hello and welcome to insight ophthalmology this is dr amrit and today we are discussing a very important topic on refraction that is the astigmatism let us have a look at the first circle if we consider this to be a perfect sphere all the meridians whether it is 90 degrees 180 degrees and other meridians they are all going to be equal and they will have equal refractive power what i mean to say is if the rays of light are passing through these meridian doesn't matter from where they enter all of them are going to converge at a single point focus and that point focus let us represent it by a however if we consider these two ovoid surfaces obviously their meridians will not be equal okay like in the first one the vertical meridian is going to be more curved and the second one the horizontal meridian is going to be more curved so obviously if the light is going to pass through such a meridian they will not get focused at one single point instead they will get focused at two points and instead of forming a single point focus they will form something which is called line focus so what is astigmatism astigmatism is a type of refractive error in which the refraction will vary along different meridians why because different meridians will have different refractive power because of that we will not have a point focus but instead we will have a focal line what is meant by this line focus let us consider this astigmatic surface which seems to be compressed from the either sides in such a way that the horizontal meridian will be more curved compared to the vertical meridian so when the rays of light are going to pass through this horizontal meridian they are going to get converged to a point say a now when the same rays of light are going to pass through the vertical meridian they are going to come to a point that is b which is present behind that of a and why does it happen because the power of the vertical meridian is less because it is less curved so what do we understand from this that we do not have a single focus from this astigmatic surface instead we are having something a and we are having another focus b right and the focus over here will be a line joining this a and b right and this line is called the line focus right so in an astigmatic surface what we get is a line focus and we do not get a point focus that we see in a perfect spherical surface right so in a perfect spherical surface if we have two meridians like this all the meridians will be actually equal and therefore the refraction also will be equal so all the rays of light which are going to pass through such a meridian they are going to come to focus at a single point say a and therefore in a spherical surface we will have a point focus but in an astigmatic surface what do we have is we will have a line for broadly our astigmatism can be divided into two types that is the regular astigmatism and the irregular astigmatism so what is this regular astigmatism a regular astigmatism is when the refractive power will change uniformly from one meridian to the another so what i mean to say is if this is an astigmatic surface and these are the two principal axes the power the power along the axis will be changing uniformly that means suppose this is plus 2 and say this is about plus 4 the change of power will be very very uniform so in between this whatever meridians are present they will have plus 3 say plus 3.5 and plus 3.75 so on and so forth right so there is a smooth transition of power from one meridian to another in case of regular astigmatism however when this does not happen the astigmatism is called irregular astigmatism so in an irregular astigmatism the power will not change so smoothly so what i mean to say is that so if this is plus two and this is plus four what happens is one meridian in between can actually have plus five okay and one million in between can actually have minus one right so this surface will not be smooth right and even the transition between this axis and this axis is not uniform so such a surface is said to have irregular astigmatism the regular astigmatism again can be further subdivided so it is divided into four types number one is with the rule astigmatism number two is against the rule astigmatism number three is oblique astigmatism and number four is by oblique astigmatism first let us discuss about with the rule astigmatism in withdrawal astigmatism the two principal meridians will be placed at 90 degrees to one another that means 1 will be 90 degrees and the other will be 180 degrees so our two principal meridians are nothing but the vertical meridian and the horizontal meridians that we should remember is that is the vertical meridian is going to be more curved compared to the horizontal meridian in case of with the rule astigmatism okay so let us exam let us consider this diagram over here so suppose we had a perfect sphere like this and we compress the perfect sphere from either side vertically so what do we get we'll get something like this okay so in such a structure when you compress a ball from up and down what happens is that the vertical meridian of that ball will become more curved as seen in this diagram and the horizontal would be less curved right so such a kind this is is called the with the rule astigmatism right and why is it called withdrawal astigmatism because it is very common and it is normally seen in young people and why is it seen normally in young people is because if this is our eyes and these our lids okay and these are our lids what happens is the lids will create pressure from either side on the eye because of which the vertical meridian of the eye will become more curved compared to the horizontal meridian and therefore in young people usually we have with the matism rule to correct this with the rule astigmatism there are two ways in which we can give cylindrical lenses along different axis to correct with the rule astigmatism so the first thing that we should remember is that in the in this with the rule astigmatism basically the vertical meridian is more convex right so how do we correct this more convex vertical meridian by decreasing the power of this vertical meridian and how do we decrease the power of the vertical meridian by giving a minus cylinder okay the minus cylinder has a concave surface that is why i have drawn a concave cylinder over here right so we have to give this concave cylinder along the vertical meridian so where will the axis of this concave cylinder be it will be at 180 degrees okay so the first way of correcting with the rule elastic metasm is by giving a minus cylinder at 180 degrees right yeah now what is the second way of correcting the withdraw elastic methodism is by correcting this flatter meridian that is the horizontal meridian now this horizontal meridian is flatter that means it is weaker so what do we have to do we have to make it stronger by giving it a plus cylinder the plus cylinder has a convex surface therefore it will give its more converging power and make this flatter meridian more stronger or at least equal to that of the vertical meridian so if we are giving a cylinder at 180 degrees that means at the horizontal meridian where will the axis of that cylinder be the axis of that cylinder will be in a direction perpendicular so the axis will be at 90 degrees right so one more way of correcting with the rule elastic magnetism is by giving a plus cylinder lens at 90 degrees so these two points are very important and we must remember that there are two ways in which we can correct with through elastigmatism that is by either giving a minus cylinder at 180 degrees or by giving a plus cylinder at 90 degrees the next type of regular astigmatism is against the rule astigmatism the against rule astigmatism is totally opposite to that of with the rule astigmatism that means