the lens is a transparent biconvex structure which is placed in between the iris in front and the wiess Chamber behind it is of utmost importance within the human eye it transmits and refracts the light and as a matter of fact it actually contributes to about 35% of the refractive power of the eye the remaining uh the remaining refraction that is about 65% actually comes from the cornea apart from that the human lens also helps in accommodation and also absorbs the harmful UV light of about less than 350 nanom in wavelength so what is meant by AIA AIA simply put is the absence of lens from the eye sometimes however the lens might get subluxated or dislocated from its normal position and in such cases when there is absence of lens from the pupilary area that is also referred to as AIA now let us see what are the causes of AIA based on the various causes the AIA can be actually divided into VAR types we have surgical AIA we have traumatic AIA congenital AIA and postd dislocation AIA or the optical AIA first let us discuss what is meant by surgical AIA whenever the lens is removed surgically it leads to a surgical AIA or absence of lens now this procedure is known as lens sect okay now this is usually done in case of congenital and infantile catract which is uh which usually require this lensectomy within about 6 to 18 months of life in order to prevent amblyopia development similarly in pediatric and adult traumatic cases most of the time a primary primary lens sectomy is necessary first to allow for complete healing before we proceed with a secondary procedure of I implantation right so in most of the trauma cases the first surgery that is done is just a lensectomy apart from that the surgical AIA could also be a unintentional complication intraoperatively for examp example in case of a posterior capsular rupture or the PCR the second type of arakia is the traumatic AIA now traumatic AIA basically occurs after trauma sometimes there will be a deep penetrating wound through the cornea and that might cause damage to the lens and the surrounding lens structure and sometimes what you'll see is the Extrusion of the lens through the cornea okay so that leads to traumatic AIA apart from that post- trauma very L the lens M matter gets absorbed on its own and there might also be a dislocation of the lens known as the traumatic dislocation of the lens now have a careful uh view of these images so in the first picture you can see that there is actually this Coral tear over here and sometime because of this cornal tear the lens which is inside obviously it will become traumatic it'll develop traumatic cataract but also because of the pressure changes within the eye because of the tear and the cornea what is going to happen is that the lens will actually come out and that is called traumatic Extrusion okay in the second picture you can see that the lens is not really sitting behind the iris instead it has come into the anterior chamber now that is called the anterior dislocation of the lens leading to this segment of a FIA apart from that in the second in the third picture you can see that the lens has actually been dislocated within the vitus cavity and is actually sitting on the retina instead of sitting at its location in the patella fora right so that that is traumatic arakia coming to the third type of arakia and that is the congenital Aria now conal arakia means that the lens is absent right from birth okay now this is an abnormality which uh develops during the fourth or fifth week of the fetal development because this is the time when our lens development takes place okay now here the genes which are involved is the fox E3 Gene and the PX six Gene apart from that also there will be sometimes rubella infection that can lead to a development of kidal AIA now kidal AIA can be of two types we can have a primary congenital AIA or we can have a secondary congenital let is try to understand what is meant by primary congenital AIA and what is secondary congenital AIA in primary congenital AIA it basically results from the failed induction of the lens pleer and therefore the entire lens is going to be abs okay now if you do not understand what exactly is a lens pord and you want to know more about about it in detail I'm going to put a link in the comment in the comment section or maybe in the description box regarding a video which is already present on the development of the human lens on the channel so that is your primary congenital AIA in which the induction of the lens bord is itself absent and therefore the lens is absent whereas secondary ofia is when the lens pord has developed but it gets resolved for some reasons before birth so here only the remnants of the lens such as the lens capsule will be present and the entire other parts of the lens would be absent now let us talk about the optical AIA now Optical AIA can also be told also be called as the post dislocation AIA here the lens is basically absent from the pupilary area either because of sublation or dislocation of the lens and this is also known as ectopia lentis that means the lens is not really present in its real position it is present in some topic position okay now this can occur secondary to trauma a very important cause of optical aeia as we have already seen in the previous uh some of the pictures apart from that you can also have dislocation post some systemic disorders that can cause weakening of the zonules and ultimately leading to the malposition of the lens apart from that we have various genetic mutations as well for example in Marin syndrome homos syura wheel marani syndrome all of these can actually lead to dislocation or subluxation of the lens leading to Optical a FIA now you can see this first picture over here which actually indicates that there is a superior