Refractive Error 3

Learning Aim

• Definition of Astigmatism

• Conoid of Strum

• Types of astigmatism

• Correction of astigmatism

• Aphakia and its correction

Astigmatism

• In astigmatism, light rays do not focus to a single

point.

• Regular astigmatism

• Irregular astigmatism

• With-the-rule astigmatism

• Against-the-rule astigmatism

• Regular astigmatism: the principal meridians of the

astigmatism have constant orientation across the pupil,

and the amount of astigmatism is same at every point.

• Irregular astigmatism: The orientation of the principal

meridians or the amount of astigmatism changes from

point to point across the pupil.

• Regular astigmatism is correctable by cylindrical lenses.

Regular astigmatism: Types

• With the rule astigmatism : The vertical meridian is

steepest. The correcting plus cylinder is used at or near axis

90°.

• Against the rule astigmatism : The horizontal meridian is

steepest. Correcting plus cylinder is used at or near 180°.

• Oblique astigmatism: Principal meridians do not lie at or

close to 90° or 180°.

Conoid of Strum

• The complicated geometrical envelop of pencil

of light rays refracted by a spherocylindrical

lens is called conoid of Strum.

Formation of Circle of Least Confusion

Conoid of Strum

Conoid of Strum

• Compound hypermetropic astigmatism

• Simple hypermetropic astigmatism

• Mixed astigmatism

• Simple myopic astigmatism

• Compound myopic astigmatism

Astigmatism

• Symptoms: blurred vision , eye strain.

• Complication: Amblyopia

• Correction: Cylindrical lens

• Surgery: Astigmatic LASIK

Correction of Astigmatism

• Glasses

• Contact lenses

• Corneal surgeries

• Intraocular lenses

Aphakia and pseudophakia

• Aphakia is absence of crystalline lens from the

pupillary area.

• Pseudophakia refers to an eye implanted with

intraocular lens in place of the natural lens

Aphakia

• Etiology: Surgical, traumatic.

• Refractive status: highly hypermetropic.

• Correction: Intraocular lens, contact lens, and

refractive surgery, and spectacle lenses.

• Magnification

• Roving ring scotoma

• Jack-in-the-box phenomenon

• Problem of binocular vision

Disadvantages of correction of Aphakia with Glasses

Points to Remember

• Definition, symptoms, optics, and types of

astigmatism

• Conoid of Strum

• Correction of astigmatism

• Problems of correction of aphakia with glasses