irregular astigmatism


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astigmatism

 [ah-stig´mah-tizm]
an error of refraction in which a ray of light is not sharply focused on the retina, but is spread over a more or less diffuse area; it is due to differences in curvature in the refractive surfaces (cornea and lens) of the eye. adj., adj astigmat´ic. Its exact cause is not known; some common types of astigmatism seem to run in families and may be inherited. Probably everyone has some astigmatism, since it is rare to find perfectly shaped curves in the cornea and lens, but the defect is rarely serious. If the refractive error is troublesome, corrective lenses may be needed.
compound astigmatism that in which both principal meridians are either hyperopic (compound hyperopic astigmatism, with rays coming into focus behind the retina) or myopic (compound myopic astigmatism, with rays coming into focus in front of the retina).
corneal astigmatism that due to the presence of abnormal curvatures on the anterior or posterior surface of the cornea.
hypermetropic astigmatism hyperopic astigmatism.
hyperopic astigmatism that in which the light rays are brought to a focus behind the retina.
irregular astigmatism that in which the curvature varies in different parts of the same meridian or in which refraction in successive meridians differs irregularly.
lenticular astigmatism astigmatism due to defect of the crystalline lens.
mixed astigmatism that in which one principal meridian is hyperopic and the other myopic.
myopic astigmatism that in which the light rays are brought to a focus in front of the retina.
regular astigmatism that in which the refraction changes gradually in power from one principal meridian of the eye to the other, the two meridians always being at right angles; this condition is further classified as being against the rule when the meridian of greatest refractive power tends toward the horizontal, with the rule when it tends toward the vertical, and oblique when it lies 45 degrees from the horizontal and vertical.

ir·reg·u·lar a·stig·ma·tism

astigmatism in which different parts of the same meridian have different degrees of curvature.

ir·reg·u·lar a·stig·ma·tism

(ir-reg'yū-lăr ă-stig'mă-tizm)
Astigmatism in which different parts of the same meridian have different degrees of curvature.
References in periodicals archive ?
From this we confirm that distinct keratometer powers along significantly oblique meridians contribute to irregular astigmatism. Corneal power for rectangular meridians is augmented by paraxial asymmetric power in (7).
On irregular astigmatism, we plot the trigonometric factor in (8) which is seen not to exist when [beta] = [alpha].
In this way, it has been proven that SCLs have a similar behavior to RPGCLs and thus can mask mild or moderate irregular astigmatism. However, a greater thickness causes a decrease in the oxygen permeability (DK) of the lens, a situation that encourages the development of possible complications.
Frequent indications for RGPCL are KC, PMD, postsurgical ectasia, irregular astigmatism, or orthokeratology (OK).
(13) Visual outcome following treatment of traumatic corneal perforation may not be optimal due to the presence of irregular astigmatism. Titiyal et al.
There is an increase in myopia and irregular astigmatism and vision cannot be adequately corrected with spectacles or contact lenses.
(7) Decreased VA, reduced quality of vision or induction of irregular astigmatism indicates the need for urgent repositioning of the flap.
The importance of monitoring ingrowth is important for preventing advanced keratolysis that tends to spread over the entire lap, resulting in hyperopia, irregular astigmatism and decreased vision.
The hallmark of keratoconus is central or paracentral thinning, irregular astigmatism and apical protrusion; clinical examination and topography aid in the diagnosis.
c) It presents with hypermetropia and irregular astigmatism
a) It is a one-position keratometer b) It is a two-position keratometer c) It can be used to measure tear break-up time (TBUT) d) It can be used to measure irregular astigmatism
Despite recent advances in refractive surgical procedures, a small proportion of patients still achieve sub-optimal results for a variety of reasons including decentred ablation zones (see Figure 5a), residual ametropia, induced irregular astigmatism and occasionally anisometropia.