Transcript astigmatism
Astigmatism is a type of refractive error
wherein the refraction varies in the
different meridian.
Astigmatism is a defect in which light
entering the eye is unable to be brought to
a single focus on retina resulting in blurred
vision at all distances.
It simply means that there is a variation
and disturbance in the shape of the
cornea or lens. It is usually due to an –
*Irregularly shaped cornea
* Scarring of the cornea
*Abnormities in the crystalline lens
**Some eyes conditions are seen along with
astigmatism as
Nystagmus
Cataract
Ptosis
Retinitis pigmentosa
Albinism
Inflammation
degeneration
Symptoms:-
Eye strain
discomfort
headache
dry eye
sensitivity to light
Blurred vision
Classification of astigmatism
Pathological
Physiology
regular
Irregular
compound
simple
myopic
myopic
mixed
hypermetropic
Hypermetropic
With the rule
With the rule
With the rule
With the rule
Against the rule
With the rule
Against the rule
Against the rule
Against the rule
Against the rule
Type of astigmatism :there are two type of astigmatism:-
1.
Regular astigmatism
Irregular astigmatism
2.
Regular astigmatism
The astigmatism is regular when the
refractive power changes uniformly from
one meridian to another
1.
2.
3.
A etiology :Corneal astigmatism
Lenticular astigmatism
Retinal astigmatism
corneal astigmatism
it is the result of abnormality of
curvature of cornea
It is usually congenital
acquired corneal astigmatism is often
irregular
Causes
Congenital curvature astigmatism
Keratoconous
Trauma to the cornea or lens
After cataract operation
Lenticular astigmatism
Mainly due to
*congenital abnormality of lens
*refractive index of lens
*congenital or traumatic subluxation of the
lens
Retinal astigmatism
*Due to oblique placement of macula
* It can be corrected by visual aid
type of regular astigmatism
It is subdivided into four type ;–
With-the-rule
astigmatism
Against-the-rule astigmatism
Oblique astigmatism
bi-oblique astigmatism
With-the-rule astigmatism:
Refractive power of the vertical meridian
is greatest
.The vertical meridian is steepest than
horizontal meridian.
Eyes see vertical lines more sharply
than horizontal lines.
Requires concave cylinders at 180+ /-20
Or convex cylinders at 90+/-20 degrees
Against-the-rule astigmatism:
the refractive power of horizontal meridian is
greatest than vertical meridian.
Horizontal meridian is steepest than vertical
meridian.
Eyes see horizontal lines more sharply than
vertical lines.
Requires concave cylinders at 90 +/-20degrees
Or convex cylinders at 180+/-20 degrees.
Oblique astigmatism:
In which the two principle meridians lies
somewhere between the axis defining
either with-the-rule or against-the-rule
astigmatism.
Complementary (45 in one meridian,135
in other meridian)
Bi –oblique astigmatism
Principle meridian are not at right angle
One at 30 , other at 100
optics of regular astigmatism
Light entering the eye cannot converge to
a point focus but form focal lines
Configuration of rays refracted from the
astigmatic surface –( sturms conoid) .
Lens with cylinder produces an astigmatic
focus .this astigmatic focus contains
vertical line ,corresponding to the focus of
the horizontal principal meridian .and
horizontal focal line corresponding to the
focus of the vertical principal meridian .
The region between these two lines is
known as the conoid of sturm .
At the dioptric mid –point between these
two focal line , the astigmatic focus forms
a circular patch known as circle of least
confusion
a. Simple hypermetropia astigmatism
When refracted rays from one meridian
takes focus on the retina and refracted
rays from other meridian takes focus
behind the retina.
b)Simple myopic astigmatism
When refracted rays from one meridian
takes focus on the retina and refracted
rays from other meridian takes focus in
front of retina.
c) Compound hypermetropic
astigmatism:
In this condition of astigmatism refracted
rays from both meridians take focus behind
the retina.
d) Compound myopic astigmatism:
In this condition of astigmatism refracted
rays from both meridians take focus in
front of retina.
e) Mixed astigmatism: When refracted
rays from one meridian take focus in front
of retina and refracted rays from other
meridian take focus behind the retina.
b) Irregular astigmatism:
Irregular differences in curvature or
refractive index of the optic components
or misalignment of any optical component
may lead to irregularity.
It is often the result of trauma,
pterygium,
corneal scars,
and complication of refractive surgery.
Another not uncommon condition is
conical cornea (keratoconous)
Diagnostic
Objectively:
With the help of retinoscopy
Autorefractometer
Keratometer
Corneal topography
Subjectively:
By Jackson’s cross cylinder
By astigmatic fan
By stenopaeic slit
management
CONSERVATIVE MANAGEMENT:
Prescription eye wear
Spherical soft contact lenses
Spherical RGP contact lenses
Toric soft contact lenses
SURGICAL MANAGEMENT
1.
2.
3.
4.
5.
6.
7.
Conventional LASIK
Astigmatic Keratotomy
Photorefractive Keratectomy
PAI LASIK
LASEK
Orthokeratology
Phakic Intraocular Lens(PIOL)
Physiological astigmatism
Almost all have minor degree of
astigmatism {physiological }
Diameter of cornea in vertical axis is
11.5mm and in horizontal meridian is
12mm.
This means horizontal meridian is flatter
than vertical.
Because diameter is inversely proportional
to curvature.
Occur as lid press on anterior corneal
surface.
Make vertical meridian steeper
Only small astigmatic error
Usually 0.12D
NO TREATMENT REQUIRED,
life is an echo ,all comes back ,the good
,the bad, the false and the true ,
so give the world the best u have and the
best will come back to u .
For Further Queries Contact :
Ms. Priyanka Singh
Head – Optometry Service
Email – [email protected]