hi my name is Con clus and you're watching the video deviation in incompetence to business in this video we'll be discussing the features of the deviation in patients with neurogenic pauses or mechanical restrictions okay now in incompetence to business we have now a distinction between what we consider the primary deviation and the secondary deviation so depending on which I is fixing there is a difference in the deviation we see now you've already studied concomitance trismus and in concomitance trismus we didn't really feature or discuss um primary and secondary deviations because we know that with conc competency the deviation is the same no matter which eye is fixing so with incompetence tosas the primary deviation is the deviation when the patient is fixing with the unaffected or sound eye so obviously in this instance if you had for instance a left Superior blink PA ear if the patient was fixing with a right eye the unaffected eye this is the instance where you're looking at the primary deviation however if you have the patient fixing with the left eye the affected eye you now have the secondary deviation and if we look at the image over here what we see is that the primary deviation so in this instance um we have a patient who has a right affected eye and the patient is fixing with the left eye um we can see that the EO deviation here is much smaller than the EO deviation when the patient is made to fix with the right eye the affected eye now why is it that there is an increase in the size of the deviation when the patient fixes with the affected eye let's take an example of a right lateral rectus paing so in this particular image if we had a right latus pausy if the patient was asked to fix with that right eye the affected eye what would have to happen is that in order for the eye to AB duct to come into primary position excessive amounts of innovation will need to go to that affected muscle and as such as excessive amounts of innovation go to the lat rectus they will do the same to the uh syes the medior rectus of the other eye and therefore what we end up seeing is a larger deviation once the patient is fixing with the affected eye the more recent the deviation the more distinction you'll see between the primary and secondary deviation uh the more long-standing the deviation becomes the more concomitant we see and less difference between the primary and secondary deviation another feature to think about is that the deviation in Prime position might alter dependent on whether a abnormal head posture or compensatory head posture is present so if we look at the um patient over here to the to the right the patient is tilting their head to the right and when we take a closer look at the eyes with the abnormal head posture we can see that there's very little deviation in that position as compared to when the patient is in primary position we can see that there is certainly a vertical deviation and in this instance a left hyper tropia we'll talk more about abnormal head postures and what drives an abnormal head posture in a later video but clearly what you can see here is that the patient is tilting their head to achieve bsv another important feature to be aware of in relation to neurogenic poses is that the eye will be deviating prime position opposite to the action of the paused muscle now what does this mean what it means is that if for instance the aductor is pausy so the Lal rectus the eye will be aded ducted in primary position so an eye that's ad deducted means an esotropic eye and this is because if your latar rectus is pausy the ipsilateral antagonist remains unopposed and then therefore you'll see that with the lateral rectus paid the ipsilateral antagonist the medial rectus will adduct the eye further and we'll see an esotropia let's do a more complicated example a left Superior BL p z so the first thing to think about is what is the action of that particular muscle and the action of that particular muscle is that it causes depression a eduction and Incycle rotation of the eye okay so we now should see a deviation that represents the opposite of the action of the extraocular muscles so what deviation do we expect well we expect that the eye will be elevated because the superior BL is a depressor we expect that the eye will be ad deducted because the super bque is an abductor and we expect that the eye will be a Cy rotated because the superior BL is an incy Rotator so what does this mean for the cover test Well what we'll see is a left esotropia with a left on right or a left hypertropia now on cover test generally you won't see the um exyle rotation so I've not documented that here okay so this is generally what we expect to see now not all patients will always present to you with this textbook deviation but it's important to be aware of the principles behind what to expect in Prime position now moving on to what do we expect to see of the deviation when we move the patient into different positions of gaze well generally with a neurogenic pausy what we expect to see is that the deviation will be greatest in the direction of action of the affected affected muscle and then it will be leased in the direction directly away or in the opposite um field of action of the affected muscle so if we have a look here at the um patient to the right what we can see is that in Prime position we can see that there is an EVO EO deviation we move the patient into right gaze we see a minus 4 a deduction of the right eye and if we were to measure the deviation here we would note that there would be an increase in the size of the EO deviation and then if we had a look over into left gaze we can see that there is very little devation there and if we were to measure it perhaps we' either see that the patient um has no manifest deviation or has very little um a little amount of ET in that position so we have a distinctly different size deviation in each position and in this image what we see is the deviation is greatest in the direction of action of the affected muscle so where the lat rectus or the right latus is working and least in the directly um opposite position to the field of action of the affected muscle which in this instance is left gaze Let's do an example of a vertical muscle let's say we had a pausy of the left Superior Blake now the left Superior Blake's field of action is in dextr depression so in right gaze and down and so what we would expect is that the deviation would be greatest in this position however would be least in the opposite direction we labor elevation so LIF and upgaze okay moving on to Mechanical restrictions the deviation in Prior Inquisition can be different to that of what we expect to see in neurogenic posy so in neurogenic poses we've already discussed that if you work out which is the affected muscle generally the deviation will be in the opposite direction to the action of that muscle now for mechanical restrictions we may not see that and generally you may see very little um a very little deviation or no deviation in Prime position despite marked limitations of eye movement so say for instance you have marked limitation of elevation um as we do in the example to the right we can see here that the patient has marked limitation of the right eye it's not going past midline yet what we see in primary position is very little deviation on the on the other hand for neurogenic pauses the deviation in Prime position is relative to the extent of the pausy so the greater the extent um the larger the deviation so the deviation in primary Position will be greater for aus4 limitation as compared to a minus1 limitation and again if we have a look at the image here of a patient with a neurogen pausy we can see that there's an obvious esotropia that is relative to the um deviation sorry that is relative to the limitation we see of the right eye another aspect of mechanical restrictions to be aware of is that mechanical restrictions can often cause reversal of the deviation and what we mean by reversal is you'll see an actual uh reversal of the direction of the deviation so say you had a right hyper deviation you can see that change to a right Hyo deviation in a specific position of gaze let's have a look at the deviation or the patient here to the to the right what we see is in primary position looks relatively straight and as we as the patient looks up we have limited elevation of the right eye and what we see is a left on right and then if we take the patient into Dow gaze we see there's limitation of the right eye um in Dow gaze and we see reversal of height we now see a right on left now this theoretically defies herring's law and sherrington's law so what we're seeing is usually a mechanical restriction in these particular instances just be aware though that there are several instances where the patient may have a neurogenic pausy annual Ser versal of height we'll talk more about these as we actually look specifically at um different types of neurogenic posies but some examples of these are bilateral Superior Bic POS for instance will cause reversal of height and a third nerve posy can do this also so in summary some key features that we'll observe in patients with neurogenic Poes is that the secondary deviation will be greater than the primary deviation deviation will be greatest in the direction of action of the pausing muscle and the deviation in Prime position is generally opposite to the action of the posing muscle okay and from mechanical restrictions um two main things um that distinguish a mechanical restriction from a neurogen py is that you may see very little deviation in Prime position and there may be no relationship between the amount of a limitation that you see and the deviation that you see in Prime position and also that um mechanical restrictions are likely or more likely to cause a reversal of deviation okay that brings us to the conclusion of this video thank you for watching