Spectacle Correction of High Refractive Error

Aphakia

Absence of lens from the eye

Caused by surgery, trauma, or congenital

Lens

Biconvex shape → High plus

Absence results in high hyperopia

Problems with aphakic specs

• Increased RIS

• Decreased FOV

• Ring scotoma

• Increased ocular rotations

• Increased lens aberrations

• Motion of objs in FOV

• Appearance

• Convergence demands

Ring scotoma

Prismatic effect of the high plus lens creates an angular gap in obj space

Angular size of the gap is equal to prismatic effect at the edge of the lens (found with Prentice’s Rule)

Pts with high myopia in their specs…

Will have diplopia at edge of specs from minification

Peripheral FOV

Field subtended at entrance pupil

Macular FOV

FOV for the moving eye subtended at the center of rotation

Center of rotation of the eye

Close to geometric center of globe

Aphakic pts macular FOV and peripheral FOV

Both decreased → Ring scotoma

Macular decrease is bigger than peripheral

Prentice’s Rule

Determines prismatic effect

Jack in the Box Effect

Ring scotoma moves in the opposite direction of the eyes and same direction as the head

Stationary objects will pop in and out of the scotoma as the eye moves

Ring scotoma causes difficulty mostly at…

Intermediate distances

Ring scotoma size and position depends on…

• Lens power (higher → bigger)

• Vertex distance (farther → moves in)

• Lens size (bigger → moves out)

• Pupil size (bigger → smaller)

• Lens thickness (thicker → bigger)

• BC (bigger → bigger)

Increase in ocular rotations

Caused by prismatic effect of lens

Prismatic deviation depends on lens power and angular separation of the 2 alternately fixed points

H: Image moves down → Increased rotation

M: Image moves up → Decreased rotation

Increased lens aberrations

Plus lenses above +8.00D → Oblique astig and curvature of image can be eliminated through aspheric surfaces (BOTH cannot be eliminated at the same time)

Aphakic specs will experience pincushion distortion

Types of aspheric lenses

• Continuous: Steeper curvature and flattens in periphery

• Zonal

Pincushion distortion

Peripheral is magnified

Distortion

Results from changing mag across the FOV of the lens

Shape and/or size are not faithfully reproduced by the image

Control requires steeply curved lenses or use of aspheric surfaces

Swimming motion

Against motion when gaze is held steady and head is moved, caused by prismatic effects of lenses

Can be avoided by only moving eyes which can cause distortion

Appearance of wearer

Aphakic lenses make pts eyes look highly magnified

Demands on convergence

If OC of aphakic lenses are placed in front of pupil at distance the pt will receive BO prism when looking at near

Increased demand for near convergence

How to decrease convergence demands

Decenter the lens nasally → Slight BI effect at distance

Very important for those who are exo at near

Parameters of aphakic lenses

• Vertex distance

• Pantoscopic tilt

• Centration

• Lens weight and thickness

• Frame adjustment

Vertex distance

Aphakic lenses are high powered → Create significant changes in effective power

Pantoscopic tilt

Want pts visual axis to coincide with the optic axis of the lens → Receive true power of lens

Drop OC 1mm for every 2 degrees of tilt

Centration

Lenses should be center in front of pupils

Choose frame that has same PD as pt

Lens weight and thickness

Use plastic and small eye size to minimize weight

Use high index (will double chromatic aberration), lenticular design (cosmetic issue), and knife edges to minimize thickness

How to increase FOV for aphakic lenses

• Decrease vertex distance

• Increase lens size

Will increase size of ring scotoma but will also move it to periphery

Multifocal segments

Need some sort of multifocal because they have no accommodation

Round or flat top segments, fit seg as high as possible to crease BU prism → Forced to look down and use bifocal

Trifocals not recommended

UV protection

Lens typically absorbs UV light

Provide UV protection

Unilateral aphakia

Use CL

Pt will not achieve binocular vision with spec lenses because mag difference is too extreme

Aniseikonia will be an issue

High hyperopes

Specs for high hyperopes have the same considerations as aphakia

Lenses for high myopia

Minify everything

RIS and pts eyes smaller

How to reduce minification in high myopia

• Reduce vertex distance

• Fit CL

FOV in high myopia

Increase macular and peripheral FOV

Problems with high myopia lenses

• Minification of pts eyes

• Thick edges

• Multiple ring effect

How to make a minus lens thinner

• Small lens size

• Frame PD = Pt PD (dist)

• Use HI

Myodisc

Lenticular lens

Small concave disk ground on the back surface of a pl lens

Lighter and thinner than equivalent corrected curve minus lens

Lenticular lenses

Used with myopia above -10.00D

May have cosmetic issues (ex. myopic rings)

Myopic rings

Issue with high minus lenses

Light coming in at periphery is reflected forward

Ways to minimize edge reflections

• Edge coating and painting

• Semitransparent edge

• Buffing the edge

• Tinting

• Hide-a-Bevel