temporal dislocation of the lens you can see the stretch zial over here so this patient actually has a partial tic segment so this inferior segment will be AIC and the superior segment is actually having the lens okay this is partial AIA in a patient with Marin syndrome because of superior temporal dislocation of the lens okay now another thing I would like to tell you is that in Marin syndrome usually you will have a superior sublation of the lens whereas in case of homo syura you will have an inferior sublation of the lens now sometimes what will happen is that this disease is has a progressive nature and the entire lens might actually get dislocated either into the anterior chamber or it can get get dislocated in the posterior chamber as you can see in the second picture the lens is now sitting right on top of the retina and the same is seen on an MRI over here okay so you can see the lens sitting on the retina instead of being present somewhere here having understood the various causes of AIA now let us try to understand the various Optics of an AIC eye now what happens in a normal eye is that we have two uh we have two structures which are dealing with the refraction so we have the Cora which has a power of about 44 diopters and we have the lens which has a power of about plus 16 diopters okay so the lens actually is separating the Aquis humor from the vus humor okay now what exactly happens in an earth fic eye is that the dioptric aparatus of the eye is now changed to a single refractive surface so we have just the conia left which is single-handedly managing the refraction and therefore the power of the eye will also now reduce so as you can see over here initially we had the lens which was contributing to about plus 16 diopters but in an earth eye it's only the cornea which is imparting a power of about plus 44 diopters to the eye also the lens between the Aquis and the witus humor is now gone and they have uniform refractive index and they actually act as a single apparatus now this actually leads to the various changes in the eye for example the focal length of the eyeball the anterior focal length of the eyeball as you can see is normally about 15.7 mm from the anterior Coral surface and from the and the posterior focal length from the an Coral surface is about 24.13 mm right now however in igia you can see that the focal lengths are actually going to increase the anterior focal length now has increased from 15.7 mm to 23.2 mm and the posterior focal length has also increased to 31 mm and you can see that the posterior focal point is now forming somewhere behind the eyeball because the length of the eyeball itself is about 24.1 3 mm right so what exactly is happening in an earth ey is that because the focal point is getting shifted behind the eyeball the eye is also becoming highly highly metropic okay so why is it happening it's happening because the power of the lens is reduced we have just the cornea present dealing with the refraction so almost about we have lost about 16 diopters of the power obviously this will vary from Patient to Patient but the but an aake guy has actually lost a signific ific an amount of its focusing power right now apart from that you can see the focal point has also shifted behind as so therefore these two factors will contribute to the development of hyper metropia in an fic eye in addition to hypermetropia there's always some amount of astigmatism in all these cases of AIA especially if the patient has a surgical AIA now what exactly happens uh is that in these surgeries usually there will be a corneal or a coral scleral section or an incision that has been made okay and usually it actually differs from surgery to surgery suppose a patient underwent a f emulsification without any suture so that becomes a sutur lless surgery so in a sutur lless surgery the incision itself will actually flatten the Meridian okay the incision alone actually flattens the meridian so for example if you have a f emulsification surgery and the incision has been taken in the superior quadrant then then post post surgery there will actually be flattening of the cornea in that Superior Meridian okay so what exactly is happening the uh the vertical Meridian or the Superior in inferior Meridian actually will become flattened out because of the incision which has been taken superiorly in F emulsification and the horizontal Meridian will now be more steeper and this type of astigmatism is known as against the rule astigmatism okay so at this point I would like to assume that all of you know what is meant by against the ru astigmatism and what is with the ru astigmatism and even if you don't know about it you don't worry I will put a link in the comment section or the description box for you to refer to that okay so usually uh pure incisal surgery where there is no suture placed like in falsification there will be against the ru astigmatism if the incision has been made in the superior quadrant okay and this incision this astigmatism is of about 1 to 1.5 diopter however if there is a surgery in which the sutures have been placed sutures usually lead to steepening of the meridians okay so you can remember sutures have an s in it and steepening also has an s in it so sutures lead to steepening and incision leads to flattening okay so in an sics surgery or an extra capsular surgery if you have put uh if there were sutures which were placed Forst surgery then there will be steep leing in that Meridian for example if you have carried out a superior sics with suture placement at the end of the surgery it will lead to basically steepening of the vertical Meridian compared to the horizontal Meridian and this will cause with the rule astigmatism and this astigmatism is usually of the order 1 to three diopter okay so this is not