Corneal mapping – topography and tomography of the cornea

The cornea, being the outermost layer of the eye, plays a crucial role in the transmission and focus of light. Its structure, transparency and shape significantly affect the overall visual acuity. Corneal topography and tomography are advanced imaging techniques widely used in ophthalmology to assess the curvature, power and thickness of the cornea, thereby aiding in the diagnosis, management and monitoring of various corneal diseases.

Two types of corneal mapping – how do corneal topography and tomography work?

Corneal topography is a non-invasive procedure that provides a detailed description of the anterior corneal surface. Using a device called a topograph, a series of concentric rings of light are projected onto the cornea. The reflections are then captured and digitally analyzed to create a detailed map of the curvature of the cornea.

On the other hand, tomography of the cornea goes deeper and provides a more comprehensive analysis of the cornea. Tomographers use advanced technologies such as Scheimpflug imaging and optical coherence tomography (OCT) to offer three-dimensional images of the cornea, which provide insight into the posterior corneal surface and corneal thickness (pachymetry) apart from the anterior surface, hence giving a more holistic assessment.

Diagnosis and management of diseases using corneal mapping

These techniques have revolutionized the diagnosis and management of various corneal conditions.

1. Keratoconus: Keratoconus is a progressive eye disease that leads to thinning and bulging of the cornea, resulting in a conical shape. In early keratoconus, topography shows an asymmetric bowtie pattern with tilted radial axes, while tomography reveals adhesions and curling on the posterior surface before anterior surface changes are apparent. Regular follow-up with these imaging techniques allows monitoring of disease progression and timely intervention.

2. Corneal ectasia after LASIK: Post-LASIK ectasia, a rare but serious complication of LASIK surgery, also appears as curling and sticking of the cornea similar to keratoconus. Here, tomography is particularly helpful in identifying subtle changes in the posterior corneal surface and corneal thickness, thus allowing early diagnosis.

3. Corneal degeneration: Corneal degeneration, such as Fox's endothelial degeneration and keratoconus, can significantly change the shape of the cornea. Tomography can help identify typical patterns associated with these conditions and guide therapeutic strategies.

4. Contact lens fitting: both topography and tomography provide important data for custom contact lens fitting, especially in patients with irregular corneas due to diseases such as keratoconus or after corneal transplantation.

Corneal mapping in various corneal diseases

In keratoconus, the topography typically shows an asymmetric bowtie pattern with tilted radial axes or a 'hot spot' of curling on the corneal map. Tomography reveals early adhesions and height changes in the posterior surface, even before the anterior surface shows changes.

In post-LASIK ectasia, similar changes are noted, although usually focused on the area of ​​laser ablation. The changes can be subtle at first, but progression leads to thinning and steepening of the cornea, similar to keratoconus.

In Fox's endothelial dystrophy, tomography shows thickening of the cornea, especially in the center, due to failure of the endothelial pump leading to corneal edema. Early diagnosis helps in correct treatment.

Conversely, in epithelial basement membrane dystrophy (EBMD), topography may show irregular astigmatism without significant changes in tomography (in the posterior face).

Summary

Topography-type corneal mapping and corneal tomography have revolutionized the assessment of corneal health, providing important insights that guide the diagnosis and management of various corneal diseases. These non-invasive techniques have particularly excelled in the detection of early keratoconus, post-LASIK ectasia and various corneal atrophies, thus allowing timely intervention and improving the patients' visual outcomes. The importance of these diagnostic tools continues to grow as our understanding of the cornea deepens, and technology continues to advance.