in against the rule astigmatism it is the horizontal meridian which is more curved compared to the vertical meridian so it is as if if you take a circular ball and then compresses from either sides what happens is the vertical the horizontal meridian will become more curved compared to the vertical meridian and this is what happens in a surface which is showing against the rule astigmatism now with the elastic mechanism was more common in young people however against the rule astigmatism is more common in case of old people and why is it so because as we age the tone of our eyelids will decrease and as i already told you it is the eyelids which are going to compress the eyeball and make it to have with the real astigmatism now in an old person because the tone of the eyelids will decrease they will not be able to compress the glow from top and bottom and instead the other sides that means the horizontal compression will take over and what will happen is that the uh horizontal meridian will become more convex compared to the vertical meridian right so this is against the rule astigmatism now let us see how should we correct against the rule astigmatism now in against the rule astigmatism our more powerful meridian is which meridian it is the horizontal medium so in order to make this horizontal meridian more weaker we have to add a minus cylinder because a minus cylinder will have a concave surface right along the horizontal meridian so if you are giving minus power along the horizontal meridian where is the axis present the axis is always perpendicular to the plane where we want to give the power so the axis is at 90 degrees right so the first way of correcting against the rule astigmatism is minus cylinder at 90 degrees okay now what is the second way of correcting against the elastic magnesium it is by giving a plus cylinder at 180 degrees how let me tell you now we know that the vertical meridian against the rule astigmatism is little weaker compared to the horizontal meridian so how do we make it stronger by giving a plus cylinder because the plus cylinder will have a convex surface and this convex surface will help to increase the converging power so if you are giving power along the vertical meridian where is the axis present the axis is at 180 degrees right so plus cylinder at 180 degrees the next type of regular astigmatism is oblique astigmatism in a normal astigmatism like the with the rule and against rule astigmatism we had two principal medias that means we had one 180 degrees media and we had one more media which was at 90 degrees however this is not the case in obligation the axis the principal meridia are present obliquely in obligation however they will not be like vertical and horizontal however they will still be perpendicular to each other they will still be perpendicular to each other so say if one is 45 degrees the other will be 135 degrees so these two meridians are not 90 degrees or 180 degrees but still the difference between them is 90 that means still they are perpendicular so such an astigmatism is called oblique astigmatism example of oblique astigmatism is say one axis is at 30 degrees and one axis is at 120 degrees now again they are not 90 and 180 but the difference between them is 90 degrees that means they are still perpendicular to each other the last type of regular astigmatism is the bio-oblique astigmatism in by oblique astigmatism the two principal meridians are not even at right angles to each other that means one can be 30 degrees and the other can be 100 degrees the difference between them is about 70 they are not at right angles to each other so such an uh astigmatism in which the axes are oblique that means they don't have principal meridians as vertical and horizontal but instead oblique but they will not be at 90 degrees to each other so certain astigmatism is called by oblique astigmatism now let us discuss about the refractive types of regular astigmatism based upon the position of the two focal lines in relation to our retina okay the regular astigmatism is further classified as simple astigmatism compound astigmatism and the mixed astigmatism if this is considered to be our retina the two and this is our cornea there are two principal meridians the vertical and the horizontal the horizontal meridian will focus one at one point and the vertical medium will focus at another point that is a and b say respectively now based on where this points a and b are formed in relation to this retina we have different types of astigmatism and that is what i am going to explain to you in simple astigmatism one meridian is always going to focus on the retina okay so in simple astigmatism one meridian is always going to focus on the retina now you might ask what about the other meridian so in other meridian in simple astigmatism the other meridian can focus either in front of the retina or it can go and focus behind the retina based on which the simple astigmatism is again of two types simple myopic astigmatism in which the other meridian is forming a focus in front of the retina that means one meridia is actually myopic and the other meridia which is forming on the retina is actually normal meridia right so you can remember it as in simple astigmatism one meridia is normal so one meridia is forming the focus on the retina however the other meridia can actually be myopic or it could be hypermetropic if it forms an image in front of the retina it is called simple myopic astigmatism or if the if that meridian is focusing behind the retina then it is called simple hypermetropic astigmatism but the point to be remembered is that in simple astigmatism one meridian is always normal and forming a focus on to the retina coming to compound astigmatism now in compound astigmatism none of the meridian is normal okay none of the meridian is normal however these two meridians will act similarly the two meridians will act similarly that means one meridian is also forming in front of the retina that is a and the other meridian is also forming in front of the retina that is b okay so both the meridians are acting together they are behaving in a similar way now since these two meridians both of them are acting myopic therefore this is called compound myopic astigmatism okay now similarly what if these two compoundly active meridians both of them focus behind the retina what does it mean that both of them are acting hyper metropic so in such a case the astigmatism is called compound hypermetropic aztecmatism so at this point i want you to remember that in simple astigmatism at least one meridia was forming on the retina however in compound astigmatism both the meridians are going to form either in front of the retina or behind the retina based on which it will be called compound myopic astigmatism or compound hypermetropic astigmatism now what about mixed astigmatism so mixed astigmatism also both the meridians will be abnormal however one meridia is going to form in front of the retina and the other meridia is going to form behind the retina that means none of the meridian is normal none is forming on the retina however one is forming in front and one is forming behind the difference from the compound astigmatism is that that they are not acting compoundly they are acting differently one is acting myopically and one meridia is acting in a hypermetropic fashion because of this mixed action it is called mixed astigmatism i hope that is clear to you so this was about the types of astigmatism and in my next video i will be giving you certain practice exercises and telling you how by reading a prescription we can actually tell what type of astigmatism it is and we can tell what type of astigmatism it is with respect to with the rule or against the rule astigmatism so i hope it was helpful thank you and have a nice day