a hard and fast rule this examples are basically considering that the patient was ametropic at the beginning of the surgery another problem that that the patients with AIA basically face is that of problem with the neor and this happens because of the loss of accommodation as we know that the lens is basically involved in accommodation that is the ability to increase its curvature or power when dealing with NE now as the lens is gone so is the accommodation and that becomes a major problem in treating accommod uh in treating a FIA other problems that you see with a FIA is that you have to understand that it can cause complex mechanical and biochemical changes in the vitus and also in the Aquis humor or in the anterior segment structures usually when there's surgical AIA intraoperatively there might be a complication known as the posterior capsular rupture where there's a discontinuity in the Integrity of the posterior capsule and because of that what happens is that you might see the loss of Vitus that means the vitus which is usually Limited in the vitus humor with behind the lens might actually find its way through the posterior capsular rupture into the anti interior chamber such a wi when it moves forward it actually causes some traction on the retina and it can actually lead to retinal detachment as well now this Vitus when it has moved from the vitus humor it can enter the anterior chamber and as it moves into the anterior chamber it can block your tripic measure work leading to the secondary gloma and also it can cause damage to the coral endothelium which is present here which which can cause the coral decompensation okay so definitely changes will occur in the vitus itself leading to Vitus degeneration and later on because of the traction the patient can actually develop a retinal detachment so these are some long-term complications associated with AIA now what are some of the signs and symptoms that you see in an AIC patients symptoms are very obvious the patient is definitely going to have defective vision and the vision will be defective both at far as well as near the far vision is defective because of the high hypermetropia and the accommodation is also affected leading to the defective Vision at near apart from that as we know that the lens basically can absorb the UV light and also infrared rays and therefore since there's absence of lens excessive entry of UV and infrared Rays occur in the into the eye leading to a a different types of Hues to the image so if you get a reddish Hue to the image that is known as the atropia and if you see a bluish Hue to the image in an EIC patient that's called as the copia now coming to the signs that you see basically in a patient of FIA now usually these patients if they have undergone an S sics surgery or an extracapsular surgery there will be a limal incision as you can see over here right obviously this is an interoperative picture but even postoperatively such patients when you invert uh when you actually retract the upper eyelid you might be able to identify a limble scar or a limble incision so this is a cornea you're going to see incision over here uh assuming that the surgery was carried out from the superior quadrate apart from that the anterior chamber in case of an AIA is usually a deep anterior chamber and why does that happen usually you have this lens which is sitting between the vitus cavity and the iris and when the lens is taken away as in case of AIA what is going to happen is that there is no support to the iris and therefore the iris is basically going to move backwards leading to increased depth of the anterior chain chamber and therefore in case of AIA and even in sudia you will have deep anterior chamber okay now apart from that what you see is since there's no support to the iris because of the lens being gone here you are going to see some Tremors in the iris as the patient moves his eyeball from left to right position or up and down you're going to observe that there is some tremulous of the iris and this is known as iridodonesis coming to the other signs that you see in a FIA is the color of pupil usually in pseudo FIA when there's an intraocular lens which is present in the eye uh when you actually shine light with your torch or when you observe this patient under a slit lamp microscopy what is going to happen is that you will see a shining reflex that is going to come from the lens that means the lens is going to shine back at you by reflection at the inocular lens surface however in case of a fic patient there's no I present and the pupilary area is actually empty and therefore the color of the pupil will appear to be jet black in color apart from that the number of Perkin images can also guide you in diagnosis uh the pering images in case of a normal eyeball is in normal eye are basically four in number whereas in AIA it is only two in number also in fundus examination you will observe that there will be a small hypermetropic disc the reason is because in AIA you have high hypermetropia and therefore the disk in these patients also will appear smaller in size refraction obviously will reveal High hyper metropia so this is what I was talking about the parin images in a normal eye there are basically about four parin Images the first image comes from the anterior surface of the cornea the second image comes from the posterior surface of the cornea the third peringer image comes from the anterior surface of the lens and the fourth comes from the posterior surface of the lens now when you have a FIA this entire lens is gone and therefore the number of peren images that you're going to see are only two one coming from the anterior surface and the second coming from the posterior surface of the cornea if you take a look at uh this these comparative pictures