hello and welcome to Insight Opthalmology this is Dr Amrit welcoming you to the squint series again today we are studying how to measure deviation using the prism cover test so first uh let us try to understand the Optics of the prism cover test before that as I explained to you in my previous video in which we studied measuring of deviation using the Crim skis modified Crim skis and the hbur conal reflex test so there we studied that how a prism if you take it bends the light always towards the base of the prism however the image is always shifted towards the apex of the prism so that is one point that you must remember now let us go into the Optics of the prism cover test now over here let us imagine that it is uh it is as if you are looking at an object which is of yellow color here so this is your right eye which is represented in od and then you have your left eye which is represented in OS okay so here as we know phobia is slightly temporal so the part of the retina which is nasal to phobia is represented in pink color and the part of the retina which is temporal to the phobia is represented in the blue color now what if the uh left eye was actually having esod deviation or the esotropia or an inward deviation what happens is that the image of the object in the right which is looking straight forward which does not have any sort of squint is being formed at the phobia however if you look at the left eye the left eye has esotropia or inward deviation and therefore you can see the image is actually formed on the nasal half of the retina that is in the Pink Zone so this is very important point now what will happen if we Now cover our right eye if you cover the right eye the right eye will not be able to take fixation so now the onus of fixation lies on the shoulders of the left eye so left eye now is supposed to take fixation however the left eye was inwardly deviated so therefore when you put a cover in front of the right eye the left eye will now move outwards in order to take fixation and by taking fixation what I mean is that the image of the object will be now formed on the phobia initially it was formed somewhere here on the nasal part of the but after you force the left eye to take fixation it will form on the phobia and because of the headings law what will happen to the right eye the right eye also will show a corresponding inwards movement now so basically what is happening the sensory origin of this reflex movement is because of the stimulation of the peripheral retinal area in the deviated eye in this case it is the left eye which was deviated okay and once we uh force it to take fixation this eye is going to turn in such a way okay so it's not just about esotropia any eye would squint okay when you force it to take fixation the eye is going to turn in such a way that the fixated object will now move towards the fobia right so I hope that is uh clear to you now and this movement which the deviated eye makes is called a redressal movement so in this case the left eye actually moved outwards in order to take fix ation outward means that it is moving towards the left side or it is moving temporally in order to bring back that image onto the FIA now this movement is called the redressal movement okay and this redressal movement is more of a quantitative movement what I mean to say is that the the amount of redressal movement that happens in the deviated or squinted I is directly proportional to the amount of squint and it is directly proportional to the distance of the fobia from the stimulated peripheral area so suppose here this was the point where the image was being formed somewhere on the Nal retina okay this was where the image was formed however this was where the phobia was so if the distance between these two points is more obviously the redressive movement will also be more okay so in more simpler terms the more is the squint the more peripheral is the image form and more is the redressal movement that the eye has to make in order to bring that image onto the phobia so I hope that redressal movement concept is clear to you now now what if we use a prism in front of that esotropic left eye okay and as I told you in the previous video what type of prism are you going to use here in esotropia where is the ey deviated the eyes is deviated inwards and therefore you are going to place the apex of the prism also inwards and the base will be outside so therefore the type of prism that you're using here is a base out prism now what did I tell you about prism a prism will bend the ray of light towards the base okay so towards this side so now you can see the initial image which was formed of the nasal retina is now formed closer to the FIA okay because of the bending of rays which is happening towards the base of the prism right so as we start putting the prism initially we will put prisms of say you know lesser power say initially we put some five prism de opter base out prism so because of the bending say the image now shifts here but still it is present on the nasal half of the retina so now you so still you are going to see some amount of redressal movement towards outside now suppose we increase it to 10 prism diopter again the uh the redressal movement will decrease that means the the image is moving closer to phobia but it is still not on the phobia okay so now we're going to suppose we increase it to about 15 prism diopter the image is shifted exactly on the fobia now is there any need for the eye to show any redressal movement no if the image is formed on the FIA there will be no redressal movement and that is the spot okay where the prism strength will be equal to the amount of deviation so I hope that you uh that uh understood this concept of the prism cover test okay so you're going to place the prisms of increasing power in front of the deving IE and check for the movement of the redressal and that movement of redressal is going to decrease uh as you keep on increasing the power and at one particular point the movement is going to totally stop and that is a point where the prism strength is equal to the amount of deviation now what if you uh were to check whether the amount of deviation that we have measured is correct or not so in that case now what we'll do is we'll start uh we'll increase the amount of prism further so suppose we got no movement at 15 prism diopter we going to increase it to 20 prism diopter based out prism so suppose this prism is 20 prism de opter now what happens happens is at 15 prism de opter the image was being formed at the fobia but now at 20 prism de opter the image will move further