Astigmatism Review: Regular vs Irregular; Correction; Magnitude & Population Data

Page 1: Regular vs Irregular Astigmatism

• Regular astigmatism: symmetric around the visual axis; bow-tie pattern on corneal topography; the power of each meridian changes as the meridians traverse $180^ ext{o}$.
• Irregular astigmatism: lacks symmetry; does not show a regular geometric pattern.

Page 2: Bow-tie irregular astigmatism and vector planning

• Vector planning can be applied to the two corneal meridian divisions in bow-tie irregular astigmatism with asymmetry and/or nonorthogonal relationships; allows separate optimization for each meridian division.
• This becomes more important as lasers are developed that can treat discrete portions of the cornea.

Page 3: Orientation-based classification (with the rule, against the rule, oblique)

• With the rule (WTR): greatest power in the vertical meridian; flat curve along the horizontal meridian. In other words, the flat surface is along the horizontal axis.
• Against the rule (ATR): greatest power in the horizontal meridian; flat curve along the vertical meridian.
• Oblique astigmatism: major meridians at oblique angles, not horizontal or vertical; greatest power lies between $30^ ext{o}-60^ ext{o}$ or between $120^ ext{o}-150^ ext{o}$.
• Correction methods include eyeglasses, contacts, or refractive surgery.

Page 4: Optical correction and focus of the book

• Glasses do not adequately correct irregular astigmatism; rigid contact lenses are often prescribed for irregular astigmatism.
• The text emphasizes laser and surgical approaches on the cornea and lens, with less detail on glasses or contacts.

Page 5: Spectacles and contact lens corrections (rules)

• In with-the-rule astigmatism, a minus cylinder is placed in the horizontal axis.
• In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis.

Page 6: Demographics: age-related patterns

• Children tend to have with-the-rule astigmatism; elderly people tend to have against-the-rule astigmatism.
• With-the-rule astigmatism is more common than against-the-rule astigmatism.
• Generally, with-the-rule astigmatism is visually more favorable than against-the-rule astigmatism.
• Oblique astigmatism is the least favorable for perception.
• Yervald's rule (1890) linked with distant-vision preference for with-the-rule astigmatism.

Page 7: Magnitude and clinical significance

• One diopter or more of astigmatism is typically considered significant; some patients are bothered by lesser amounts.
• Up to $20\%$ of eyes with cataract have at least $1.5\ \,\mathrm{D}$ of corneal astigmatism.

Page 8: Population-based prevalence

• Most refractive studies use manifest refraction; significant astigmatism defined as cylinder $\ge 1\ \mathrm{D}$; seen in roughly $10\%\text{ to }50\%$ of patients, varying with age and race/ethnicity.
• A key population study (2008) with $n = 12{,}010$ from NHANES (1999–2004): for people aged 20+, astigmatism of $\ge 1\ \mathrm{D}$ found in $36.2\%$ (95% CI: $34.9\%\ -\ 37.5\%$).
• Figure 1-4 shows results by age and race/ethnicity.
• Conclusion: The majority of people worldwide have refractive error; astigmatism of magnitude relevant to refractive and cataract surgery is found in more than a third of prospective patients.