The
astigmatic eye is unique in the sense that it has two far point planes, one for
each of the two principal meridians of the refractive error. In simple myopic
astigmatism, one plane is located at infinity and the other is at a finite
distance in front of the eye. In compound myopic astigmatism, the two far point
planes are located at different distances in front of the eye.
 |
| Fig:- Astigmatism |
Similarly, in
simple hyperopic astigmatism, one plane is located at infinity, and the other
is located behind the eye, whereas in compound hyperopic astigmatism, both
planes are located at different distances behind the eye. In mixed astigmatism,
one far point plane is located in front of the eye, and the other is located behind
the eye. To correct astigmatic refractive error, cylindrical lenses are used.
Such lenses create two line images, each with a specific orientation. Patients
with astigmatism cannot achieve perfect retinal clarity by holding an object at
any single position, as the myope is able to do. Neither will accommodative
effort help the astigmatic patient achieve proper focus as it will for the prepresbyopic
hyperope. For patients with astigmatism, optical correction
is the only option for creating sharp retinal image.
Non-uniform
image magnification and minification by a spherocylinder lens sometimes causes
meridional aniseikonia which produces disturbances in spatial orientation. A
common complaint is that flat surfaces appear tilted. However, almost always,
the patient will rapidly adapt to these initial symptoms, that ceases to be
annoying. If not, simply reducing the amount of astigmatic correction, while
maintaining the spherical equivalent, is often all that is necessary to
alleviate such symptoms. The following factors must be kept in mind for astigmatic
patients:
 |
| Fig:- Astigmatic Eye |
•
Astigmatism may be of two types: Regular and Irregular.
Irregular
astigmatism is difficult to correct with lenses
•
Astigmatism is usually because of unequal radius of curvature of cornea
•
Cornea at birth is usually perfect sphere
•
By age of 4 years it loses its spherical properties
•
Horizontal axis, i.e. vertical radius of the cornea becomes more steeper than
horizontal radius of the cornea (WTR)
•
With the advancing of age, anatomical structure and lid tension have been
postulated as a cause of corneal flattening above and below the horizontal
meridian (ATR).
 |
| Fig:- Normal Eye |
In
with-the-rule astigmatism, the focusing elements of the eye require more
convergent power in the 180° meridian, which is corrected by plus cylinder axis
90° (or minus cylinder axis 180°). In against-the-rule astigmatism, the
focusing elements of the eye require more convergent power in the 90° meridian,
which is corrected by plus cylinder axis 180° (or minus cylinder axis 90°). JCC,
Stenopaic Slit and Fan and Block Tests can be applied to perform the refraction
for astigmatic patient. However, no single technique serves adequately in all
situations. The optometrist
must be able to
conduct a suitable alternative procedure.
0 Comments