the contact lens exam
Features to Consider in Choosing a Biomicroscope
BY HARUE MARSDEN, OD, FAAO

Using a biomicroscope in contact lens practice is paramount to comprehensive care and lens fitting. The slit lamp examination requires an instrument that can comfortably, conveniently and effortlessly allow you to evaluate the health of patients' eyes and the fit of their contact lenses.

It All Starts with the Optics

The optical properties of the biomicroscope can affect your visual comfort depending upon whether the oculars use parallel or convergent optics. Some slit lamps claim variable interpupillary distances of 47mm to 85mm. However, with parallel optics it's oftentimes more difficult to fuse the image, especially if you have a narrow interpupillary distance or poor divergence ranges.

Variable magnification is beneficial to appropriately assess details of the cornea. Some bio-
microscopes have two magnifications (10x and 16x) while others offer three to six various magni-fication levels (6.3x, 10x, 16x, 25x, 30x, 40x). Additionally, you can replace the eye pieces with oculars of various magnification or reticules for measurement.

The capability of your biomicroscope to convert to an ocular photography system may also be a feature to consider when selecting a slit lamp. Many slit lamps have adapters that allow you to attach a commercially available camera either to the beam splitter or to the ocular itself.

Positioning is Important

The ability to position the lamp housing in front of the oculars without obstructing your view is also important in the optical system of the biomicroscope. To be able to adequately manipulate the illumination, slit width and filters improves your efficiency. Each slit lamp features a slit that rotates to assist in measuring toric lens rotation or corneal vascularization. Some are located just below the light source while others are on the lamp housing itself. A protractor is generally included to help assess rotation in degrees.

Also consider the ability to vertically and horizontally adjust the lamp for a smooth, efficient exam. With some biomicroscopes, you make all adjustments by moving the joystick front to back and side to side, with vertical adjustments made by rotating the joystick clockwise and counter clockwise. In other instruments, the vertical adjustment is on an outer ring surrounding the joystick that you rotate to adjust the lamp up and down.

Nuances for the CL Exam

You should be able to easily control the illumination while conducting the slit lamp examination. Some slit lamps have multiple specific illuminations located on the console with the power switch, while others have a rheostat that is conveniently located adjacent to the joystick.

Colored filters aid in examining the cornea and fitting lenses. The red-free (green) filter can accentuate corneal pigmentation and vessels, which can aid in measuring vascular encroachment. You can use the cobalt blue filter with sodium fluorescein to detect corneal staining and also to assess the fitting relationship of a GP lens. You can enhance this evaluation by placing a yellow filter over the slit lamp objective lenses.

The Right Slit Lamp for You

Careful consideration of the optical properties and physical manipulation of the biomicroscope can improve your efficiency in conducting an examination.

Dr. Marsden is an associate professor at the Southern California College of Optometry and is the chief of the Cornea and Contact Lens Service.