Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Chapter 2

Figure 2-17: Shows a “multiple exams view” of left both eyes of the same patient, a 38 year old woman prior to LASIK surgery. Corneal topography remains a routine exam for preoperative and postoperative assessment of the refractive patient. This report shows normal, spherical (round), corneas in both eyes (44 D at vertex, and mostly green colour in the map). The colour zones are approximately circular in shape. Notice that lid aperture is not the same in both eyes, thus making it more difficult to map superior corneal periphery in left eye.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Regular astigmatism (with-the-rule) gives an oval axial corneal map, being the most common deviation from optically perfect spherical (round) cornea. If the bow tie is vertical (the long axis is near the vertical meridian) in an axial map, it represents a cornea having with-the-rule-astigmatism.

If the bow tie is horizontal, it represents an “ against-the-rule” astigmatism, ninety degrees rotated when compared to a with-the-rule astigmatism.

When the bow tie is diagonal, it represents a cornea having an oblique astigmatism. The shape and colours of the bow tie are influenced by the rate of peripheral corneal flattening, and the appearance is influenced by the scale interval chosen by the explorer. The bow tie may be symmetrical or asymmetrical along the perpendicular meridian: one half of the bow tie is significantly larger than the other,

the corneal apex being located in the direction of the larger bow half, slightly decentered form the visual axis.

In the normal eye, nasal cornea is flatter than temporal. The nasal side of a healthy corneal map becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. There is a physiological astigmatism of around 0,75 diopter. Physiologically, the axis may not be the same superiorly than inferiorly. In an axial map, the rate of flattening is greater when the colour scale interval is larger, and there are many colour zones. A focal steepening inferiorly may exist due to the lower tear meniscus.

Generally, the two eyes of the same subject are very similar, and present a mirror image of each other (Figures 2-18 and 2-19). This

Subjects Index

Help ?

Figure 2-18: Axial diopter displays are showed for both right and left eyes. The patient suffered from regular astigmatism (with-the-rule), that gives an oval corneal map, being the most common deviation from optically perfect spherical (round) cornea. The long axis is near the vertical meridian. The shape and colours of the bow tie are influenced by the rate of peripheral corneal flattening: notice the nasal peripheral flattening in left eye (purple colour). This binocular report form Dicon’s CT-200 topographer shows pupil size and simulated keratometry of both eyes. RE size pupil is 4.03 mm, and astigmatism 3.12 D at 8 º. Notice that the two eyes present a mirror image of each other: this phenomenon is called enantiomorphism.

FUNDAMENTALS ON CORNEAL TOPOGRAPHY

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

phenomenon is called enantiomorphism. The knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing to the map of contralateral eye.

Small changes in corneal shape do occur throughout life:

•in infancy the cornea is fairly spherical,

•in childhood and adolescence, probably due to eyelid pressure on a young tissue, cornea becomes slightly astigmatic with-the-rule

•in the middle age, cornea tends to recover its sphericity

•late in life, against-the-rule astigmatism tends to develop

Short-term fluctuation and diurnal variations are not rare, and usually remain unnoticed by individuals with normal corneas. Some conditions like corneal dystrophies, ocular hypotony, radial keratotomies or contact lens use can make them apparent.

Table 6:

Factors that Slightly Affect the Normal Curvature of the Cornea

Lid closure during sleep time Tear film quality

Lid pressure on the cornea (weight, exoftalmos)

intraocular pressure Menstruation Pregnancy

Comparing Displays:

Maps can be compared directly only on the same scale, when taken with the same instrument, and preferably by the same explorer. It is not a good idea to compare maps taken with different instruments: every instrument uses a different measuring

Chapter 2

Figure 2-22: 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. Picture 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.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

BRIEF ATLAS OF CORNEAL TOPOGRAPHY

SPECIAL TOPOGRAPHIC CONDITIONS

Figures a1 to a20:

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: picture shows the axial map of a right eye normal round cornea, with concentric green rings in an absolute scale. Notice 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.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

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. Notice that colour scale may confuse the explorer.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

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. Picture 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). Notice 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 page 35)

Subjects Index

Help ?