3-D tomography

Three-dimensional (3-D) tomography offers multiple advantages over 2-D topography for corneal analysis, and yet uptake of the superior diagnostic technology has been surprisingly slow, said Michael W Belin MD, in a keynote address at the 2nd EuCornea Congress.

[caption id='attachment_671' align='aligncenter' width='600' caption='Figure 1 – A 4-map composite display showing early ectatic change on the posterior corneal surface (lower right) in spite of a normal anterior curvature (upper left) and anterior elevation (upper right) (OCULUS Pentacam)'][/caption]

'Ophthalmologists have used 2-D surface evaluation of the cornea for over 20 years and are comfortable with it. However, while ophthalmologists are usually on the cutting edge and early adopters of new technology, there is a surprising reluctance to let go of this old and more limited technology,' said Dr Belin, professor of ophthalmology and visual science, University of Arizona, Tucson.

[caption id='attachment_672' align='aligncenter' width='600' caption='Figure 2 – A subtraction map of the posterior corneal surface. The map on the left is the post-LASIK posterior elevation. The map in the middle is the preoperative map. The difference map (far right) shows a significant change representing early identification of post-LASIK ectasia'][/caption]

'3-D tomography has been embraced in other areas of ophthalmology, and it is time to apply it to evaluation of the cornea in order to truly understand pathology that exists and improve diagnosis.'

Tomography, whether generated via Scheimpflug photography or ocular coherence tomography, uses a series of cross-sectional images to recreate the anterior segment in three dimensions. Placido-based corneal imaging only provides information on the curvature of the anterior corneal surface, while 3-D tomography analyses the anterior and posterior surfaces and allows creation of a full pachymetric thickness map. It also provides better coverage of the cornea, often from limbus to limbus, whereas images from reflective technology cover at most 60 per cent of the cornea, Dr Belin explained.

[caption id='attachment_673' align='alignright' width='400' caption='Figure 3 – Placido overlay showing the area of corneal coverage. The lower elliptical area represents the location of the area of thinning seen in true Pellucid Marginal Degeneration. This area is significantly outside the area of Placido coverage (TechnoMed C-Scan)'][/caption]

Using a series of clinical examples, Dr Belin illustrated how the benefits of 3-D corneal analysis allow for improved diagnosis of keratoconus, pellucid marginal degeneration (PMD), and post-LASIK ectasia. He also discussed its use for planning intracorneal ring (ICR) surgery and identifying corneal oedema.

For diagnosis of keratoconus, 3-D tomography is clearly superior to anterior surface analysis because posterior surface abnormalities and/or changes in pachymetric distribution typically predate changes in the anterior surface (Figure 1). Dr Belin highlighted use of 3-D tomography in a patient with so-called 'unilateral keratoconus' whose disease was apparent in one eye based on anterior surface mapping, but whose fellow eye had normal anterior curvature, elevation and curvature indices. 3-D tomography revealed an ectatic region on the posterior elevation map along with an abnormal progression index.

'If this patient presented for laser vision correction, most surgeons would not have operated on the 'normal' eye because of the pathological fellow eye. But what if both eyes looked like the so-called normal eye on screening with 2-D corneal analysis?'

The ability of 3-D tomography to image the posterior corneal surface also accounts for its advantage over Placido-based analysis for diagnosing post-LASIK ectasia. Dr Belin explained that early diagnosis of post-LASIK ectasia couldn't be done using anterior surface analysis only because the anterior surface has been surgically altered and changes to it are a late sign of ectasia development. A standard pachymetry map is also not useful for diagnosing post-LASIK ectasia because the cornea has been surgically thinned and so its pachymetry is no longer normal.

'The earliest changes from post-LASIK ectasia are seen in the posterior corneal surface, and that is the best time to intervene. If our goal in performing crosslinking is to halt ectasia before vision loss, waiting for the patient to become symptomatic with anterior surface changes is really too late,' Dr Belin said.

He added that the best way to screen for post-LASIK ectasia using 3-D tomography is with a posterior elevation subtraction map created using pre- and postoperative images (Figure 2).

'As we have published, changes in the posterior corneal surface do not occur after uneventful LASIK or PRK,' Dr Belin said.

The superiority of 3-D tomography over 2-D analysis for diagnosing PMD relates to the greater corneal coverage achieved with the former technique. Using 3-D tomography, the pachymetric map offers the best way to diagnose PMD, which will be identified as a band of thinning 1-2mm from the inferior limbus.

'A standard Placido system not only can't image where the pathology is in eyes with PMD (Figure 3), but it loses all data in the area of pathology. Therefore, it cannot be used reliably for diagnosis,' Dr Belin said.

For guiding ICR surgical planning, 3-D tomography is better than anterior surface analysis because the tomographic elevation map more accurately identifies the location of the cone than an anterior curvature map. In addition, the pachymetric map generated with 3-D analysis more accurately identifies the thinnest point than a standard pachymetric map and therefore enables safer determination of channel depth.

Dr Belin explained that standard pachymetric maps look at the normal to surface tangent whereas with 3-D analysis also allows the computation of thickness determined based on the minimal distance.

'These two measures can be very different in an ectatic eye,' he said, illustrating the point with a case in which the thinnest point identified by 3-D analysis was 23 microns less than that read on a standard pachymetric map.

Dr Belin concluded by showing how the pachymetric progression map can be used for assessing corneal oedema in addition to diagnosing corneal thinning disorders. In eyes with oedema due to endothelial dysfunction, the pachymetric progression map shows flattening from the centre to the periphery. This finding often predates changes on specular microscopy and thereby may allow early detection, Dr Belin said.