Short-sightedness and astigmatism
Myopia and astigmatism are frequently associated, and can be corrected with glasses, lenses, or operation of refractive surgery. The chosen correction depends on the level (strength) and the origin (horny or crystalline) of astigmatism. Refractive surgery is effective to correct astigmatism associated with myopia: in most situations, astigmatism correction is accomplished at the same time as that of myopia.
Effect of in the myopic astigmatism
Visual blurring caused by myopia is accentuated by those associated with astigmatism : However, this is not really noticeable and it is not possible to distinguish the specific effect of blurring the astigmatism of one related to myopia for a short-sighted and have astigmatism subject-related. However, if it corrects only nearsightedness, and astigmatism remains, the latter will affect visual perception.
Depending on the degree of uncorrected astigmatism, there will be a reduction in Visual acuity (loss of tenths), either a light duplication around bright lights or subtitles. Despite a sharp close or equal to 10/10. The correction of myopia and astigmatism is therefore necessary to ensure optimal vision. This correction can be made through the following features:
-Glass of glasses: nearsightedness and astigmatism are corrected by a sphero-cylindrical glass : the "sphere" is a concave glass (negative power) that corrects nearsightedness, and the 'cylinder' a glass o-ring, whose power is variable depending on the orientation of the considered Meridian: maximum zero next to the axis 'long' of glass, in the against axis.
Correct myopia and astigmatism with glasses glasses, to conduct a review of the refraction, and use spherical and cylindrical glasses on a mount to test (or an automated mount). The cylindrical glasses correct myopic astigmatism, and are oriented based on the axis of astigmatism measured by auto refractometer.
– O-ring Contact lens : she must also present a stabilization system, to stay properly oriented on the cornea.
– LASIK/PKR : can the ablation profile issued to correct nearsightedness and astigmatism at the same time. Depending on the degree of astigmatism, the thickness and the regularity of the cornea, the realization of a PKR or a LASIK may be preferable.
– Cataract surgery : in case (more than 1 diopter) significant corneal astigmatism, the laying of a toric implant can be proposed. The character 'o-ring' of the implant to correct astigmatism. The toric implant should be steered accurately during surgery.
Association of myopia and astigmatism
The association between myopia and astigmatism is relatively frequent. The role for astigmatism in the appearance or the evolution of myopia is not shown. Some studies suggest that the presence of astigmatism accelerates the progression of myopia. It seems the high myopia (more than 6 dioptres) comes more readily says astigmatism «» direct "(the cornea is more cambered on that horizontal vertical meridians: the centerline of the concave cylindrical glass correction test is oriented in a horizontal direction).
Origin of astigmatism associated with myopia
Eye astigmatism is mainly caused by the cornea, which has a certain degree of 'toricite '. The astigmatic patient's cornea is similar to a toric surface because it has one variable bend according to the meridians.
Some cataracts cause the appearance of a refractive astigmatism related changes who sit at the level of the lens. The lens is the lens inside the eye: located behind the cornea, the lens allows, by deforming to see net close)accommodation). One speaks then ofastigmatism internal. "Corneal" astigmatism and 'internal' astigmatism combine to give since the incident light suffers the effect of the cornea and the front lens "refractive" astigmatism, "to impress' the retina (see" ") Eye and optical »).
Refractive astigmatism is the one that is measured as the degree of optical correction of astigmatism to prescribe to correct vision at best. Thus, when it undergoes an eye exam with letters to identify with the use of a correction in glasses, we measure refractive astigmatism. The specific measure of corneal astigmatism requires conducting a corneal topography, or a measure known as "k" (refracto-keratometer auto).
Short-sightedness and astigmatism: explanations
One can understand this optical defect (myopia and astigmatism) as that of myopia whose degree would be different depending on the concerned (meridians) axes. When an astigmatism is associated to myopia, this means that some meridians are 'more myopic' than others. The greater the difference, the more astigmatism. Called the association of myopia and astigmatism of the «» compound myopic astigmatism ».
For whether of astigmatism is present in a lens correction, we must look at the formula correction (or refraction). Myopia is the first turnover, preceded by a negative sign (ex:-3), astigmatism is the second digit that axis is associated with (ex:-1 x 0 °). In this example,-1 corresponds to the 'magnitude' of astigmatism, and 90 ° is his "orientation" (axis). We must understand this expression (-1 x 0 °) as «» the meridian of axis at 0 ° has an optical power or vergence which is lesser of a diopter than has the Prime Meridian perpendicular axis "(i.e., the Meridian located at 90 ° in this example). There are several ways to write the formula of myopia associated with astigmatism, according to the choice of a sign that is used to express the difference of optical power between the meridians.
In summary: The degree of astigmatism is simply the difference between the meridians of extreme vergence (low, high). Astigmatism of 3 diopters means that there is a difference of 3 diopters between the most powerful Meridian and the meridian of the less powerful. The axis used for the notation of astigmatism (in degree) is one of the most powerful Meridian (notation in positive cylinder) or the less powerful (notation in negative cylinder).
