Astigmatism

Astigmatism is a refractive error in which the cornea (or less often the lens) has unequal curvature in different meridians, so light cannot focus to a single point on the retina and images are blurred or distorted at all distances. It is very common, with some degree present in about 30 to 60 percent of people. Most astigmatism is regular and correctable with lenses, but irregular astigmatism (keratoconus, scars, surgery) needs special lenses or treatment.

• Types: with-the-rule (steeper vertical meridian, common in the young), against-the-rule (steeper horizontal meridian, common in the elderly), oblique; regular (principal meridians at right angles) and irregular (distorted corneal surface).
• Causes: congenital and genetic corneal shape, eyelid pressure from chalazion, ptosis or haemangioma, pterygium, corneal scars, ulcers, keratoconus and other ectasias (pellucid marginal degeneration), cataract surgery, corneal grafts, trauma, contact lens warpage, dry eye.
• Consequences: blur, glare, eye strain, amblyopia in children if uncorrected (meridional amblyopia), reduced quality of vision.
• Keratoconus onset is typically in the teens to 20s, with progressive thinning and cone formation; it may worsen with eye rubbing and atopy.

• Blurred or distorted vision at distance and near, ghosting, haloes and starbursts at night, headaches, eye strain, squinting, tilted head, eye rubbing, difficulty reading or driving at night.
• Children may be asymptomatic.
• Examination: reduced acuity improved with cylindrical lenses, scissoring reflex on retinoscopy, irregular red reflex, oval or irregular keratometer mires, Munson sign and Fleischer ring in keratoconus.
• Red flags: rapidly increasing astigmatism, unstable refraction (keratoconus), monocular double vision, corneal thinning or scarring, pain, redness, discharge, a history of eye rubbing in atopy, astigmatism in an infant (amblyopia risk), post-surgical change.

• Visual acuity, retinoscopy, autorefraction, subjective refraction with cross cylinder.
• Keratometry, corneal topography and tomography (Scheimpflug or optical coherence tomography), pachymetry; required before surgery and contact lens fitting, and to detect keratoconus early.
• Slit lamp for corneal scars, ulcers, pterygium and lid lesions; cycloplegic refraction in children.
• Differentials: lenticular astigmatism, keratoconus, corneal scars, dry eye, pterygium, cataract, uncorrected myopia or hyperopia.

• Optical correction:
  • Spectacles with cylindrical lenses, with a full time wear in children with significant astigmatism.
  • Toric soft contact lenses for regular astigmatism.
  • Rigid gas permeable, scleral or hybrid lenses for irregular astigmatism and keratoconus.
• Children: correct astigmatism that is symptomatic or at amblyopia risk (about 1.50 to 2.00 dioptres or more at age 4 to 6, higher threshold in infants if with-the-rule (often resolves), lower threshold for oblique or against-the-rule); treat amblyopia with patching or atropine penalisation of the better eye.
• Refractive surgery (stable refraction for at least 1 to 2 years, age 18 or over, normal corneal topography): LASIK, PRK, SMILE with astigmatic correction; toric implantable collamer lens or toric intraocular lens at cataract surgery; limbal relaxing incisions.
• Keratoconus and ectasia: stop eye rubbing; treat allergy (olopatadine 0.1 percent twice daily); corneal collagen cross linking with riboflavin and ultraviolet A light to halt progression (specialist, ideally before significant loss of vision); intracorneal ring segments; corneal transplant (deep anterior lamellar or penetrating keratoplasty) in advanced disease.
• Treat contributing conditions: remove pterygium, treat chalazion in children, manage dry eye.
• Review every 6 to 12 months (more often in children and in suspected keratoconus); refer to ophthalmology for progressive change, irregular astigmatism, amblyopia, or surgery.