in the first picture you can see that there are actually Four pering images and this is actually a case of pseudo FIA now in pseudo FIA also you're are going to see four pering images and the third and the fourth pering image are going to come from the anterior and the posterior surface of the intraocular lens okay now the second picture is that of an AIC patient you can see there's only one pering image over here which is seen however if you actually were to move your slit beam from side to side you can actually appreciate two pering images in case of a FIA I'm so sorry this is AIA and not FIA right so in AIA you have two pering images and in case of pseudo FIA we see four pering Images now that we understand u a fake here we have to now deal with the treatment of AIA and the main Optical principle behind treating AIA is to use certain sort of convex lenses of appropriate poers so that the image can be formed on the retina we know that in AIA what is happening there is high hypermetropia and the image is being focused behind the eyeball because of the increase in the uh focal length of the eye and also because of the decrease power of the eye and therefore we have to find ways to supplement uh the eyeball with this excess with this additional power so there are various modalities of correcting AIA we have AIC spectacles contact lenses inocular lens and also refractive Coral surgery and to tell you the standard of care right now is contact lenses and inocular lens and the best one are actually the intraocular lens however in this video we are going to actually um focus on the disadvantages of using Earth F spectacles because these actually form a very important uh mcqs and also high eeld point for your entrance examinations so AIC spectacles were uh very commonly used for treating AIA in the past and even now when surgery is contraindicated when the patient cannot really wear contact lenses then also spectacles has uh can be used for treating the FIA now prescribing AIC uh refraction or AIC glasses is not so easy now there is a misconception that goes around that earthy glasses are just plus 10 convex glasses okay no that is not true the prescription actually can vary from Patient to Patient it all depends on the previous refractive status of your patient for example if there's a patient who is who was already myopic before becoming AIC now post AIA this patient might need a much lesser power than that of an ametropic aric patient so in such patients your plus 10 de opter glasses are not going to work okay so therefore it is advisable that for every tic patient proper refraction is carried out and not just a u a common glass is given to everyone okay so we need proper refraction and the refraction should be done for near as well as far okay so not to forget that the the aric patient also have problem with accommodation and therefore a near ad for them is also very very uh important now let us first try to enumerate the problems with the Earth spectacles the first problem is image magnification then we have sperical aberration chromatic aberration limited field of vision Prisma IC effects of the glasses high power glasses rowing ring scotoma also known as the Jack in the Box phenomenon and of course the Cosmetic problem and the heaviness of the glasses okay so now we are going to discuss all these problems one by one now let us try to understand the first complication that can actually occur with Earth spectacles and that is that of the image magnification so looking through the AIC glasses the image is actually magnified by 33% that means the image is about 1/3 times larger than the normal image okay also this higher magnification to the image also results in a better performance or an enhanced performance of the standard visual Equity test okay for example if there's a patient who's wearing Earth spectacles and the results are about 6x9 on the visual Equity testing snance chart then this will be equivalent to a 6x2 of an ametropic patient also what happens is that in the first picture what I uh I'm trying to depict over here here is that as the image basically moves farther away from the eye the visual angle that it actually extends on the eye also decreases and the image which is magnified it actually it actually extends a much larger visual angle at the eye and this basically gives an illusion that that artificially magnified object is closer to the eye than it really is okay and so therefore this is going to lead to a lot of judgment problems for the patient so there's a there's a false partial orientation of the visual objects the objects will appear to be much more closer to the eye than they really are and this is happening because of the increased visual angle that is subtended at the eye by the enlarged object or the enlarged images and therefore these objects are going to be judged to be nearer than they are leading to um misjudgment in these OIC patients adding to their misery right it actually you know takes the patient months of trial and error to figure out the perfect hand ey coordination the patient is uh visually uncoordinated he's going to knock and fall often fall also more often so quite a problem right so as we talk about a spectacle magnification it is very important to understand two important terms and these are spectacle magnification and relative spectacle magnification so first let us try to understand what is spectacle magnification this is a very important McQ which is asked so the optical correction of ametropia is actually associated with the change in the size of of the image right at the retina so the the ratio between the corrected and the uncorrected image size is known as the spectacle magnification that means suppose you have this man okay and initially he had some problems say he had uh say