Astigmatism: Refractive Error and Correction

Study Notes

Astigmatism is a refractive condition where a point focus of light cannot be formed on the retina, resulting in multiple foci.
It occurs due to variation in powers in different meridians of the eye.

Thomas Young: had a crude measurement of his own astigmatism (about 1.75 D).
George Biddle Airy: first to correct astigmatism using a spherocylindrical lens (USA, 1828).
John Green: produced the first distance test chart for astigmatism, including the clock dial chart.
H. Knapp: credited with the first trial case containing cylindrical lenses and suggested the method for locating the axis.

Total Astigmatism: the sum resultant astigmatism of all factors of the refractive system.
Corneal Astigmatism: variation of curvature throughout different meridians of the cornea.
Lenticular Astigmatism: astigmatism due to unequal curvature of lens surfaces or layers.
Resultant Astigmatism: in relation to the total refracting system.

Regular Astigmatism: each meridian of the cornea is uniform, with two principal meridians at right angles to each other.
Irregular Astigmatism: curvature is not uniform, and principal meridians are not at right angles to each other.

Direct/With-the-Rule: curvature of greatest power near the vertical meridian (60-120°).
Inverse/Perverse/Against-the-Rule: curvature of greatest power near the horizontal meridian (0-30°, 150-180°).
Oblique: greatest power and least power lies within 30-60° and 130-150°.

Compound Hyperopic Astigmatism: two principal meridians fall beyond the retina, corrected with plus sphere and plus cylinder.
Simple Hyperopic Astigmatism: one meridian falls on the retina, and the other falls beyond the retina, corrected with simple plus cylinder.
Compound Myopic Astigmatism: two principal meridians fall in front of the retina, corrected with minus sphere and minus cylinder.
Simple Myopic Astigmatism: one meridian falls on the retina, and the other falls in front of the retina, corrected with simple minus cylinder.
Mixed Astigmatism: one meridian falls behind, and the other falls in front of the retina, corrected with one sign combined with an opposite sign.

Symptoms: blurred vision, transient blurring, frontal and temporal headaches, tearing, and smarting.
Signs: tilting of the head, squinting, wrinkling and furrowing of the brows, and holding reading material too close.

Pre-school years: globe increases by 5mm on average between birth to 3 years.
School years: increase of 1mm axial length from 3-13 years old.
Myopia: develops around 5-6 years old and may remain or increase.
Hyperopia: develops around 5-6 years old, with +1.50 D hyperopia remaining.
Spherical refraction (+0.50-+1.25 D) at 5-6 years old may lead to emmetropia at 13-14 years old.
Prevalence of myopia is lower in rural regions compared to urban regions, supporting environmental exposure's impact on myopia development.

A refractive condition where the near point of accommodation recedes due to advancing age, making it difficult or impossible to see at near without corrective lenses.
Often occurs between 38-40 years old, but can appear as early as 40 or as late as 48-50 years old.
Not a disease, pathological condition, or anomaly, but an expected and inevitable part of the healthy aging process in humans.