Astigmatism when applied to the
cornea means that the cornea is not spherical like the top of
a ball, but oblong like the back of a spoon. Lenticular astigmatism
means that the natural
crystalline lens of the eye is not perfectly symmetrical front
and back, but the crystalline lens is misshaped similar to the back
of the spoon example.
Astigmatism, no matter the
source, causes light passing through the cornea to focus at more
than one location on the
retina, causing blurred vision and sometimes ghosting. Astigmatism
refractive error that can be corrected with glasses, contacts,
refractive surgery like
PRK, but there are important limitations on the correction with
P-IOLs do not have astigmatic correction available, so a P-IOL
cannot correct astigmatism in the cornea nor in the lens.
It is currently impossible
to correct lenticular astigmatism at the crystalline lens without
removing the lens and replacing it with a new plastic or silicone
intraocular lens (IOL). It is impossible to just fix the crystalline
lens; the crystalline lens must be replaced. This replacement is
the same process as
cataract surgery, but when there is no cataract it is called
Refractive Lens Exchange (RLE). Due to other considerations,
RLE is not always appropriate to correct purely refractive errors.
IOLs do not have astigmatic
correction available, so an IOL cannot correct astigmatism in the
cornea, but can resolve astigmatism in the lens by replacement.
Corneal based refractive
surgery is able to correct corneal induced astigmatism at the source.
It is also possible to correct for lenticular astigmatism by "overcorrecting"
in the cornea to accommodate the lenticular astigmatism. In most
refractive surgery cases, the desire is to correct all astigmatism
no matter the source. This provides the patient with the most clear
Think of the eye as having
two lenses, one is the cornea, the other is the crystalline lens.
If the lens has an error of, say -1.00
diopter astigmatism, then the cornea could be changed +1.00
diopter to provide zero net astigmatism and a clear view. This is
like saying that if you can't raise the bridge, lower the water.
If the patient has lenticular
astigmatism and the lenticular astigmatism is corrected at the cornea,
then when the crystalline lens is replaced with an IOL for RLE or
cataract surgery, the corneal correction that includes the correction
for the lenticular astigmatism becomes an "overcorrection" at the
There are many ways to accommodate
this situation and much depends upon the individual patient's circumstances.
Obviously, someone with significant lenticular astigmatism could
simply not have refractive surgery. If the patient is nearing the
age of cataract development or is fully presbyopic, RLE is a possible
option. The patient could have only the astigmatism in the cornea
removed, but this may provide poor vision because the astigmatism
in the lens may cause blurry vision or ghosting. The patient could
have both corneal and lenticular astigmatism corrected at the cornea,
and then have the cornea corrected again after RLE or cataract surgery.
Lenticular astigmatism is
easily diagnosed before surgery and is just one of the many considerations
that must be evaluated for every patient. A patient with lenticular
astigmatism should discuss the situation with his or her doctor
and come to a decision that best fits the individual patient's needs