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OV2018 binocular vision > extraocular muscle 3 > Flashcards

extraocular muscle 3 Flashcards

1
Q

what is Sherrington’s law?

A
  • whenever an agonist receives an impulse to contract, an equivalent inhibitory input is sent to its antagonist, which relaxes and actually lengthens
  • so when an impulse is sent to the right lateral rectus to contract , inhibitory impulse is sent to right medial rectus to relax
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2
Q

what is ipsilateral muscles ?

A
  • muscles in the same eye

- e.g. right lateral rectus and right medial rectus

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3
Q

what is contralateral muscles ?

A
  • muscles in opposite eye

- right superior rectus and left inferior oblique

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4
Q

what are synergists ( also known as yoke muscles ) ?

A
  • muscles that are working together in order to pull eyes in same direction
  • e.g. left lateral rectus and right medial rectus
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5
Q

what are contralateral synergists ?

A
  • two muscles moving the eye in the same direction
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6
Q

what is Hering’s law?

A
  • when an impulse is sent to a muscle to contract a simultaneous and equal impulse is sent to its contralateral synergist to also contract
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7
Q

what is Hering’s law properties ?

A
  • binocular law ( 2 eyes together)
  • aids maintenance of BSV
  • applies solely to the extra ocular muscles (EOM)
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8
Q

what are the two types of normal binocular eye movements ?

A
  1. conjugate movement (version)

2. disjugate movement (vergence )

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9
Q

what is conjugate movement (version)?

A
  • two eyes move in the same direction so that the visual axis remain more or less parallel
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10
Q

what is disjugate movement (vergence) ?

A
  • two eyes move in equal but opposite directions so that the visual axes do not remain parallel
  • convergence ( right eye looking to the left and left eye looking to the right)
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11
Q

what happens in disjugate movement ?

A
  • hering’s law still applies
  • muscle paring have changed
  • convergence both MR
  • divergence both LR
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12
Q

what happens when looking in any direction of gaze ?

A
  1. agonist ( the muscle producing the movement contracts)
  2. contralateral synergist ( muscle in other eye pulling in the same direction as agonist )
  3. direct ( or ipsilateral ) antagonist
    - the maximally relaxing muscle
    - primary action is in the opposite direction to agonist
  4. contralateral antagonist
    - maximally stretched muscle in other eye
    - primary action in the opposite direction to the contralateral synergist
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13
Q

what happens when px looks to the right ?

A
  1. right LR contracts (agonist)
  2. contralateral synergist LMR contracts
  3. ipsilateral antagonist RMR relaxes
  4. contralateral antagonist LLR relaxes
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14
Q

what happens when px looks to left ?

A
  1. right MR contracts (agonist)
  2. contralateral synergist LLR contracts
  3. ipsilateral antagonist RLR relaxes
  4. contralateral antagonist LMR relaxes
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15
Q

what happens when px looks into dexto elevation ( right and up ) ?

A
  1. right SR contracts ( agonist)
  2. contralateral synergist LIO contracts
  3. Ipsilateral antagonist RIR relaxes
  4. contralateral antagonist LSO relaxes
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16
Q

what happens when px looks into dexto depression ( right and down) ?

A
  1. right IR contracts (agonist )
  2. contralateral synergist LSO contracts
  3. Ipsilateral antagonist RSR relaxes
  4. contralateral antagonist LIO relaxes
17
Q

what happens when px looks into laevo elevation ( left and up) ?

A
  1. RIO contracts (agonist )
  2. contralateral synergist LSR contracts
  3. ipsilateral antagonist RSO relaxes
  4. contralateral antagonist LIR relaxes
18
Q

what happens when px looks into laevo depression ( left and down)?

A
  1. RSP contracts ( agonist )
  2. contralateral synergist LIR contracts
  3. ipsilateral antagonsit RIO relaxes
  4. contralateral antagonist LSR relaxes
19
Q

what happens if one muscle has defective input ?

A
  • the muscle will have reduced innervation
  • this muscle will not be effective as it should be
  • it will under-act compared to the other eye
  • there will be a consequent or sequential effect on other muscle actions ( muscle sequelae )
20
Q

what is muscle sequela ?

A
  • muscle sequelae develops to restore concomitance ( make things better)
  • first and second step happens quickly the rest takes months and years
  • muscle sequelae develops with time
  • the spread of muscle sequelae varies between patient
21
Q

how does a muscle sequelae happen when you have a right superior rectus palsy?

A
  1. primary under-action
    innervation to muscle reduced ( RSR)
  2. over action of the contralateral synergist ( left inferior oblique)
    - when the affected eye is forced to look at the target. the unaffected eye will move further than the affected eye to try to move both eyes into that location
  3. over-action ( and contracture) of the ipsilateral (direct) antagonist
  • right inferior rectus will start to contract and over act because nothing is acting against it
  • the muscle will have normal input but less opposition than normal, so the muscle will over-act and eventually contract
  1. secondary inhibitional palsy ( under-action) contralateral antagonist
    - when the palsied eye is forced to fixate. the over-action ( and contracture) of the ipsilateral antagonist ( RIR ) means the contralateral antagonist will relatively under-act ( LSO)
22
Q

what happens in R IR palsy ?

A
  1. R IR is under action
  2. over action of contralateral synergist L SO
  3. over-actions ( and contracture) of the ipsilateral antagonist ( RSR )
    - contracture of ipsilateral antagonist (right SR) is vertically opposite muscle
  4. under-action of contralateral antagonist ( left IO)
23
Q

what happens in L LR palsy ?

A
  1. L LR is under action
  2. over-action of contralateral synergist (R MR)
  3. over-action and contracture of the ipsilateral antagonist (L MR)
  4. under-action contralateral antagonist ( R LR )
24
Q

what happens in RSR palsy?

A
  1. R SR is under-action
  2. over-action of contralateral synergist (L IO)
  3. over-action and contracture of the ipsilateral antagonist ( right IR)
  4. under action contralateral antagonist ( left superior oblique )
25
Q

what happens in a muscle sequelae following a muscle palsy ?

A

. step 2: its contralateral synergist will over-act ( due to herings law)

. step 3: its direct antagonist will over-act ( due to sherrington’s law)

. step 4: the contralateral antagonist will under-act ( due to Hering’s law)

26
Q

how to investigate binocular eye movements?

A 
. field of BSV ( only HES) 
. synoptophore ( only HES) 
. ocular motility 
. diplopia charts 
. hess charts
27
Q

what is the fourth nerve

A
  • fourth nerve palsy is the trochlear nerve which supplies superior oblique
28
Q

how to to interpret a fourth nerve palsy?

A
  • inside green line is field of BSV
29
Q

what is binocular field of fixation ?

A
  • small central field more useful than larger field displaced upwards
  • a narrow field indicates a mechanical problem
  • a good fusion range can increase the size of the field
  • enables progression to be monitored
  • uniform and repeatable record
  • plots a larger area than the hess chart

OV2018 binocular vision (15 decks)

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