Tip:
Highlight text to annotate it
X
Keratoconus and irregular cornea. We define
corneal ectasia as a protrusion of the cornea as a result
of a progressive thinning of the corneal thickness and a progressive
increase of the corneal curvature. Generally they are bilateral
and asymmetric, and can be classified in primary and secondary ectasias.
Within the primary we have: keratoconus,
pellucid marginal degeneration and keratoglobus. And within
the secondary or post traumatic: postLASIK iatrogenic ectasia
and traumatism. Keratoconus is a corneal disease
characterized by a progressive evolution. During this evolution
the cornea adopts an irregular conical shape due to the alteration
of the internal structure of the corneal tissue. The main
anatomic signs are: corneal thinning of its
central or paracentral area and a posterior float elevation
associated to a high irregular astigmatism which causes ocular
aberrations that cannot be corrected with glasses. In this
pathology the corneal biomechanics can be altered. In the
biomechanics there are other factors that are taken into account:
intraocular pressure, atmospheric pressure, lid tension,
extraocular muscle tension and ciliary muscle tension.
And also intracorneal factors such as: corneal
thickness and the crosslinking of collagen fibers.
The etiology or keratoconus cause in unknown and the role
inheritance is not clearly defined. What we know is that the interior of
the cornea has a large number of collagen fibers, which are
proteins that conform the internal corneal tissue structure
affording it flexibility and elasticity. These
collagen fibers are arranged in an orderly network.
If fiber networks suffer breaks, this leads to
corneal ectasias, in between which we find keratoconus.
Keratoconus can be classified in four grades:
Grade 1 or subclinical: there would be a posterior
corneal surface elevation with conserved visual acuity
and K´s lower than 45 diopters. At grade 2
the elevation would be seen on both the anterior and posterior corneal surface.
Visual acuity would be altered and the K´s would be between
45 and 52 diopters. There are also
Vogt’s striae. At grade 3, in addition to the above
we would find K´s between 52 and 55 diopters.
And at grade 4 the K´s would be in excess of 55 diopters
and we would find scarring and corneal opacities.
Keratoconus diagnosis
An early diagnosis of this pathology is very important
and it is identified using a topographer which evaluates corneal
surface, specially of the posterior corneal surface.
Orbscan is a good topographer to carry out this
diagnosis. Orbscan uses a slit-scan
to measure the elevation and the Placido rings to measure
the curvature. The combination of topography with elevation and curvature
has great potential for detecting corneal anomalies, adapt
contact lenses and planning refractive surgery.
To carry out a good diagnosis
its four maps are required. The one for anterior elevation,
posterior elevation, Keratometric and Pachymetry.
In addition, it provides other data such as the measurement white to white
the pupil diameter, the thinnest point and the kappa angle.
With the Orbscan the elevation maps are constructed
in function of a reference surface. This is a sphere which contains
the majority of the corneal surface and the points of this sphere
are green. The elevated points above the
reference surface would be represented by
yellow, orange and red colours. And the depressed points
under the reference surface in blue tones.
The Keratometric map would show
the power and radius of curvature. Red colours
would indicate a small radius and a high power and
the blue colours a high radius and a low power.
On the Pachymetry map the red would indicate
a thinner cornea and the blue a
thicker one. This is very useful for finding
the thinnest areas of the cornea. What order should
be followed to analyse the maps? First and foremost, it is important to analyse
each of the maps independently. Secondly,
correlate the maps to resolve any query and finally
analyse the four maps as a whole. We start off with
the elevation map of the anterior surface. The highest point exceeding 30 µ and the difference between the highest point and the most depressed exceeding 50 µ would suggest an ectasia.
The highest point exceeding 30 µ
and the difference between the highest point and the most depressed exceeding
50 µ would suggest an ectasia. As regards the elevation map
of the posterior surface, the most important thing for the early detection of keratoconus
as it is the first thing which is affected in this pathology.
There is a suspicion of corneal ectasia when the highest point exceeds 50 µ
and the difference between the highest point and the lowest
exceeds 100 µ. On the Keratometric map
the K values must be constant at the main meridians
due consideration must be given to K´s exceeding 48 diopters.
On the Pachymetry map, pachymetries of less than 475 µ
are suspected of ectasia.
The thinnest point would coincide with the highest point, more curved
and it is usually inferior, temporary displacement.
The four maps must be correlated and analysed
as a whole. The highest point of the anterior surface
must coincide with the highest point of the posterior surface.
That of maximum curvature and the thinnest. It is usually
inferior, temporary displacement.
Pentacam is also a topographer for corneal study
This topographer uses a rotating Scheimpflug camera
to produce high-resolution 3-D images of the
anterior segment of the eye in 2 seconds.
Obtaining up to 25,000 points of the corneal elevation map.
By means of the QS parameter it allows
the quality of the test to be ascertained.
Here we can see a 3-D image of the anterior pole
taken by Pentacam. Pentacam, as with
Orbscan, uses four maps. The anterior elevation one,
posterior elevation, curvature and pachymetry.
It uses the same colour code as Orbscan but different
elevation parameters. In this case, the elevation of the posterior surface exceeding
18 µ in the central zone of 4 to 5 mm would be
suspected of ectasia. Pentacam carries out a
pachymetric progression profile based on normal corneas.
Thin corneas would be represented above
the normal curve and thick corneas would be represented below the
normal curve. By using a pachymetric progression profile
we can also identify the location of the thinnest points in the cornea.
Another parameter to take into account is the rate of progression. Normal corneal should be
under 1,2.
The pachymetric progression of a keratoconus would be shown
in a curvatura with points represented over the normal curve.
The rate of progression would be above 1,2.
Pentacam uses a Belin / Ambrosio analysis to calculate
the corneal elevation. It uses a reference sphere with a radius depending on the layers.
Belin / Ambrosio analysis, optimises the sphere
not taking into account the central areas, 4 and 5mm.
In this way, pathologies stand out.
In the diagram we can see that with the red colour spheres the
keratoconus would be much more highlighted.
If we suspect of keratoconus this analysis can help to identify it.
With the standard display, on the first representation,
there can be doubts, but with the optimised Belin / Ambrosio display
keratoconus is evident. Moreover, it also differentiates
both spheres making the result conclusive.