Corneal topography gives the detailed curve map this doctor wants. A special camera projects a pattern of concentric lighted rings, called a Placido disk, onto the front of the cornea. The instrument reads the distortions of the reflected rings. Software turns those distortions into a map of corneal curve and power. The map marks steeper areas, flatter areas, and irregular spots across the whole surface.
A keratometer samples only a small central zone, so it can miss an uneven surface. Topography covers a much wider area, which is why it is used for fitting complex contact lenses and for following corneal disorders and transplants. It is also used in calculations before refractive surgery, because that surgery changes corneal shape. One camera, the Pentacam, uses a different optical method. It can also measure anterior chamber depth, corneal power, and the density of a corneal opacity. Whatever the camera, the printed result is a color-coded map that the surgeon reads next to the patient's refraction.
Match the test to the question being asked. Curve of the cornea means topography, thickness means pachymetry, and cell count means specular photography. Knowing which corneal test answers which question is a central skill in the biometry content area of the COA exam.
Incorrect Answers
- A. Ultrasonic pachymetry reports corneal thickness in micrometers at the spot the probe touches. It says nothing about the shape or curve of the surface.
- C. Specular photography magnifies and counts the cells of the corneal endothelium, the innermost cell layer. A cell count says nothing about where the cornea is steep or flat.
- D. A-scan biometry measures the axial length of the globe for intraocular lens power. It uses sound waves to find distances inside the eye, not surface shape.
Testing Pearls
- Corneal topography projects a Placido disk of rings and maps the reflected pattern.
- The map shows corneal curve and power, including steep, flat, and irregular zones.
- Main uses: complex contact lens fitting, corneal disease and transplant follow-up, and refractive surgery planning.
- The three corneal tests split cleanly: pachymetry for thickness, topography for curve, specular microscopy for cells.
- A keratometer reads only a small central zone, so it can miss an uneven or scarred cornea.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
