Corneal Topography
Corneal topography maps the shape of the front surface of the cornea, and often the back surface and thickness, using a camera and computer. The cornea provides about two thirds of the focusing power of the eye, so small changes in its shape change vision. Topography measures the curvature at thousands of points, and shows the result as a colour map, which finds irregularities that cannot be seen at the slit lamp or measured by routine tests.
Several technologies are used. Ring based (reflection) systems project rings of light onto the cornea, and measure the shape of their reflected image on the front surface. Rotating or slit scanning camera systems image the front and back surfaces, and measure the thickness over the whole cornea (tomography).
Optical coherence tomography of the anterior segment, and high frequency ultrasound, are other methods. Results show axial or sagittal curvature, tangential curvature, elevation maps (the height of the surface compared with a best fit sphere), pachymetry (thickness) maps, and indices of irregularity and of keratoconus risk.
The test takes 1 to 3 minutes per eye, and is non-contact. The normal cornea has a mean curvature of about 43 to 44 dioptres and a central thickness of about 520 to 560 micrometres. Topography finds subclinical keratoconus before it is visible, and screens candidates for laser refractive surgery, in whom unrecognised keratoconus can lead to ectasia (progressive bulging and loss of vision) after surgery, which occurs in about 0.02 to 0.6% of procedures.
Indications.
- Suspected or known keratoconus and other corneal ectasias (pellucid marginal degeneration, post-surgical ectasia), and monitoring of progression.
- Screening before laser refractive surgery and lens implant surgery.
- Unexplained irregular astigmatism, poor best corrected vision, or distorted vision.
- Fitting of contact lenses, especially in irregular corneas and after corneal surgery.
- Cataract surgery planning, including toric lens and astigmatism correction, and assessment of the effect of surgery on astigmatism.
- After corneal transplantation, trauma, scarring, or infection, and in corneal dystrophies and degenerations (pterygium, Salzmann nodules).
- Planning of corneal cross-linking and monitoring of its effect.
- Myopia control and orthokeratology monitoring.
Not indicated when the cornea cannot be imaged because of severe surface disease or opacity.
Contraindications and precautions.
- No contraindications. The test is non-invasive and contact free.
- Soft contact lenses are stopped for at least 3 days to 2 weeks before measuring, and rigid lenses for several weeks, since lens wear distorts the corneal shape (warpage) and leads to false results.
- Tear film problems, dry eye, and recent eye drops, and ointment, change the surface, and the test is done before anaesthetic drops and tonometry.
- Patients who cannot fixate, who have tremor or nystagmus, or who cannot place the head on the rest, give poor results.
- Pterygium, scarring, severe dry eye, and ptosis can block the view.
- Eye rubbing, which is linked with keratoconus, is noted, and the patient is advised to avoid it.
Preparation and technique.
The patient sits with the chin and forehead against the rest, and is asked to blink once or twice and then keep the eye wide open, looking at a fixation light. The operator aligns the instrument and captures several images, choosing a scan that is well focused, centred, with good coverage and a complete tear film.
The software builds the colour maps, which include the curvature maps, elevation maps of the front and back surfaces, and thickness maps. The test is repeated if the scan has gaps or poor quality scores, and the quality of the data is checked before the result is used. Both eyes are measured, and the values are compared with the other eye and with normal values for the instrument.
