Control the beam. Examine the eye. Find the pathology.

Illumination techniques

Diffuse illumination

Wide beam, diffuser in, low magnification, the arm well off to the side.

The overview: lids, lashes, conjunctiva, the pattern of redness, gross corneal opacity, the position of the pupil. Start every examination here.

Direct focal illumination

A moderate beam, 2 to 4 mm, at 30 to 45 degrees, focused on the structure.

Surface detail with texture and depth: an infiltrate, keratic precipitates on the endothelium, a lens opacity. The workhorse technique.

Parallelepiped

A beam 1 to 2 mm wide at about 45 degrees: the lit block of cornea has a front face, a back face and a body.

Where in the cornea a lesion sits: the epithelium, the stroma or the endothelium, because the block has depth you can read.

Optical section

The narrowest beam, well under a millimetre, at 45 to 60 degrees, high magnification.

A cross section of the cornea and lens, like a histological slice: corneal thickness and thinning, the depth of an ulcer, the anterior chamber depth, and the lens layers from capsule to nucleus.

Retroillumination

A small beam sent through the pupil to bounce off the fundus, the arm nearly in line with the microscope, the structure of interest viewed against the red reflex.

Anything that blocks or scatters the returning light: posterior capsule opacity, iris transillumination defects, corneal deposits and fine vessels, dendrites as dark branching lines.

Specular reflection

The angle of the beam equal to the angle of view, so the corneal surface acts as a mirror.

The endothelial mosaic, the tear film, and surface irregularity. Used for the endothelium before cataract surgery.

Sclerotic scatter

A broad beam onto the limbus with the microscope focused on the central cornea, out of the beam.

Light carried across the cornea by internal reflection lights up any opacity from within: subtle oedema, a faint scar, the haze of a contact lens overwear.

Cobalt blue and fluorescein

Fluorescein on the tear film, the cobalt blue filter in, a wide beam and moderate magnification.

Epithelial defects glow green: abrasions, dendrites, punctate keratopathy, a leaking wound (Seidel test). Also the tear break-up time and the fit of a rigid lens.

The examination, front to back

1. 1. Eyelids and lashes

   Lid position, margin, meibomian orifices, lash line; evert the upper lid if a foreign body is suspected.

   Best with: Diffuse illumination

2. 2. Conjunctiva

   Bulbar and tarsal surfaces; the pattern of injection (diffuse, ciliary flush, sectoral); follicles versus papillae; discharge.

   Best with: Diffuse, then direct focal

3. 3. Cornea

   Clarity, epithelial integrity, infiltrates, oedema, deposits, vessels; measure any lesion and state its depth.

   Best with: Parallelepiped, optical section, cobalt blue with fluorescein

4. 4. Anterior chamber

   Depth, then cells and flare in a short bright beam against the pupil; anything settled below (hypopyon, hyphaema).

   Best with: Short narrow beam, high magnification, room dark

5. 5. Iris and pupil

   Shape, colour, the pupil's roundness and reaction, synechiae, nodules, transillumination defects, rubeosis at the margin.

   Best with: Direct focal, retroillumination

6. 6. Lens

   Through the dilated pupil: the capsule, cortex and nucleus, the type and density of any opacity, the position of the lens.

   Best with: Optical section, retroillumination

7. 7. Anterior vitreous

   Behind the lens: cells, pigment (tobacco dust), haemorrhage.

   Best with: Optical section pushed back, high magnification

The controls

• Beam width

  From a full diffuse flood, through a broad beam and a narrow parallelepiped, down to an optical section a fraction of a millimetre wide.

• Illumination angle

  The angle between the illumination arm and the microscope, usually 30 to 60 degrees for the cornea, near zero for retroillumination from the fundus.

• Beam height

  The length of the slit, shortened to a small square for looking into the anterior chamber.

• Illumination intensity

  The lamp's brightness, with a neutral density filter for the photophobic patient.

• Magnification

  Usually 10x for the survey and 16x or 25x for detail; higher powers narrow the field and shorten the depth of focus.

• Filter

  Cobalt blue excites fluorescein so an epithelial defect glows green; red-free (green) light darkens blood and vessels so they stand out; the yellow barrier filter sharpens fluorescein against the blue.