away from the f and now it has entered on the temporal aspect of the retina right so at this point if the if we do our alternate cover test the movement of redressal is going to get reversed okay that means instead of moving outwards now the ey is going to show an inward movement so what does that mean it means that after the after one point okay we have by increasing the prism uh the strength of the prism we have actually induced exotropia in this patient and now the ey is actually showing a reversal of movement okay uh it is called reversal because it is in the opposite direction to the movement which was seen previously right and this is called a point of reversal so while doing a cover test if you see a point of reversal that means change in the direction uh uh of the redressal movement you it gives you an idea that you have actually exceeded your exact point so at this point now you're going to again go back to your previous uh strength of the prism so in this case we go back to 15 prism deopa and this is how you cross check your uh uh Prism Power okay so let us now summarize this concept of prism cover test or the redress and movement so ressle in the prism and the cover test is basically a psycho Optical reflex that occurs when the eye is forced to take fixation and here we're talking about the deviated eye right so as the deviated eye takes fixation the peripheral retina is is going to get stimulated and there will be a redressal movement in that deviated eye in order to bring back the image onto the fobia now this amount of redressal movement will be proportional to the amount of squint obviously and now you're going to keep on increasing the power of prism plac in the front of the deving eye start with the low Prism Power and then keep on increasing as you increase the power of the prism the redressal movement is going to keep on decreasing and as the image is going to come exactly onto the fobia you will see that at one point there will be no redressal movement okay and that is your Prism Power now if you were to go one step further and increase the Prism Power by one more step what will happen the the redressal movement will now reverse okay so you will see a point of reversal at that point you know that there's no uh use of increasing more uh no use in increasing more Prism Power so now you're going to come back and choose the prism where you did not see any movement so so that is your prism cover test so in prism cover test basically we are measuring the angle of deviation using the prisms and simultaneously we are doing an alternate cover test okay so that was the optical principle right now the thing is before you start doing the prison cover test you are always supposed to do your cover uncover and Alternate cover test the alternate cover test specifically you're supposed to do because that will help you establish the nature of the deviation you should know first first of all whether you're dealing with an esotropia an exotropia or you're dealing with a hyper or a hypertropia because if you do not know what kind of squint you are dealing with how are you going to place the prism in front of the eye now as I told you in the previous video also when you're dealing with an esotropia that is an inward deviation you're supposed to place the prism in such a way that the Apex is toward deviation right so in esotropia you put a base out prism in exotropia you're supposed to put a base in prism in hypertropia you will put a base down prism in hypotropia that is downward deviation you will put up a base up prism and in all these you're going to put the prism in front of the I where you're supposed to measure the deviation so here you're putting in front of the deviated IE okay now one more important concept is that when you have put up a prism that eye is not going to take fixation the other eye without the prism is the eye which is going to take fixation and the I which is present below the prism is the I that you are measuring the deviation of so I hope that is clear to you so the fixing eye is the eye without a prism and the movement of the eye behind the prism is neutralized so you're checking the deviation of the i in which there is prism okay now always always carry out your prism cover test at distance and also at uh near fixation it is very very important that you carry out at distance as well as near fixation moreover you're supposed to carry out in all the nine gazes of the uh nine positions of the Gaze now if you're doing it at a distance make sure that the distance fixation uh you give him at least a 6x9 visual Equity symbol okay and if the patient is preliterate patient or kids then you can use maybe some electronically operated moving mechanical toys or some projected moving cartoons also for near fixation usually the distance is about 303 cm and the fixation targets are these tongue depressors or these ice cream sticks onto which you can actually stick uh these accommodative targets now for the purpose of prism cover test either you can use these loose prisms or you can use the pris prisms which are mounted on a bar right so loose prisms are like this you can actually pick pick up one one prism at a time put it in front of the patient based on how you're going to put it uh based on the type of squint okay that is BAS in base out base up or base down and the prism bar cover test the in prism bar cover test basically you're going to use these prism bars okay so prism bars are nothing but the prisms are going to be mounted in such a way like suppose we have this horizontal bar in the horizontal bar the prisms are mounted in such a way that the base is going to be present horizontally or laterally right like this so this is how you have the horizontal prism bars whereas in the vertical bars the prisms are going to be oriented in such a way that the bases are going to be either up or down right so that those are your vertical prism bar so you can also hold it in such a way that if you want to use it as a base up prism you're going to hold it in one way and then if you want a base down prison you're going to turn them down and then use it on the patient so let us see how we carry out this prism cover test suppose this is the patient that we want to test in figure a you can see that the right eye is showing what so if you're looking at the patient always this is your right eye and this is your left eye okay I hope that is uh clear to you so in right eye you can see that there is an esotropia obviously there is this hur's cornal reflex test also if you carry out you can see that the reflex is temporal that means we are