hello and welcome to insight ophthalmology this is dr amrit and today we are talking about the visual pathway lesions there are three important points that we should know with respect to the visual path relations the first point is that the visual fields and the retina have inverted and reversed relationship so what does it mean is the superior part of the visual field will be represented in the inferior part of the retina whereas the inferior part of the visual field will be represented in the superior part of the retina similarly the temporal part of the visual field will be represented in the nasal part of the retina and the nasal part of the temporal field will be represented in the temporal part of the retina this is what is meant by the reversed and the inverted relationship between the visual fields and the retina the second thing that you should know is what is the meaning of homonymous okay because this term is used when describing the visual field autonomous means that both the eyes are involved with the same laterality that means if you consider this to be the visual fields of right and left eye in both the eyes either the right side of the visual field is affected or it is the left side of the visual field which is affected okay so this is what is meant by homonymous that means the same side is involved in both the eyes third point is that what is meant by congruence field effect or the incongruence field effects okay so the congruence field effect means that the visual field effect is alike in both the eyes an incongruous field effect means that if the field effect is different in two eyes that means if we consider this to be the visual field of two eyes if both the eyes have similar type of involvement in both the fields of vision then it is called congruence suppose this is right eye this is left eye and both of them have similar halves which are affected so that means it is congruence however if the defect looks somewhat like this only the upper half is involved in one eye and the other eye the entire half is involved so these two defects cannot be superimposed on each other because they are not alike they are different so such a field effect is called incongruence field effect now let us discuss these lesions of the visual pathway at different levels so first starting with the optic nerve whenever an optic nerve of one side is involved either because of the optic atrophy or because of traumatic evaluation of the optic nerve or transition of the optic nerve it will lead to almost a complete blindness of the same side okay so it will lead to complete blindness of the same side so as shown in the diagram and represented by number one if the left side optic nerve is affected there will be a complete blindness on the left hand side along with that the ipsilateral direct light reflex will be gone and counter lateral consensual light reflex will be gone so what do i mean to say is that if you shine a torch in the left eye this side the left sided pupil will not react to light along with that the right side pupil also will not react to the light which is shown in the left eye but if you show the light in the right eye then it will still react because the right sided optic nerve is normal right so you know so it will revise the right the left sided optic nerve lesion will lead to left side total blindness along with that ipsilateral light reflects affection and contralateral consensual light reflex will be affected now let us look at a lesion of the optic nerve very close to the chiasma or we can say almost at the point where the chiasm is starting represented by number 2 in this diagram this diagram over here represents the crossing of various fibers in the chiasm so if we look at this over here is the optic nerve and it is divided into four five quadrants that is the upper temporal quadrant the lower temporal quadrant the upper nasal quadrant and the lower nasal quadrant and in the center we have the macular fibers running through so as we already know that in the optic chiasm it is only the nasal fibers which are going to cross and the temporal fibers do not cross and they are going to pass just like that so the same thing can be seen that the lower temporal fibers are passing uncrossed in the chiasm and the upper temporal fibers are also passing uncrossed in the optic chiasm and even the macular fibers are going uncrossed in the optic chiasm however what we should know and we should notice here it is that the lower temp the lower nasal fibers that is represented by here so this is the lower quadrant and it is present in the nasal side so it is these fibers the lower nasal fibers they are going to come in a way that they before crossing they will form a genu or they will actually take a bend in the contralateral optic nerve and then they are going to cross to the opposite side now because they are forming a loop towards the opposite side before they actually cross in the optic chiasm this is called the willy brand's knee okay so this is called the will brands knee so whenever there is a affection of the optic nerve near the optic chasm so what might happen is that this willy brand knee also might get affected okay corresponding defect to this willie brand's knee now what is that defect to the calibrant's knee i already told you that the will brand's knee is carrying the fibers from the lower nasal part of the optic nerve and i already told you that the representation is reversed and inverted that means the lower nasal fibers are carrying information from the upper fields and the nasal fibers are carrying information from the temporal field and moreover because crossing is present they will this fibers will belong to the opposite side so counter lateral upper