14 - Vertical Imbalance Flashcards
What is vertical Imbalance?
When different prismatic effects are present at different positions of gaze, resulting in a power difference in power between right + left lenses
When does vertical imbalance cause the most issues?
When looking down to read or doing close work for long periods of time
The differential prismatic effect caused by looking down (below OCs) is called vertical imbalance T/F
True
When should the need to correct vertical imbalance be considered in someone who is Anisometropic?
When they are switching from single vision to multifocals
Practice examples of finding vertical imbalance for sphere only prescriptions
How do we check for vertical imbalance?
1)Hand them a reading card and ask them to read it
2)Watch what the patient does when reading the card
3) If px drops eyes to read, they are use to reading with vertical imbalance, no need to worry
4)If the px drops their head to read (reading through distance OCs). The need to compensate for the vertical imbalance needs to be considered.
What are the 8 ways we can correct vertical imbalance?
Contact Lenses
2 separate prs of glasses
Dropping the MRP height
Raising the seg height
Fresnel press on prism
Slab off (Bicentric grinding)
Dissimilar seg heights
Compensated ‘R’ seg heights
Characteristics of ‘Contact Lenses’ for correcting Vertical Imbalance? 3
-Best Option
-OC moves with the eye
-Contact lenses remove lens-induced prismatic difference and vertical imbalance.
Characteristics of ‘2 separate pairs of glasses’ for correcting Vertical Imbalance? 2
-Anisometropia is rarely a problem in separate pairs as they can drop.
head rather than eyes.
-no need to look below the OC.
Characteristics of ‘dropping the MRP height’ for correcting Vertical Imbalance? 4
-Not Optically Sound
-Dropping OC reduces distance to the reading level so prismatic effect is lowered.
-It moves the imbalance to the distance portion.
-Dropping the MRP in multifocal for borderline cases of imbalance may be successful but not best options.
Characteristics of ‘raising the segment’ for correcting Vertical Imbalance? 2
-Raise the seg height without moving the Distance OC (Specify to the lab).
-Raising the seg heights means will not have to look down as far to the reading.
Characteristics of ‘Fresnel Press On Prism’ for correcting Vertical Imbalance? 3
-Fresnel can be used at lower point of the lens to counteract the prismatic effect.
-Not permanent solution – trial basis.
-Used at back of lens simulates a slab-off.
Characteristics of ‘Slab Off (Bicentric Grinding)’ for correcting Vertical Imbalance? 3
-Most common Option
-Identified by a horizontal line across one lens at the level of the segment top.
-Reverses the vertical prismatic effect in the lower half of one lens beginning at the level of the bifocal segment line.
Characteristics of ‘Dissimilar seg Heights’ for correcting Vertical Imbalance? 2
-Uses the vertical imbalance induced at the reading segment to counteract the vertical imbalance induced.
-Prismatic effect at the reading portion is separate (it’s a miniature lens).
Characteristics of ‘Compensated ‘R’ seg heights’ for correcting Vertical Imbalance? 5
-R (Ribbon) is flat at the top and bottom.
-Resurfaced creates prism in the seg which can counteract the vertical imbalance.
-Moves the seg OC up in one lens and down in the other
-Upper limit for the amount of prism depends on power add.
-Only used when amount needed to be corrected is 1.50 Prism Dioptres
