Hi everybody and welcome to this webinar on the equine Fundus examination. Hopefully by the end of this you'll be feeling a lot more confident in attempting to examine the equine fundus and hopefully also interpreting what you see. So this is the basic outline of this webinar.
Firstly, unfortunately we're going to briefly go over some anatomy of the fundus, which will hopefully help you to better understand what you're seeing. Then we'll quickly go over when you would or should be performing an examination of the fundus. And then for the bulk of the webinar, I'll talk about the different techniques that you can use to examine the fundus and hopefully give you a few tips to make it easier.
And at the end, I'll spend some time going over some photographs of what a normal equine fundus looks like, and then also some relatively common abnormalities, and incidental findings that you'll likely see in time if you're performing fundic exams relatively regularly. But this isn't a webinar on interpreting, interpreting your findings, that's a bit beyond the scope of this. Maybe we can do that another time, but yeah, the focus is on the process and technique of viewing the fundus or what we call fundoscopy or funduscopy, depending on who you ask.
As, just a brief disclaimer, there's a fair few images in this presentation which I've kind of pilfered from various sources, because there's lots of beautiful pictures out there, but I've referenced them on the slides. Most of them are from Brian Giler's equine Ophthalmology textbook, which is a great book, to get if you're interested in equine eyes. So let's get started.
So in talking about the fundus of the eye, it basically means the back of it. So, the back of it over here, so the optic nerve head, the retina, and something called the choroid amongst some other things. And all of these things are in what we call the posterior segment of the eye, which is, divided up here compared to the anterior segment of the eye.
And some people think that the retina and fundus are the same thing, but they're not. So the retina is part of the fundus. But there are many other components to the fundus as well.
And horses have what's called a poor angiotic fundus where there are only small little vessels visible around the optic disc, which is in stark contrast to dogs and cats, for example, if you've ever looked at their fundus during vet school, for example, they've got these really big beautiful branching vessels in the fundus. And it's important to remember this because if you're expecting to see lots of blood vessels, when you look at the back of a horse's eye, you'll you'll be disappointed, but that's perfectly normal in the horse. There is an exception to that rule, which I'll discuss towards the end of the presentation.
Most horses also have what's called a tapetum, which is what gives them that kind of cat's eye reflection, in the dark when you shine a light at them in the field, for example, and it allows the eye to maximise the amount of light available in low lighting conditions. So it's really useful for animals that, you know, are active or feeding in the evenings or at nighttime. Although technically you can see straight through to the fundus, if you shine a light at the eye and look through the pupil, you won't be able to appreciate any detail.
Oh sorry, Dawn, I'm just, I think I've skipped a slide there. Don't worry, I'll just make a note of the time. Yeah, perfect.
So we'll go on from The discussion about animals being active in the evening time. The pups. Taking a closer look at the layers of the fundus.
Because this is really important to get your head around. So this is a schematic, which is not to scale, by the way, but it represents a cross section through the fundus. So imagine that this side, here, where this, lightning bolt is, is innermost.
So that's kind of where the vitreous of the eye is, so the jelly of the eye. And then as we move this way, away from the lightning bolt, we're moving away from the centre of the eye. So, once the light hits the eye, after it's kind of travelled through the pupil and through the vitreous, the first layer that it will hit is the retina.
And sometimes we call it the neurosensory retina. And this is a really complex structure with many layers, within it itself, which you can appreciate here, from this, histology section here. And you guys don't need to know about all of these layers, but just to appreciate how organised and complex it is, so that area in the black box at the top.
There is the retina. And the second layer of the retina contains what are called, the retinal ganglion cells, which form the optic nerve, which is an extension of the brain, and then the photoreceptors or our rods and cones, which are right down the bottom here. Are kind of at the the base of the retina, so the light has to travel all the way through the other layers to get to the rods and cones.
So the next layer of our fundus after the retina is something called the retinal pigment epithelium, which is often abbreviated to RPE. And this is a single layer of cells that has a really important role in maintaining the health of the rods and cones of our photoreceptors. And the name is a little bit misleading.
So, because it's called the, even though it's called the retinal pigment epithelium, this layer can be pigmented or non-pigmented. And that's really important because if it's pigmented, it will hide the layer underneath, which is that bright reflective tapetum, which is the green layer here on the schematic on the left. Whereas if the RPE isn't pigmented, you'll be able to see that underlying tapetum and therefore the torpeal reflection.
And there's often a combination of pigmented and non-pigmented RPE in an eye. And then under the reflective tapetum, we have what's called a choroid, which is a really vascular layer with lots of blood vessels that bring oxygen and nutrients to the eye. And then finally we have the sclera, which is the white shiny, fibrous tunic of the eye, which you can actually just see with the naked eye, it's the the white of your eye basically.
Now this photo on the right is, it's actually a dog, or a cat, I'm not sure which, but I think it's really helpful to understand these layers a bit better. So this creamy pinky tissue down there where the arrows pointing to, that's our neurosensory retina, and it's completely detached, so it's come away from the underlying RPE and it's just hanging on at the optic nerve there. So imagine that normally this layer, it's kind of thin, slimy, it's pretty transparent when it's spread out.