temporal field effect will be seen that means that if this will brings knee is affected in the left side we will see an upper temporal defect like this okay we will see an upper temporal defect like this okay but up along with that along with the affection of the vibrancy even the optic nerve is getting affected so what happens when the optic nerve is getting affected we will see total blindness on the side where the optic nerve is affected right so such a kind of field effect where we have ha almost entire entire field of defect present on one side and on the counter lateral side we have only the upper temporal field effect present this defect is called the junctional scatoma okay this field effect is called the junctional scatoma so let me explain this to you in this diagram so in this diagram also the lesion represented by number two we can see that the left sided optic nerve is totally affected so on the left side we will see a total field effect however the in the right side we can see that the lower nasal fibers are coming and they are crossing and forming the vilbrin's knee in the optic chiasm and they might also get affected when the lesions are affecting the optic nerve near to the optic chasm so therefore we will see a defect on the counter lateral upper temporal field okay like this and this is called junctional scotoma it's called junctional skatoma and it represent a lesion present in the optic nerve near to the chiasm the third lesion that is the lesion of the optic chiasm okay and let us talk about the sagittal uh lesions which will affect the sagittal the optic chiasm right represented by number three over here now as we already know that in the optic chiasm it is the nasal fibers which are going to cross and the temporal fibers do not cross and just pass uncross so whenever there is a lesion affecting the central part of the optic chiasm it will affect the nasal fibers which are crossing so that will affect which part of the field it will affect the temporal parts of our field that means the temporal part of the left eye and the temporal part of the right eye will be affected now since both the temporal parts are affected such a lesion is called bi-temporal and because the halves are affected it is called hemi and anopia means we can't see so a lesion of the optic chiasm which affects the central optic chasm will lead to bi-temporal hemianopia along with that there will also be bi-temporal hemianopic paralysis of the pupillary response that means the pupillary response on the temporal part of the pupil will be absent along with that we will see optic atrophy also now let me tell you one more point is that the optic nerve lesion the optic chiasm lesion and also the optic tract lesion and sometimes the lateral geniculate body lesion can lead to optic atrophy also however when the lesions are behind the lateral geniculate body that means in the part of visual pathway which is present behind the little geniculate body the op the optic radiations and the visual cortex usually the optic atrophy is absent so the optic atrophy is present only in the lesions which are which are present in front of the lateral geniculate body the same thing applies to the pupillary reflexes also the pupillary reflex will be affected up to the level of the optic tract because i already told you in the previous video that the pupillary reflex fibers will come out of the optic tract and go to the superior colliculus right so since they have already crossed before the level of the lateral geniculate body anything which will affect the visual pathway after the level of optic uh tractor after the level of lateral geniculate body will not affect the pupillary reflexes right now uh what are the lesions which can affect the optic chiasm from the central part so the lesions are the supracellular supracellular aneurysms tumors of the pituitary craniopharyngioma supracellular melange third ventricular dilatation due to hydrocephalus now in order to remember this you should actually remember the relations of the optic chiasm which i explained to you in the previous video so what i told you was that there is a depression in the sphenoid bone in the base of the skull and that depression is actually called a cella turcica so in that cella tursica what we have is our pituitary gland above the level of the pituitary gland is our optic chiasm right and above the optic chasm we have our third ventricle right and on the either side of the optic chiasm what are we having is we have the blood vessels so any of these structures when they get affected so either if it's a craniopharyngioma which is coming from the third ventricle or it is the dilatation of the third ventricle they will come and press upon the superior aspect of the optic chiasm similarly next we come to the optic chasm lesion involving the lateral part of the optic chasm which is represented by number four over here so it is the lateral affection know that the lateral part of the optic chasm will be carrying the temporal fibers which are not getting cross and the central part is carrying the nasal fibers which will get cross so since the lateral affection will cause will affect these temporal fibers the field of vision which will be affected will be nasal so it will lead to nasal field effects on both the eyes and because it affects the nasal fields of both the eyes it is called by nasal hemi and opia along with the binisal hemianopia we will also have by nasal hemianopic paralysis of the pupillary reflex that means only the nasal part of the pupils will not react and along with that we will have optic atrophy now let me tell you a little bit about the optic atrophy which is present in this uh opticalism lesion is that they will not affect the