A brief introduction to the slit lamp exam
The slit lamp instrument is indispensable to any proper eye exam. You may know it as that obnoxious light that dazzles your vision with blood vessels and blinds you for a few minutes. But these minor annoyances can be tolerated when we consider that the slit lamp exam is the best way for your optometrist to examine the front portion of your eye, monitoring its health and detecting various diseases that can effect your vision.
In this article we’ll review the anatomy of the front portion of the eye, the mechanics of the slit lamp instrument, and some common conditions that can be found during the slit lamp exam.
The anterior segment of the eye
We’ve already discussed the retinal photo, which gives the optometrist a peek into the posterior, or back surface, of the eye. By contrast, the slit lamp exam allows the eye doctor to visualize the anterior, or front surface, of the eye — the part we see when looking in the mirror, or into other people’s eyes. Whereas the posterior segment of the eye translates incoming light into electrical signals that are sent to the brain, the anterior segment is responsible for focusing the light onto the retina in the first place.
The eyelids and eyelashes are external structures which protect the anterior segment of the eye. The nerves that innervate the eyelids are involved in multiple reflexes that automatically close the eye in response to potentially harmful stimuli, such as when the eye is touched or when we’re startled. The eyelid is also responsible for replenishing the entire eye with tears. Eyelashes catch debris and particles that would have otherwise entered the eye and provide some sensitivity to nearby objects, not too different from whiskers on a cat.
The most superficial portion of the anterior segment is the cornea. This is a thin layer of skin that refracts — or bends — light as it enters the eye. The cornea is one of the few areas of the body that doesn’t rely on blood vessels for its oxygen supply, receiving it directly from the air and tears surrounding it.
Beneath the cornea are the iris and pupil. The pupil is responsible for collecting light and constricts under high-light conditions. The iris surrounds the pupil and adjusts its size. The pupil also constricts as objects are brought closer to the eyes, in an important reflex known as accommodation.
Between the cornea and the iris is a space called the anterior chamber. It’s filled with a fluid called the aqueous humor, whose pressure is routinely measured to monitor for a disease called glaucoma. Techniques for measuring this pressure include the dreaded air-puff test, a hand-held device that uses a small probe, and a method called palpation in which the optometrist will touch the cornea directly.
Just below the pupil is the lens, an elastic substance which further refracts the light entering the eye. Whereas the refractive power of the cornea is fixed, the shape of the lens can be altered by muscles known as ciliary bodies in order to change the extent of refraction. In that sense, they’re our own natural pair of eyeglasses, whose ‘prescription’ changes depending on the distance to which we’re focusing.
Finally, the conjunctiva is a thin membrane which covers the whites of your eyes, called the sclera. This structure has many blood vessels which can be seen with the naked eye, and is also an important barrier between the eye and the outside environment.
The instrument
The slit lamp instrument is essentially a giant microscope that gives a magnified view of the eye. The optometrist brings it up to the patient, inviting them to place their head on the chin-rest, forehead against the bar, and look off beyond their ear. The instrument consists of two parts. A light-emitting source emits a narrow but intense beam of light that gives the instrument its name. There is also the microscope itself, which is oriented horizontally and can be adjusted by the optometrist using a joystick.
When used by itself, the slit lamp allows the anterior portion of the eye to be viewed. However, the optometrist can also use an additional, hand-held lens such as a 90 diopter in order to look through the pupil and into the posterior portion of the eye. However, this part of the eye is typically visualized using a camera that captures a fundus photo.
The earliest technology resembling the slit lamp was an early incandescent light-bulb attached to a vertical adjustable column. Over time, this set-up was improved by the biggest names in the industry, including Zeiss and Haag-Streit.
Major remarks, common disorders and conditions
The optometrist first notes the status of the lids and lashes. Common disorders of the lids include ptosis, or drooping lid. In addition, the meibomian glands —which provide the eye with a lubricant oil that prevents tears from evaporating— may be blocked, causing dry eye. This can also result in a reddish boil called a chalazion. Generalized inflammation of the eyelids is known as blepharitis and is a fairly common condition.
Next, the optometrist checks the conjunctiva. Two common observations are an injection, which refers to engorged blood vessels not caused by an underlying disease, and an edema, which refers to swelling caused by irritation of the eye. Finally, a viral or bacterial infection can cause inflammation called conjunctivitis, better known as pink-eye due to the reddish appearance of the sclera.
The optometrist then notes the tear prism, a layer of tears that sits just above the cornea. The optometrist can also stain the eye with a dye called fluorescein in order to visualize the tear prism and note the tear breakup time: the time it takes for the tears to evaporate. A brief tear breakup time can be an indication of dry eye.
Normally, the anterior chamber is free of any debris. The optometrist confirms this by saying that the anterior chamber is deep and quiet. Next, she notes the status of the cornea. Injuries to the eye can result in a corneal laceration which is visible during the slit lamp exam. In addition, keratoconus describes a condition whereby thinning of the cornea causes it to bulge outwards.
Finally, the optometrist checks the health of the lens by looking for cataracts: clouding on the surface of the lens. Cataracts are a leading cause of vision loss in the US, and their onset is associated with blurred vision and poor visual acuity. A patient who has undergone cataract surgery has their lens replaced by an artificial lens called a posterior chamber interocular lens, whose presence is sometimes noted by the optometrist.
Conclusion
The slit lamp instrument allows the optometrist to assess the front portion of the eye. This area of the eye is responsible for protecting the eye and refracting light onto the retina. Common diseases of the anterior eye such as blepharitis and cataracts can easily be detected during this crucial part of the exam.
