Astigmatism (from the Greek “a” meaning absence and “stigma” meaning point) is a refractive error (ametropia) that occurs when parallel light rays entering the non accommodating eye do not converge to focus at a single point on the retina

The cornea of the normal eye generally has a uniform curvature, resulting in equal refracting power over its entire surface. Most astigmatic corneas are are healthy, yet in some astigmatic individuals the cornea is irregular , resulting in irregular astigmatism. When the curvature is greater in one meridian (plane) of the eye compared to the other meridian, much like a rugby ball, a "splif" focus results.
Light rays refracted by this cornea are not brought to a single point of focus and the retinal image from objects both distant and near are blurred or distorted and may appear broadened or elongated.

Signs and symptoms

Astigmatism
Distortion or blurring of images at all distances is one of the most common astigmatism symptoms. This may happen vertically, horizontally, or diagonally. There can be indistinctness of objects, circles become elongated into ovals and a point of light begins to tail off.

Symptoms of eye strain such as headaches, photophobia and fatigue are also among the most common astigmatism symptoms, including the reading of small print during near work.

Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as:

  • squinting
  • eye discomfort
  • blurred vision
  • headaches
  • distortion in the visual field
  • monocular diplopia
  • glare sensitivity
  • reduced night vision

  • Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches.

    Detection and Prevention

    Astigmatic evaluation
    VA assessment A visual acuity assessment generates a vision score and is notoriously recorded as a fraction such as 20/40. The top number is the standard distance at which testing is performed at 20 feet. The bottom number is the smallest letter size the patient is able to see correctly. A person with 20/40 visual acuity would have to get within 20 feet of a letter that should be seen at 40 feet away. Normal distance visual acuity is 20/20.
    Keratometry 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 the cornea's surface. This measurement is particularly critical in determining the proper fit for contact lenses.
    Corneal Topography A computerised instrument used to provide detailed mapping of the shape of the cornea, more commonly used in eye disease management and before surgical procedures.
    Auto-refractor An automated instrument which estimates the focusing power of the eye based on how light travels to the retina whilst the patient should be in a relaxed state of accommodation.
    Retinoscopy A hand held instrument called a retinoscope is an objective method used to accurately focus light on a single point on the retina in order to create clear vision when the patient is viewing an object at a specific distance
    Refraction This is the most accurate method to measure the visual impact of astigmatism. During a comprehensive eye examination the eyecare professional uses a specialised instrument called a phoropter to subjectively measure how light focuses at the back of the eye onto the retina.


    Combined with other tests performed, the eye doctor determines the correct cyl lens power required to treat astigmatism and promote clear, comfortable vision or enter into discussions about other treatment options.

    Stats and causes

    While much research has been carried out into the prevalence and changes in astigmatism throughout life, questions still remain of the causes of astigmatism.

    Astigmatic distortion
    Astigmatism (more than 0.5 diopters) is a commonly encountered refractive error, accounting for about 13% of the refractive errors of the human eye. It is commonly encountered clinically, with prevalence rates up to 30% or higher depending on the age or ethnic groups.

    Human infants exhibit both high prevalence and high degrees of astigmatism, largely corneal in origin and lessens in prevalence and amplitude over the first few years of childhood.

    Children as young as preschool age may exhibit visual deficits caused by astigmatism, although astigmatism has not been fully investigated in preschool children, its prevalence is reportedly greater in infants than in schoolchildren and among ethnicity groups.

    Treatment

    The various modes of non surgical treatment of astigmatism include, spectacles, contact lenses and treatment of the cause. A cylindrical spectacle lens is the simplest and safest means of correcting the refractive error resulting from astigmatism. However as most astigmatic individuals are also either hyperopic or myopic, they require correction in more than one meridian. To provide the correction they need, a lens formed from the combination of cylinder and sphere is generally required, called a sphero-cylindrical lens.

    Irregular astigmatism occurs when when the principal meridians of the cornea, as a whole, are not perpendicular to one another. Although all eyes have a small amount of irregular astigmatism, this term is clinically used only for grossly irregular corneas such as those occurring with keratoconus or corneal scars. Cylindrical spectacle lenses can do little to improve vision in these cases, and so for best optical correction, rigid contact lenses are more commonly needed.

    Custom designed soft Toric contact lenses or soft disposable lenses often provide another means to correct astigmatic refractive errors. These contact lenses offer good centration when properly fitted, a flexible wear schedule and improved comfort compared to hard lens designs. Regardless of the design chosen, adequate contact lens movement is essential for comfortable wear and maintenance of corneal integrity.