Corneal topography test

The cornea is the transparent front window of the eye and contributes much of its focusing power. An irregular corneal shape can blur or distort vision.

Corneal topography maps the curvature and optical power of the front surface of the cornea. Conventional mapping based on reflected rings does not measure corneal thickness or the back surface. Corneal tomography and pachymetry provide these additional measurements and are often used alongside topography, particularly when assessing keratoconus or suitability for refractive surgery.

During a corneal topography examination, the patient rests their head on a chin rest in front of a scanning device. The device projects a series of concentric light rings onto the corneal surface. These light rings reflect off the cornea and are captured by a digital camera integrated into the device.

A computer software processes the image of the reflected rings. The distribution, spacing, and shape of these rings provide precise information about the corneal curvature and its light refracting power. The results are displayed as a color coded map, where different colors (typically blue and purple for flatter areas, and red for steeper areas) indicate the degree of light refraction across various regions of the cornea.

Corneal topography is used for a wide range of clinical purposes:

Diagnosis and Monitoring of Corneal Diseases: The test is essential for diagnosing keratoconus, a condition where the cornea thins and becomes cone shaped, leading to blurred vision. In diagnosed cases of keratoconus, repeated topography allows for monitoring disease progression and deciding on appropriate treatment, such as corneal cross linking to stabilize the cornea or fitting specialized rigid contact lenses.

Assessment of Astigmatism: Astigmatism is a common refractive error caused by an asymmetrical corneal shape. Corneal topography helps in diagnosing and quantifying astigmatism, enabling precise prescription of eyeglasses, contact lenses, or planning for the implantation of astigmatism correcting intraocular lenses during cataract surgery.

Planning Laser Vision Correction Surgery: Before laser eye surgery to correct refractive errors, topography is used with corneal tomography and thickness measurements to assess the corneal shape and identify irregularities or areas that are too thin, which might preclude safe surgery.

Fitting Specialized Contact Lenses: In cases of corneal irregularity, such as keratoconus or high astigmatism, standard contact lenses may not provide optimal vision correction. Corneal topography provides the necessary information for manufacturing and fitting custom rigid gas permeable or scleral contact lenses tailored to the unique corneal structure.

Post Corneal Transplant Monitoring: After corneal transplant surgery, periodic topography helps assess the shape of the transplanted cornea, identify potential complications, and monitor the healing process.

Corneal topography usually needs little preparation. Contact lenses can temporarily alter the shape of the cornea and affect the measurements. Ask the clinic how long to leave them out before your appointment. The interval depends on the lens type, wear pattern and purpose of the examination, and some lenses need a longer break or repeat measurements to confirm stability.

Topography itself does not usually require dilating drops. Bring relevant previous measurements and medical documents. If other examinations are planned at the same visit, follow the clinic's separate preparation instructions.

The test itself is quick, usually lasting about ten minutes, and does not involve direct contact with the eye. It is non invasive and does not cause pain or discomfort. Since there is no contact with the eye and no pupil dilation is required, there are no known risks associated with the examination.

After the test, you can immediately resume your daily activities without any restrictions. No recovery time is needed. The test results are typically available instantly as a color printout and are provided to your treating ophthalmologist for interpretation and further management.