Astigmatism is a very common eye problem that produces blurry vision.
It is caused by an imperfection in the shape of either the cornea or the lens – the 2
structures responsible for focusing images of visual objects.
Ideally, both the cornea and the lens have a curvature of a perfectly round sphere.
They refract light evenly - all light rays converge on a single point on the retina,
producing a sharp image.
In astigmatism, either the cornea or the lens is misshaped.
Most commonly, the cornea is oval-shaped instead of round; it bends light rays coming in different
planes differently, creating multiple focal points, hence multiple images of the same
object, causing blurry vision.
Both near and far vision can be affected.
Astigmatism is assessed using lines, called meridians, that divide the arc of the cornea
into sections, much like the meridians on a globe.
A perfectly shaped cornea has a consistent curvature in all meridians.
With astigmatism, some meridians have a steeper, or flatter, arc than others.
Astigmatism is classified as corneal or lenticular depending on the structure that is misshaped.
It is also specified as regular or irregular: - Regular astigmatism is when the principal
meridians – the steepest and flattest curves - are perpendicular to each other.
This type is typically present from birth and may be inherited.
- Irregular astigmatism is when the principal meridians are not perpendicular, or the surface
of the cornea or lens is irregular or bumpy.
This type is most commonly caused by injuries, scarring, conditions such as keratoconus,
or previous eye surgery.
The most common type is corneal regular astigmatism; it is also the easiest to treat.
Astigmatism is common in children, but because children don’t usually realize they have
blurry vision, a regular eye checkup is important to detect the condition.
Signs to watch out for include blurred or distorted vision, eyestrain or discomfort,
headache, and squinting.
Astigmatism is treated with corrective lenses or refractive surgery.
Corrective lenses are designed to counteract the optical error produced by the misshaped
cornea or lens.
They can be worn in the form of regular glasses, or contact lenses.
There are several types of contact lenses: - Toric lenses are soft contact lenses that
correct astigmatism.
Unlike a regular contact, a toric lens is shaped like a small slice of a donut.
This shape creates different refractive powers on different planes, thus compensating for
the different curvatures of an astigmatic cornea or lens.
A toric lens must be worn in the right orientation to be effective.
Fortunately, most manufacturers have come up with some design feature that allows the
lens to automatically rotate to the correct position once inside the eye.
- Gas permeable lenses are rigid lenses that retain their shape on the eye surface.
They can be of regular or toric shape.
Rigid lenses can also be worn at night to reshape the corneal surface, much like dental
braces for teeth, in a procedure called orthokeratology.
- There are also hybrid lenses, essentially a hybrid between soft and rigid lenses.
Refractive eye surgeries use laser to reshape the cornea and can treat most cases of astigmatism,
corneal and lenticular.
Several types of refractive surgeries are available, each have their pluses and minuses.
Lenticular astigmatism can also be treated with lens replacement, in a procedure similar
to cataract surgery.
This procedure is particularly helpful when the patient also has cataract, as it can fix
both problems at once.
Patients with corneal astigmatism and cataract may benefit from a cataract surgery with toric
intraocular lenses.