the contact lens exam
Four Secrets to Using a Slit Lamp to its Fullest Capabilities
BY CRISTINA M. SCHNIDER, OD, MSC, FAAO
When I think about performing biomicroscopy on an eye, I always think of a quote from one of my favorite authors, the late Douglas Adams of The Hitchhiker's Guide to the Galaxy: "A scientist must be absolutely like a child. If he sees a thing, he must say that he sees it, whether it was what he thought he was going to see or not. See first, think later, then test. But always see first. Otherwise you will only see what you are expecting. Most scientists forget that."
Check Your Expectations
Replace the word "scientist" with "biomicroscopist" and you have my advice for becoming a champion slit lamp driver. My personal belief is that the biggest disservice we do in our early procedures, methods and ocular anatomy classes is to tell students that the cornea is clear. Hence the wisdom of Douglas Adams's quote: If you expect to see nothing, that's exactly what you'll see.
1. As any intrepid explorer would, begin with a "big picture" look: low magnification, moderate beam width. Save the higher magnification for the end or when you find something fascinating to observe.
2. Start from a known location, such as the tear film, to orient yourself. Look for the tiny specs of debris and fringes of the lipid layer and pull them into sharp focus before progressing. With low magnification, your depth of field should let you sufficiently observe all corneal layers in good focus.
3. Drive your slit lamp like a high performance sports car -- not like the family station wagon. Ideally, biomicroscopy is a dynamic yet controlled process. Scan in the area that the light source directly illuminates as well as to each side in the indirect and retro-illumination areas as you move the observation system across the cornea. At the same time, move the light source from the far left side to the far right side and back regularly.
You'll detect most interesting things as the light flashes by, often in the "pseudo-retro-illumination." By the way, because slit lamps are inanimate objects incapable of independent movement, there's no need to keep a strangle hold on the base of it. Place one hand on the joy stick to drive the observation system and the other on the light tower to create the dynamic lighting effects so critical in picking up the "fun" stuff.
4. Different objects require different lighting conditions: direct for opaque lesions (scars, pigment) and indirect and/or retro-illumination for refractile (translucent) lesions.
My favorite and probably the most overlooked illumination is specular reflection. It can tell you much about surfaces (the tears, a contact lens, the lid margins, the lids and the endothelium). Just watch the reflection move across the surface as you move the light tower. If it's jumpy, bumpy or gets deflected, then the surface isn't smooth. Watch it "run" along the lower lid margin of a nice healthy eye, vs. bumping along the pits and scars seen with chronic meibomian gland dysfunction. Or watch the reflex go from shiny to matte as a contact lens dries up after the blink.
For your next patient, see if you can find something on the cornea (a nerve fiber, a cob-web like nerve plexus opacity, a blood vessel, endothelial pigment, etc.). You should even be able to see individual punctate dots of corneal staining in white light without fluorescein if you're looking.
By the way, for you Hitchhiker's Guide fans, the ultimate answer to life, the universe and everything is...42.
Dr. Schnider is director of Academic Affairs at Vistakon. She's a diplomate in the Cornea and Contact Lens Section of the American Academy of Optometry and has lectured and published internationally on issues related to cornea and contact lenses.