he had hyper metropia and the size of the image initially was say x at the retina now you have corrected this patient you have given him the convex glasses and now the size of the image on the retina is y so your spectacle magnification now would be corrected image size by uncorrected image size that is y / X okay so I hope that is clear so now let us try to understand what is meant by relative spectacle magnification so actually speaking clinically the relative spectical magnification is much more useful to us because it actually compares the corrected ametropic image size with an ametropic image size that means a normal patient okay so you actually comparing the corrected image size in the patient to a normal patient with a normal image size so that is called the relative spectacle magnification so in the spectacle magnification you were you were actually comparing the image size in the same patient one image one image size was the corrected image size and the other was the uncorrected image size whereas in the relative spectacle magnification you you actually compare the corrected image size of a patient with the normal image size of a normal person so this relative spef relative spectacle magnification actually is very important and if you actually see in case of a spect iCal correction it is about 1.36 and whereas in case of a contact lens the RSM value is about 1.1 okay now this becomes actually important you can actually compare over here this is the normal size and this is the size of the image that is going to happen with the contact lens that is what going to be achieved with the contact lens and this is the size of the image that's is going to be achieved using the spectacle lens so if you carefully observe here the size of the image is maximum with the spectacles and it depicts that the magnification that you achieve with spectacles is the maximum compared to the contact lens and then the inocular lens okay so the advantage of using a contact lens or the advantage of using an intraocular lens over the spectacles is that these systems whether it is a contact lens or an inocular lens they will actually become an intregal part of the optical system of the eye itself the spectacles are always plac at a distance from the eyee whereas the contact lens are going to be S sitting directly on the surface of the eye the inocular lens is going to be sitting within the eye right so they're actually becoming an integral part of the optical system of the eye and therefore the magnification that these U modalities of treatment offer will be much less compared to what a spectacle gives right so the use of contact lens and interocular implants will basically reduce the RSM value to 1.1 to1 for a contact lens the RSM is about 1.1 and for an inocular lens it is almost Unity that means there is no image magnification that occurs another point is spectacles give about 33% image magnification and contact lenses basically causes about 10% image magnification now another complication that can happen with a FIA and also with treating a FIA with spectacles is that of anisometropia and anonia so now let us see what are those basically we know that in a fake eye there's a total lack of accommodation right apart from that what is happening is we have this relative spectacle magnification value of about 1.33 because of which the image in an earth eye after spectal correction is going to be about oneir larger than the image in the normal eye right so this isad this will definitely cause a a difference in the size of the image the image from one eye which is a fake is going to be much more larger compared to the image in the other eye this will lead to anonia and because this an isocon it is going to become extremely difficult for the brain to fuse these images and therefore the patient is going to suffer with diplopia okay so there's going to be double double with spectacles for the Earth K glasses so I hope that is clear now another point that we must remember is that an Isom metropia basically means difference in the power between two eyes right so each one diopter of this difference or each one diopter of an Isom metropia usually corresponds to to approximately 1% difference in the relative magnification between the eye and the visual system can actually compensate for about 3% to 5% of relative magnification whereas when we are talking about Earth FIA there's actually about 33% magnification okay so that is definitely a problem now so basically when you have a fia the patient is going to suffer with an Isom metropia okay that is difference in the refractive power of the two eyes and when you're going to treat the AIA with spectacles what is going to happen there's going to be difference in the magnification and because of that there is going to be an isoc Coria and because of this an isoc Coria the patient is going to land up with diplopia and difficulty infusion of these images now another problem that occurs with using these Earth spectacles is the spherical and chromatic aberration so we'll try to understand these one by one first let us see what is meant by the chromatic abberations so failure of the lens to f Focus all the light to one single point uh is known as the chromatic aberration now this basically occurs because of the changing refractive index of the lens with the wavelength of the light so as you can see over here the red light which is passing through the lens is actually coming at a focus at a much farther away Point compared to the blue light if if there was no chromatic operation all these lights irrespective of their wavelength will come to focus at one single point but that really doesn't happen the refractive index of the transparent materials actually decreasing with the increasing wavelength so what I mean to say is that