Convergence Flashcards
What is disjugate eye movement?
The eyes move in the opposite relative direction
What is conjugate eye movement?
The eyes move in the same relative direction
What is ‘normal convergence’?
Near point of convergence of less than or equal to <10cm without effort is considered normal
What’s the ‘near triad’?
Convergence = closer = fatter lens (accommodation)
Far objective = no accommodation = lens thinner
Near objective = pupil miosis
What unit is convergence measured in?
- Prism Dioptres
- Degrees
- cm
- Meter angle (equivalent to dioptre of accommodation used in clinical research)
What do we need to know to work out required convergence?
Need to know a person’s IPD and know the target distance
What’s the average adult IPD in females and males?
52 - 76mm
Female average is 62mm
Male average is 65mm
When do we use zero convergence?
When looking at a target at infinity or with an IPD of 60mm at 6cm
How much convergence is required for a distance target?
Less convergence
How much convergence is required for a wider IPD?
More convergence required
How much convergence is required for a narrower IPD?
Less convergence required
If 1PD of convergence moves an image 1cm at 1m, how many cm does an image move at 0.5m and 2m?
0.5m = 0.5cm
2m = 2cm
If a target is 1m away and IPD is 6cm, how much must the target move?
To view target 1m the image must move 3cm + 3cm
RE converges 3PD for 3cm
LE converges 3PD for 3cm
So total convergence required is 6PD
If a target is 0.5m away and IPD is 6cm, how much must the target move?
As 1PD of convergence moves an image 0.5cm at 6m this means we’d need 12PD convergence in total (6PD in each eye)
If a target is 2m away and IPD is 6cm, how much must the target move?
As 1PD convergence moves an image 2cm at 2m, 6/2 = 3PD so 1.5PD in each eye
What are the 4 types of convergence?
Tonic Convergence
Accommodative Convergence
Proximal Convergence
Fusional Convergence
What is Tonic Convergence?
Passive state of vergence of the eyes in the absence of a stimulus maintained by the tonus of the EOMs (which are never without neural activity)
It’s the ability to hold position of convergence without a target when the anatomical position of rest is divergence. When awake this new tonic convergence is seen due to innervation of eye muscles; it can be eliminated by occlusion or deep anaesthesia and decreases with age
What is Accommodative Convergence?
Convergence is induced when accommodation occurs; it’s induced by ‘blurred retinal image’
Accommodation Convergence/ Accommodation
Measures changes in AC in PD induced when the patient exerts or relaxes 1PD of A; normally 4:1
What is Proximal Convergence?
When convergence is stimulated by the awareness of nearness of an object.
Why might measurements on a synoptophore differ compared to a prism cover test?
Due to proximal convergence; from over-convergence on synoptophore due to artificial viewing causing an increased ET angle and decreased XT angle (but Georgievski, 1995 found no difference on PCT on synoptophore vs. free space)
What is Fusional Convergence?
Stimulated by disparate (separate) images to maintain bifoveal fixation and avoid diplopia (to ensure that the images hit the corresponding retinal areas in each eye)
- Positive (convergence) = when insufficient convergence from other components
- Negative (divergence) = when too much convergence from other elements
What is Relative Fusional Convergence?
Allows accommodation or convergence to be flexible e.g. in refractive error or heterophoria
It’s the maximum vergence movement enabling a patient to see a comfortable clear image and is represented by the first point of blur and so can be improved with exercises. Measured by the prism fusion range and it’s convergence response in excess of accommodation or vice versa
What’s near point of convergence (NPC)?
NPC is the closest point at which an object can be seen as one during bifoveal vision and doesn’t change with age
How is near point of convergence measured?
Free space or the RAF rule
