Slit-Lamp Examination
The slit lamp examination is the main way of viewing the front of the eye in detail. A slit lamp is a microscope with a bright light that can be narrowed to a thin beam, which gives an optical section through the eye. It shows the eyelids and lashes, conjunctiva, sclera, cornea, anterior chamber, iris, lens and anterior vitreous at magnifications of 6 to 40 times.
The patient rests the chin and forehead on supports, and the examiner moves the beam across the eye while viewing through the microscope.
Different illumination techniques are used: diffuse illumination for an overview, a direct beam for optical section (depth of lesions, corneal layers), retroillumination to show opacities against the red reflex, indirect and sclerotic scatter for corneal haze, and a specular method for the endothelium.
Fluorescein dye with cobalt blue light shows corneal epithelial defects, and a contact or hand held lens extends the view to the angle (gonioscopy) and posterior pole.
The examination takes 5 to 15 minutes. It is the basic tool in the diagnosis of a red eye, corneal and conjunctival disease, cataract, uveitis, and contact lens problems. It also measures intraocular pressure with an applanation tonometer attached to the instrument, and it is used for the assessment of the depth of the anterior chamber and the risk of angle closure. In an eye clinic, most conditions of the front of the eye can be diagnosed with this instrument alone.
Indications.
- Red or painful eye, discharge, light sensitivity, foreign body sensation, and trauma.
- Reduced vision, to look for corneal scars, cataract, or other causes.
- Contact lens wearers with pain or redness, and after corneal surgery.
- Uveitis and suspected inflammation, with systemic disease (spondyloarthritis, sarcoidosis, juvenile arthritis, Behçet disease).
- Pre-operative and post-operative assessment of cataract and glaucoma surgery.
- Intraocular pressure measurement, and angle assessment.
- Screening in diabetes and glaucoma, with dilated examination of the lens and fundus.
- Systemic disease with eye signs: thyroid eye disease, Wilson disease (Kayser-Fleischer ring), and drug toxicity (amiodarone, chloroquine, steroids).
Not applicable to patients who cannot sit at the instrument, for whom a hand held slit lamp is used.
Contraindications and precautions.
- No absolute contraindications.
- A suspected open globe injury: the examination is gentle with no pressure on the eye, and no contact tonometry. A hand held instrument or CT is used if needed.
- Active infection (conjunctivitis, keratitis): the chin rest and forehead bar are cleaned between patients, and hands are washed, to avoid spread, particularly of adenovirus and herpes.
- Pupil dilation is avoided in a patient with a narrow angle until risk is assessed, and neurological pupil checks are done before dilation in head injury.
- Patients with kyphosis, tremor, or learning difficulty need adapted positioning.
- Fluorescein is avoided in soft contact lens wearers while the lens is in place, since it stains the lens.
Preparation and technique.
The patient is seated and comfortable, with the chin on the rest, the forehead against the bar, and the eyes level with the marker. The room is dimmed. The examiner begins with low magnification, and examines the lids, lashes and tear film, then the conjunctiva and sclera, the cornea (epithelium, stroma, endothelium), the anterior chamber (depth, cells and flare), the iris and pupil, and the lens.
The beam height, width and angle are varied. Fluorescein drops or strips are used to show defects, and the peripheral chamber depth method assesses the angle. After topical anaesthetic, applanation tonometry is done. Dilating drops are given when the lens, vitreous and fundus are to be examined.
