Astigmatism Diagnosis, Treatment, Prevention

Astigmatism is not a disease but a common vision condition that causes blurred vision. Most people have some degree of astigmatism. Small amounts of astigmatism do affect vision but may not require treatment. Larger amounts can cause distorted vision, eye discomfort and headaches.

This refractive error occurs when the front surface of the eye (cornea) or the lens inside the eye is slightly irregular or cylindrical in shape. An irregularly shaped cornea or lens prevents light from focusing properly on the retina, resulting in vision being blurred or distorted at all distances.

Types of Astigmatism

Axis of the principal meridian

Regular astigmatism – principal meridians are perpendicular. (The steepest and flattest meridians of the eye are called principal meridians.)

• With-the-rule astigmatism – the vertical meridian is steepest (a rugby ball or American football lying on its side).^[rx]
• Against-the-rule astigmatism – the horizontal meridian is steepest (a rugby ball or American football standing on its end).^[rx]
• Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.^[rx]

Irregular astigmatism – principal meridians are not perpendicular.

Focus of the principal meridian

With accommodation relaxed

Simple astigmatism

• Simple hyperopic astigmatism – first focal line is on the retina, while the second is located behind the retina.
• Simple myopic astigmatism – first focal line is in front of the retina, while the second is on the retina.

Compound astigmatism

• Compound hyperopic astigmatism – both focal lines are located behind the retina.
• Compound myopic astigmatism – both focal lines are located in front of the retina.

Mixed astigmatism

• Focal lines are on both sides of the retina (straddling the retina)

Causes of Astigmatism

Your eye has two structures with curved surfaces that bend (refract) light onto the retina, which makes the images:

• The cornea, the clear front surface of your eye along with the tear film
• The lens, a clear structure inside your eye that changes shape to help focus on near objects
• In a perfectly shaped eye, each of these elements has a round curvature, like the surface of a smooth ball. A cornea and lens with such curvature bend (refract) all incoming light to make a sharply focused image directly on the retina, at the back of your eye.
• If either your cornea or lens is egg shaped with two mismatched curves, light rays aren’t bent properly, causing a refractive error. This makes a blurry image. Astigmatism is a type of refractive error.
• Astigmatism occurs when your cornea or lens is curved more steeply in one direction than in another. You have corneal astigmatism if your cornea has mismatched curves. You have lenticular astigmatism if your lens has mismatched curves.
• Either type of astigmatism can cause blurred vision. Blurred vision may occur more in one direction, either horizontally, vertically or diagonally.
• Astigmatism may be present from birth, or it may develop after an eye injury, disease or surgery. Astigmatism isn’t caused or made worse by reading in poor light, sitting too close to the television or squinting.

Symptoms of Astigmatism

Here are some symptoms of astigmatism:

• Blurring of small print, difficulty reading
• Double vision
• Inability to see both near and distant without squinting
• Headaches
• Eyestrain
• Squinting
• Distorted or blurred vision at all distances
• Difficulty driving at night
• Objects both near and far appear blurred
• Constant squinting of the eyes
• Difficulty driving at night

Diagnosis? of Astigmatism

This examination may include

• Visual acuity – When you read letters on a distance chart, you are measuring your visual acuity. Visual acuity is given as a fraction (for example, 20/40). The top number is the standardized testing distance (20 feet) and the bottom number is the smallest letter size read. A person with 20/40 visual acuity would have to get within 20 feet to read a letter that should be seen clearly at 40 feet. Normal distance visual acuity is 20/20.
• Keratometry/Topography – A keratometer is the primary instrument used to measure the curvature of the cornea. By focusing a circle of light on the cornea and measuring its reflection, it is possible to determine the exact curvature of that area of the cornea’s surface. This measurement is particularly critical in determining the proper fit for contact lenses. A corneal topographer, which is gaining use, generates a contour map of the cornea and provides even more detail of the cornea’s shape.
• Refraction – Using an instrument called a phoropter, your optometrist places a series of lenses in front of your eyes and measures how they focus light. This is performed using a handheld lighted instrument called a retinoscope or an automated instrument that evaluates the approximate focusing power of the eye. Based on your responses, the power is then refined to determine the lenses that allow the clearest vision. Despite improved technology, patient input remains integral in determining vision needs.
• Corneal topography – This advanced technology provides the most detailed information about your cornea’s shape. The doctor will tell you to look at something specific. Meanwhile, the device collects thousands of tiny measurements. A computer then builds a color map of your cornea from the data. Your doctor will refer to it if he plans surgery for astigmatism or cataracts. He might use it to fit your contacts. It can also help diagnose keratoconus, a disease that causes large amounts of astigmatism.

