Astigmatism: the refractive power of the eye differs between meridians, so a point is imaged as two focal lines rather than a point (see the conoid of Sturm in Astigmatic Lenses)
Most astigmatism is corneal. The lens contributes a small amount, usually against the rule, which partly offsets the cornea in young people

## Regular and irregular astigmatism

Regular: the two principal meridians (strongest and weakest) are 90° apart. It can be corrected fully with a cylindrical lens
Irregular: the principal meridians are not at right angles, or the power varies irregularly across the pupil

Causes: keratoconus and other ectasias, corneal scars, pterygium, poorly healed grafts, decentred refractive surgery, lens subluxation
It cannot be corrected with spectacles. A rigid contact lens replaces the irregular cornea with a smooth tear-filled surface (see Contact Lens Optics)
Signs: a distorted keratometer mire, distorted Placido rings, a scissoring reflex on retinoscopy

Regular astigmatism is oblique when the principal meridians are not near 90° and 180°

## With and against the rule

Type	Steepest meridian	Correcting cylinder	Typical age
With the rule	Vertical (90° ± 30°)	Plus cylinder at 90°, or minus cylinder at 180°	Children and young adults; the upper lid presses on the cornea
Against the rule	Horizontal (180° ± 30°)	Plus cylinder at 180°, or minus cylinder at 90°	Older adults; the cornea gradually flattens vertically
Oblique	Between 30° and 60°, or 120° and 150°	Cylinder at an oblique axis	Any age; often less well tolerated

Remember: a plus cylinder's axis lies along the steep meridian; a minus cylinder's axis lies along the flat meridian
Worked example: keratometry reads 42.00D at 180 and 44.00D at 90. The vertical meridian is 2D steeper: 2D of with-the-rule corneal astigmatism, corrected by −2.00 × 180 (or +2.00 × 90)

## Classification by the focal lines

With accommodation relaxed, each principal meridian forms a focal line. The type is named by where those lines fall relative to the retina

Simple: one line on the retina. Simple myopic (the other in front) or simple hypermetropic (the other behind). Example: plano / −1.00 × 180
Compound: both lines in front (compound myopic) or both behind (compound hypermetropic). Example: −2.00 / −1.00 × 90
Mixed: one line in front of the retina and one behind. Example: +1.00 / −2.00 × 180, which is +1.00D in one meridian and −1.00D in the other

To classify a prescription, work out the power in each principal meridian (sphere, and sphere + cylinder): both minus is compound myopic, both plus is compound hypermetropic, one of each is mixed, one zero is simple

## Symptoms and effects

Blurred vision at all distances, asthenopia and headache, head tilt (with oblique axes) and squinting to make a stenopaeic slit
Patients accommodate, when they can (not in myopic astigmatism), to bring the circle of least confusion onto the retina, which gives the best uncorrected vision. The spherical equivalent places it there
Uncorrected astigmatism of more than about 1.50D in early childhood risks meridional amblyopia

## Correction

Spectacles: a sphero-cylindrical lens. A new or changed cylinder, especially at an oblique axis, causes meridional distortion: tilted floors and slanted walls. Older patients tolerate large changes poorly (see Prescribing Spectacles)

Ways to reduce it: under-correct the cylinder while keeping the spherical equivalent, rotate the axis towards 90° or 180°, use a short vertex distance, or change gradually

Contact lenses: rigid lenses correct corneal astigmatism through the tear lens. Soft toric lenses need stabilisation (prism ballast, thin zones) to stop rotation (see Contact Lens Optics)
Surgery: toric IOLs, limbal relaxing incisions, arcuate keratotomy, excimer laser

Rotation of a toric IOL away from its intended axis loses about 3.3% of the cylinder correction per degree. At 30° off axis, all the correction is lost and the cylinder changes axis

Surgically induced astigmatism: a corneal incision flattens the cornea in its own meridian and steepens it 90° away (coupling)
A cataract incision placed on the steep meridian reduces pre-existing astigmatism
Tight sutures steepen the cornea in their meridian; removing a tight suture flattens it
Toric IOL planning uses total corneal astigmatism, including the posterior cornea, which adds about 0.3–0.5D against the rule

Measuring astigmatism: keratometry and topography measure the corneal component (see Keratometry and Topography). Retinoscopy finds the total refractive astigmatism (see Retinoscopy), and the Jackson cross-cylinder refines its axis and power subjectively (see Subjective Refraction). The difference between refractive and corneal astigmatism is the lenticular, or residual, astigmatism. It matters when fitting rigid contact lenses.