Subjective Refraction Technique: Astigmatic Dial

Excerpt

Astigmatism is defined as a condition where there is an unequal curvature of the front surface of the eyes. Astigmatism can occur in the cornea (corneal astigmatism), the crystalline lens (lenticular astigmatism), or, less frequently, in the retina.

Astigmatism is usually congenital but may change in location and amount as the individual ages. A rare occurrence is when excessive congenital astigmatism results from a toric growth of the crystalline lens onto the underlying capsule in a condition known as lenticonus.

It is commonplace to see the terms "meridians" and "axes" when dealing with this topic. An astigmatic axis is the exact location of astigmatism on the 360-degree scale, while a meridian is a plane along which that power exerts its optical characteristics.

Types of Astigmatism Encountered in Refractive Errors (See Images)

Simple Myopic astigmatism - One axis is on the retina while the other is in the front of the retina (in the hypothetical vitreous space).

Compound myopic astigmatism - Both axes are in front of the retina (in the hypothetical vitreous space).

Simple hyperopic astigmatism - One axis is on the retina, while the other is behind the retina.

Compound hyperopic astigmatism - Both axes are behind the retina.

Mixed astigmatism - One axis is in front of the retina, and the other is behind the retina.

Astigmatic dials are instruments used for the detection and quantification of ocular astigmatism. They usually consist of black lines arranged radially (i.e., Fan dial, Sunburst dial, and Clock dial) or in a cross, as in the Cross dial (see image). The dial may be stationary and fixed in one position or rotate around an axis. They are usually made of plastic as an accessory to a distance wall-mounted chart, which can have a digital chart component.

Twelve radial lines comprise the astigmatic dial. The lines may be single in some models or consist of three parallel lines per clock hour. The sunburst dial consists of a semi-circle facing upwards like a sunrise, with the radiating lines 5 degrees away from each other, running from 0 to 180 degrees. The Lancaster dial consists of two lines perpendicularly bisecting each other similar to the cross dial.