Motor functions: fusional reserves & prism tests Flashcards
define motor fusion
ability to align the eyes so that sensory fusion can occur
what does motor fusion occur with
vergences
describe how motor fusion occurs with vergences
the eyes must look towards/face the object of regard so that the image can fall on the fovea
eg at near eyes must converge
what must the eyes do in order to maintain BSV
move together
what is the stimulus which creates retinal disparity outside panum’s areas
diplopia or confusion
and when the object of regard falls out of panum’s area
what is vergence
when the eyes move in opposite directions to one another
both visual axis needs to converge or diverge
what is version
when the eyes move in the same direction
how much degrees of fov does the fovea have
6
how much prism dioptre is equivalent to 6 degrees of foveal gov
1
how is the image placed nicely on the fovea and achieve bsv
when the images are very close to, it is bought about by motor fusion, you can then do final fine tuning by sensory fusion
when is motor fusion completely suspended
typically when the eyes converge on a plane 1 meter away
but others converge more or even diverge, so theres varience between individuals
what are the 2 resting positions of mergence system
- distance
- near
what does distance vision require as a resting position
active divergence
what does near vision require as a resting position
active convergence
what must both eyes do when converging and diverging in order to achieve bsv
co-ordinated fashion
list the three motor fusion for distance
- position of anatomical rest
- fusion free position
- distance fusion
describe the position of anatomical rest
- no input from etraocular muscles
- usually this position when closed eyes or in dark
- mostly eyes are divergent
describe the position of fusion free position
no fusion, but the patient is awake
for some it an be slightly divergent or convergent
describe the distance fusion
usually takes this position when there is an object to look at.
it is a distance fusion when the visual axis becomes parallel and you get bsv
how is the fusion free position maintained
by tons of extra ocular muscles
list the two types of motor fusion near
- proximal convergence
- accommodative convergence
what is proximal convergence
awareness of near object, so have to converge
what is accommodative convergence
the blurred image (when we immediately look from distance to near) so must accommodate and converge to make the image clear
what fusions must you need to get bsv at near
motor & sensory
what are prisms used to assess
strength
if a prism is placed before the eyes of a person with strong bsv, list the things which will occur
- initially diplopia is experienced
- prism base out, eyes must converge to take up fixation
- prism base in, eyes must diverge to take up fixation
- prism base up before one eye and prism base down before the other measures vertical vergence
what does prism base out do to the image, and what does the eye have to do to achieve single vision/fixation
moves image temporally
eye has to move in
what does prism base in do to the image, and what does the eye have to do to achieve single vision/fixation
moves image nasally
eye has to move out
describe why an exodeviation is measured with a prism base out
we want to measure how good they are at controlling the exodeviation and how good they are at converging
what is the level of control of latent deviation if has good motor fusion
good control
what is the risk of latent deviation if has poor motor fusion
latent deviation can become manifest
what type of control does someone have over their latent deviation if it is small
good control
what type of control does someone have over their latent deviation if it is large
bad control even if they have good motor fusion due to it being a large deviation
what stimulates motor fusion
diplopia
what is the amplitude of motor fusion
if the image falling on corresponding retinal areas is shifted, upper limit possible of vergences in which diplopia can not be avoided
what is the strength of bsv indicated by
amount of motor fusion possible
the more prism that can tolerate…
the better their motor fusion, and less likely for their deviation to become manifest
what does the essential strength of motor fusion indicate
the strength of BSV
what is horizontal and vertical fusion assessed with
prism bar
or
Risley prism and fixation target
what are the fixation targets when using the prism bar and Risley prism when horizontal and vertical fusion is assessed
distance - snellen chart
near - budgie stick
why is a letter on a snellen chart used instead of a light, when assessing fusion
as letter produces more incentive to fuse a detailed target
which types of fusion is assessed with a synoptophore
- horizontal
- vertical
- cyclofusion (sagittal axis)
what does the prism fusion range measure
patients ability to maintain sensory fusion through a range of vergence movements
which sensory fusion range does base out prism measure
convergent fusion range
which sensory fusion range does base in prism measure
divergent fusion range
which sensory fusion does base up prism measure
supravergence fusion range