Ratings of compound myopic astigmatism
Astigmatism associated with myopia can be expressed by different notations, as the magnitude of astigmatism is expressed in positive or negative. For example:-1 (-2 × 0 °) is a notation in negative cylinder. It means that the horizontal Meridian (0 °) has a 'deficit' of optical power towards the Meridian that is perpendicular (90 °) - astigmatism is associated here with myopia of - 1 d. Because of the association of this myopia-1 d (we can call it 'associated with myopic sphere of 1 D'), we can consider that this optical defect is equivalent to a myopia of 3 diopters next to the vertical (90 °) Meridian and a diopter adjustment next to the horizontal Meridian (0 °). It is true that the formula of writing (which is a form of prescription erected on conventions) suggests the opposite...
We can write this astigmatism associated with myopia in "positive cylinder", as follows:-3 (+ 2 × 90 °) the vertical (90 °) Meridian has an excess of power of + 2 D towards the perpendicular Meridian (0 °): myopia is of 3 diopters next to the vertical Meridian, and a diopter adjustment next to the horizontal Meridian. There are formulas for conversions to convert a formula called positive negative cylinder cylinder.
Special clinical situations
The presence of a slight astigmatism (less than one diopter) is very common in short-sighted. It is rare to do not measure a quarter (0.25) or a half (0.50 diopter of astigmatism associated with myopia. Some situations duopied cause the appearance of a particular astigmatism:
It is detected in childhood, at the same time as nearsightedness, and don't is growing little or no during growth. There are unilateral or bilateral forms.
Keratoconus is a Pathology caused by the repeated eye rubbing (according to the author of this site, is not a primitive dystrophy but indeed a mechanical pathology, favored by the allergy, and more generally anything that can push patients to rub their eyes, often since adolescence). In the advanced forms of Keratoconus, astigmatism can reach several diopters (up to 7 or 8 D), which complicates the prescription glasses glass and forces to wear a correction in o-rings or rigid lenses. This astigmatism is linked to the deformation of the cornea, and is associated with an "irregular" component in the beginner forms of Keratoconus (ex: infra clinical Keratoconus, which meets in patients who are 'a bit' rubbing eyes), astigmatism is often oblique direction (more camber Meridian is close to the axis 45 ° to the left eye, and 135 ° to the right eyes), or reverse (more camber Meridian is close to the horizontal direction).
After a trauma resulting in a plague of the cornea, the appearance of an astigmatism is common. The cataract surgery performed in "manual" way, in a technique called 'extra capsular', is often the cause of a pronounced astigmatism, initially related to the placement of sutures, then later next to the scar the cornea is released.
Modern surgery is performed through small incisions (2 mm or less), which allows to limit so-called "induced" astigmatism, which meets when the incisions are wider (3 mm and more). When an astigmatism is present before the surgery, he must determine whether it is from the cornea or the lens, or a combination of cristallinien and corneal astigmatism. To a toric implant allows to correct the severe corneal astigmatismes, and all or part of the myopia. It takes a standard biometric calculation and order a toric implant, which neutralizes the corneal astigmatism.
In case of important post operative astigmatism, needed a corneal topography. Corneal astigmatism is not at the origin of refractive astigmatism, should suspect a displacement of the implant.
Subluxation of the lens
The displacement of the lens is original traumatic ('coup' on the eye) or associated with a general disease (ex:) Marfan syndrome). Astigmatism can reach a relatively high value, and evolve according to the displacement of the Crystalline lens.
The cornea transplant (keratoplasty keratoplasty) often causes an astigmatism pronounced, associated to the myopia. The withdrawal of the son can reduce the astigmatism, but wearing a lens correction is often necessary.
Surgical correction of astigmatism associated with myopia.
Many pages of the site detail the principles and surgical methods of correction of an optical defect involving short-sightedness and astigmatism.
It is done most often jointly to myopia, the correction of astigmatism is programmed into the software that drives the excimer laser. The distribution of spots in case of pronounced astigmatism is special: many spots are delivered along the flat (see:) profile of simple myopic astigmatism laser ablation). The realization of a Oval flap (or elliptical) is indicated, if the femtosecond laser allows this modality. When possible, the LASIK technique allows corrections of astigmatism greater than the PKR (lower risk of slot correction). The use of a technique of Iris recognition is recommended, to align the treatment of astigmatism on the desired axis. This technique is a mapping of the iris with a topographer to effecuter (the patient sits and the head is prop on a horizontal Chin rest), and transmit it to the excimer laser. A comparison between the transmitted image and the image "seen" by the laser at the time of issuing the correction excimer is performed. If the eye rotated (the patient goes under the laser and the position of the head can slightly be changed), a correction and a realigement of the axis is made to match the axis which was that of the eye "sitting".
It is necessary to provide for the installation of an O-implant to correct the corneal component of astigmatism (the Cristallinienne component is removed during surgery).