we know that if you remember this acronym whip gar the red light actually has the maximum wavelength and what happens is that as the wavelength of the light increases the refractive index of the material through which it is passing for it will basically go down right and as the refractive index goes down the bending ability of that material for that light Ray will also go down and therefore you can see that the red light doesn't Bend as much as the blue light which has a shorter wavelength and therefore the blue light is going to come at a focus much more earlier Point compared to the red light right so that is what is known as your chromatic aberation now what what really happens because of chromatic aberation is that the chromatic aberation is going to manifest itself as this fringes of color along along the boundaries that separate the dark and bright parts of the images so if you look at this this is a bird but to a patient who is a faking and wearing a faking spectacles so he will suffer with chromatic abberation and the border of this bird is going to look as if this there are fringes of colored uh bands along the border right so that is known as the chromatic aberation so all these aberations are quite pronounced with higher power glasses and since in AIA we are using really high power plus uh lenses therefore we have these spicular liberations and chromatic liberations now the question is what is meant by spherical liberations now if you would carefully observe here here also what is happening the light rays that are striking a spherical surface in this case this lens okay and the light rays which are actually present at the ends of the lens that is off center so there are some rays which are present which are passing through the center of the lens these are called the axial rays and then there are rays which are present on the other side on The Eccentric portions the peripheral portion of the lens and those are called the paraxial Rays now if you would see the central Rays or the axial Rays basically are coming to focus at a point whereas the paraxial rays are basically bending too much right so the parial rays are actually coming at focus at different points right so this is called as the spherical aberration okay and this is called as a postive spherical aberration if if the peripheral rays are bent too much and that means they come to focus at a point ahead of the normal focus and it is called a negative spherical aeration if the Rays will come to focus behind normal focal point that means if the rays are bending too less right so that is called as a positive or A negative spherical aberration now when we are talking about the earth fake glasses usually what we are going to see is a positive spherical aberation so I hope that is clear okay so this is one problem that occurs with the uh aake glasses now the question is how does it really manifest itself now sperical aberration they manifest in a patient who has a fake as Distortion so they are going to see Distortion of all the objects which are present in the periphery now as a result of these spherical libration which are induced by the strong convex lenses used in our fakey glasses the straight lines are actually going to appear as if they are curved okay so this is the Distortion that I'm talking about in case of the earthy glasses um as a matter of fact for an example a newly corrected AIC person who passes through a doorway the doorway will appear to him as if it is curved inwards and it as if there's a very little narrow space for the person to travel through it right so definitely U I can understand you will also say that it's fine you know it is just an illusion it's not reality but definitely it can be very very disturbing for a new a fake patient right another effect that can occur or another problem that can occur uh in a fak place patients because of spical liberations and because of the Distortion is is the pin cushion effect okay so usually there are different ways in which an image can get distorted usually as I told you that in case of an AIC patient or in case of very high plus lenses or convex lenses the for example this door example for as a matter of fact we took what was happening was the door would appear as if it's bent inwards right the same thing happens is that the Distortion this sort of distortion in which the straight lines are actually bent inwards is known as the pin cushion Distortion okay it looks as if there's a pin cushion right so it resembles the shape of the pin cushion and therefore it is called a pin cushion effect or pincushion Distortion and this is seen with high power convex lenses whereas when you talk about the concave lenses suppose you have a patient who's using who has to use a really high power concave prescription which is a negative prescription then in those cases the patient will observe what is known as a barrel Distortion now in Barrel Distortion the lines will be distorted in such a way that they will appear to be curved towards the outside okay so that is called a barrel Distortion now it is called Barrel Distortion because it resembles a barrel right so this is a very high yield point that remember that in a convex lens in a high power convex lens and also in Earth correction with spectacles you're going to see a pin cushion effect whereas in a concave lens high power concave lens we will see what is known as the barrel shape Distortion now the question is is there a way the patient can actually prevent uh suffering from this Distortion yes there is only if he can actually keep his head straight looking always through the optical center of the lens not really you know moving the Rays along the par aial part so what I mean to say is as I told you that if this is a lens the axial rays will focus on the focal point