Astigmatism – Definition, Etiology, Classification, Diagnosis and Non-Surgical Treatment

1.1 Definition Astigmatism (from the Greek “a” meaning absence and “stigma” meaning point) is a refractive error (ametropia) that occurs when parallel rays of light entering the nonaccommoding eye are not focused on the retina [American Academy of Ophthalmology (AAO), 2007]. Astigmatism occurs when incident light rays do not converge at a single focal point. The cornea of the normal eye has a uniform curvature, with resulting equal refracting power over its entire surface. Most astigmatic corneas are normal also. In some individuals, however, the cornea is not uniform and the curvature is greater in one meridian (plane) than another, much like a football as a rugby ball. Light rays refracted by this cornea are not brought to a single point focus, and retinal images from objects both distant and near are blurred and may appear broadened or elongated. This refractive error is called astigmatism [AAO, 2007]. As concept astigmatism is at least 200 years old; as name, more than 150. Javal ascribes the concept to T. Young in 1800 and the name to W. Whewell. Bannon and Walseh give an early history [Harris, 2000]. Total astigmatism can be divided into corneal (or keratometric) astigmatism, lenticular astigmatism, and retinal astigmatism. Most astigmatism is corneal in origin. Lenticular astigmatism is a result of uneven curvature and differing refractive indices within the crystalline lens [Abrams, 1993]. It is well accepted that there is some relationship between the eye’s corneal and internal astigmatism. In 1890, Javal proposed a rule that predicted the total astigmatism of the eye based on the corneal astigmatism [Grosvenor, 1978; Read et al, 2007]. Javal’s rule states: At = k + p(Ac) Where At is the total astigmatism and Ac is the corneal astigmatism. The terms k and p are constants approximated by 0.5 and 1.25, respectively. Grosvenor, Quintero and Perrigin [Grosvenor et al, 1988] suggested a simplification of Javal’s rule and proposed a simplified Javal’s rule of At = Ac – 0.5, that was supported by Keller and colleagues [Keller et al, 1996]. It should be pointed out to the reader that with modern topographers and aberrometerss it is possible to measure corneal and internal astigmatism and such estimations like Javal’s rule are clinically less relevant. Kelly, Mihashi and Howland [Kelly et al, 2004] suggested that the horizontal/vertical astigmatism compensation is an active process determined through a fine-tuning,