# Regular and irregular astigmatism

THEastigmatism is a common eye defect. When we talk aboutastigmatism and its correction, we under generally the 'regular' character of astigmatism; regular astigmatism is often defined as the component of astigmatism is it possible to fix by a glass of bezel, in contrast to astigmatism said irregular.

## Origin of irregular astigmatism

Irregular astigmatism is observed in some eye conditions, which may involve the cornea or the lens: irregular deformation of the cornea (ex: Keratoconus, corneal SCAR), asymmetry (ex: decentering after) LASIK), generates of irregular astigmatism of corneal origin.

Internal irregular astigmatism can be caused by an ectopy of the lens (Marfan disease), cataracts (inhomogeneity of the cristallinien kernel), a displacement or a toggle of the implant (tilt) after cataract surgery.

Irregular astigmatism brings together the optical aberrations that it was not possible to study in detail before the widespread use of the eye OPD. Thanks to the OPD, irregular astigmatism study corresponds to the collection and analysis of so-called aberrations of "high level". The type and rate of these aberrations are specified by the study of ocular wave-front, that allows to split the aberrations that make it up in terms of different degrees. The coupling of the aberrometric to the corneal topographer (ex: topo aberromere OPD - Nidek, or iTrace - SCAN Tracey) allows to study specifically the aberrations induced by the front of the cornea (from the measurement of topography of the cornea). By subtracting the aberrations of the cornea to the entire eye, we can estimate the rate of aberration of internal origin.

## Regular astigmatism

Regular astigmatism is a regular change in the defocus with azimuth. The defocus is quantified through the terms of degree 2 in the classification of Zernike. The decomposition into polynomials of Zernike phase shifts of the Wavefront attributes to regular astigmatism two components: Z(2,-2) and Z (2.2). These components are 'independent' (mathematically, it is said that they are orthogonal: they allow to break down all regular astigmatism in a combination of these two terms).

The value of the coefficients C(2,-2) and C (2.2) which weigh these terms depends on the direction (in degrees) and the magnitude (in power) of regular astigmatism:

In case of astigmatism direct or inverted (oriented exactly 90 ° vs 0 °), only the C (2,2) component has a value non-zero (negative if direct astigmatism, positive if astigmatism reverses). The value of C (2, -2) = 0

In case of astigmatism oblique (oriented to exactly 45 ° vs 135 °), only the C (2, -2) component has a value not zero, and the component C (2,2) = 0.

The astigmatismes focused on other areas (ex: 25 ° vs 115 °) result in a combination of values non-null for C(2,-2) and C (2, 2).

The magnitude of the regular astigmatism is equal to the square root of the sum of the squares of the components C(2,-2) and C (2, 2).

Regular astigmatism is corrected in glasses, refractive surgery (LASIK, PKR) and cataract (toric implant) surgery.

Astigmatism correction is based on the use of a device that generates an astigmatism of same magnitude but oriented at 90 ° of astigmatism to be corrected.

## Irregular astigmatism

It corresponds to the phase shifts caused by aberrations of degree greater than or equal to 3. A moderate rate of irregular astigmatism is measured for human eyes, and this rate increases with the dilation of the pupil. The most frequently recovered aberrations are of type coma: Z (3, -1) and Z (3,1), trefoil: Z (3, -3) and Z (3.3), Z (4.0) and Z (6,0) spherical aberration.

Irregular astigmatism is not correctable by glasses. It can be reduced by wearing rigid contact lenses, a laser photoablation custom, or the treatment of the cause (ex: biased implant).

Keratoconus, corneal () deformationcaused by the eye rubbing According to the author of this site) causes of the type coma aberration because corneal asymmetry causes an asymmetric phase shift. The optical path from the fovea is short in region where the cornea is less arched (in superior) and longer in the region where the cornea is more arched.

The schema below below represents schematically the emerging wave front of an affected eye of Keratoconus, which is a variation of optical path from the fovea:

## Keratoconus: regular and irregular astigmatism

Some clinical examples illustrating these differences.

It is a patient with Keratoconus stable, cataract surgery and received a toric implant insertion to correct the regular component from astigmatism. In post operative, Visual acuity is 10/10 without correction by far, and Parinaud 3 without correction closely. The aberrometrique study allows to split the consequences of regular astigmatism corneal (and its compensation by the implant), as well as the persistence of the effects associated with irregular astigmatism (see also:) example review aberrometrique and Keratoconus)

The topo-aberrometrique with OPDSCAN III study to explore the relative contributions of the cornea with implant (internal):

This almost total regular astigmatism correction is borne out by the study of the position of the implant (toricite) astigmatism corneal.

The map of corneal phase shift induced by the toricite corneal (regular component) is represented here, in microns.

The representation of the regular component of the toric implant-induced astigmatism reveals a similar magnitude, and direction at 90 °.

The orientation of the implant (its the most powerful Meridian is aligned with the corneal Meridian the least powerful) can be quantified by the study of the components of astigmatism (Zernike polynomials: Z(2,-2) and Z (2,2).)

map OPD, which shows the refractive fluctuations within the pupillary area, reveals however the presence of an increase in the negative power in lower. This refractive asymmetry is the consequence of the corneal asymmetry (Keratoconus).

The OPD map restricted to only optical aberrations of high level confirms the responsibility of the aberrations of high degree in the genesis of the lower power increase.

The aberrations of high degree study confirms the presence of a high rate of type coma aberration: this aberration represents the essence of responsible "irregular" astigmatism aberrations

The effect of ocular components and their contributions to the regular and irregular astigmatism can be studied using the maps "Optical quality" (optical quality):

Ultimately the toric implant helped neutralize the regular component of corneal astigmatism. After implantation and orientation of it, the optical quality of the eye remains tainted by the component "irregular" astigmatism, represented mainly by the coma aberration, consequence of the corneal asymmetry (Keratoconus).

## Internal irregular astigmatism: ectopy of the lens

The topo - aberrometrique (OPD SCAN) map obtained at a patient presenting a total eye astigmatism at the same time of corneal and internal origin;

After dilation slit lamp examination reveals the origin of internal regular and irregular astigmatism: there is a sub luxation of the lens induced by rupture of the zonule in lower - in a context of Marfan syndrome. The zonule is an elastic ligament, which keeps the lens in a State of tension at rest. The partial breakdown of the (elastic tissue affected by Marfan disease) zonular fibers causes local relaxation of the lens with localized bulge. This bulge causes an overall increase in the vergence in the vertical axis, more marked next to the disinsertion rupture.

The following image shows the overlap between the map OPD and the photographic image of the review to the slit lamp:

See also: rotation of toric implant in a patient suffering from Keratoconus and cataract surgery

[…] Regular and irregular astigmatism [...]

Hello

I have an irregular astigmatism diagnosed by Dr. Saad. I will have rigid lenses but in the meantime it only gets worse. How to slow his progression? For three days, I see the ground distorted and it seems to me far. It's even worse without glasses. Can the irregular astigmastisme be the cause?

Thank you

Kind regards