What Is a Slit-Lamp Exam?

A slit lamp is one of the devices that your doctor may use for checking the health of your eyes. Getting regular eye examinations can help identify serious eye problems at an early stage when you might not have any symptoms and your vision isn't affected.

When diagnosed early, eye diseases are usually more treatable. A slit lamp exam can be used to help in diagnosing conditions such as glaucoma, cataracts, macular degeneration, a detached retina, and more.

What Is a Slit Lamp?

A slit lamp is a microscope with a bright light that can be focused into a thin beam. It allows your eye doctor to taker a close look at the structures at the front and inside of your eye.

A slit-lamp examination is a non-invasive test, and it is not harmful. A slit lamp can only be used to observe your eyes—it is not used for a therapeutic procedure.

What Conditions Can a Slit-Lamp Exam Diagnose

Verywell / Theresa Chiechi

What to Expect During the Exam

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

Your eye doctor 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. This is not uncomfortable and it helps keep your head positioned properly for the exam.

With the slit lamp, your doctor can see the following structures of your eye:

  • 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 that 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 make abnormalities in the cornea and frontal areas more visible. Your eye doctor 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, your tears wash the dye away. 

Dilating drops might be placed in your eye so your doctor can see the structures in the back of your eye, including the retina and optic nerve. Dilating your pupil allows more light into your eye, which is helpful for identifying 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 eye doctor 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, dilating drops don't cause adverse effects. However, the dilating drops can temporarily make your vision blurry and your eyes sensitive to light. This will wear off after a few hours.

  • You should arrange for someone to drive you home because it isn't safe to drive with blurry vision.
  • Wearing sunglasses will 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 fluid pressure in the eye, which is an emergency. 

What Your Results Mean

After your slit lamp exam, your doctor might tell you that your eyes appear healthy and no diseases have been detected. 

The exam can also detect a problem with your eye or eyes. The slit lamp can help your doctor diagnose several issues, including:

  • Cataracts
  • Injury or disease of the cornea
  • Damage to the sclera
  • Dry eye syndrome
  • Retinal detachment
  • Retinal vessel occlusion
  • Macular degeneration
  • Glaucoma
  • Uveitis
  • Bleeding or a foreign body in the eye

This is not an exhaustive list of eye conditions. Other diseases of the eye can be 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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  1. National Eye Institute. Get a dilated eye exam. Updated August 2, 2019

  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