dealing with esotropia now if you put a cover in front of the left eye what is happening to the right eye the right ey is moving outwards to take fixation that again confirms that you're dealing with a case of eso deviation if you put a cover on the right eye again what is happening on the alternate cover test the left eye is now moving outwards that means you're dealing with a alternating esotropia now what you do you start doing your prism cover test because we're dealing with an esotropia we're going to put a base out prism with the Apex towards the deviation and now you can see that as you put cover as you keep on Switching the cover between the right eye and the left eye simultaneously you are also checking the movement of the uh right eye behind the prism you can see the movement is outward so at one point as you can see in the figure F the reflex is in the center but obviously what we are looking here is the redressal moment there's no redressal movement so that is your point of neutralization in the prism cover test now this very important point that the prism cover tests are supposed to be performed in all the positions of the Gaze right so there are two ways in which you can do either the fixation tar Target you can put straight up it can remain fixed in the primary position and you can actually manipulate the patient's head in such a way that the eyes move in all the nine positions of the Gaze or what you can do is you can take the fixation Target from one position to the another just like you check for the movements of the eye okay and then you carry out your deviation uh you carry out your prism cover test in all the gazes of course the first test is much more the first way is much more convenient now the reason we are doing it is because if the angle of deviation is equal in all the nine position of gazes it means that we are dealing with a coment squint okay so in Comet and squint basically there is no paralysis of any muscle there is no restriction of any muscle and in simple words it is just a sort of balance which is Disturbed between the two eyes however if there is any disparity in the angle of deviation in different position of the gazes it means that we are dealing with an incompetent squint okay so if the angle of deviation is equal in all the position of gazes we are dealing with a coment squid if the angle of deviation has a disparity in different position of gazes it means that we are dealing with an incompetent squint so I hope that is clear now an incompetent squint can be restrictive squint it could be paralytic squint okay that's a topic for discussion in another video now what if your patient has a refractive error that means the patient is wearing glasses do you carry out prism cover test with glasses or without glasses the answer is both you carry out the prism cover test with glasses and then you measure your uh angle of deviation at distance and also at near similarly you also carry out it without glasses at distance and at near the reason is because when you actually carry out the deviation at distance fixation with the patient wearing his full correction the accommodation is totally relaxed and whenever you carry out the uh whenever you measure the angle of squint when your accommodation is controlled when the fusion is controlled Fusion how are we taking care here by doing the alter alternate cover test properly right so when your angle of deviation is measured in such a way that there is no influence of the accommodation or the fusion we are basically measuring the static or the basic deviation okay and however if the correction of the refractive error is not uh adequate okay or we are measuring the angle of deviation without the correction then the commodation is obviously not controlled and then the angle that we get the angle of squid that we get is called the dynamic angle of squin now usually for the purpose of surgeries it is the basic deviation or the static deviation which is measured if you followed the entire video till this moment give yourself a pat on your back and now let us understand what is primary and secondary deviation so what is meant by primary deviation now basically you have two eyes one eye is a fixing eye and the other eye has the deviation also called The V ey or the parai right now if you are measuring the deviation of the partic eye and the good eye or the nonp partic eye is fixating on the object that means you are measuring the primary deviation however if you are measuring the deviation of the nonp partic ey by forcing your squinted eye or partic ey to take up the fixation then that is called a secondary deviation so just know these sentences because now I'm going to explain it to you in detail so let us see what is primary deviation suppose these are your eyes and the right eye seems to have some sort of squint mostly an esotropia right now if you were supposed to measure the right eye deviation how are you going to do it you're going to place a prism in front of the right eye and that would be a Bas or prism so here you're trying to measure me the deviation in the right eye which has a squint right so that's a partic ey so this is called a primary deviation measurement however now if you place a cover on the left eye what are you doing you're forcing the right eye which was the partic eye to take fixation now the right eye was already a weak eye that had squint it was inwardly deviated now as you put a cover in front of the left eye the right eye has to take up fixation to bring the object onto the fobia that means the right ey lateral rectus has to do so much work to pull it outward so that it becomes straight right so now the fixating eye is which eye it is the right eye but because of the headings law what will happen the medial rectors of the left eye will also get that strong Innovation because of the herrings law right the left uh the right eye lateral rectus had to do so much work to pull the right eye toward the center and therefore the medial rectors will also get that strong Innovation and what will happen to the left eye behind the cover the left eye will now assume this ESO deviation right so what has happened here the IE which was originally having the problem that is the right eye which was a partic or squinting eye has now become straight and has become the fixing eye and now you have deviation in the left eye now if you were to measure the deviation in this left eye by putting a base out prism in front of the left eye you are going to measure what is called a secondary deviation or a normal I deviation so I hope that is clear right now obviously if I