Introduction to Refractive Error and Prescription Writing

Introduction to Refractive Error and Prescription Writing Walter Huang, OD Yuanpei University Department of Optometry

Vision • Optics • Cornea • Aqueous humor • Lens • Vitreous humor • Retina

Refractive Error • The result of a mismatch between optics and the growth of the eye • It is due to a combination of genetic and environmental influences • It is NOT considered an eye disease • Treatment includes spectacles, contact lenses, and refractive surgery

Types of Refractive Error • Emmetropia • Myopia • Hyperopia • Astigmatism • Presbyopia

Emmetropia • The average emmetrope has a VA of 20/20 or better

Myopia • When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus in front of the retina

Myopia • Blurry vision at distance • Clear vision at near

Myopia • It is corrected by divergent or minus lenses • Power of corrective lens needed can be estimated by finding the far point where the patient can achieve clear vision • Example • Far point is at 20cm • Focal length of corrective lens needed = 20cm • Power of corrective lens needed = 1/f =1/0.2m = -5.00D • Unit for Power = Diopter (D)

Hyperopia • When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus behind the retina

Hyperopia • Blurry vision at distance and near • Intermittent blurring of vision

Hyperopia • It is corrected by convergent or plus lenses • A young patient with low hyperopia can accommodate to focus the distant image on the retina • Since accommodation decreases with age, a low hyperopic patient tends to wear corrective lenses for near work at an earlier age

Astigmatism • When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina

Astigmatism • It is due to a distortion of the cornea and/or lens • The refracting power is not uniform in all meridians • The principal meridians are the meridians of greatest and least refracting powers • The amount of astigmatism is equal to the difference in refracting power of the two principal meridians

Astigmatism • Distorted vision • Letter confusion • P versus F • A versus R • H versus N

Astigmatism • It is corrected by cylindrical or spherocylindrical lenses

Presbyopia • Presbyopia = “old man’s eye” (Latin)

Presbyopia • Decrease in the amplitude of accommodation or loss of accommodative ability with age

Presbyopia • It is a natural part of the aging process • The onset of presbyopia is at approximately 40 years of age and over though it may be earlier in low hyperopes

Presbyopia • Blurry vision at near • Difficult or impossible to accommodate sufficiently for near work

Presbyopia • It is corrected by convergent or plus lenses for near work only (near Add)

Types of Lens • Spherical lens • Cylindrical lens • Spherocylindrical lens

Spherical Lens • A plus or minus lens where the refracting power is equal in all meridians • Diopter Sphere (DS) is the measuring unit used to differentiate the spherical lens from lenses with cylindrical component • Power cross and prescription writing for a spherical lens require the specification of the spherical power component only

Spherical Lens • Power cross: • Prescription form: +2.50DS

Cylindrical Lens • A flat or plano (pl) axis meridian perpendicular to a power meridian

Cylindrical Lens • Diopter Cylinder (DC) is the measuring unit used to differentiate the cylindrical lens from lenses with spherical component • Power cross and prescription writing for a cylindrical lens require the specification of both the cylindrical power and axis components

Cylindrical Lens • Power cross: • Prescription form: -4.00 x 180

Spherocylindrical Lens • A toric lens consists of two perpendicular principal meridians • Power cross and prescription writing for a spherocylindrical lens require the specification of the spherical power, cylindrical power, and axis components

Spherocylindrical Lens • Power cross: • Prescription form: • +3.00 -1.00 x 180 • +3.00/-1.00 x 180

Prescription Writing • Example 1 • Power cross: • Prescription form: +1.00 -0.50 x 120

Prescription Writing • Example 2 • Power cross: • Prescription form: -3.00 -0.50 x 084

Prescription Writing • Example 3 • Power cross: • Prescription form: +1.25 -2.50 x 005

Rules for Power Cross • Specify both power and axis • Power is always represented by plus or minus sign in front and contains two digits after the decimal point • Power is presented in 0.25D steps

Rules for Power Cross • Axis meridian starts counter-clockwise from 0 to 180

Rules for Power Cross • The cross orientation is drawn to the actual meridian • When axis is at the 0 to 180 horizontal, use 180 instead of 0 • Degree notation may or may not be used for axis • If degree notation is NOT used for axis, three digits must be used for axis, except in the case of 0

Rules for Prescription Writing • Always include power, cylinder, and axis, except for spherical lenses (specified as DS) • Degree notation is NOT used for axis

Minus versus Plus Cylinder • In Optometry, prescription writing is in minus cylinder (-cyl) form • Sphere and axis specified is the most plus principal meridian

Minus versus Plus Cylinder • In Ophthalmology, prescription writing is usually in plus cylinder (+cyl) form • Sphere and axis specified is the most minus principal meridian

Conversion between Minus and Plus Cylinder • Be sure to know how to convert between minus and plus cylinder form and back

Conversion between Minus and Plus Cylinder • Example • Minus cylinder form: +1.00 -3.00 x 180 • Plus cylinder form: -2.00 +3.00 x 090

Introduction to Refractive Error and Prescription Writing