Overview
This lecture explains astigmatism, a common refractive error in the eye, its types, characteristics, and methods of correction.
Understanding Astigmatism
- Astigmatism is a refractive error where refraction varies along different meridians of the eye.
- In a perfect sphere, all meridians have equal refractive power and focus light to a single point (point focus).
- In astigmatism, different meridians have different curves and thus different refractive powers, causing light to focus as lines (line focus) instead of a point.
Types of Astigmatism
- Two major categories: regular astigmatism (uniform power change between meridians) and irregular astigmatism (non-uniform changes).
- Regular astigmatism subdivides into: with-the-rule, against-the-rule, oblique, and bi-oblique astigmatism.
Regular Astigmatism Subtypes
- With-the-rule astigmatism: vertical meridian more curved than horizontal; common in young people due to eyelid pressure.
- Correction: minus cylinder at 180° or plus cylinder at 90°.
- Against-the-rule astigmatism: horizontal meridian more curved than vertical; more common in older adults.
- Correction: minus cylinder at 90° or plus cylinder at 180°.
- Oblique astigmatism: principal meridians are perpendicular but not aligned with 90°/180° (e.g., 45°/135°).
- Bi-oblique astigmatism: principal meridians not perpendicular (e.g., 30° and 100°).
Refractive Types of Regular Astigmatism
- Simple astigmatism: one meridian’s image focuses on the retina; the other focuses in front (myopic) or behind (hypermetropic).
- Compound astigmatism: both meridians focus in front of (myopic) or behind (hypermetropic) the retina; none focus on the retina.
- Mixed astigmatism: one meridian focuses in front, the other behind the retina; neither focus on the retina.
Key Terms & Definitions
- Astigmatism — A refractive error with different power in various meridians, causing line instead of point focus.
- Regular astigmatism — Refractive power changes uniformly across meridians.
- Irregular astigmatism — Refractive power changes unevenly, causing distorted vision.
- With-the-rule astigmatism — Vertical meridian more curved than horizontal.
- Against-the-rule astigmatism — Horizontal meridian more curved than vertical.
- Oblique astigmatism — Principal meridians at oblique angles (not 90°/180°), but still perpendicular.
- Bi-oblique astigmatism — Principal meridians not perpendicular.
- Simple, compound, mixed astigmatism — Classification based on focal line positions relative to the retina.
Action Items / Next Steps
- Review practice exercises (to be given in the next session).
- Learn how to interpret astigmatism prescriptions and classify the types.