entire optic nerve since only a part of the fibers are getting involved that means either the temporal or the nasal the optic atrophy will also be partial optic atrophy right so what are the lesions which can cause this optic chiasm affection from the lateral side so it could be either the distension of the lateral ventricle so the lateral ventricle can the third ventricle sorry can get distended and then can extend downwards from the either side and affect the optic chiasm laterally or it could be say an etheroma of the carotid which are present on the either side and they can go and press on the lateral parts of the optic chiasm next we will discuss about the optic tract lesions which is represented by the number 5 in this diagram as we know that the optic tract will carry the uncross temporal fibers and the cross retinal fibers the uncrossed temporal fibers will lead to a nasal defect in the same eye and the cross nasal fibers will cause a temporal defect in the opposite eye now since the same side of both the eyes is getting affected it is called a homonymous type of hemianopia uh and one more thing that we should know is that the optic tract is the first site in the visual pathway from which the homonymous representation will actually start and if we see from the optic tract onwards that is the optic tract lateral geniculate body optic radiations the homonymous defects will start from the optic tract this homonymous hemianopia is actually incongruous and i already told you in the beginning of the video that incongruous means when the visual field effects are not similar now you might ask that these two visual defects in the two eye they are actually looking similar but in reality that is not true in reality we do not have these perfect circles the visual fields look somewhat like this in both the eyes that is it is not exactly oval it is somewhat triangular because the nasal part of the fields are smaller compared to the temporal part of the field so if in one eye the nasal part of the field is affected in this manner and in the other eye the temporal part of the field is affected these two field effects will not look similar as is seen in this easy representation right it is diff it is different so because they are not similar and they are different this type of homonymous hemianopia which is seen in the lesions of the optic tract it is called incongruence homonymous hemianopia so that is very important to remember the second point is what we will see in in terms of pupillary response is a hemianopic pupillary response what does it mean is that whenever the light is shown in the part of the pupil which is affected we will not have any light response we will not have any constriction when we show when we show light in the part of the pupil which is affected whereas when we put light in the non-blind part of the pupil the other half of the pupil there will be pupillary reaction now because such a hemianopic pupillary response is present it is also called vernicase pupil okay so wernicke's pupil is a feature of lesion of the optic tract now along with that there will also be optic atrophy which can be seen and sometimes because of the close association with the midbrain which is present somewhere here because of the close association with the midbrain the third nerve palsy can also be present and ipsilateral hemiplegia also might be present because of the close relationship with the third of nucleus and the corticospinal tract now what are the lesions which can cause this optic tract lesions so these are number one they could be the tumors of the thalamus now in my previous video i told you how the optic tract is very closely present in relation to the thalamus or it could be the posterior cerebral artery aneurysms or the superior cerebellar artery aneurysms we know that the posterior cerebellar the cerebral artery will be coming like this and the the superior cerebellar arteries will be going somewhere like this so the aneurysms of these arteries can also affect the optic tract along with that infections like syphilis and tuberculosis they can also have a role in optic tract lesions next in line are the lesions of the lateral geniculate body which is represented by number six in this diagram so lateral cynically body are the endings of the optic tract and the arrangement of the fibers is also the same so they are also carrying the same uncrossed fibers from the temporal retina and causing a nasal effect in one eye the lateral eye and the cross fibers which are nasal fibers and which will cause a temporal field effect in the opposite eye so they since they are also affecting the same side of the visual field in both the eyes they will also cause homonymous hemianopia of the incongruous variety but there is a difference between the visual field effect of the optic tract and the visual field effect of the lateral geniculate body although both of them are causing homonymous hemianopia and the difference is that in the lateral geniculate body there will be sparing of the pupillary reflex and why is it so because as we have already seen that the pupillary fibers will come out of the optic tract and will go to the superior colliculus present in the midbrain much before the lateral geniculate body so the lateral geniculate body does not receive any pupillary fibers because of which there will not be any affection to the pupillary reflex in case of lateral geniculate body lesions also there might be a partial optic atrophy present in case of lateral geniculate body now there are certain advanced visual field effects which can be of congress variety also like the sector nopia and the quadruple sector nopia which can be seen in lesions of the lateral geniculate body however they