Astigmatism occurs when different refractive errors are present in different lens/eye meridians. Astigmatism requires cylindrical rather than spherical lenses for correction. The prescription typically includes the dioptre value correction and the cylindrical axis of the required lens. i.e. Dioptres of Cylinder DC: -1.5 x 75 [dioptres x degrees]. Astigmatism is most often combined with a spherical error, myopia or hypermetropia.
The equivalent spherical error is taken as the mathematical sum of any spherical refraction error plus half that of the cylindrical refraction error.
For instance an applicant with the following prescription:
DS -4.0 / DC -2.5 x 150 can be estimated to have a total error: -4.0 + (-2.5 x 1/2) = -5.25
One should be suspicious of keratoconus in applicants with worsening astigmatism. Keratoconus can be missed at the early stage, only to compromise a flying career later.
Thus an applicant with worsening astigmatism should be assessed for keratoconus. If present, the condition may progress and eventually render an applicant ineligible some years later, despite currently meeting the corrected visual acuity standards. In case of doubt a yearly optometrist or ophthalmologist report including corneal topography mapping should be obtained. The applicant should be well informed of the potential for future certification difficulties.
A special eye report must or should be provided in the following circumstances:
An applicant with astigmatism who meets the uncorrected or corrected visual acuity standards may be considered as not having a condition that is of aeromedical significance.
An applicant with astigmatism, who only meets the visual acuity standards while using spectacles, may be considered as having a condition that is not of aeromedical significance if the certificate is endorsed with: