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Forty-nine cases of primary keratophakia and 13 cases of secondary keratophakia were analyzed for postoperative astigmatism. For primary cases, the surgically induced astigmatism was 1.55 D, whereas for secondary cases it was 0.19 D (insignificant). There was a tendency for both procedures to induce against-the-rule astigmatism, and both procedures were found capable of producing irregular astigmatism.
PURPOSE: To determine whether there is inherent corneal astigmatism in patients with Fuchs' heterochromic iridocyclitis (FHI). PATIENTS AND METHODS: A case-controlled study of 48 patients with unilateral FHI was conducted. These patients had no history of previous surgery or previous diagnosis of glaucoma or cataract. The controls were the fellow eyes of the patients, and the cases were the FHI eyes. All eyes were refracted, and the results were analyzed with descriptive and parametric statistics. RESULTS: Twenty-one women and 27 men were studied. The mean age was 39.5 years. The mean keratometric value for the astigmatism in the FHI eye was 2.2 diopters (D) (standard deviation +/-1.19) and 0.96 D (+/- 0.64) in the controls (p= 0.000). Refractive astigmatism was 1.44 D (+/-1.45) in FHI eyes and 0.38 D (+/-0.62) in the controls (p= 0.000). CONCLUSIONS: Patients with unilateral FHI have inherent corneal astigmatism in the disease eye.
Data provided by a practicing optometrist in Gallup, New Mexico indicate a greater than expected frequency of astigmatism among first- and second-grade Zuni and Navajo Indian children. Zuni children displayed a greater frequency of high astigmatism (3.00 D) than Navajo children. In an effort to understand the etiology of this prevalence of high astigmatism, a study of corneal astigmatism in 22 Zuni families was undertaken. The results suggest both genetic and environmental causes. A very high frequency of albinism was also observed among the Zuni.
Astigmatism was measured retinoscopically for 709 subjects residing in an institution for the mentally retarded and for 56 noninstitutionalized mentally retarded subjects at a community training center. About 60% of the subjects were between 15 and 45 years old. Compared to an intellectually normal sample of young men, astigmatism in the mentally retarded was greater in frequency and degree. For subjects with greater degrees of mental retardation, the prevalence of high astigmastism was significantly greater for whites than for nonwhites. There was no difference in astigmatism between institutionalized retardates and those living in a community setting, a finding that implies that the institutional environment was not the cause of the astigmatic differences between retarded and normal subjects.
Contradictory views exist concerning astigmatism and its causes. Most consider it to be a structural anomaly following genetic and age patterns. Other attribute an undefined functional origin to the various forms of astigmatism. An investigation has revealed a relationship between changes in astigmatism and an observable triad consisting of eye scan, head movement, and head posture. This relationship is described and a model developed. Indications also exist that changing elements within this triad can have a positive effect in reducing astigmatism.
Corneal toricity was measured using a keratometer with the lids in the normal position, and with the lids retracted. Lid retraction usually changed the measured toricity. In corneas with more than 1.00 D of with-the-rule astigmatism, the change was in the direction of less with-the-rule astigmatism. In corneas with astigmatism between 1 D with-the-rule and 1 D against-the-rule, the change was not systematic. The results compare with the original experiment of Masci (1965), who also found that corneas with more with-the-rule toricity show a greater reduction in toricity when the lids are retracted. Clearly the pressure the lids exert on the eyeball is a major factor influencing astigmatism.
We compared the theoretical peripheral power errors and oblique astigmatism of eyes corrected with commercially available intraocular lenses (IOL's) with experimental data of normal phakic eyes. The peripheral power errors and the oblique astigmatism of the pseudophakic eye are larger than those of phakic eyes. The most sensitive component of the optical system of the pseudophakic eye to the peripheral power errors is the shape of the IOL, but only if the lens is away from the iris. The corneal and the retinal surfaces do not affect the peripheral power errors and oblique astigmatism significantly. A plano-convex lens with the flat surface facing the cornea gives the least peripheral power errors and oblique astigmatism and thus the value closest to the experimental data of phakic eyes. However, this design does not give the minimum spherical aberration. Therefore, eyes corrected with IOL's are expected to have poorer peripheral retinal image quality than normal phakic eyes.
