Slit Lamp Examination

A slit lamp examination is a routine eye test in which a doctor uses a specialised microscope with a narrow beam of bright light to look closely at the structures of your eye.

The doctor sits across from you and shines a thin, controlled strip of light — the 'slit' — into your eye. This beam acts like a cross-section, letting the doctor see layers of the eye in detail: the cornea (the clear front surface of the eye), the lens inside the eye, the iris (the coloured ring around the pupil), the anterior chamber (the fluid-filled space between the cornea and the iris), and sometimes the back of the eye. It is one of the most thorough ways to inspect the eye without any cuts or injections.

A slit lamp examination is used whenever a doctor needs a detailed look at any part of the eye, whether to diagnose a new problem, monitor a known condition, or check the eye after surgery or an injury.

• Cataracts (clouding of the lens that blurs vision)
• Glaucoma (a condition where pressure inside the eye can damage the optic nerve)
• Corneal ulcers, scratches, or infections on the front surface of the eye
• Dry eye syndrome (when the eye does not produce enough tears or tears evaporate too quickly)
• Uveitis (inflammation inside the eye)
• Macular degeneration (breakdown of the central area of the retina at the back of the eye), when used with an additional lens
• Diabetic retinopathy (damage to blood vessels in the retina caused by diabetes), when used with an additional lens
• Foreign bodies in the eye, such as dust or metal fragments
• Contact lens–related complications
• Routine pre-operative and post-operative eye checks

There are very few situations where a slit lamp examination cannot be performed. However, it may be difficult or impossible if the patient cannot sit upright or keep their head still, for example due to a physical disability or severe discomfort. In those cases, a portable version of the instrument may sometimes be used instead.

A slit lamp examination is a non-invasive (no cutting or entering the body) diagnostic test and carries almost no risk for the vast majority of patients.

• Temporary light sensitivity or mild discomfort from the bright beam — this passes within minutes
• If dilating eye drops (drops that widen the pupil to allow a better view of the back of the eye) are used, vision may be blurred and the eyes sensitive to light for several hours afterwards
• Rarely, dilating drops can trigger a sudden rise in eye pressure in people who already have a very narrow drainage angle in the eye — your doctor will assess this risk beforehand
• If a contact lens or special gel is placed on the eye during the examination, there is a very small chance of minor surface irritation

Slit lamp examination usually requires no special preparation. You do not need to fast, stop any medications, or change your diet beforehand. However, there are a few practical things to keep in mind.

If you wear contact lenses, your doctor will usually ask you to remove them before the examination. If dilating eye drops are likely to be used, it is strongly advisable to arrange for someone else to drive you home, since your near vision and light sensitivity may be affected for several hours. Wearing sunglasses after the appointment can help with the light sensitivity.

No laboratory tests or imaging scans are needed before a slit lamp examination. Your doctor may review your general eye history and any previous prescriptions, but no special pre-tests are required.

The examination itself typically follows these steps:

• You sit in a chair facing the slit lamp instrument and rest your chin on a chin rest and your forehead against a support bar to keep your head steady.
• The doctor dims the room lights and adjusts the lamp to direct the narrow beam of light at your eye.
• You are asked to look straight ahead or in different directions as the doctor examines each part of the eye in turn.
• If the doctor needs to see the back of the eye, dilating drops may be placed into your eye. You then wait, usually for some minutes, for your pupil to widen fully before this part of the examination continues.
• In some cases, the doctor places a special lens gently on the surface of the eye after applying numbing drops (anaesthetic drops that briefly remove sensation from the eye surface). This allows an even closer view of certain structures.
• The doctor records their findings. The whole examination is usually completed within a short session, though timing can vary depending on what needs to be assessed.

Because a slit lamp examination is non-invasive, recovery is immediate for most people. If no dilating drops were used, you can usually resume all normal activities, including driving, straight away. If dilating drops were used, your vision may remain blurred and your eyes sensitive to bright light for several hours, so you should not drive until these effects have fully worn off.

• Rest your eyes if they feel tired or mildly irritated after the examination — this usually settles on its own within a short time.
• Wear sunglasses outdoors if dilating drops were used, until your pupils return to their normal size.
• If a contact lens was placed on your eye during the examination, your doctor may advise waiting a short while before putting your regular contact lenses back in.
• Your doctor will discuss the findings with you at the end of the appointment or at a follow-up visit, depending on whether further tests are needed.
• If the examination reveals a condition that needs treatment, your doctor will explain the next steps — this may include further diagnostic tests, a referral to another specialist, or a treatment plan.
• Attend any follow-up appointments your doctor schedules, especially if you have a condition such as glaucoma or diabetic retinopathy that requires regular monitoring.