Astigmatism is a common refractive error that results in distorted or blurred vision at any distance. This condition occurs when the cornea, the clear front surface of the eye, or the lens inside the eye is curved more like a rugby ball than a sphere. Instead of focusing light onto a single point on the retina, the irregular shape causes light to focus at multiple points. Astigmatism can definitively affect only one eye, a condition sometimes called unilateral astigmatism.
Why Astigmatism Can Develop in Only One Eye
The development of astigmatism is attributed to the irregular shape of the cornea or lens, which can manifest asymmetrically between the eyes. While many cases are present from birth and linked to inherited factors, these genetic influences do not always affect both eyes equally. The degree of curvature can simply be greater or more pronounced in one eye, leading to a significant difference in refractive error.
Acquired causes provide a clear mechanism for astigmatism to develop solely in one eye, often due to an event localized to that side. For instance, a direct physical trauma or injury to one eye can result in scarring or changes to the corneal tissue, altering its smooth, spherical shape. Furthermore, surgical interventions, such as cataract surgery, can induce or change the existing astigmatism in the operated eye, especially if a standard intraocular lens is used instead of a toric lens.
Certain progressive eye diseases also commonly present with an unequal impact on vision, highlighting the potential for unilateral progression. Keratoconus, a condition where the cornea thins and bulges outward into a cone shape, frequently starts or progresses more aggressively in one eye than the other. This localized thinning creates highly irregular astigmatism, which is often confined to the affected eye. These localized events explain why one eye can have a significant astigmatic prescription while the other remains unaffected or only mildly impacted.
Recognizing the Vision Shift
When astigmatism is present in only one eye, the symptoms experienced often relate to the brain’s attempt to reconcile two different images. Common symptoms include blurred vision, eye strain, and headaches, stemming from the distorted light entering the single affected eye. The visual information from the astigmatic eye appears stretched or indistinct, while the other eye provides a clear image.
The brain naturally favors the clearer image from the unaffected eye, a process known as visual compensation. This tendency can mask the underlying problem, making the condition subtle and difficult to detect, particularly in adults. However, this compensation leads to eye fatigue and discomfort as the visual system strains to merge the mismatched inputs. The discrepancy in image quality impacts binocular vision, specifically affecting depth perception and spatial awareness.
In children, unilateral astigmatism carries the added risk of amblyopia, commonly known as lazy eye. If the brain consistently ignores the blurry input from the highly astigmatic eye to avoid confusion, the neural pathways for vision in that eye fail to develop properly. Early detection is paramount for children, as treatment during the developmental period can prevent this permanent reduction in visual acuity. Parents should be watchful for signs like frequent squinting, head tilting, or difficulty with reading and concentration, as children often cannot articulate that their vision is blurry.
Correcting Asymmetric Vision
Diagnosis begins with a comprehensive eye examination that includes specialized measurements of the eye’s shape. Instruments like the keratometer measure corneal curvature, while a corneal topographer creates a detailed contour map of the surface. These measurements allow the eye care professional to precisely quantify the degree and axis of the irregularity, determining the exact cylindrical power needed to correct the refractive error.
The primary goal of treatment for unilateral astigmatism is to balance the visual input between the two eyes, ensuring both transmit a clear image to the brain. Eyeglasses are the simplest option, utilizing a specialized cylindrical lens to counteract the irregular curve in the affected eye. Contact lenses, particularly toric soft lenses, often provide superior correction for higher degrees of astigmatism because they conform closely to the eye, offering a wider field of view without the distortions seen at the edge of spectacle lenses.
For stable cases, refractive surgery options like LASIK or PRK offer a permanent solution by using a laser to reshape the cornea. This procedure aims to eliminate the irregular curvature so the eye can focus light correctly without the need for external corrective devices. Regular eye exams are important, regardless of the correction method, to monitor the stability of the astigmatism and ensure the most appropriate prescription is maintained, particularly if underlying conditions like keratoconus are present.