What is a Slit Lamp?

A slit lamp is a microscope with a bright light used during an eye exam. It gives your ophthalmologist a closer look at the different structures at the front of the eye and inside the eye. It’s a key tool in determining the health of your eyes and detecting eye disease.

Before the exam. There is no special preparation needed before a slit lamp exam. However, your eyes will be dilated (widened) with dilating drops. If your eyes are dilated, you should not drive after your exam. Your vision will be blurry, and your eyes will be highly sensitive to light for several hours. Bring sunglasses to the exam and plan to have someone drive you home.

The exam. Your doctor will have you sit in the exam chair in front of the slit lamp. You will be asked to place your chin in the chin rest and your forehead against the forehead band. This keeps your head steady during the exam. Your doctor may use eye drops that contain a yellow dye to help see problems with the front of the eye. Dilating drops may also be used to widen your pupil for a better look at the back of the eye. Your doctor will sit down facing you and look through the microscope at your eyes. He or she will then turn on the slit lamp and focus a narrow, high-intensity beam of light towards your eye. Although the light is very bright, it will not cause any damage to your eye and should not cause any pain. Here's what your doctor will look at:

• The sclera. The white part of the eye. The sclera is made up of tough, fibrous tissues and forms a protective outer layer of the eye. Inflammation and discoloration of the sclera can be detected during a slit lamp exam. These can be signs of scleritis, an autoimmune disorder that can lead to vision loss. Allergies and infections of the conjunctiva (the thin, transparent tissue covering the sclera), called conjunctivitis or pink eye, can be diagnosed as well.
• The cornea. The cornea is the clear, dome-shaped window of the front of your eye. Dry eye, a problem with the eyes’ tears and tear film, can be found looking through the slit lamp. Sometimes, a build-up of abnormal material in the cornea is seen during a slit-lamp exam. This can be a symptom of corneal dystrophy, which causes blurry vision and vision loss if not treated early. You may be given an eye drop with a yellow dye (fluorescein) during this portion of the exam. This helps your ophthalmologist highlight certain areas and find injuries to the eye like a corneal abrasion, or corneal infections like herpes keratitis.
• The lens. This is the clear part of the eye behind the pupil that focuses light on the retina so you can see. Cataract (when the eye's lens becomes cloudy) can be diagnosed during the exam. This is a normal part of aging and will require surgery once it interferes with your daily activities.

The slit lamp also gives a detailed view of the back of the eye as well. To do this, your ophthalmologist will dilate (widen) your pupils with dilating eye drops. With your pupils fully dilated, he or she can see:

• The retina. The retina is the layer of nerve cells lining the back wall inside the eye. It senses light and turns it into visual messages so you can see. A torn or detached retina can be found during an exam, which usually requires treatment to prevent vision loss. A slit lamp exam can also help diagnose age-related macular degeneration, which affects your detailed, central vision, as well as inherited retinal diseases, like retinitis pigmentosa.
• The optic nerve. A nerve at the back of your eye that connects to your brain. Glaucoma is a disease that slowly damages the optic nerve and causes vision loss if not diagnosed and treated early. A slit lamp exam is a key part of making the diagnosis.