Slit lamp examination: what it is, what happens and why it is used

A slit lamp combines a microscope with a light source that is shone through a slit. NHS England describes it in a patient leaflet as having two parts, a very bright source of light shone through a slit and a microscope. Together they let the examiner look at the individual parts of the eye in detail, in particular the retina at the back of the eye.

You sit in front of the instrument and rest your chin on one support and your forehead against another, which keeps your head steady while the examiner adjusts the light and focus. The examination is a way of looking at the eye, not a treatment.

Moorfields Eye Hospital describes its use in examining the eyes with a microscope called a slit lamp in the assessment of keratoconus, and the Royal Wolverhampton NHS Trust says the doctor will examine your eye under a special microscope, the slit lamp, as part of the diagnosis. In other words, the same basic instrument is used in many different eye clinics.

Most people having NHS diabetic eye screening have digital photographs of the back of the eye. NHS England explains that you need a slit lamp appointment if the screening team could not get a clear view of the back of your eyes by photography. Wirral University Hospital describes slit lamp biomicroscopy as the technique used when an adequate retinal examination cannot be obtained using digital photography.

One common reason is a cloudy lens. Wirral explains that the lens of the eye can get too cloudy to see the back of the eye, which may be because of a cataract, and asks people to say if they have had cataracts removed recently or are going to have them removed soon, because that may allow clear photographs again.

The purpose of the examination is to look for diabetic retinopathy, which NHS England describes as damage to the small blood vessels in the retina that can leak or become blocked and can affect sight. It notes that screening can detect the condition before you notice any changes to your vision. Screening is offered every 12 months to anyone with diabetes aged 12 and over.

The NHS England leaflet sets out the steps. Drops are put in your eyes to temporarily make your pupils larger. They may sting and blur your vision slightly. You are asked to sit in the examination chair and rest your chin and forehead on the support. The appointment lasts about 40 minutes. The screening team writes to you and your GP within 6 weeks with the result.

The purpose of the slit lamp appointment is simply to give the screening team a clear enough view to judge whether there are signs of diabetic retinopathy when photographs have not.

• Bring all the glasses and contact lenses you wear, with lens solution for contacts.

• Bring sunglasses, as eyes can feel sensitive after the drops.

• You may want to bring someone with you.

• Do not drive after the appointment until your vision returns to normal.

• Tell the team if you have had cataracts removed recently or have surgery planned.

The slit lamp is a basic tool in eye clinics, not only in screening. For keratoconus, where the cornea thins and bulges into a cone shape, Moorfields says that the eye doctor or optometrist will examine your eyes with a slit lamp, and that you might also have a corneal topography scan, which it describes as a quick, painless photo that checks the shape and thickness of your cornea in detail. The Royal Wolverhampton leaflet likewise says the doctor will examine the eye under a slit lamp and arrange a special scan.

Because it allows the individual parts of the eye to be looked at in detail, the slit lamp is used for different questions in different clinics. Which structures are examined depends on why you were referred, and your clinician will explain what they are looking for.

Eye drops may affect your vision for a few hours, so NHS England advises not driving until your vision returns to normal. If there are no changes, or only mild diabetic changes, and clear photographs are still unlikely, the leaflet says you will be invited for another slit lamp appointment in 12 months. If there are signs of serious changes, you will be referred to a specialist eye doctor at your nearest hospital ophthalmology clinic.

Remember that diabetic eye screening does not replace a regular eye examination. NHS England stresses that screening does not look for other eye conditions and that you should continue to visit your optician regularly as well.

NHS England also lists ways to help protect your eyes: keep blood sugar and blood pressure at agreed levels, keep cholesterol at agreed levels, attend all screening appointments and get professional advice about any new sight problems.