Makindo Medical Notes

In astigmatism, the front surface of the cornea is curved more in one direction than in another, leading to blurred or distorted vision. Regular eye examinations are essential for proper diagnosis and management.

About Astigmatism

Astigmatism is a common refractive error caused by irregularities in the curvature of the cornea or lens. Instead of being perfectly spherical, the cornea has an uneven shape, which prevents light from focusing correctly on the retina, resulting in blurred or distorted vision. Astigmatism can occur in combination with myopia (nearsightedness) or hyperopia (farsightedness).

• Astigmatism involves irregularities in the radius of curvature of the cornea, leading to distorted vision.
• It represents a variation in the normal structure of the eye's refractive surfaces.

Aetiology

The development of astigmatism can be attributed to both congenital and acquired factors. Understanding the underlying causes is essential for effective management and prevention.

• Congenital: Astigmatism can be present at birth due to inherited irregularities in corneal shape.
• Acquired: Trauma or injury to the eye can alter the corneal curvature, leading to astigmatism.
• Other Causes: Conditions such as keratoconus, where the cornea becomes progressively thinner and cone-shaped, can induce irregular astigmatism.

Clinical Presentation

Astigmatism manifests with a variety of visual symptoms that can significantly impact daily activities. The severity of symptoms often correlates with the degree of corneal irregularity.

• Blurry Vision: Difficulty seeing fine details clearly at all distances.
• Distorted Vision: Objects may appear wavy or bent, similar to looking into a distorted, wavy mirror.
• Eye Strain: Persistent effort to focus, leading to discomfort and fatigue, especially during prolonged visual tasks.
• Headaches: Often resulting from prolonged eye strain and attempts to compensate for the blurred vision.
• Difficulty with Night Vision: Increased glare and halos around lights, making night driving challenging.
• Double Vision: In severe cases, patients may experience double vision due to unequal image focusing.

Differential Diagnosis

Several ocular conditions can present with symptoms similar to astigmatism. Differential diagnoses include:

• Keratoconus: A progressive thinning and cone-shaped deformation of the cornea, leading to irregular astigmatism.
• Corneal Scarring: Resulting from infections, injuries, or surgeries, which can cause irregular corneal surfaces.
• Cataracts: Clouding of the lens can induce refractive changes and astigmatism-like symptoms.
• Dry Eye Syndrome: Can cause fluctuating vision and discomfort, mimicking astigmatism symptoms.
• Posterior Capsule Opacification: A common complication after cataract surgery, leading to blurred vision.

Investigations

Accurate diagnosis of astigmatism involves a combination of clinical evaluation and specialized diagnostic tests to determine the type and severity of the refractive error.

• Comprehensive Eye Examination: Includes visual acuity testing using a Snellen chart to assess clarity of vision.
• Refraction Test: Performed using a phoropter or autorefractor to determine the precise prescription needed for corrective lenses.
• Keratoscopy: A manual method to evaluate the curvature of the cornea by projecting a pattern of lines onto the corneal surface.
• Corneal Topography: A computerized imaging technique that maps the surface curvature of the cornea, providing detailed information about irregularities.
• Pachymetry: Measures corneal thickness, which is crucial in diagnosing conditions like keratoconus.
• Slit-Lamp Examination: Allows for detailed visualization of the cornea, lens, and other ocular structures to identify any underlying abnormalities.

Management

The management of astigmatism focuses on correcting the refractive error to improve visual acuity and reduce symptoms. Treatment options vary based on the type and severity of astigmatism.

• Corrective Lenses:
  • Spectacle Lenses: The most common and non-invasive method to correct regular astigmatism. Cylindrical lenses compensate for the uneven curvature.
  • Contact Lenses:
    • Rigid Gas Permeable (RGP) Contacts: Provide clearer vision by maintaining their shape on the cornea.
    • Toric Contact Lenses: Specifically designed to correct astigmatism by having different powers in different meridians.
    • Soft Contact Lenses: Available in toric designs but may not be suitable for severe or irregular astigmatism.
• Refractive Surgery:
  • Laser-Assisted In Situ Keratomileusis (LASIK): Uses an excimer laser to reshape the cornea, correcting the refractive error and reducing or eliminating the need for glasses or contact lenses.
  • Photorefractive Keratectomy (PRK): Similar to LASIK but involves reshaping the cornea's surface without creating a corneal flap.
  • Refractive Lensectomy: Involves removing and replacing the eye's natural lens with an artificial intraocular lens (IOL), suitable for cases with significant refractive errors.
  • Intrastromal Corneal Ring Segments (Intacs): Implanted into the cornea to flatten its curvature, beneficial in conditions like keratoconus.
• Corneal Collagen Cross-Linking (CXL): Primarily used for keratoconus but can help in stabilizing corneal curvature in progressive astigmatism cases.
• Topography-Guided Treatments: Customized laser treatments based on detailed corneal topography to address irregular astigmatism.
• Orthokeratology: Specially designed rigid contact lenses worn overnight to temporarily reshape the cornea and reduce astigmatism during the day.
• Regular Monitoring: Essential for progressive or irregular astigmatism to assess changes in corneal shape and adjust treatment accordingly.

Prognosis

The prognosis for individuals with astigmatism is generally excellent with appropriate corrective measures. Regular eye examinations and timely interventions can effectively manage symptoms and prevent complications.

• Corrective Lenses: Provide immediate improvement in vision quality with minimal side effects.
• Refractive Surgery: Offers long-term correction with high satisfaction rates, though it carries risks such as dry eyes, glare, and potential overcorrection or undercorrection.
• Progressive Astigmatism: Requires ongoing monitoring and potential adjustments to corrective lenses or treatments to maintain optimal vision.
• Irregular Astigmatism: May require specialized contact lenses or surgical interventions for effective management.

Conclusion

Astigmatism is a prevalent refractive error that can significantly impact visual clarity and quality of life. Early detection through comprehensive eye examinations and appropriate management strategies are essential for optimal visual outcomes. Treatment options range from corrective lenses to advanced surgical interventions, tailored to the individual's specific type and severity of astigmatism. With timely and effective treatment, most individuals with astigmatism can achieve clear and comfortable vision.

Revisions

• Last updated: 27/04/2024

References

• American Optometric Association. Astigmatism. Available at: https://www.aoa.org
• Mayo Clinic. Astigmatism. Available at: https://www.mayoclinic.org
• Kanski, J. J., & Bowling, B. (2011). Clinical Ophthalmology: A Systematic Approach. Elsevier Health Sciences.
• Sheppard, R. J., & Wolffsohn, J. S. (2018). Digital eye strain: prevalence, measurement and amelioration. BMJ Open Ophthalmology.
• National Eye Institute (NEI). Astigmatism. Available at: https://www.nei.nih.gov
• British Journal of Ophthalmology. Management of Astigmatism. Available at: https://bjo.bmj.com/