Corneal topography, tomography and specular microscopy
- Examination
Topography maps the shape of the cornea; specular microscopy counts its inner cells. Both matter before refractive, cataract and corneal surgery.
See it in 3D: Cornea
What it is
Placido-disc topography analyses rings reflected from the tear film to map the curvature of the front of the cornea. Scheimpflug tomography and anterior segment OCT also map the back surface and the thickness (pachymetry). These maps detect early keratoconus, screen candidates for refractive surgery and guide astigmatism correction. Specular microscopy photographs the corneal endothelium to measure cell density, variation in cell size and shape, and guttae.
Normal values and scales
- Keratoconus: inferior steepening, an asymmetric bow-tie pattern, raised back-surface elevation and a thin point displaced downwards.
- Healthy central corneal thickness is a little over half a millimetre; a thin cornea gives falsely low applanation pressures.
- Endothelial cell density falls with age; a healthy adult has a few thousand cells per square millimetre, and the cornea swells when density falls very low.
- Larger, more varied cells (polymegathism, pleomorphism) mean a stressed endothelium.
- Guttae appear as dark spots on specular microscopy and are the hallmark of Fuchs endothelial dystrophy.
How it is done
- Ask the patient to stop contact lenses beforehand (rigid lenses for longer than soft ones), because they temporarily change the corneal shape.
- Seat the patient at the instrument, chin and forehead on the rest, looking at the fixation light.
- Ask for a blink and then for the eyes to stay open; capture several images of each eye and repeat if the tear film is dry or broken.
- Read the curvature map (warm colours are steep), the elevation maps of the front and back surfaces and the thickness map.
- Specular microscopy: take a non-contact image of the central endothelium; the software counts cells in a set area and describes their size and shape.
- Compare both eyes and earlier scans to judge progression.
How to interpret it
- Topography is mandatory before laser refractive surgery: an early, subclinical keratoconus is the main cause of ectasia after LASIK.
- In Fuchs dystrophy, a low cell count and a thick cornea before cataract surgery predict corneal swelling afterwards; combined cataract and endothelial graft may be planned.
- A teenager whose astigmatism keeps changing, or who sees poorly despite new glasses, needs topography for keratoconus.
Next step / Initial management
- Adolescent with rising or irregular astigmatism, or poor vision with glasses: refer for topography; progressive keratoconus is treated with corneal cross-linking.
- Patient interested in refractive surgery: topography and pachymetry come before any decision.
- Morning blur, glare and guttae at the slit lamp: refer for specular microscopy and pachymetry before cataract surgery.
- Keratoconus or allergic eye disease: advise against eye rubbing and treat itching.
Test yourself
Think of the answer, then tap the question to see it.
What topographic pattern suggests keratoconus?
Inferior steepening with an asymmetric bow-tie, a displaced thin point and raised back-surface elevation.
Why is topography mandatory before LASIK?
To detect early keratoconus, the main risk factor for ectasia after surgery.
What does specular microscopy measure?
The density, size and shape of the corneal endothelial cells, and guttae.
Connections
See it in 3D1
The structures of Durgical's 3D anatomy where it is found.
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: Corneal Topography (StatPearls Publishing (NCBI Bookshelf), 2023); Specular Microscopy (StatPearls Publishing (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.