Slit lamp exam: How it works and what to expect
What is a slit lamp exam?
A slit lamp exam is a test that allows your eye doctor to check your eye health. The slit lamp, a special microscope with an adjustable beam of bright light, allows the eye doctor to view all the parts of the eye.
During a slit lamp exam, an eye doctor can check general eye health and diagnose various eye conditions. This test can help your eye doctor spot signs of problems so you can begin treatment as soon as possible.
How does a slit lamp exam work?
A slit lamp exam is an important part of a comprehensive eye exam. Here’s a step-by-step guide to how a slit lamp exam works:
- Dilating eye drops – Before the exam, the eye care professional may place a drop of special dilation solution in your eyes. It typically takes 15 to 20 minutes for the medication to take effect.
- Patient positioning – The eye doctor may ask you to sit in a chair in front of the slit lamp and place your chin in the chin rest and forehead against the forehead support.
- Turning on the light beam – Once you’re in position, the eye doctor may sit in front of you, turn on the light and shine it into your eye while looking through the slit lamp lenses to see your eye.
- Examining the front of the eye – The eye doctor may first use the slit lamp to look at the anterior segment of the eye. This includes the white of the eye (sclera), the cornea and the lens of the eye. Your eye doctor may also put a drop of dye on your eye (fluorescein eye stain) to check for corneal scratches, infection or irritation caused by contact lenses.
- Measuring eye pressure – Using a small instrument called a tonometer that attaches to the slit lamp, the eye doctor may gently touch the surface of the eye (after applying numbing drops) to measure intraocular pressure (IOP). This applanation tonometry test is the standard method used to screen for glaucoma. There are other techniques that use an air puff or briefly tapping the eye with a tiny, lightweight probe to measure your eye pressure.
- Examining the back of the eye – The eye doctor may use a special handheld lens to look at the posterior segment of the eye. This may include examining the retina of the eye and the optic nerve, the nerve that sends signals from your eye to your brain to create visual images.
The slit lamp is a key part of a comprehensive eye exam that helps eye doctors visualize all the parts of the eye in detail and diagnose a variety of eye conditions.
Note: For infants, young children or patients who can’t be positioned at a standard chin rest, the eye doctor may use a handheld, portable slit lamp instead, often with the patient seated on a caregiver’s lap.
Preparing for the exam
Learning what a slit lamp test involves can help you to prepare for the exam. Dilated pupils allow more light in, so you may experience light sensitivity (photophobia) afterward. Consider bringing sunglasses to your appointment or ask if your eye doctor provides disposable sunglasses to wear home.
The dilation eye drops may also cause blurry vision for a little while. For this reason, it’s recommended to arrange for a ride home from your eye exam. Driving with blurry vision is not recommended for safety reasons.
Finally, the side effects of the dilation drops may affect your ability to work, especially if your job involves reading fine print or working with screens. Consider taking a few hours off from work after a slit lamp exam.
What happens during the exam?
During a slit lamp exam, the eye doctor asks you to rest your chin on a chin rest and your forehead against a forehead band. The eye doctor typically starts with one eye and does a full slit lamp exam on that eye before moving to the other eye.
The eye doctor may start the exam by sitting in front of you and shining a beam of light into your eye. They then may look through special lenses to see the front and back of your eyes. During the exam, they may ask you to look up, down and to the sides to allow for a better view of the eye.
During the slit lamp exam, the eye doctor may begin by looking at your eyelids and eyelashes before moving on to the conjunctiva, sclera, cornea, iris, lens and anterior vitreous, an area right behind the lens, where they look for signs of retinal detachment. The eye doctor may move the light closer to and further from your eye and may change the width of the light beam.
If the eye doctor uses dye during the slit lamp exam, they may insert the dye and then ask you to blink so that the dye will spread across your tear film. This allows the dye to help them see your cornea better to check for scratches or other issues. To see the back of your eye, they may use a special handheld lens in front of your eye.
What conditions can a slit lamp exam detect?
A slit lamp exam can detect signs of a variety of eye conditions. These may include:
- Blepharitis – A condition that causes eyelid swelling and irritation that may be caused by a clogged oil gland or overgrowth of the bacteria normally found on the eyelid.
- Cataracts – A clouding of the clear lens of the eye, caused by breakdown and clumping of protein in the lens, that is more common with age.
- Corneal injury – An abrasion or scratch on the cornea, which can be caused by a fingernail or rubbing your eye, can lead to infection if left untreated.
