Ординатура / Офтальмология / Английские материалы / Astigmatism – Optics, Physiology and Management_Goggin_2012

It is important to remark; the selection of the Zernike terms to be included in the correction is not trivial. Only Zernike terms considered not clinically relevant or minor clinically relevant can be excluded from the correction, but they must not be necessarily excluded. Actually, single Zernike terms considered not clinically relevant will only be disabled when they represent an extra tissue for the ablation, and will be enabled when they help to save tissue for the ablation.

In this way, particular cases are represented by the full wavefront correction, by disabling all not clinically relevant terms, or by disabling all high order terms.

The selection process is completely automatically driven by a computer, ensuring systematic results, and minimization of the amount of tissue to be ablated, simplifying the foreseeable problems of manually selecting the adequate set of terms.

Fig. 17. Optimised Aberration Modes Selection. Based on the wavefront aberration map, the software is able to recommend the best possible aberration modes selection to minimise tissue and time, without compromising the visual quality. Notice that the wavefront aberration is analysed by the software showing the original ablation for a full wavefront correction and the suggested set of aberration modes to be corrected. Notice the difference in required tissue, but notice as well that the most representative characteristics of the wavefront map are still presented in the minimised tissue selection.

A critic to this methodology is the fact that it does not target a diffraction limited optical system. That means it reduces the ablated tissue at the cost of accepting a “trade-off” in the optical quality. However, there are, at least, three criteria (chromatic blur, depth of focus, wide field vision) favoring the target of leaving minor amounts of not clinically relevant aberrations. There are, as well, no foreseeable risks derived from the proposed minimization functions because they propose ablation profiles simpler than the full wavefront corrections. Some drawbacks and potential improvements may be hypothesized:

There may be a sort of “edge” problem considering the case that a Zernike term with DEq of 0.49 D can be enabled or disabled, due to its expected minor clinical relevance, whereas a Zernike term with DEq of 0.51 D shall be corrected.

It is controversial, as well, whether the clinical relevance of every Zernike term can be considered independently. The visual effect of an aberration does not only depend on it but also in the other possible aberration present; e.g. a sum of small, and previously considered clinically irrelevant aberration, could suppose a clear loss of overall optical quality.

A possible improvement comes from the fact that current selection strategy is in an “ON/OFF” fashion for each Zernike term, better corrections and higher amounts of tissue saving could be obtained by using a correcting factor F[n,m] (range 0 to 1) for each Zernike correcting a wavefront of the form:

n 0 m n

However, this would correspond to a much higher computation cost.

Another possible improvement would be to consider possible aberration couplings, at least, between Zernike modes of the same angular frequency as a new evaluation parameter. New algorithms and ablation strategies for efficiently performing laser corneal refractive surgery in a customized form minimizing the amount of ablated tissue without compromising the visual quality are being developed. The availability of such profiles, potentially maximizing visual performance without increasing the factors of risk, would be of great value for the refractive surgery community and ultimately for the health and safety of the patients.

8. References

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15

Measurement and Topography Guided

Treatment of Irregular Astigmatism

Joaquim Murta and Andreia Martins Rosa

Hospitais da Universidade de Coimbra,

Portugal

1. Introduction

Corneal astigmatism occurs when one corneal meridian has a different refractive power from the orthogonal meridian. In regular astigmatism, the two meridians are at 90º from each other, such as a sphere having a cylinder superimposed on its surface. In irregular astigmatism the two meridians are not at 90º from each other or the cornea curvature is not axially symmetric. Irregular astigmatism can be imagined as a sphere, with or without a cylinder on its surface, and several other different shapes superimposed on it. In irregular astigmatism the same meridian has different degrees of curvature, making it impossible for a spherocylindrical lens to correct such irregularity. (1)

The diagnosis of irregular astigmatism can be suspected when there is loss of spectacle corrected visual acuity but preservation of vision through pinhole or while wearing rigid gas-permeable contact lenses. Other clues for the presence of irregular astigmatism are difficulty in determining the axis of astigmatism during manifest refraction, a significant amount of astigmatism at automated refraction not accepted by the patient and achieving the same visual acuity despite correction of the cylinder in different axis. Patients complain of bad quality of vision resulting from glare, halos, distortion of image and monocular diplopia. The diagnosis can be confirmed with a topographic examination.