which sensory fusion does base down prism measure
infravergence fusion range
list the steps of how to measure prism fusion range
- patient sits head erect
- patient directed to appropriate letter 6/60
- prism bar placed before one eye
- strength increased slowly (start with 2 prism as thats the smallest)
- continue increasing prism strength until patient reports diplopia
- ask if they can make the image single (as with time they can maintain fusion & bsv through a new more prisms)
- continue to increase prism
- note maximum amount of prism before diplopia is recorded - break point
what is the break point
the maximum amount of prism before diplopia is reported
what may a patient complain of during relative accommodation
blur
when does relative accommodation occur
normally when vergence changes accommodation changes by a linked amount
give examples of how when vergence changes, accommodation changed by a linked amount
- convergence increases, accommodation increases
- divergence increases, accommodation decreases
list the types of accommodations relative to convergence
- positive relative accommodation
- negative relative accommodation
prism bar is asking to change vergence without….
changing accommodation, which can do initially but as increase the prism, their accommodation is going to kick in and will report that the target is blurred
what do you record when recording the prism fusion range of a px
- break
- distance
- base direction
give an example of recording a prism fusion range of a px
near PFR
base in 18
base out 28
what happens to the eyes when placing a base out prism infront of left eye
- left eye moves in
- right eye will move out and then back in (herrings law) to achieve bsv
what happens in prism fusion range when measuring convergent amplitudes (base-out)
- continued innervation present when prism is removed (px still has active, tonic convergence as they don’t recover immediately after the prism base out removal)
- reduces divergent amplitude if measured directly afterwards (but it isn’t truly reduced though, only for that time)
in which order do you measure the prism fusion range
- base-out
- base-up right eye
- base-in
- base-down right eye
in this order
repeat for near using suitable target
what happens when some patients do not report diplopia when testing prism fusion range
- they will either suppress image of deviated eye or don’t appreciate/notice the diplopia (which can be overcome by placing red filter infant of eye which allows them to notice the diplopia)
at which distance can prism fusion range diplopia be determined objectively at
near
what should be done when a px cannot notice diplopia during prism fusion range test
place red filter infant of eye
what happens to the eyes when a patient cannot take the prism which is placed infant of them during measuring prism fusion range
eyes will make a version movement instead
so this is the objective point of break point
and if px doesn’t notice diplopia, or doesn’t tell us, place a red filter infant of eye to help them notice
list the steps of how to measure vertical fusion range
- direct patient to a 6/60 letter
- place prism bar in front of one eye, prism base down
record this prism power at just before the break point - repeat with either
prism base down in front of other eye
or
prism base up in front of same eye - repeat with a suitable letter target at patient’s preferred near working distance
list the prism fusion ranges and corresponding break points
- distance base-in = 5 - 10 (10)
- distance base-out = 15 - 20 (20)
- near base-in = 15 - 20 (20)
- near base out = 35 - 40 (40)
at which distance is prism fusion range greater at in comparison with at horizontal
greater at near than at distance
at what vergence is prism fusion range better at
better at converging that diverging
which base direction is greater in prism fusion range
base out is larger than base in
which factors causes variances in prism fusion range
- alertness
- toxicity
what is our poorest fusion range
our ability to diverge at distance
what is our best fusion range
ability to converge at near
what is exophoria
tendency for the eye to move out
what do you want to investigate on an exophoria px
how well they can control the tendency of the eyes to converge
what do you want to investigate on an exophoria px at near
interested at base out fusion range
what is the vertical prism fusion range distance and near
3 prism base up and down
what is the torsional fusion range
between 8 - 22 prism dioptres
what is the analysis of prism fusion range compared with
normals
what does percival state
that convergent and divergent fusional reserves should be balanced
one should be no less than half of the other
which prism fusion range and at which distance does the percival not work for
distance as distant base in = 5-10 (5 = 1/2 of 10)
and does take into account latent deviation
what does sheard state
opposing fusional reserve to blur point should be twice degree of phoria
e.g. if 10 dioptre exophoria, you need 20 dioptres of base out convergent prism fusion range to cope
what may insufficient fusional reserves result in
- motor fusion breaking down
- diplopia
- asthenopic symptoms
fusional amplitudes should be in..
