Astigmatism is a common condition where the eye’s shape, most often the cornea, has an imperfection in its curvature, causing blurred vision at any distance. Instead of having a uniformly spherical surface like a basketball, the eye’s front surface is curved more steeply in one direction, resembling the shape of a football. This irregularity prevents light from focusing precisely onto a single point on the retina, causing the light rays to bend unevenly. This refractive error makes seeking a professional check necessary for diagnosis and treatment.
Common Signs Indicating a Check is Needed
The most noticeable indicator of astigmatism is blurred or distorted vision, which affects both near and far objects. This distortion can make objects appear shadowed or stretched, and it may be more pronounced in specific directions, such as seeing vertical lines more clearly than horizontal ones. The eye attempts to compensate for this constant blur, often leading to significant eye strain and discomfort.
The continuous effort to focus frequently triggers persistent headaches, particularly after visually demanding tasks like reading or working on a computer. Individuals with uncorrected astigmatism often find themselves squinting to temporarily clarify their sight. Difficulty with night vision, manifesting as glare or halos around lights, is another common symptom prompting a professional eye examination.
Preliminary Self-Screening Tests
Before a professional appointment, some simple tools can offer a preliminary indication of astigmatism, though they are not a substitute for a diagnosis. A popular method involves using a radiating lines chart, which consists of lines radiating outward from a central point like the spokes of a wheel. Individuals with astigmatism may perceive certain lines on this chart as appearing darker, sharper, or clearer than others.
This difference in clarity occurs because the irregular curvature of the eye focuses light from one meridian more effectively than others. Another tool, the Amsler grid, which is primarily used to monitor the macula, can also sometimes reveal vision problems if its straight lines appear wavy, distorted, or missing. However, these self-administered tests only suggest the possibility of an issue and cannot determine the severity or exact prescription needed.
Comprehensive Professional Diagnostic Tools
A comprehensive eye examination uses multiple instruments to accurately diagnose and measure astigmatism. The process often begins with a standard visual acuity test, typically using the Snellen chart, to determine the level of vision correction required to reach optimal clarity. Following this, objective measurements of the eye’s curvature are performed using a keratometer or corneal topographer.
The keratometer measures the curvature of the central cornea by projecting light rings onto its surface and analyzing the reflection pattern. This instrument provides two distinct “K” values—one for the steepest curve and one for the flatter curve—quantifying the difference in corneal power that causes astigmatism. Corneal topography creates a detailed, color-coded map of the entire corneal surface, which is useful for detecting irregular astigmatism that a keratometer might miss.
The final step in determining the precise prescription is the subjective refraction assessment, often assisted by a phoropter. This instrument contains numerous lenses that the eye care professional changes while asking the patient which combination offers the clearest vision. The professional also uses retinoscopy, an objective technique involving shining a light into the eye to observe how light reflects off the retina. This helps determine the general refractive error, including the orientation of the astigmatism, before subjective refinement.
Interpreting the Examination Results
The results of the professional examination are translated into a prescription containing specific numerical values that define the astigmatism. These values include the Cylinder and the Axis.
Cylinder (CYL)
The Cylinder (CYL) value, measured in diopters, indicates the severity or amount of lens power needed to correct the irregular shape of the eye.
Axis
The Axis, expressed as a number between 1 and 180 degrees, specifies the orientation or location of the astigmatism. This angle tells the lens manufacturer precisely where the cylindrical power needs to be placed on the corrective lens to neutralize the irregular curve.
A prescription with both a Cylinder value and an Axis measurement confirms the diagnosis of astigmatism and directs the creation of custom lenses. Based on the severity, treatment options range from corrective lenses, such as eyeglasses or toric contact lenses, to refractive surgery like LASIK.