Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Chapter 3: Fundamentals on Corneal Topography

Figure 21: A diopter difference map is useful to assess the validity of the different exams with the same fixation performed in the same session. Low differences due to tear film irregularities, lid aperture and blinking is acceptable. In case of difference between maps taken at the same moment, they need to be repeated, after a few blinks form the patient. If significant difference persists, try instillating a tear substitute in both eyes and wait a few minutes. Should differences persist, repeat the exams in a few days. Image shows a left eye with regular (wit-the-rule) high astigmatism : both axial diopter maps were taken in the same session: differences exist between the exams. Eye fixation is the same (center): differences a attributable to different lid aperture and form blinking. Axial diopter difference (down, with a square grid overlay) shows that differences are almost non significant (around 0.25 - 0.50 diopters), but exist. Such differences are physiological: difference maps allow validation of various exams taken in a same session.

45

Figure 22: Difference maps ease the astigmatism progression follow-up . Tangential diopter displays show right eye maps of a 22 year old myopic patient referred for refractive surgery. To our surprise, neither glasses nor contacts had astigmatism. The existence of astigmatism was ascertained with the keratometer, subjective refraction and skiascopy. Corneal topography was performed and helped the demonstration of its existence. Figure shows a difference map between two exams taken with a 3 months delay (see the dates of the exams). Tangential diopter difference is 0 (green), meaning that no changes have occurred in that period of time. The first impression is that the guy never had good refraction, but new topographic exams will be performed 6 months and one year later, before refractive surgery is decided, so as to make sure that no keratoconic formation is on the way.

Figure 23: Difference maps are extremely useful in the keratoconus progression follow-up . Axial diopter displays show right eye maps of a 21 year old myopic and astigmatic patient with oscillating visual acuity. The existence of a keratoconus in his right eye was ascertained with the corneal topography performed in his first visit (map on the left). Figure shows a difference map between two exams taken with a 9 months delay (see the dates of the exams). Axial diopter difference is not 0 (yellow-orange, around 1 diopter progression), meaning that changes have occurred in that short period of time. The first impression is that the guy has a progression that will be confirmed in future topographic examinations, and we suspect that a keratoconic formation is on the way in his left eye (apparently normal in the last topographic examination).

46

Chapter 3: Fundamentals on Corneal Topography

BRIEF ATLAS OF CORNEAL TOPOGRAPHY

SPECIAL TOPOGRAPHIC CONDITIONS

Figures 1 to a 20:

All maps have been taken with a KERATRON™ Corneal Topographer (Optikon 2000® S.p.A, Italy - Europe). The corneal maps are courtesy of:

Istituto Scientifico Ospedale San Raffaele - Milano (Prof. Brancato - Dr. Carones) Ospedale Fatebenefratelli - Roma (Prof. Neuschüller - D.ssa Cantera)

Centro Oculistico - Rovigo (Prof. Merlin - Dr. Camellin)

Clinica Oculistica Universitaria - Padova (Prof. Bisantis)

University of North Carolina - Chapel Hill (Prof. Cohen - D.ssa Tripoli) University of California - Jules Stein Institute - Los Angeles (Dr. Maloney)

We want to specially thank them as well as the manufacturer of the Keratron™ videokeratoscope, Opticon 2000® S.p.A., for the permission to reproduce them.

Figure a1: Map of a Normal Round Cornea

There is a wide spectrum of normality. No human cornea demonstrates the kind of regularity found in the calibration spheres of a topographer: the eye is not polished glass-made. Normal corneal topography can take on many topographic patterns: Figure shows the axial map of a right eye normal round cornea, with concentric green rings in an absolute scale. Note that the nasal side of this healthy corneal map becomes blue more quickly than temporal side, indicating that the nasal cornea is flatter than the temporal. In the central 3 mm zone, there is a small amount of astigmatism (1 D displayed), which is within normal limits, and does not mean that the patient needs to be corrected with this astigmatism.

47

Section II: Topography

Figures a2 and a3: Normal Cornea with Astigmatism According to the Rule

Regular astigmatism (with-the-rule) gives an oval axial corneal map, being the most common deviation from optically perfect spherical (round) cornea. Observe that the bow tie is vertical (the long axis is near the vertical meridian) in an axial map, representing a cornea having with-the-rule-astigmatism. Figure displays an axial curvature map of a -3.7 D regular astigmatism in an adjustable scale. Always check the scale in which the map is offered: colour differences do not always mean a difference in dioptric or radial values, but can mean a difference in the scale used by the explorer. Note that a simulated keratometric overlay is displayed at the centre of the bow tie.

Modern topographers run under Windows™ operating system, and are easy to use. Most software enables to enlarge desired areas for better explanation to the patient and to better view the details. Figure shows an enlarged area of a with-the-rule astigmatism with an absolute scale.

48

Chapter 3: Fundamentals on Corneal Topography

Figure a4: Topographic Map of Astigmatism Expressed in Heights

Representation of the topographic map of an astigmatism (-3.75 D at 176º) expressed in height (in microns). The yellow area corresponds to a sphere with a defined radius, while orange-red and green-blue areas correspond to either elevation or flattening of the cornea. Note that colour scale may confuse the explorer.

