What Is a Slit-Lamp Exam?

Slit-lamp examination

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A slit lamp is a vital tool in detecting eye disease and checking the health of your eyes. Receiving a regular eye examination can help detect serious eye problems at an early stage when there may be no symptoms or your vision isn't affected.

When diagnosed early, it is easier to treat concerns with your eyes. Eye examinations can look for various conditions such as glaucoma, cataracts, macular degeneration, detached retinas, and more. However, it is not possible to examine the eye without specific equipment.

What Is a Slit Lamp?

The eye is a small and complex structure that requires specific tools to examine it in detail. One of these tools is called a slit lamp (also known as a biomicroscope). A slit lamp is a microscope with a bright light that can be focused into a thin beam.

It allows your ophthalmologist (eye doctor) to taker a close look at the structures at the front of and inside the eye-especially the cornea. It will enable your opthalmologist to look for any problems or abnormalities in your eye. 

What to Expect During the Exam

You do not need to do any specific preparation before attending a slit lamp examination. However, during the test, the opthalmologist may place drops in your eyes to dilate (widen) your pupils.

This can make your eyes blurry and sensitive to light, so it is recommended that you take a pair of sunglasses with you and arrange for someone to drive you home from the exam. 

The opthalmologist will ask you to sit in a chair and place your head on a piece of equipment that will keep your head steady. Your chin will rest on a chin rest, and your forehead against a forehead strap. It is not uncomfortable and just helps you to keep your head still.

The eye doctor will begin to examine your eyes with the slit lamp, specifically looking at the:

  • Sclera: White part of your eye
  • Cornea: Clear, dome-shaped lens in front of the eye
  • Lens: A part of the eye behind the pupil that focuses light
  • Eyelid: The thin layer of skin that opens and closes over the eyeball
  • Iris: The colored part of the eye which regulates the amount of light that enters the eye
  • Conjunctiva: A thin membrane that covers the inner surface of the eyelid and the white part of the eyeball
  • Retina: A sensory membrane that lines the back wall inside the eye
  • Optic nerve: The nerve that connects the back of your eye and transfers visual information from the retina to the brain

A yellow dye called fluorescein is used to help examine the front of the eye. It helps to make any abnormalities in the cornea and frontal areas more visible.

Your opthalmologist can add the dye as an eye drop or by using a tiny strip of paper stained with the dye to touch the white of your eye. When you blink, the eye easily washed the dye away in your tears. 

To see the back areas of your eye, like the retina and optic nerve, dilating drops may be used to widen your pupil. By dilating (widening) your pupil, it allows more light into your eye. This helps your opthalmologist to check for eye problems like glaucoma or age-related macular degeneration.

After the dilation drops are added, it takes about 15 to 20 minutes for them to work. Once they are working, your opthalmologist will examine your eye with the slit lamp again, using a different lens to look at the back of the eye.

The examination is not painful, but you may find it uncomfortable when the eye drops are added to your eye. 

Side Effects

In most cases, there are no side effects. However, the dilating drops can make your vision blurry and your eyes sensitive to light. This will wear off after a few hours.

It is best to be prepared and organize for someone to drive you home. Wearing sunglasses will also protect your eyes from sunlight until the sensitivity wears off.

In very rare cases, symptoms like nausea, vomiting, eye pain, or dizziness might occur when dilating drops are used. If this happens, return to your eye doctor right away as it could be a sign of increased pressure of the fluid in the eye, which is an emergency. 

What Your Results Mean

If your results are normal, it means that the eye structures appear healthy and no diseases have been detected. 

If your results are abnormal, it indicates a problem with your eye or eyes. The slit lamp can detect several issues, including:

This is not an exhaustive list of eye conditions. There may be other diseases of the eye that are diagnosed during a slit lamp exam. 

A Word From Verywell

Most eye problems can be detected early before they cause symptoms or problems with your vision. However, the Centers for Disease Control and Prevention estimate that 93 million adults in the United States are at high risk for severe vision loss. Still, only half visited an eye doctor in the past twelve months.

An eye examination is safe, painless, and only takes about an hour of your time. It is beneficial to be proactive and have regular eye examinations to help keep your eyes in good health. 

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Article Sources
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  2. Dartt D, Besharse J, Dana R. Encyclopedia Of The Eye. Chapter Title- Imaging Of The Cornea. Amsterdam [Netherlands]: Elsevier/Academic Press; 2010:263-268.

  3. American Academy of Ophthalmology. What is a slit lamp? Updated April 23, 2018.

  4. MedlinePlus. Slit-lamp exam. Updated February 28, 2019.

  5. Centers for Disease Control and Prevention. Fast facts of common eye disorders. Updated June 9, 2020