Muscle sequelae Flashcards
what is muscle sequelae
Sequence of extra ocular muscle adaptation following muscle weakness or limitation. Relates to Hering’s and Sherrington’s laws of innervation
what is donders law ?
To each position of the line of sight belongs a definite orientation of the horizontal and vertical retinal meridians relative to the co-ordinates of space irrespective of the route taken (BIOS, 2012).
A definite and invariable amount of torsion occurs for a given position of gaze, irrespective of how that position was reached
what is sherringtons law of reciprocal innervation
A law of reciprocal innervation: whenever an agonist receives an impulse to contract, an equivalent inhibitory impulse is sent to its antagonist which relaxes (BIOS, 2012).
When increased innervation is sent to a muscle to contract, decreased innervation goes to its direct antagonist. This means the direct antagonist muscle is relatively relaxed. This is a uniocular law, which ensures smooth accurate movements are made to take up fixation of a targe
what is herrings law of equal innervation
A law of ocular motor innervation whenever an impulse for the performance of an eye movement is sent out, corresponding muscles of each eye receive equal innervations to contract or relax (BIOS, 2012).
When an impulse goes to a muscle causing it to contract, a simultaneous and equal impulse is sent to its contralateral synergist. This is a binocular law which aids the maintenance of BSV
what is the difference between underaction and restriction
Underaction: Reduced ocular rotation which improves on testing ductions, often associated with neurogenic palsy.
Restriction: A term to describe abnormal ocular rotation where the movement does not improve fully when testing ductions and is often associated with mechanical aetiology.
what is an overaction
Overaction: Excessive action of a muscle caused by increased innervation as a consequence of palsy or limitation to the ipsilateral antagonist or contralateral synergist.
describe the pattern of muscle sequelae
Primary underaction (limitation)
a. This is the primary affected muscle
b. It can be an underaction or a restriction
2. Overaction of the contralateral synergist (Hering’s law of equal innervation)
3. Overaction (or contracture) of the ipsilateral antagonist (Sherrington’s law of
reciprocal innervation)
- Secondary inhibition of the contralateral antagonist (Hering’s law of equal innervation)
a. Sometimes called the secondary inhibitional palsy
b. Occurs because the overacting ipsilateral antagonist requires less
innervation, less innervation is therefore sent to the contralateral antagonist
what factors affect the development of muscle sequelae
In the beginning the overaction of the contralateral synergist may be all you see, with the rest of the muscle sequelae taking time to develop.
How significantly affected the primary underacting muscle is will also affect the degree of muscle sequelae. If a slight primary underaction is present – then the overaction of the contralateral synergist will also be slight, however if the primary underacting muscle is completely paralysed and has no function, then the overaction of the contralateral synergist (and the subsequent pattern of muscle sequelae) will be much greater.
Usually the patient will fix with their unaffected eye, but not always! Visual acuity, pre-existing strabismus and other factors may mean the patient chooses to fix with their affected eye. Contracture of the ipsilateral antagonist should not develop as much in a patient that fixes with their affected eye.
what happens when muscle sequelae develops
As mentioned above, muscle sequelae takes time to develop. Over time the deviation will look less incomitant and will become more concomitant.
When a deviation is very incomitant the deviation in primary position will measure a different amount fixing with either eye (FEE) in primary position (remember - the fixing eye is the eye not behind the prism).
Primary deviation – the angle measured when fixing with the unaffected eye Secondary deviation – the angle measured when fixing with the affected eye
The secondary deviation will be greater than the primary deviation in an incomitant deviation, i.e. the deviation will be greatest fixing with the affected eye in primary position. As muscle sequelae develops this difference between the primary and secondary deviation reduces and may become concomitant over time.
As a result of these changes over time the muscle sequelae can be useful in determining whether a palsy is of recent onset or is long standing.
what is a diplopia chart
A diplopia chart provides a drawn record of the separation of diplopic images in nine positions of gaze, as described by the patient (subjectively). The patient wears red and green goggles & views a linear light source, which is presented vertically at 50cm. The patient is then asked to describe the position of the images they see and you draw it on a chart.
when are diplopia charts useful
A patient is bedridden or has impaired mobility preventing them being able to perform other tests such as Lees Screen
● There is a subtle bilateral 4th nerve palsy, i.e. in a head injury where the bilateral 4th nerve palsy can be very asymmetrical. The less obvious of the 4th nerve palsies can be easily missed if a small amount of reversal of the torsion or the vertical deviation is missed in extreme depressed positions.
how to interpret a diplopia chart
Important to consider where there is the widest separation of images – indicating the position of gaze where the affected muscle is acting.
Is the horizontal diplopia crossed (heteronymous) or uncrossed (homonymous)
Remember a higher eye will see a lower image and an extorted eye will see an intorted image.
how to interpret a hess chart
eed to think about which is the field of the right eye and the field of the left eye
- the field showing the movements of the RE has been plotted with the left eye fixing - the field showing the movements of the LE has been plotted with the right eye fixing
Consider:
1. Comparisonofthetwocharts-position,shape&size
2. Sizeoftheoverallfield
a. Smaller field = affected eye
3. Primary position deviation
a. primary deviation Vs secondary deviation
b. size of deviation (1 square of the chart = 5 degrees of deviation)
4. Underactions (inward displacement)
a. look for largest underaction - primary affected muscle
b. deviation in other secondary and tertiary positions of gaze (30 degrees
excursion)
c. muscle sequelae - develops over time
5. Overaction(outwarddisplacement) a. Look for largest overaction
6. Musclesequelae
7. Differentialdiagnosis
a. mechanical Vs neurogenic
b. longstanding palsy with developed muscle sequelae may look like a
concomitant deviation – difficult to identify original primary affected
muscle
8. Equalfieldsize?
9. A or V pattern - sloping / twisted field, not torsion
10.Sequential Hess charts
what is a agonist
the primary muscle
what is. antagonist
eom whose action opposes the contracting muscle
