4.1.1: Anomalies in an Rx & implements course of action

Question 1

What is anisometropia and when is it problematic?

Answer 1

Anisometropia becomes problematic when the difference is >2.00D.
When looking away from the optical centre of the lens (e.g. looking down to read), issues
can include:
* Blurred Vision
* Double Vision
* Unable to tolerate reading for long periods as a result of >1D of VDPE (vertical differential prismatic effect)

Question 2

What happens for each dioptre of difference in power between the eyes? How much VDPE can people tolerate?

Answer 2

For each dioptre difference in power between the eyes, there is 1 dioptre of
vertical differential prismatic effect at the near visual point (10mm below, 2mm in from
the optical centre).
According to British standards, patients can tolerate 1 dioptre of VDPE, but the tolerance
is much greater horizontally (10 BO – 4 BI at dist, 7 BO – 7 BI at near)
Actual Vertical Prism Reserves range between 2-4PD

Question 3

What is prentice rule? How does it work?

Answer 3

P= C x F
P = Prism at the Near Visual Point
C = Centration in cm (ALWAYS 1CM)
F = Power along the vertical meridian
E.g.
RE: +4.25/-0.50x90
LE: +1.50/-0.50x90
* Don’t include the cyls in this example as they are at an axis of 90, they are actually acting at 180, (90 deg to the meridian stated in the rx) making them horizontal -> for this equation, we focus on the vertical meridian ONLY.
* P=CxF
o RE: 4.25 BU
o LE: 1.50 BU
o Total: 2.75 Base UP RE - It’s labelled as RE as it has the highest power of prism to begin with

Question 4

What is anisometropia? What is aniseikonia? What are the symptoms of aniseikonia?

Answer 4

Anisometropia:
Where the prescriptions between R&L are different.
Aniseikonia:
For patients with anisometropia of more than 2, they may experience a difference in the
size and shape of visual images and magnification: Aniseikonia.
Symptoms of Aniseikonia include:
* Visual discomfort
* Visual distortion
* Difficulty with stereopsis/BV.
* Dizziness, H/A, Nausea.

Question 5

What are the single vision dispensing solutions of anisometropic patients?

Answer 5

Solution:
* Aspheric Lens to the Most Positive Powered Eye
o The retinal image size is reduced as the aspheric surface is approx. 2.00D flatter.
* Iseikonic lens to the Least Positive Eye
o The thickness is increased by increasing the base curve (to approx 8.00D)
to increase the spectacle magnification of the least positive eye.
Spectacle Magnification = Shape Factor x Power Factor
Shape Factor: a ratio of surface curvatures that describe the shape of the lens and
typically ranges from values of -2 to +2
Power Factor: The dioptric power of the lens considering the power and vertex distance
of the lens.

Question 6

What are other single vision solutions for anisometropic patients?

Answer 6

• give the px two pairs for different distances, as the optical centres will be set for
  viewing that particular distance. Ultimately this eliminates vertical differential
  prismatic power.
• Advise the px to tilt their head downward when reading in order to view through
  the optical centre of the lens – however this is only successful for quick reading,
  as may give px neck pain due to the uncomfortable head position.

Question 7

Describe bifocal dispensing for anisometropic patient?

Answer 7

• Slab off near-portion for the most negative eye as it induces the most base down
  prism
  o Most ideal for minus lenses as the base up prism induced by the slab-off
  neutralises the base down prism of the main lens) (minimum slab off
  available – 2 dioptres base down)
  o The most negative eye induces the largest amount of base down prism
  which can be removed by slabbing-off.
  o The slab-off line should be set at the lower limbus.
  When minus lenses are slabbed off base down prism is removed. When positive lenses
  are slabbed off base up prism is added.
• Unequal Round Segs
  o Larger segment given to the most positive eye. It works on the principle
  that a larger segment will induce the most base down prism
  Formula used:
  Segment radius (cm) x near add = amount of relative vertical prism overcome
  o Most ideal for Positive lenses: the base down prism induced in the
  segment eliminates the base up of the distance power in the main lens)
  o Only successful in round segs.
  o To find out how much difference in size to give:
  Difference in seg diameters (mm) = 20 x differential prism / Add

Question 8

What are other options for anisometropic patients?

Answer 8

CLs
Fanklin split
Prism control bifocals - prism in near seg
Thinning in one lens