Another name for this instrument is "Bio-Microscope", which is a more self-explanatory term, and definitely sounds more modern and scientific. The instrument consists of a binocular-type eyepiece for the examiner to look through and a high-intensity light source, which illuminates the eye. One can vary the degree of magnification from seeing the entire eye and eyelids in the same field to honing in on one small structure and magnifying it by up to 40 times.
The main advantage of this microscope is that the beam of light can be made into a slit, which has the advantage of adding depth of field to the observers view. To illustrate this point, imagine standing directly in the path of a beam of sunlight. You may see particles floating in the beam, but to judge their distance away from you, or the distance separating the particles, cannot be done with any accuracy. But if you stand to one side of the beam, the particles can be seen more easily, and you will also more easily be able see how far they are separated from each other. The slit lamp therefore allows the observer to position his eyes to one side and move the light beam around to obtain a "side" view. To use a more practical example: imagine an eye with a brown-coloured foreign body on the cornea. It may be easy to see the foreign body reflected in the light, but not so easy to assess just how deep the object is situated. By using the slit lamp with the beam angled to one side, you will be able to see with great accuracy whether the foreign body is superficial or whether it has penetrated to a deeper section of the cornea.
Use and preparation
You will sit on a chair with your chin and forehead resting against the instrument-support, which is attached to the slit lamp. No preparation is needed. Very young children may feel a bit apprehensive and need a bit of time to get used to the instrument. With a bit of coaxing and parental help, things should however go smoothly. Small infants may be examined by letting them stand on their mothers lap while she is seated on a chair
While the slit lamp is mostly used to study the front sections of the eye, i.e. the eyelids, the coats of the eye (conjunctiva, and sclera), the cornea, iris, pupil and lens, its range can be extended to include the deeper structures of the eye (including the retina). This is achieved by means of additional lenses, which are either held by the examiner or placed in direct contact with the eye. Injuries, various types of inflammation, infections, ulcers, growths and tumours may be evaluated on the surface of the eye and the positioning of contact lenses can be assessed with greater accuracy. If we look deeper into the lens of the eye we may see inflammatory cells in the chambers of the eye, e.g. iritis or cataract formation. We may even see the Fundus of the eye.
By adding a dab or drop of Fluorescein dye and using a blue filter, one may find smaller defects that are not immediately apparent. Thse defects, unlike the normal tissues, will be stained bright blue. Adding a drop of anaesthetic, one can also measure the eye pressure by means of the "tonometer", which is attached to a small side-arm of the slit lamp. In order for the pressure to be measured, the tonometer must make contact with the cornea. You may be aware of a light touch; it will however not be painful.
In the presence of a very hazy cornea or a very dense cataract, a view of the deeper structures may be obscured, even when using a very bright light. In such cases other examination procedures like ultrasound may be necessary.
Slit lamp examination does not involve any risk. There may be some slight discomfort due to the bright light used, but this doesn’t last very long. The small risk in using anaesthetic or dilating drops has already been covered.