- normal range
- related to heterophoric position for the distance at which the vergences are measured
give an example of fusional amplitudes, of 10 dioptre exophoria at distance
fusional reserves 14 prism base out and 14 prism base in
- but as 10 prism base in required to correct heterophoria fusional reserves beyond heterophoric position are 4 prism base in and 24 prism base out fusion range
- this suggests that reduced base out (convergent fusional reserves) are adequate to control deviation
what type of device is a synoptophore
a haploscopic device
how does the synoptophore work
- each eye looks through a tube and a mirror on each eye piece reflect the image on the slide into the eye
- eyepiece lenses are +6.50DS and targets mounted at focal length
- 3.00DS inserted to simulate near viewing conditions
in the synoptophore, which power lens in inserted to simulate near viewing conditions
-3.00DS
what does the synoptophore estimate
prism fusion range of an individual
how must each image be in comparison to one another in a synoptophore in order to measure fusion range
similar
in a synoptophore, which different size slides are used subtending angles of
- 1 degrees at nodal point
- 3 degrees at nodal point
- > 5 degrees at nodal point
which size targets should be used comparable with visual acuity in a synoptophore
smallest targets, e.g. high va’s should use small target
which size slides are better used for exotropias in a synoptophore
larger
why are larger size slides better used for exotropias in a synoptophore
they are more likely to suppress
in a synoptophore if in one eyepiece is a lion and the other is a cage, what will ox see if they can fuse the images
lion behind a cage
which type of patients is a 20 prism base out test used on
- children
- adults unable to co-operate with prism fusion range
what is the 20 prism dioptre test assessed with
single base out prism placed before one eye and then the other
what is child attention attracted by in a 20 dioptre prism test
near object e.g. toy at 33cm
how much you work during the 20 dioptre prism test
quickly
what is the 20 dioptre prism test useful in proving the presence of binocular single vision in
- suspected pseudo strabismus
- child with poor fixation in which results on cover test are inconclusive
what must you encourage a px to do if a 20 prism base out prism in not overcome
encourage to fuse images
what is important to note from the 20 prism base out test
- recovery movement as prism is removed
- speed
- accuracy
what happens to the eyes in a 20 dioptre base out prism test which produces a normal response
eye under the prism moves in so the other eye moves out then in (herrings law) to take fixation
record as 20 dioptre base out overcome
what happens to the eyes in a 20 dioptre base out prism test with the right eye suppressed
base out prism placed infant of left eye, both eyes make a version movement which in theory should give diplopia, but because they’re suppressing information from the right eye, the brain is no longer aware that the image is not on the fovea so the right eye doesn’t come back in
what happens when the 20 dioptre base out prism is over the suppressing eye
no movement at all
what is the 4 dioptre prism base out test for
macula suppression used when microtropia (small angle manifest strabismus) is suspected
what happens during a 4 dioptre prism base out test
- px is directed to an appropriate letter target at distance or near
- prism is placed before right eye, image displaced outside macula area
both eyes make versional movement towards apex of prism to left
left eye then converges (which is more difficult to see with 4 dioptre prism base out as it is much smaller than 20) to re-establish foveal fixation
what do microtropia patients develop in order to avoid diplopia
ARC
a microtropia patient is…
rewired so that they think the deviating eye is looking straight ahead, and has a small area of central suppression which helps them look ahead
