GP lens eye Rounds

ASTIGMATISM: Definition

Jessica Conroy OD, FAAO

 When thinking about corneal rigid gas permeable (GP) lenses, “Astigmatism” is defined in terms of both “residual astigmatism” and “corneal astigmatism.”

• Recall: Corneal astigmatism is the difference between the two principle keratometry meridians.
  • • Low corneal astigmatism is defined as being <2.00D.
    • High corneal astigmatism is defined as being >2.50D.
• Recall: Calculated residual astigmatism is the difference between corneal astigmatism and refractive astigmatism. Actual residual astigmatism is the amount of astigmatism refractive error present when a contact lens is placed on the eye (i.e., the amount of astigmatism in an over-refraction).²
  • • Low Residual Astigmatism is defined as being <0.50D.

References

1. Mickles CV, Gordon A. Optical Considerations in Contact Lens Practice. In Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 5th ed., Wolters Kluwer, Philadelphia, PA, 2020:36–67.
2. Bennett ES, Henry VA, Kinoshita BT, Lampa M. Correction of Astigmatism. In Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 5^th ed., Wolters Kluwer, Philadelphia, PA, 2020:388–439.