6. Measure of phoria & tropia

Question 1

How accurate is the prism cover test? and does it provide dissociation?

Answer 1

Accurate to 2 dioptres
It provides complete dissociation

Question 2

In which case are PCT and Maddox rod not suitable?
And so what should you use instead?

Answer 2

When vision is poor don’t use PCT and Maddox rod.
Instead estimate the tropia using corneal reflections.

Question 3

How is dissociation achieved in PCT?

Answer 3

Through alternating cover test

Question 4

For exo deviations place the prism base…

Answer 4

IN

Question 5

For eso deviations place the prism base…

Answer 5

OUT

Question 6

For hyper deviations place the prism base…

Answer 6

DOWN

Question 7

For hypo deviations place the prism base…

Answer 7

UP

Question 8

For which type of patients is maddox rod not suitable?

Answer 8

Those with abnormal retinal correspondence (strabismus)

Question 9

If the line is to the left for the Px which type of phoria is it?
Which way should you put prism base?

Answer 9

To the left= exophoria
Prism base in

Question 10

If the line is to the right for the Px which type of phoria is it?
Which way should you put prism base?

Answer 10

To the right= esophoria
Prism base out.

Question 11

If the line is seen above for the Px in RE which type of phoria is it?
Which way should you put prism base?

Answer 11

Above= Right hypophoria
Base up in RE
or
Base up in RE and Base Down LE

Question 12

If the line is seen below for the Px on RE which type of phoria is it?
Which way should you put prism base?

Answer 12

Below= Right hyperphoria
Base down in RE
or
Base down in RE and base up in LE

Question 13

What does disjunctive mean? Give an example.

Answer 13

Movement in opposite directions such as convergence and divergence.

Question 14

What is Tonic convergence? Explain how.

Answer 14

Even if there is no fusional stimuli the tonic convergence will keep our eyes aligned due to midbrain nerual activity.

Question 15

What types of vergence keeps our eyes straight?

Answer 15

Tonic convergence
Accommodative convergence
fusional convergence
proximal convergence

Question 16

When would there be no fusional stimuli?

Answer 16

In the dark.

Question 17

What does AC/A stand for ?

Answer 17

Accommodative convergence/ accommodation ratio.

Question 18

What is included in the near triad?

Answer 18

Vergence, accommodation and pupil constriction.

Question 19

What is the normal AC/A ratio?

Answer 19

2-3:1

For every 1 dioptre sphere of accommodation, approx 2-3 prism dioptres of vergence occurs

Question 20

What is the response AC/A ratio?

Answer 20

Where the magnitude of vergence is measured according to 1 unit change in accommodation. Measured objectively using an optometer.

Question 21

What is the stimulus AC/A ratio?

Answer 21

Where the magnitude of vergence is measured according to 1 unit change in accommodative stimulus.

Question 22

What are the clinical measurement methods for AC/A ratio?

Answer 22

Heterophoria method
Gradient method

Question 23

How to carry out Heterophoria method?

Answer 23

Change the distance of the fixation target from infinity to 33cm.

Question 24

How to carry out the gradient method?

Answer 24

Using spherical lenses to increase/decrease amount of accommodation.

Use the equation:
AC/A =
PCT with lenses - PCT without lenses / D
Distance (6m) = PCT with accommodative target.
Insert -3.00DS.
PCT repeated until target seen clearly

Near (33cm) = PCT with accommodative target.
Insert +3.00DS
Repeat PCT until target seen clearly.

Precautions:
Patient should be fixated on letters they can read so they are actaully accommodating.
May be easier to start with 1.00DS lens as it may be difficult to overcome 3.00DS to be able to see the letters still.

Question 25

What is the relationship between AC/A and age?

Answer 25

AC/A remains fairly constant with age. Decline at age 50. Linked to presbyopia and increased exophoria with age.