Refractive Errors: Myopia, Hyperopia and Astigmatism

What is refraction

Change in direction of light as it passes obliquely from one optical medium to another of a different refractive index

ALSO

Process of measuring and correcting the refractive error of the ey

Refraction of eye

Human eye refracts at 2 ocular structures:

• Cornea

  • Fixed focusing

  • Power approx. +40D

  • 2/3 of total refraction

Lens

• Variable focusing power - accommodation

• Min power approx. +20D

• (Max power approx. +33D – when accommodating)

• 1/3 of total refraction

Total power of eye approx. +60D

3 factors determine the eyes ability to focus light

Shape of the cornea  Power of the cornea

Power of the lens

Length of the eye

Why does corneal shape dictate power

smaller radius of curvature = steeper = more powerful = shorter focusing distance

corneal shape dictate power

flat cornea = bigger ROC

cornea - allows light pass through - hits it obliquely inside cornea = refractive index = 1.4

air = 1

Cornea surface power

N2 = RI of cornea

N1 = RI of air

Steeper - smaller ROC = more powerful

Focal length

The shorter the focal length the more powerful the surface/lens

Steeper (smaller radius of curvature) = more powerful = shorter focal length

Emmetropia

Occurs when there is “matching” between the power of the eyes optical system (i.e. lens and cornea) and the length of the eye

Light focused on the retina – the eye is emmetropic

Results in clear vision

Objects at infinity

• Focus at the retina

• No accommodative effort

• 60D

  • 40D cornea (7.65mm)

  • 20D lens

    

Refractive error

Occurs when there is a “mismatch” between the power of the eyes optical system (i.e. dioptric power of lens and cornea) and the length of the eye

Light is not focused on the retina – the eye is ametropic

Often results in blurry vision

Object at infinity

• Light not focusing at the retina

• No accommodative effort

• Blur circle projected

3 types of refractive error (ametropia)

Myopia

Hyperopia (Hypermetropia)

Astigmatism

No refractive error = emmetropia

Myopia

Object at infinity (parallel light)

Focused in front of the retina

Blur circle projected

• D blurry

• N - clear

Object nearby (divergent light)

Focus on retina

No accommodative effort

Occurs when:

refractive myopia

• Cornea is too curved

• binatiLens is too powerful

Eye is too long - axial myopia

Or a combination

Correction

• Parallel light focus before the retina

• A blur circle is imaged on the retina

• Use a negative spherical lens to bring the light into focus on the retina

Hyperopia

Object at infinity

• No accommodative effort

• Light focussed behind retina

• Blur circle

N - blurry

D - clear

Occurs when:

refractive hyperopia

• Cornea is too flat

• Lens is too weak

Eye is too short - axial hyperopia

Or a combination of these things

Correction

Light focuses behind the retina - blur circle

Use a positive lens to bring image into focus on retina

Accommodation

Change in the refractive power of the eye by a change in shape of the crystalline lens

Associated with convergence

Young hyperope (pre-presbyopic)

• Effort of accommodation - can use accommodation to bring behind of retina to in front of retina

• Brings the image to focus on the retina

Astigmatism

Light is incident on toric cornea and/or lens, this produces two foci meaning an elliptical blur is imaged on the retina

Occurs when:

• Cornea and/or the lens have different curvatures in different meridians.

Types:

• Regular astigmatism

  • Axis with greatest curvature and axis with least curvature are at 90o angle to one another

• Irregular astigmatism

  • Irregularities in the curvature conform to NO particular geometry e.g keratoconus

Toric/Astigmatic Surface

The two principal meridians (which are located 90 degrees apart) have different curvatures

The radius of curvature of a surface determines the power, so a toric surface will have different power/ focal strength in the two meridians

The astigmatism is expressed as the difference between the 2 powers in dioptres.

With the rule astigmatism: steeper vertically.

Against the rule astigmatism: steeper horizontally.

Correction of Astigmatism

• Refractive error of the eye produces two foci

• An elliptical blur is imaged on the retina

• A combination of spherical and cylindrical lenses used to focus the light on the retina

A combination of spherical and cylindrical lenses used to focus the light on the retina

spherical lens

• A lens where all meridians have the same refractive power

• Spherical lenses have power in all meridians therefore all light rays will be refracted by a spherical lens

Cylinder

• curved = refractive power

• A lens where one of the principal meridians has zero refractive power

  Below examples:

  • Curved in the horizontal plane - producing refractive power

  • No curve in the vertical plane - giving no refractive power

correction of Astigmatism

A combination of spherical and cylindrical lenses used to focus the light on the retina

Spherical lenses have power in all meridians therefore all light rays will be moved by a spherical lens

Cylindrical lens has power in one principal meridian and no power in the other principal meridian.

This means light passing through the meridian with power is refracted, whilst light passing through the zero-power meridian is not refracted.

The position of zero power is called the cylinder axis and this has to be orientated correctly when correcting astigmatic prescriptions.

Summary of correction of ametropia/ refractive error

• Measured in dioptres

• A dioptre (D) is a measurement of the ability of a lens to CONVERGE OR DIVERGE light.

• The amount of power, in dioptres, required to bring the light into focus on the retina

  • Correcting myopia - negative lens

  • Correcting hyperopia - positive lens

  • Correcting astigmatism - correct one meridian with a sphere and the other with a cylinder. The cylinder must be orientated correctly.

Recording spectacle prescription:

The spectacle prescription is recorded in terms of:

• Sphere

• Cylinder

• Axis

E.g +3.00/-2.00 x 125

It can be recorded in negative-cylinder form and positive-cylinder form. To switch between the two forms transposition takes place.

When you wish to transpose a spectacle prescription:

Add the sphere and cylinder powers together - ensure you take sign into account

Change the sign of power of the cylinder.

Change the axis by adding 90 (if less than 90) or subtracting 90 (if greater than 90).

Rx +3.00/ -2.00 x 125 +1.00 / +2.00 x 35

Cross Line Chart

A cross line chart is sometimes used to record spectacle prescription in terms of dioptric power alone.

Methods for correcting refractive error…

• Spectacles

• Contact lenses

  • Rigid gas permeable

  • Excellent optics

  • Keratoconus /high cyls.

  • Tolerance needs to be built up

• Soft

  • Daily disposables, Monthly

  • Spherical/Toric/Presbyopic correction

  • Infections

ADV CL

• Wider Field of View.

• Minimum difference in retinal image size in refractive anisometropia.

• Minimises aberrations.

• Appearance

• Correction of complex prescription

DIS - CL

• Wider Field of View.

• Minimum difference in retinal image size in refractive anisometropia.

• Minimises aberrations.

• Appearance

• Correction of complex prescriptions