- Dry eye disease – A condition caused by a disruption to the tear film of your eyes that may result in eye dryness and irritation.
- Glaucoma – An eye condition that involves damage to the optic nerve and can lead to vision loss, especially if not diagnosed and treated early. Typically identified using the slit lamp together with an eye pressure test, gonioscopy, visual field analysis and imaging studies, such as photography and optical coherence tomography (OCT).
- Age-related macular degeneration (AMD) – A condition more common in older adults that affects cells in the macula, a part of the retina, and can cause vision loss in later stages. The disease can be confirmed and monitored with a dilated retinal examination and additional retinal imaging, such as OCT and fluorescein angiography (FA).
- Uveitis – An inflammation of the uvea, the middle layer of the eye, that can cause vision loss if not diagnosed and treated promptly.
A slit lamp is an important diagnostic tool for the detection of a wide variety of eye conditions and issues.
Risks and limitations of a slit lamp exam
A slit lamp exam typically carries no risk, considering it is a non-contact procedure. Some steps that may accompany it, such as applanation tonometry or fluorescein staining, briefly touch the surface of the eye but are considered safe when performed correctly.
In rare cases, some patients may also experience side effects from the eye dilation drops. These may include dry mouth, headache, stinging in the eye, nausea or vomiting. Other side effects, more common in children, can include confusion or difficulty breathing.
The slit lamp exam has a few limitations. Its usefulness depends on the skill of the provider, and it offers a better view of the front of the eye than the back of the eye. Some patients may experience photosensitivity to the light beam. This may require the eye doctor to modify the exam.
Advancements in slit lamp technology
Slit lamp technology is always evolving. Newer slit lamps may use digital technology and high-resolution images. And recent innovations are moving toward allowing eye doctors to use slit lamp technology to diagnose eye conditions more easily and quickly with the help of artificial intelligence (AI) and other technology.
AI integration
The integration of AI with slit lamp technology can help eye doctors flag potential eye issues quickly. AI can be used to analyze images from traditional slit lamps and may also be integrated into new slit lamp technology. For example, a low-cost portable slit light scanning device with integrated AI is being studied. It may help to expand access to eye diagnostics in areas where resources are limited.
Complementary imaging technologies
Many eye doctors pair the slit lamp exam with OCT, a noninvasive imaging test that creates detailed cross-sectional images of the cornea, optic nerve or retina. OCT can pick up subtle structural changes, such as early glaucoma damage and macular disease. These signs aren’t always visible through the slit lamp alone.
A related technique, OCT angiography (OCTA), maps blood flow in the retina without dye or needles and is increasingly used to help identify abnormal blood vessel growth in people with AMD and diabetic retinopathy.
Mixed reality tools
Researchers are also studying the use of mixed reality (MR) tools combined with slit lamp technology. This allows an eye doctor to wear an MR headset, either in the room with the patient or remotely, to get better views of slit lamp images. In the future, eye doctors might be able to wear these headsets while performing eye procedures.
Frequently asked questions about slit lamp exams
What should I avoid before a slit lamp exam?
Alcohol can affect blood pressure and cause eye dryness or irritation, so it’s a good idea to avoid drinking for about 24 hours before any comprehensive eye exam that includes a slit lamp test. It’s also a good idea to steer clear of anything that may cause eye strain, such as hours of screen time without a break.
How much does a slit lamp exam cost?
A slit lamp exam is part of a comprehensive eye exam. The cost of a comprehensive eye exam varies by location and whether you have vision insurance.
Does insurance cover a slit lamp exam?
Vision insurance typically covers an annual comprehensive eye exam, which includes a slit lamp test. Depending on the insurance, you may have a copay or coinsurance. Medical insurance may cover an eye exam if it’s being used for a medical problem related to the eyes.
Are there any risks with a slit lamp exam?
A slit lamp exam itself is noninvasive and carries essentially no risk. Some patients may experience side effects from dilation eye drops. These are uncommon but may include dry mouth, eye stinging, headache, nausea and vomiting.
What follow-up care is needed after a slit lamp exam?
There is no follow-up care required after a slit lamp exam unless your eye doctor diagnoses an eye condition. In that case, they will explain the treatment and follow-up for your specific condition.
The importance of regular eye exams
Regular comprehensive eye exams are an important part of taking care of eye health. A slit lamp exam is one component of a comprehensive eye exam. If it’s been a while since you had an eye exam, you may want to call your eye doctor to schedule a visit soon.
