Astigmatism
Astigmatism is a refractive error caused by an irregular shape of the cornea or our natural lens. If the cornea and the eye lens are not evenly and regularly curved, but more bulging or flattened in one meridian, light rays in the eye refract irregularly and do not focus to a single point on the retina as with clear vision, but dissipate instead. This causes diopter, and astigmatism is just one type of diopter that patients call cylinders.
Astigmatism can occur with hyperopia or myopia. It creates blurred vision at all distances, i.e. the light refracts irregularly at a specific meridian. As a result, people suffering from astigmatism often tilt their heads to find an adequate position to see clearly.
Astigmatism is often present since birth, but it can also be acquired later due to eye degeneration, injuries or inflammatory corneal diseases.
Types of Astigmatism
Regular astigmatism is an eye condition when two principal meridians of the cornea are at the angle of 90 degrees one with another (i.e. perpendicular to each other). The meridians of the smallest and the largest curvatures are usually vertical and horizontal.
Oblique astigmatism is an eye condition where meridians of the smallest and largest curvature are perpendicular to each other but are neither vertical nor horizontal but rather at an “oblique” angle.
Irregular astigmatism occurs when 2 meridians are not perpendicular to each other. This type of astigmatism is often found with keratoconus or it occurs as a result of a corneal injury or inflammation.
The Symptoms of Astigmatism
As the consequence of the constant eye strain to create a clear image, signs, and symptoms of astigmatism are as follows:
- Blurriness or distortion in the vision
- Excessive squinting when looking at close and faraway objects
- Eyestrain
- Headache
- Dizziness and irritability
- Overall fatigue
A person with astigmatism squints when looking at both close and faraway objects. While reading, these people, as well as those with myopia, bring the objects very close to the eyes. Also, very often they don’t wear any kind of eye correction.
People with higher degrees of congenital astigmatism often don’t even know they have a blurred vision because they never had a clear vision in the first place. If they don’t wear any kind of correction in childhood while their vision is still developing, the weaker eye will become “lazy” (amblyopic), and the dominant eye overtakes the function of the eyesight.
Diagnosis of Astigmatism
To diagnose astigmatism, the ophthalmologist examines the eyes and performs a refraction test. Determining the error with children and people who cannot answer the questions is done with a technique called retinoscopy, which is a test by shining light through the pupil to see how it reflects off the retina.
Measuring light reflection off the corneal surface is done with an instrument known as a keratometer. This device quantifies the amount and orientation of corneal astigmatism, i.e. the angle at which it is situated. The corneal surface is measured with special instruments; most often the corneal topography is created (the map of the corneal surface).
Also, modern instruments create a display of the corneal surface in the shape of a relief map of different colors (video keratograph). This examination uses topopographers, tomographers and anterior segment OCT.
Treatment of Astigmatism
The possible solutions to correct astigmatism are the following:
- Prescription glasses with cylinders
- Contact lenses
- A laser intervention
- An intraocular lens implant
Very often the patients with astigmatism do not wear glasses or lenses for vision correction. Prescription glasses with cylinders can be very uncomfortable, especially if big cylinders are needed. Contact lenses (toric, RGP or sclerals) are a better solution because they rest directly on the cornea and enable the correction of the whole field of vision. The problem is that the contact lenses can be uncomfortable, irritate the eye, and patients find it hard to get used to them.
Regular astigmatism is corrected with cylindrical eyeglass prescription or sphero-cylindrical eyeglass prescription (a combination of cylindrical and spherical lenses blended in one glass).
The simplest solution for astigmatism is a laser eye surgery, which permanently removes this refractive error and enables clear vision. Refraction surgery reshapes the cornea and corrects refractive errors (myopia, hyperopia, and astigmatism).
Intraocular lens implant eliminates astigmatism for good and enables clear vision both for close and faraway objects. A special artificial lens is implanted into the eye, which corrects vision and becomes a permanent part of the eye.