the red-free filter, whereas pigmented
lesions will remain dark. Some slit lamps can also be equipped with a neutral density
filter and/or a heat-absorbing filter to increase patient comfort.
7. Magnification
Most slit lamp models offer 10X, 16X and 25X
magnification, although some models extend to 40X. Most use eyepieces of 10X or
12.5X. As magnification increases, it limits the depth of focus, hence small adjustments
with the joystick of the slit lamp will keep the image in sharp focus. Furthermore,
increased magnification reduces the field of view (Table 2), allowing for greater
detail of structures. Depending on the model, magnification drums can have specified
click stops or be continuous. The continuous option, also called zoom system, has
the advantage of a smoother transition with no image loss.
If upgrading your biomicroscope is not in
your immediate plans, then try extending the magnification by simply purchasing
an additional set of oculars (for example, 20X). Returning the magnification to
the lowest setting at the end of the examination will reduce prep time for the next
patient.
8.
Illumination
Intensity
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 |
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Figure
2. Moving the observation arm from its habitual position
will enhance your view of the corneal layers. |
Novice users have a tendency to use the highest
illumination intensity during the entire examination. The problem is that these
habits are hard to break. The examination should begin under the lowest intensity
to examine external tissues and to allow the patient to adapt to the illumination,
which is even more important if the patient has undergone pupillary dilation. The
slit width is often decreased to examine details of ocular tissues under higher
magnification. Reserve higher intensity illumination for examining details and use
it for a shorter amount of time to conserve energy. You can equally reduce the slit
beam height to increase patient comfort during the use of higher illumination intensity.
Returning the intensity level to its lowest
setting following examination will prolong the longevity of the light bulb, reduce
prep time and limit photosensitivity for the next patient.
9.
Viewing
Angle
During most of the SLE, the observation arm of
the slit lamp is directly in front of the patient (Figure 2a). You can move the
illumination arm to obtain different illumination angles to assess different tissues.
Large illumination angles allow you to determine the depth of corneal and lenticular
lesions with greater accuracy because the distance between layers of these tissues
is increased under these conditions.
To increase the distance between layers even
further, make the illumination angle even larger. One way of achieving this is to
displace the observation arm from its habitual central position to a more nasal
one while extending the illumination column more temporal (Figure 2b). This will
enhance your view of the individual layers of the cornea or lens and allow you to
determine the depth of lesions with enhanced accuracy.
10.
Measuring
Eyepiece
Practitioners have traditionally made a subjective
evaluation of the height and width of lesions, nevi, staining, meniscus and neovascularization.
A measuring eyepiece or graticule can replace a traditional eyepiece to considerably
improve this measurement. The eyepiece has a linear and an angular scale, the latter
being useful for evaluating toric lens rotation with improved accuracy. With the
advent of ocular photography, many offices photograph lesions to properly document
problems and to allow for future comparisons. If this capability isn't available
in your office, then a measuring eyepiece may considerably enhance your documentation.
You can subsequently store the eyepiece for safekeeping and future use.
Final Pearls
SLEs are dynamic. You should have one hand on
the joystick and the other available to adjust the illumination arm, magnification,
lighting intensity, slit width or even to hold an accessory instrument.
Locking your biomicroscope following examination
will prevent unnecessary swinging and possible damage to the mechanics of the slit
lamp during positioning towards or away from the patient. Using a dust cover over
the slit lamp at the end of the day will help protect your investment.
Knowing the different options (filters;
illumination width, height and intensity; magnification) your slit lamp has to offer,
coupled with proper maintenance, will ensure a high image quality and help you perform
a more efficient SLE.
For references, please visit
www.clspectrum.com/references.asp
and click on document #132.
Dr. Bitton is an associate professor of
optometry at the École d'optométrie, Université de Montréal
and is the Externship Director. She is also Vice-Chair of the AOCLE.
Contact Lens Spectrum, Issue: November 2006