2. Etiology of irregular astigmatism

Causes of irregular astigmatism include ectatic disorders, nonectatic disorders, refractive surgery and corneal transplantation. (2) (3)

2.1 Ectatic disorders

Noninflammatory ectatic disorders include keratoconus, pellucid marginal degeneration (PMD), keratoglobus and posterior keratoconus. (2) (3)

KERATOCONUS is the most common ectatic disorder and it is an important cause of irregular astigmatism. It is a non-inflammatory, progressive, ectatic and thinning disease of the cornea, usually bilateral, although asymmetric, with onset at puberty. It manifests as a protusion with paracentral inferior thinning, it may be surrounded by an iron line (Fleisher’s ring), it can contain scars and fine posterior stress lines (Vogt striae). Later signs of keratoconus include Munson’s sign and Rizutti’s phenomenon. (2) (4)

Fig. 1. Keratoglobus. Notice the generalized peripheral thinning.

PELLUCID MARGINAL DEGENERATION is also bilateral, age at onset from 20 to 50 years old, most commonly found in males, with an inferior peripheral band of thinning (usually from 4-8 o’clock with 1 to 2 mm width) and protusion superior to the thinned area. It may have striae, but less frequently than keratoconus. (2) (4)

KERATOGLOBUS is a rare bilateral disorder that presents at birth with a generalized corneal protusion and limbus-to-limbus peripheral thinning, causing the cornea to assume a globular profile. Keratometry measurements can often be as high as 60-70 D. Vogt’s striae, sub-epithelial scarring, Fleischer’s ring, lipid deposition and corneal vascularisation are rarely found. (2) (4) (Figure 1).

POSTERIOR KERATOCONUS is a very rare corneal disorder, usually unilateral and nonprogressive that is present at birth. There is only an excavation in the posterior and paracentral cornea, but scarring is common. (2)

2.2 Nonectatic disorders

CONTACT LENS WARPAGE refers to the modification of corneal topography associated with all types of contact lens wear. It may manifest clinically with decreased best spectacle corrected visual acuity and irregular mires on keratometry or only on topographic examination in patients seeking refractive surgery. It may resemble keratoconus or pellucid marginal degeneration, or it may be an irregularity with no specific pattern. (3) The most important factor indicating the required time for refractive stabilization after contact lens removal is the amount of time the patient has worn contact lenses. (5) A simple way to memorize when to discontinue contact lens wear before refractive surgery is 1 week for each 5 years of use of soft contact lenses and 2 weeks for rigid lenses.

DRY EYE can cause any pattern of irregular astigmatism or it may make impossible to obtain an adequate topography. Dry eye decreases the smoothness of the epithelium, creates focal irregularities on topography and may appear similar to keratoconus and pellucid marginal degeneration. (3) These irregularities improve with artificial tear instillation prior to the examination. Wavefront aberrations are a measure of irregular astigmatism and have been shown to decrease 2 to 3 times with tear instillation. Sequential aberrometry is an useful objective method to evaluate sequential changes of visual performance related to tear-film dynamics. (6)

PTERYGIUM is a commonly occurring ocular surface disease, characterized by epithelial overgrowth of the cornea, usually bilateral. There is also an underlying breakdown of Bowman’s layer and its size is significantly correlated with the magnitude of spherical power, asymmetry, regular and irregular astigmatism. (7) (8) (9) Pterygium removal surgery improves these changes, but regular astigmatism and higher order irregularities may remain. (10) Pterygia are usually located in the nasal interpalpebral cornea, where they induce local flattening and with-the-rule astigmatism. However, as the flattening is asymmetric, irregular astigmatism can also be induced (Figure 2). Changes in the tear film, as local pooling at the head of the pterygium, also induce irregular astigmatism. (3) The development of pterygium-like lesions in axes other than the horizontal (pseudo-pterygium) are secondary to traumatic, inflammatory or vascular conditions.

Fig. 2. Pterygium causing nasal flattening of the cornea and significant irregular astigmatism in the 3 and 5 mm zones (4.2D and 10.6D).