The influence of posterior corneal surface toricity upon total corneal astigmatism was investigated in 60 young subjects (mean age 22.04 +/- 3.24 years). Both surfaces were found to be flatter horizontally than vertically. Astigmatism arising from the anterior corneal surface was therefore reduced by astigmatism of the opposite sign arising from the posterior surface. Had the toricity of the posterior corneal surface been purely governed by that of the anterior surface this reduction would have amounted to about 5%. However, the posterior surface was found to exhibit additional toricity bringing about a greater reduction of total corneal astigmatism amounting to approximately 14%.
Aphakic eyes possess considerably less oblique astigmatism than phakic eyes. The possibility that posterior iris displacement after lens extraction accounts for the observed reduction in oblique astigmatism is tested in this study. Oblique astigmatism was calculated for a phakic and an aphakic schematic eye. With no iris displacement little reduction of oblique astigmatism occurred contrary to experimental findings. However, introduction of varying levels of iris displacement reduced this aberration to values found experimentally. Posterior iris displacement (mean = 1.19 mm) was confirmed in vivo by pre- and postoperative anterior chamber pachometry in eight patients.
Astigmatism poses a number of special difficulties when attempting to achieve a desirable refractive outcome during cataract surgery and intraocular lens implantation. One of the most pertinent difficulties is that traditional Gaussian theoretical formulas are simply unable to handle general astigmatic optical systems, that is, systems in which the principal meridians of the refracting interfaces do not lie in two mutually orthogonal planes. This means that the traditional approach falls short when the cornea is treated as bitoric or when other toric optical devices are already present in the eye. In this paper, the modified step-along procedure is utilized in a demonstration of the derivation of the traditional Gaussian theoretical formulas for the determination of intraocular lens power in a stigmatic eye. Matrices are then used to derive a general formula for the determination of the power of a thin toric intraocular lens in an astigmatic eye. This formula is then modified to allow the user to determine the power of the secondary intraocular lens required to compensate for any residual refraction in a phakic or pseudophakic astigmatic eye. The formulas hold under all conditions, including the case when the cornea is treated as thick and bitoric and when other toric optical devices are already present in the eye. Worked examples are provided.
Many factors have led to the improved success rate for clear corneal grafts after penetrating keratoplasty. Unfortunately, postoperative corneal astigmatism commonly occurs and can produce significant visual impairment. Astigmatic correction may include spectacle correction or contact lenses, but if this fails, then surgical options are considered. Refractive surgical techniques such as suture removal or adjustment, relaxing incisions, wedge resections, and photorefractive keratectomy or laser in situ keratomileusis can dramatically reduce postoperative astigmatism after penetrating keratoplasty and lead to improved, functional vision. However, significant variability between results in individual patients can occur. So although general guidelines are useful, it is important to individualize and modify the planned surgery based on qualitative keratoscopy and corneal topography for the initial and subsequent astigmatic corrections.
PURPOSE: The propagation of astigmatic wavefronts through astigmatic optical systems is reconsidered in the wavefront perspective. METHODS: The stepalong method for vergences, described by 2x2 matrices, is applied and augmented to produce off-axis information like the magnification. This so-called augmented stepalong method (ASAM) is derived by applying the paraxial propagation of astigmatic wavefronts to tilted wavefronts as well. RESULTS: The features of the ASAM are discussed for a single surface, a thick lens, and a general system. CONCLUSIONS: The ASAM provides all necessary information to describe a centered astigmatic optical system in paraxial approximation.
PURPOSE: The paraxial propagation of astigmatic wavefronts through astigmatic optical systems can be described by the augmented step-along method (ASAM). Its equivalence to the linear ray optics approach is considered in detail. METHODS: The ASAM is exploited to derive paraxial ray paths through a general coaxial astigmatic system. RESULTS: Starting from the information inherent in the ASAM all 2x2 submatrices rendering the general 4x4 transference of linear optics can be generated. This proves the complete equivalence of both approaches. Additionally, we show that the symplectic relations are automatically obeyed in the ASAM. CONCLUSIONS: The ASAM offers a complete alternative to describe the paraxial optics of astigmatic optical systems. According to the ASAM, an optical system is fully characterized by the back vertex vergence and the angular magnification matrix. Hence, a complete description of the paraxial optics of an eye should not only report the state of refraction but the angular magnification matrix as well, although it is not yet very common. The magnification matrix might be important in cases of anisometropia or the design of progressive addition lenses. Yet, a simple clinical procedure to determine the angular magnification matrix is missing.