whereas the par AAL R will focus either behind or in front of the focal point so only the patient could actually keep his head straight all throughout looking through this optical center of the spectacles okay and not move his eyes only use his head move movements keeping his eyes motionless only then maybe the Distortion might not appear but definitely this is too much to ask from the patient now another problem that we deal with the high power glasses and also with our figy glasses because they are high power conx lenses is the Prismatic effect of the lens okay so what is meant by Prismatic effect we know that prisms basically have this property that whenever light rays pass through the prism what is going to happen is that the light Ray will basically Bend towards the base of the prism and the image get shifted towards the Apex now if you carefully observe the concave lens is actually a a fusion of two prisms which are stacked at their apexes that means the two uh the concave lens is basically made up of two base out prisms or base um yeah base out prisms whereas the convex lens are actually made up of two prisms with their bases together right so definitely since the lenses are made up of these prisms there will be Prismatic effect so what really happens is that in a case of a convex lens and definitely in case of a higher power convex lens this is going to be seen more that the Rays of Light which are incident upon the lenses outside the axial Zone that is the outside uh the rays which are present in the par aial zone or in the peripheral part of the lens they are going to be deviated towards they are going to be deviated inwards okay this is what happened because this is the Apex and the rays are bent where they bent towards the base of the prism right so a convex lens is acting as a prism like this that means the bases are towards each other and therefore when the Rays pass like this they're going to bend towards the base of the prism and therefore in a convex lens the Rays will bend towards the axial Zone whereas in case of a concave lens opposite happens and the Rays of Light which are falling on the peripheral portion of the lens will actually be bending away from the axis right so this is known as the Prismatic effect and this Prismatic effect is going to be much larger at the end of the lens and it basically increases towards the periphery of the lens and it is seen in case of earthia and also seen in case of the high myopic glasses right now that we understand what is meant by Prismatic effect we can understand what is meant by skotoma and why do we see a ring scotoma in case of AIA now the light which is falling at the edge of the lens will definitely be bending because of Prismatic effect and sometimes also because of the sperical liberations right now because of that what happens is that not all the light rays are going to enter inside the puper the light rays which are coming from the parial Rays that means from the periphery these are going to be bent and um ahead of the focal lens and therefore they're not even going to enter the pupil and therefore they'll not even be seen so this is going to lead to the development of scotoma and as the lens as the edge of the lens is present all around so what I mean to say this is just a cross-section that I drawn but actually if this is the eye the lens is also going to be circular like this and you actually have the lens Edge all throughout 360° and therefore you're also going to see a scotoma at 360° and this is called as the ring skotoma right so the ring skotoma is actually known as the rowing ring scotoma it is called rowing because the scotoma also keeps on shifting its position as the patient moves his eyeballs from left to right or up and down okay so it moves with the eyeball and therefore it is called as a robing ring Kota the term robing ring Kota was suggested by Welsh and it occur because of the Prismatic effect occurring at the periphery of the strong L giving to giving rise to this ring of blindness at the central field okay this Kota is usually about 15° okay so the width of this is about 15° and it is US situated about 55 to 65° from the central fixation now let us try to understand another phenomen which is called as the Jack in the Box phenomena the diagram over here depicts the ring Scoma so as you can see this is a patient who is looking straight ahead wearing an aake glasses and at the edge of the lens what is happening is because of the Prismatic effect there is going to be a creation of a ring skotoma right now if an object of Interest appears somewhere some somewhere here what is going to happen the patient is going to try to look towards this object so the patient is going to rotate the eyeball like this and as the patient rotates his eyeball towards the object what is going to happen the the scotoma is also going to shift in its position because of the shift in the position of the lens and in this case the Scoma is actually going to move in the opposite direction that is from A to B position and as that happens what is seen is now the object is now falling in the area of the skotoma and therefore the object is going to disappear again so this diagram will make it even more clearer so you can see in the first picture this is a patient who's looking straight ahead at 0° okay and this is the convex lens which is there and you have a scotoma at say about 50° to 65° right now if an object appears over here the patient would want to look at the object and therefore the patient will shift his position of gaze in by rotating the eyeball towards that heart and as the patient actually shifts his eyeball what is going to happen the lens is also going to shift and also the position of skotoma is going to shift the scotoma is going to move in the opposite