Treatment of Astigmatism

Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving eye health, refractive status, and lifestyle determine whether one option may be better than another. In those with keratoconus, certain contact lenses often enable patients to achieve better visual acuity than eyeglasses. Once only available in a rigid, gas-permeable form, toric lenses are now available also as soft lenses. Laser eye surgery (LASIK and PRK) is successful in treating astigmatism.^[rx]

People with astigmatism have several options to regain clear vision. They include:

• Eyeglasses – People with astigmatism primarily choose eyeglasses to improve their vision. The eyeglasses contain a special cylindrical lens prescription? that compensates for the astigmatism. This provides additional power in specific parts of the lens. Generally, a single-vision lens is prescribed to provide clear vision at all distances. However, patients over age 40 who have presbyopia may need a bifocal or progressive addition lens.
• Contact lenses – Some people will have better vision with contact lenses rather than eyeglasses. Contact lenses may provide clearer vision and a wider field of view. However, since contact lenses are worn directly on the eyes, they require regular cleaning and care to safeguard eye health.
  Standard soft lenses may not be effective in correcting astigmatism. However, special toric soft contact lenses can correct for many types of astigmatism. Because rigid gas-permeable contact lenses maintain their regular shape while on the cornea, they can compensate for the cornea’s irregular shape and improve vision for people with astigmatism.
• Orthokeratology – Orthokeratology (ortho-k) involves the fitting of a series of rigid contact lenses to reshape the cornea. The patient wears contact lenses for limited periods, such as overnight, and then removes them. People with moderate astigmatism may be able to temporarily obtain clear vision without lenses for most of their daily activities. Orthokeratology does not permanently improve vision. If patients stop wearing the retainer lenses, their vision may return to its original condition.
• Laser and other refractive surgery procedures – Astigmatism can also be corrected by reshaping the cornea through LASIK (laser in situ keratomileusis) or PRK (photorefractive keratectomy). PRK removes tissue from the superficial and inner layers of the cornea. LASIK removes tissue only from the inner layer of the cornea.

Refractive surgery

Refractive surgery improves vision and reduces the need for eyeglasses or contact lenses. Your eye surgeon uses a laser beam to reshape the curves of the cornea, which corrects the refractive error. Before surgery, doctors will evaluate you and determine if you’re a candidate for refractive surgery.

Types of refractive surgery for astigmatism include

• Laser-assisted in-situ keratomileusis (LASIK) – With this procedure, your eye surgeon makes a thin, hinged flap in your cornea. He or she uses an excimer laser to sculpt the shape of the cornea and then repositions the flap.
• Laser-assisted subepithelial keratectomy (LASEK) – Instead of creating a flap in the cornea, the surgeon loosens the cornea’s thin protective cover (epithelium) with a special alcohol. He or she uses an excimer laser to change the curvature of the cornea and then repositions the loosened epithelium.
• Photorefractive keratectomy (PRK) – This procedure is similar to LASEK, except the surgeon removes the epithelium. It will grow back naturally, conforming to your cornea’s new shape. You may need to wear a bandage contact lens for a few days after surgery.
• Epi-LASIK –  This is a variation of LASEK. Your doctor uses a special mechanized blunt blade — instead of the alcohol — to separate a very thin sheet of epithelium. He or she then uses an excimer laser to reshape the cornea and repositions the epithelium.

References