are quite advanced to explain and they're outside the scope of this video however if you want me to make a video on that kindly comment in the comments section we discussing the lesions of the optic radiations their features will vary depending upon the site of lesion for the purpose of simplicity let us talk about number seven first that means the fibers which are arching along the lateral ventricle inferior lobe so as i explained it to you in my previous video on the anatomy of visual pathway that few fibers of the optic radiation will arch along the later ventricle and travel in the inferior lobe of the brain that is the temporal lobe since these fibers are present inferiorly they will carry the information from the counter lateral superior part of the visual field and affection of this part of the optic radiation which is also called the mayor's loop will lead to counter lateral defect these males loop or the inferior fibers are carrying information from the superior the part of the retina they will cause a superior defect and since the left sided uh optic radiation is affected it will cause a defect on the right side so it will lead to right sided superior 1 4 defect and this is called the pie in the sky defect this spy in the sky defect is seen whenever there is a lesion in the mayor's loop that is nothing but the inferior fibers of the optic radiation traveling in the temporal lobe there were also fibers which are going to pass straight away traveling superiorly into the parietal lobe and going up to the calcane fissure so these superior fibers were carrying information from the lower part of the visual field and they are called the bomb's loop and affection of this bombs loop will cause an inferior field effect and because there is a left optic radiation is affected the right visual field of the inferior part will be affected okay and this defect is called the pi on the floor so pi on the floor is seen whenever the bombs loop is affected or the superior part of the optic radiation is affected which is shown in this number eight now what will happen when the entire optic radiation will be affected then these both the pipes will fuse with each other and we will see something which is very similar to what is seen in the lateral geniculate body lesion or in the optic tract region that is we will see incongruous homonymous hemianopia of the counterlateral side will the pupillary reflex will be affected the answer is no since the pupillary fibers have already separated right from the optic tract anything behind that will not affect the pupillary fibers and there will be no rapd there will be no effect on the pupillary reflex is now about the visual cortex lesions which are represented by the number 10 and 11. we must have a brief knowledge about the blood supply of the visual cortex so the visual cortex is getting its blood supply mainly from the posterior cerebral artery and from the middle cerebral artery so both the carotid system and the vertebral system is actually contributing to the blood supply of the visual cortex moreover the tip of the tempo the tip of the occipital lobe or the part where actually the visual cortex is present the maximum number of fibers are representing the macula in other words we can say that the macular fibers have the maximum area occupied in the occipital lobe or in the visual cortex and as can be seen clearly in this diagram that the posterior cerebral artery and the middle cerebral artery both are supplying the macular area our posterior cerebral artery supplies most of the anterior part of the visual cortex so whenever there is a stroke involving the posterior cerebral artery almost the entire visual cortex will be affected except the macular part so in such a case we will see a homonymous hemianopia but there will be sparing of the macula right so we will see a counter lateral homonymous hemianopia sparing the macula now why is the macular spare the macula is paired because the macular is also getting its blood supply from the middle cerebral artery and it is perfused even in case of a posterior cerebral artery stroke because of the bilateral because of the two blood supply dual blood supply present to the macular area there will be macular sparing okay now the next question is is this defect congress or incongruous so when we reach the visual cortex the defect will become congruence okay the defects in both the eyes will become more alike and why is it so it is it happens because in the optic radiation the fibers are actually radiating and after radiating as they reach the visual cortex they will converge to a particular point so as the fibers come together they're coming more together and they are becoming more alike the visual field is becoming more alike however in the case of optic radiation because the fibers were more apart the visual field effect was also incongruous the defect that we see in posterior cerebral artery stroke is congress homonymous hemianopia which is sparing the macula there is an injury which is affecting directly the tip of the occipital lobe as such in case of a head injury which is affecting the tip of the occipital lobe where the macular fibers are present or sometimes a gunshot injury also okay where the bullet directly goes and hits the tip of the occipital cortex so over there the maculo fibers will get affected and in such a condition what we are going to get is we will get congress homonymous macular defects on the counter lateral side okay as shown in this picture finally we will not see any optic atrophy or any problems in the pupillary light reflex in the visual cortex lesions so this was about the visual pathway lesions any doubts you can ask in the comment section thank you and have a nice day