The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R-1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images I (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 +/- 7.0 degrees (Hartinger) and 9.4 +/- 9.8 degrees (Canon). Both values were found to be significantly higher than angle alpha 5.0 +/- 1.2 degrees. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.
Transverse relaxing incisions were made in 12 eyes with a mean corneal astigmatism of 10.7D following penetrating keratoplasty. A few days later the mean graft astigmatism had been reduced to 5.5D (p = 0.0022), the reduction being significantly correlated to the degree of astigmatism before the keratotomy was done (p = 0.0044). Thereafter the corneal contour remained fairly stable through a mean follow-up period of 28.1 months, although a tendency towards a further reduction of the astigmatism was indicated. The mean corrected visual acuity improved from 0.5 before the keratotomy to 0.7 at the last control visit (p = 0.0117).
1. Orientational differences in visual resolution were measured at a number of different luminance levels on two subjects with high astigmatism that had remained optically uncorrected until the age of 10. Because of their astigmatism both of these subjects see vertical contours more clearly than horizontal contours with the unaided eye.2. The measurements were made using sinusoidal gratings generated on the face of an oscilloscope with the refractive error carefully corrected with lenses and with the gratings viewed through 3 mm artificial pupils.3. Visual resolution was found to be much better for vertical than for horizontal gratings for both these subjects under these conditions. The difference between the contrast sensitivities for vertical and horizontal gratings was even evident with gratings having spatial frequencies as low as 1 c/deg, but became progressively more pronounced at higher spatial frequencies. In one of the subjects the visual acuity (the cut-off spatial frequency) for horizontal gratings was more than 3/4 of an octave lower than that for vertical gratings.4. This is very different from the results obtained from normal subjects who typically show only a slight reduction in contrast sensitivity for oblique gratings but resolve vertical and horizontal gratings equally well.5. The quantitative differences between the contrast sensitivities for vertical and horizontal gratings of both high and low spatial frequencies cannot be accounted for by either errors of focus in one meridian or by the presence of meridional aniseikonia.6. In order to completely eliminate any optical explanations for these findings measurements of contrast sensitivity were made using sinusoidal interference fringes formed directly on the retina, thereby bypassing the eye's optics. Since the orientational differences in resolution persisted with this method it must be concluded that they are of neural origin.7. By analogy with the effects on cortical physiology that follow early selective visual deprivation in cats and monkeys, it is argued that these orientational differences in resolution are a consequence of changes induced in the neural organization of the astigmat's visual system by the distorted visual input provided by the uncorrected astigmatism early in life. It is furthermore argued that the smaller orientational differences in resolution observed in normal eyes might similarly be induced by certain asymmetries in the early visual input.
Photorefraction of a sample of 93 infants of ages 1 day to 12 months showed that 63 percent of the subjects had astigmatism of 0.75 diopter or greater, and 12 percent greater than 2 diopters. Seventy percent of these astigmatisms were in the horizontal-vertical meridians. By comparison, only 8 percent of a sample of 26 adults tested by the same method showed astigmatism (all 0.75 to 1 diopter). The high incidence of infant astigmatism has implications for critical periods in human visual development and for infant acuity.
A series of 100 cases of intraocular implants has been considered, especially with regard to the postoperative astigmatism. This was found to be far higher than in a similar series of cases subjected to the same techniques without an implant. The operative results appear to be as good as in any conventional series, except for 2 cases. The reason for the excessive astigmatism (average 2.84 D) is not clear but the following factors have been suggested: (1) faulty surgical technique, that is, irregular corneal healing; (2) astigmatic errors in the actual intraocular lens itself; (3) tilting of the intraocular lens; (4) the intraocular lens, owing to its own inherent weight, may pull on the iris and ciliary body and even distort the cornea on healing. A summation of all these factors could produce an undesirably high degree of astigmatism.