direction and now it'll be present uh in the area of 30° to 45° and as that happens what is happening the object is now falling within the area of scotoma and therefore again the patient will not be able to appreciate the object right so this is what is actually happening when the eye basically is moving the blindness ring along with it that is the roing the scotoma is also moving along with the eye so that the object object is appearing and disappearing just like a Jack In The Box and therefore this is known as the Jack in the Box phenomena okay so it's very important to know that this problem arises only in the intermediate distances and for example when the when the patient is actually in a room the area of the Clear Vision will actually be surrounded by a ring of fog and whenever the object basically Falls in that region of scotoma the patient would not be able to appreciate those objects right so after understanding the Jack in the Box phenomena another thing another problem with AIA is the heaviness of the aake glasses so these are high powered glasses high powered convex glasses they're quite heavy and the problem is that they keep on slipping down the patient's nose and therefore they also keep on altering the effective power of the lens right and when we talk about the convex lenses moving a convex glass away from the nose actually increases the power of the lens and so by some patients actually use this to their advantage to gain some sort of artificial accommodation that means for example if this is a patient okay and the patient is actually wearing about plus 10 diopters and after some time if the patient is having some difficulty in near Vision such patients actually learn on their own to move their glasses down to away from their eyes or maybe slip their glasses onto their noses now as that happens the glass basically moves away from the eyeball and as the glass basically moves away from the eyeball what is seen is that the power of the lens will basically increase and this increased power of the lens will basically help in gaining some amount of accommodation in these patients right because if you would remember from the video on accommodation accommodation is nothing but it is basically an ability of the lens to gain some extra amount of power right to gain some extra power and by moving the convex lenses away from the eyeballs the patient actually is gaining that artificial accommodation now the question is is there a way we can actually decrease the weight of the Earth glasses yes we can we can either use Plastics as the materials for the frames but the problem is that they easily scratch another means of reducing the lens weight and thickness is by the use of lenticular form glasses so the lenticular form glasses basically have this central portion in which the power resides and the peripheral portion of the lens basically is a plain carrier lens um which does not really Harbor the power of the lens okay so it just acts as a carrier the problem with the lenticular lens is that since the power of the lens is situated only in the central part of the lenticular lens the field of vision is also reduced in case of a lentic lens okay so when I talk about the glass selection first very important thing is that you have to choose your frame very carefully the frame should not be too thick the frame should not be too heavy also you should try to not choose a very large frame so whenever a large frame is used and more so you giving the patient a very thick lens the lens is going to become more thicker in the center and such a large frame will become more heavier and possibly quite harder also for the patient to tolerate okay at the same time you have to also avoid rimless or semi- rimless frames because then there will be no support to that thick lens and these are actually your lenticular glasses so lenticular glasses if you see the power is situated only in the center part of the lens and the peripheral part of the lens basically act as a carrier lens okay so now since they appear like the fried eggs they also known as the fried eggs lenses and the problem with the lenticular glasses although they decrease the heaviness of the glasses they make it lighter the problem is that they have a very reduced field of we uh field of view now whenever you are prescribing these patients it's very important to remember that there's actually an absence of crystalline lens and since our crystalline lens was associated with absorption of the U race these patients are more prone to UV damage and therefore UV protection is must in these spectacles now since we are dealing with high power fake Ki glasses the vertex distance and even the pantoscopic til any misalignment or even minor variation in these two parameters is going to actually cause um inaccurate refractive correction in these problems right so over here you can see that the pan top slit the pantoscopic Tilt angle is nothing but the tilting of the lens basically with respect to the horizontal axis this tilt is usually done to make the lenses more comfortable for the patient and uh the distance between the front of the corne and the back surface of the lens is know as the vertex distance so all those all these two parameters basically the panoscopic Tilt angle and the back vertex distance are going to control how the Earth glasses work and even a minor variation and some um in the parameters is going to lead to a lot of problems in Earth aake patients so with this we actually come to the end of the video I hope it was useful I hope you could actually understand the problems which an earth patient face with these spectacles obviously the contact lenses and iols have their advantages and disadvantages we shall be discussing them in another video till then thank you and have a nice day