Article Date: 11/1/2006

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.

Figure 1. Turn the knurled ring of the eyepiece to adjust the objectives.

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. 

TABLE 1

llumination Techniques
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.

Table 2

Effect of Magnification on Field of View*
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

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