INFECTIOUS DISEASES (keratitis) cause corneal scarring and local flattening, resulting in irregular astigmatism. It is important in these cases to differentiate the contribution of the irregular astigmatism from that of opacity in the decrease of visual acuity; a pinhole and a gas permeable contact lens will improve the irregular astigmatism but not the opacity caused by the keratitis.

IMMUNE-MEDIATED DISEASES include Mooren´s ulcer, atopic keratoconjunctivitis, peripheral ulcerative keratitis, ocular mucous membrane pemphigoid and Stevens-Johnson syndrome. (2) (4) These diseases can cause severe corneal melting and scarring, with resulting irregular astigmatism.

CORNEAL DYSTROPHIES affecting all corneal levels can cause irregular astigmatism. It is important to differentiate the relative contribution of the irregular astigmatism from that of the opacity itself. Corneal dystrophies can be divided according to the level of the cornea involved into 1) Epithelial and Subepithelial Dystrophies (epithelial basement membrane dystrophy, epithelial recurrent erosion dystrophy, subepithelial mucinous corneal dystrophy, Meesmann corneal dystrophy, Lisch dystrophy and gelatinous drop-like corneal dystrophy); 2) Bowman´s layer (Reis-Bucklers, Grayson-Wilbrandt and Thiel-Behnke corneal dystrophies); 3) Stroma (transforming growth factor beta-induced –TGFFBI- corneal dystrophies - granular dystrophy and lattice dystrophy; Macular dystrophy; Schnyder corneal dystrophy; Congenital stromal dystrophy; Fleck dystrophy; Posterior amorphous dystrophy; Central cloudy dystrophy of François and Pre-Descemet corneal dystrophy) and

4) Endothelial (Fuchs endothelial dystrophy, Posterior polymorphous dystrophy, Congenital hereditary endothelial dystrophy 1 and 2 and X-linked endothelial corneal dystrophy). (11)

CORNEAL TRAUMA is an important cause of irregular astigmatism due to scar formation and consequent local variation of the refractive properties of the cornea. Irregular astigmatism occurs relative to the type of trauma as well as with the surgical technique used in the primary repair. (12) It is important, once again, to evaluate the relative contribution of the opacity versus the optical effect of the scar tissue, before attempting laser correction of the irregular astigmatism.

2.3 Following refractive surgery

AFTER RADIAL KERATOTOMY. Healing of the RK incisions is very slow and unpredictable, often incomplete even years after surgery. (13) Healing of these incisions involves irregular fibrous tissue and epithelial plugs, leading to an asymmetric central flattening. Visual distortion and glare are more marked in patients having more than 8 incisions, incisions located inside the 3 mm central zone and hypertrophic scarring. (13) (Figure 3) There is sometimes continuous hyperopic shift that also reduces visual acuity. (13)

Fig. 3. A-Radial keratotomy incisions inducing 6.0 D of topographic astigmatism and irregularity (5.9 D in the 3 mm zone and 9.1 D in the 5mm zone) at Orbscan (B). C- It is difficult to obtain data over the radial incisions (Topolyzer).

AFTER LASIK. Irregular astigmatism can occur due to problems with the laser ablation pattern, after both myopic and hyperopic treatments, or flap related complications (Figure 4). Laser induced problems include decentered ablations, either from misalignment, involuntary eye movement or eye tracker malfunction and central islands. (14) Central islands are steep areas inside the treatment zone that can result from poor laser calibration, improper laser dynamics, central blockage of the laser treatment by laser plume, central corneal water accumulation and individual healing responses. (14) A small optical zone can also cause symptoms of irregular astigmatism, because when the pupil dilates light rays are focused differentially according to the curvature of the area they go through. Flap related complications inducing irregular astigmatism include partial flaps, buttonhole flaps, flap striae, diffuse lamellar keratitis and epithelial ingrowth. (3) Dry eye is also a cause of irregular astigmatism and it should always be considered in pre and post refractive surgery patients.

AFTER PRK. The causes of irregular astigmatism are the same as the ones mentioned for LASIK. PRK eliminates flap related complications, but stromal incursions during