Visante Omni basics
| The ATLAS Corneal Topographer provides the interpretation of data from the Placido disc image. This image can only provide the information from the front surface of the cornea. In the image below the red line is the piece of information provided by the ATLAS Corneal Topographer.
The Visante Anterior Segment OCT can only provide a map of corneal thickness referred to as the Pachymetric Map. This portion is represented by the gray fill in the middle of the image below. The Visante omni files in the gap to provide the posterior surface information (the blue line in the image below). The posterior surface, or the inside surface, of the cornea is where many times early changes occur. Ability to identify this map is essential in evaluating pre-LASIK candidates.
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The image below also illustrates this over the entire surface of the cornea.
The Visante omni is physically accomplished by networking the ATLAS Corneal topographer to the Visante Anterior Segment OCT via an ethernet CAT-5 cable. The Visante 3.0 software then combines the data from the ATLAS and the Visante.
One of the main key points here is that the placido image is the reflection off of the tear film on the cornea. Several major factors affect that. Dry eyes is easily identified by a trained specialist as there are missing segments of placido rings data. Additionally, the corneal epithelium is a dynamic tissue. What this means is that the corneal epithelium can change with changing environment. Classically, the corneal epithelium has been described as a 6 cell layer, 50 micron thick layer of tissue at the surface of the cornea. With any form of injury this tissue can be reduced in size and regenerate as allowed.
There are many states that change the size of the corneal epithelium. Dry eyes can cause erosion of the surface cell layers and therefore cause a thinning of the layer. Myopic LASIK can induce a hypertrophy of the epithelium in the centeral region of the cornea. There are studies that suggest 10% or greater hypertrophy.
The corneal epithelium is also exceptionally vulnerable to normal, yet chronic movements of the eyelid over this tissue. Much like sand at the beach that is reshaped by the waves, the surface conditions like dryness and elevations cause reshaping of the corneal epithelium. As elevations was mentioned, in certain abnormal corneal states like keratoconus, the cornea thins out and the entire stromal tissue bulges forward. The inner aspect has no capacity to regenerate or reshape. The middle segment (the stroma) also only gets thin in this case. The front surface however, has the capacity to change shape. The epithelium will redistribute such that it will thin over the main bulge and evenly distribute to the surrounding area. So in case of early keratoconus, one may see that the anterior surface placido image may seem relatively normal, yet the poster surface may show significant changes.
There are several examples presented of normal, normal toric, keratoconus, and pellucid marginal degeneration presented to make this illustration for you. There are also many case presentations that have been posted. There will be more cases to come. Stay Tuned!!!
07/04/11