were to ask you what is the primary deviation more or secondary deviation more in case of incompetent squint or a non-competent squint where you have some sort of restriction or some sort of paralysis now here suppose your right eye had some sort of restriction restriction means say the medial rectus was very very tight okay or paralysis that means the lateral rectus was not working properly so what will happen there will be so much amount of innovation going to the lateral rectors so in order to pull this esotropic eye outwards toward the center the same Innovation is going to the medial rectors as well and therefore there will be esotropia in the left ey obviously therefore because of the increased Innovation that is going in case of these non-com stabismus the secondary deviation is always going to be greater than the primary deviation that means if you were to measure deviation in a normal eye it is always going to be greater than the deviation that you measured in a partic or in a partic eye right and this is seen in case of non-competent strabismus and this happens because of the headings law of equal Innovation now here I have a question for you what will happen and what will be the relation between the primary and secondary deviation in case of a coment strabismus okay so answer that question in the comment section so now we are done with the prism cover test now the question is are there's some precautions that you need to do uh that you need to take while uh doing a prism cover test definitely prism cover test is based on dissociating the eyes and therefore you are doing an alternate cover test do an alternate cover test properly okay don't do it halfheartedly half cover on the eye because that will cause fusion and then you're not going to get the proper measurement so pre prevent Fusion by doing a continued use of alternate cover test never give the patient a chance to fuse or by using his both eyes at the same time give the patient adequate time to fixate accurately as you doing the cover test if you move the cover too quickly the patient is not going to uh fixate very properly and you're not going to get accurate results in prism cover test so always do the alternate cover test slowly and give the patient time to fixate on your target always always make sure that you are in control of the accommodation use a detailed target use a 6x9 visual Equity uh uh line on the Snelling or or an equalent chart use accommodative Targets on for the near measurements okay and make sure that when you're using those High high powered prisms like 45 and 60 you know they will reduce the clarity of the vision and it becomes very difficult for the patient always to actually fixate with those prisms also in front so it's always better that you put those prisms uh the high power prisms in front of the better High which has some amount of good visual Equity now sometimes you will have uh the angle of prism strength some sometimes you have the angle of squint which will be too large sometimes you might not have those prisms also to measure so suppose the angle is about 100 and the prisms available are only up to 60 prism deop so what are you going to do in such cases you could actually use a 60 and also a 40 okay and you can divide the prisms you can place 60 in front of one eye and 40 in front of the other eye however can you like simply add these prisms to get the angle of deviation the answer is no okay so suppose uh this was a patient who had alternating esotropic and you were using 50 prism de Ops in one eye and 20 prism de Ops in the other eye and you find that as you use these two prism there was no redressal movement so now what is the angle of deviation is it 70 the answer is no so then you have to use these tables which was given by Thomson and gon here you can see that in one eye you have used 50 and the other eye you have used 20 so you go down and see the total is 78 prism diopters okay so it is not not plain uh addition so 50 + 20 is not 70 it is 78 prism diopters so that should be very clear now apart from that you know kids they can concentrate only for a very short period of time so it's always better that in kids you carry out your test quickly and you start your measurement with an approximately the correct prism so say you carry out an hbur conal reflex in these kids and you find out that the reflex is actually present just at the margin of the pupil so you know that the HB col reflex it is about 15° so if you want to convert it into prism de opter how much it will be it will be roughly 15 into two that comes to 30 prism de Ops so you start your test using uh some somewhere around 30 prism de opter of a prism and then go up uh accordingly right so that way you will save time and you don't have to deal with the short attention span of kids now at in in the end we would like to discuss how do you go about a combined deviation that means there's also a vertical component and then there's a horizontal component obviously in such cases you cannot use your prism bars so it's better to use the loose prisms and it is always better to use these prisms in front of the same eye because they can be incompetent in such cases if you're supposed to if you were to actually you know split the prisms so it's always better that you uh mix the prism in front of same eye now the question is which component are you going to neutralize first always neutralize the larger component and then go ahead and put a prism in front of it to neutralize the smaller component coming to what are the advantages definitely it's very accurate to about two prism diopter it provides complete dissociation and therefore it measures the maximum angle of deviation so very good for measuring deviation before surgeries it is performed in free space and you can measure the deviation at different distances including the far distance and if you remember this table in which we were actually comparing uh the different types of test in the previous video we talked about the coral reflection and now the prism cover test if you see the problem is you cannot carry it out if the prism cover test if the visual Equity is too less okay if the patient is not Cooperative okay and how or if when this's presence of torsion so torsion and measuring of deviation in case of torsion it's very very difficult while doing a prism cover test uh thus prism cover test require simult perception the answer is no because you're anyways auding one ey and you have to carry out the prism cover test in all the nine positions of the Gaze so that's all for today I hope you liked it thank you and have a nice day