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The slit lamp: what each light shows

  • Examination

The slit lamp is a microscope with an adjustable slit of light. Changing the width, angle and filter of the light shows each layer of the front of the eye.

Updated

See it in 3D: Cornea · Iris · Lens

What it is

The slit lamp biomicroscope combines a binocular microscope with a light that can be narrowed to a slit and moved around the eye. It shows the lids, conjunctiva, cornea, anterior chamber, iris and lens in three dimensions and at high magnification. With a hand-held lens it also shows the retina and optic disc, and with a contact lens the drainage angle. It is the basic tool of ophthalmology and of many emergency departments.

Normal values and scales

  • Anterior chamber cells (white blood cells) and flare (protein) are graded on a standard scale and mean active inflammation.
  • The peripheral anterior chamber depth compared with corneal thickness (the Van Herick method) estimates whether the angle is narrow.
  • Pus settled at the bottom of the chamber is a hypopyon; blood is a hyphema.
  • Gonioscopy (a mirrored contact lens) is needed to see the drainage angle itself.

How it is done

  1. Adjust the chin rest so the patient's eyes are level with the mark on the frame, and set the eyepieces.
  2. Start with low magnification and a broad, diffuse light to look at the lids, lashes, conjunctiva and cornea.
  3. Narrow the light to a thin slit at an angle (optic section) to see the depth of a corneal lesion and the depth of the anterior chamber.
  4. Shorten the slit to a small, bright beam in a dark room to look for cells and flare in the anterior chamber.
  5. Use the cobalt blue filter after fluorescein to show epithelial defects.
  6. Use retroillumination (light reflected from the iris or fundus) to see corneal and lens opacities.
  7. Hold a high-power lens in front of the eye to see the optic disc and macula.

How to interpret it

  • Cells in the anterior chamber confirm uveitis; the general doctor without a slit lamp sees only ciliary flush and a small pupil.
  • A narrow peripheral chamber warns of angle closure risk before dilating.
  • A dendritic ulcer stained with fluorescein is herpes simplex keratitis and contraindicates steroids.

Next step / Initial management

  • No slit lamp available and a red flag present: refer the same day rather than guess.
  • Anterior chamber cells or a hypopyon: same-day ophthalmology.
  • Narrow angle on Van Herick: gonioscopy by ophthalmology before routine dilation where possible.
  • Learn the basic techniques: in the emergency department the slit lamp is used for foreign bodies, abrasions and the Seidel test.

Test yourself

Think of the answer, then tap the question to see it.

What do cells and flare in the anterior chamber mean?

Active intraocular inflammation (anterior uveitis): white cells and leaked protein in the aqueous.

What does the Van Herick method estimate?

The depth of the peripheral anterior chamber compared with corneal thickness, as a screen for a narrow angle.

Which filter is used to see corneal staining with fluorescein?

The cobalt blue filter.

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Connections

See it in 3D3

The structures of Durgical's 3D anatomy where it is found.

In Pharmacology1

← All of ophthalmology

Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: Slit-Lamp Biomicroscope (StatPearls Publishing (NCBI Bookshelf), 2023); Auxiliary Lenses for Slit-Lamp Examination of the Retina (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.