49

Section II: Topography

Figures a5 and a6: Keratoconus

An important indication of corneal topography is the screening of candidates for refractive surgery. It is very important to identify patients with corneal ectasia, since surgical outcomes are uncertain in most cases. Early detection of a subclinical keratoconus can save the patient of a refractive procedure (incisional or photoablative) that likely will not result in the desired visual outcome, and may result in dangerous corneal thinning. The most frequent ectatic corneal disorder is keratoconus. This condition is characterised by a corneal stromal thinning. It typically presents in early adulthood, is almost always bilateral (although can be very asymmetric), and progresses slowly over the years. Mild keratoconus cannot be detected easily at the slit-lamp, and only corneal topography can help detecting them. Some other conditions, like corneal warpage of RGP contact lenses may mimic mild keratoconus corneal maps. In most cases, the corneal thinning occurs just inferior to the corneal centre. Protrusion of this region gives the cornea an exaggerated prolapsing shape. The point of maximum protrusion is called the apex of the cone. Figure displays a typical map of a moderate keratoconus (- 5.6 D), showing a corneal steepening inferior to corneal vertex (orange-red, in absolute scale, in the shape of a pear fruit). Note the high corneal central power (around 50 D), the inferior cornea (orange) steeper than superior cornea (green), and the large difference between the power of the corneal apex and that of the periphery. (Cont. in next page).

50

Chapter 3: Fundamentals on Corneal Topography

A topographic classification of keratoconus can been established:

The comparison of representation of dioptric powers, axial (left) and local (right), of the same eye with an inferotemporal keratoconus is surprising: note the minimal extension of the corneal surface involved in the pathology, and the flattening of the adjacent area.

Figure a7: Corneal Ulcer

By quantifying the irregularity of the cornea, topography helps to determine the proportion of the visual loss of a patient suffering from a corneal ulceration or epithelial disruption close to the visual axis. It also helps to follow-up a corneal abscess or ulceration. Figure shows the true curvature map of a corneal inferior ulceration. Note the local flattening of the corneal surface (in blue), resulting form the localised depression of the ulcer, surrounded by a ring of oedematous elevated tissue (in red).

51

Section II: Topography

Figure a8: Marginal Pellucid Degeneration

Stromal corneal disease include a variety of inflammatory and non-inflammatory disorders, like Terrien’s marginal degeneration, Mooren’s ulceration, pellucid marginal degeneration an others. Figure displays true elevation map (left) and axial map (right) of a pellucid marginal degeneration, a narrow band of corneal thinning located 1-2 mm from the inferior limbus. Observe the flattening of the central cornea (true elevation and axial maps) along the vertical axis. Extensive peripheral guttering leads to irregular against-the-rule astigmatism, such as this arching inferior bow tie visible in the axial map. These topographic findings are characteristic: they help establishing diagnosis even in patients without slit-lamp typical findings.

Figure a9: Contact Lens Overuse (warpage)

Different types of contact lenses have different impact on corneal surface and different indications. We can classify them into three main groups: soft, rigid gas permeable (RGP) and hard (PMMA). The last are no longer considered suitable for making contacts, and are only prescribed in special cases. Rigid gas permeable contact lenses a relatively popular: they offer good visual performance, they can be polished, they tolerate most known cleaning solutions, and custom designs are possible. The bad side also exists, since they require individualised fitting (by means of k readings, topographic maps, …), they are not easily tolerated at first, and induce with relative ease changes of the shape of the cornea: the process of changes is termed warpage. It is thought due to mechanical pressure on the cornea, although other factors like oxygen deficiency have not been excluded. Many topographic patterns may result,

like the one in the Figure, depending upon the fit (size, curvature, …) and position of the lens. In this case, observe the inferior steepening in the axial map causing meridian asymmetry as a result of superior riding contact lens. The true elevation map shows corneal surface irregularity (orange).

Cessation of the lens wear and good ocular lubrication result in return to corneal former shape.

52

Chapter 3: Fundamentals on Corneal Topography

Figure a10: Curved Arcuate Keratotomies (Astigmatic Keratotomies)

Most refractive efforts have concentrated on altering the shape of the cornea, which is the main diopter of the eye. Topography is valuable in the preoperative assessment and planning of the surgery. Figure displays both true elevation (left) and axial maps (right) of an astigmatic patient who underwent astigmatic keratotomies. Two paired circumpherential relaxing incisions centered on the steep axis result in focal steepening (orange-red in true elevation map) and central flattening in that meridian (blue). The final result is 0.13D of astigmatism in the 3mm central cornea.

Figure a11: Keratotomy With a Resulting Ectasia

Every surgical procedure has some risks the patient must be aware of. Any kind of keratotomy (radial, astigmatic or other) may perforate the globe or result in an ectasia like the one shown in the figure. The inferior ectasia simulates an irregular keratoconus, in both true elevation (left map) and axial (on the right) maps.

53

Section II: Topography

Figure a12: Photorefractive Keratectomy (PRK)

To correct myopia, the excimer laser removes more tissue from the centre than the periphery of the treatment zone (ablation zone). The ablation profile is different for every model of laser. The map on the left represents the spherical approach (axial curvature) of a patient who suffered myopic photorefractive keratectomy. Only the "true elevation" map (on the right) shows the transition area, where dioptric powers are very high (ring in red).

Figure a13: Subtraction Map in a PRK

The most effective way of displaying the changes in a cornea that undergoes a refractive procedure are difference maps. The change induced by surgery is obtained by subtracting the preoperative map (upper small axial map) from the postoperative map (lower small axial map). The image on the right shows the result (in terms of dioptric variation, axial curvature) of a myopic PRK. In red, the ablation zone. In orange, the transition zone, which is easily delineated in the postoperative axial map which shows that the central cornea has been flattened (lower small axial map on the left).

54