That's usually sat against the whole of the back of the eye. So with it detached, it's a bit easier here to see this lovely bright tapetum. So over this whole area where you can see the blue and black, there is RPE there.
But it's only pigmented in the darker bits. And in this bit where you can see the tapetum underneath, the RPE is still there. It just doesn't have any dark pigment in it.
So this area where you can see the tapetum is called the topetal fundus, and then this darker bit here and here, where we can't see the tapetum is called the non-oppeal fundus. So when are you going to need to or want to look at the back of a horse's eye? Obviously, if an owner or trainer or whoever has noticed that a horse has kind of obvious visual deficits, for example, if they say it's bumping into things or tripping over things, then it goes without saying that you would want to look at the eye, including the fundus.
But that's actually pretty uncommon. More likely horses will present being a bit spookier than normal or losing a bit of confidence, or just general poor performance, and all of those things should warrant the look at the fundus unless there's something obviously really obvious causing the behaviour like, you know, a marked lameness. And then obviously pre-purchase exams require a funic exam as part of the general exam of the horse.
So at some point in your career as an ambulatory first opinion vet, you're going to have to look at a horse's fundus and see if you think it's normal or not. So how do we do it? There are 2 commonly used methods, which I'll go into in more detail on the upcoming slides, but in short, they're called direct ophthalmoscopy and indirect ophthalmoscopy.
With either of these techniques, it's going to be a lot easier if you have a dark room or a dark stable to perform it in. And obviously that's not always possible, but you will miss things if you try and do this in a bright stable or even worse out in the field during the day. So just warn owners that if that's kind of what they're expecting you to do, there is a chance that you will miss things.
When We're performing eye exams as ophthalmologists, we would always apply, what's called trapikamide eye drops to the eye to dilate the pupil. Because without doing that, you're going to make your life much more difficult than it has to be because the hole you're going to be looking through, so the pupil will be so much smaller if you don't apply what's called a midriatic agent. So yeah, if possible, do this.
But it's just important to bear in mind that it can take, at least 30 minutes usually to really start working. So you've got to kind of factor that into your scheduling. When I was an ambulatory vet, if I'd kind of seen a horse that, you know, for whatever reason, I came to the conclusion needed a fundus exam, I would actually usually schedule to go back for a follow-up visit.
And leave a little vial of trippicamide comes in like a little minims pipette. I'd leave one with the owner and ask them to apply it a couple of hours before I was due on the yard. So once I arrived, pupils are fully dilated, and you can just get straight on in there and it massively speeds things up.
If you do use it though, just worth mentioning to the owner that it can last up to 12 hours. So it's a really bright sunny day outside. The horse won't be able to constrict their pupil to block the light out.
So tell them either to keep the horse, stabled until the next day or put them out with a decent kind of fly mask with some UV protection on. I also tend to advise that they don't ride them until it's worn off as well because it can affect the eye's ability to focus and when humans get it, you, you're not supposed to drive. Sedation Again, yeah, I'm all for making things as easy and stress free and safe as possible for me when examining horses.
So, yeah, I would tend to sedate horses when performing an eye exam because you're getting really close up to their heads and they've got really big heavy heads and it. Just takes one noise, you know, somewhere behind them to just swing around, smack you in the face, you know, knock your ophthalmoscope out of your hands. So unless the horse or pony is, you know, an absolute angel.
Or, you know, there's some contraindication to sedating them, then yeah, I would always sedate it'll be quicker and you'll do a better job. I just tend to use ditomidine and brophenol, . But if, yeah, if I need to top them up, I just tend to go a little bit easy on the buorphannol the second time because with kind of multiple top-ups of buorphenol, you can start to get the, the torque twitches, as we call them, which, often doesn't really matter, but with the head, it does, because you really need the head to be still.
If you can't sedate the horse for whatever reason, and you find that it's just clamping its eyelids shut when you're shining your light at it, I would probably advise performing just a quick eiculo palpibral nerve block, which will just paralyse those eyelids and mean that you're not battling against those really strong eyelid muscles. And Yeah, you wouldn't usually need to do that if the horse was sedated cause their eyelids go a bit droopy anyway, but yeah, if they're not sedated, it takes 5 minutes and it's really worth doing. But yeah, the nerve blocks are a bit beyond the scope of this webinar, but I think I think I covered them in my last one if you wanted to brush up on the technique.
So let's go through these two different types of ophthalmoscopy to look at the fundus, and we'll start with direct ophthalmoscopy. Which is what most of you are probably most familiar with, because this tends to be what you're taught in vet school, and it's often all you have available to you as an ambulatory vet as well. So this here is your direct ophthalmoscope.
It's essentially a light source and a magnification in one. And the battery is in the handle. And the more modern ones, you can just kind of plug into a USB, to charge them up rather than having to remove the batteries.
And the head you can detach and change to other attachments if you want to as well. And the side, on the left goes to the horse, and the one on the right goes to your face. The decent ones often have that this little brow rest, on them, on the, the practitioner side.
So it makes it, a bit obvious which side you, you have to look through. To turn it on, you have to press this little button which is green in this one. And then you twist that little black bit next to it, which is called the rear stat, and that will turn it on but also decrease or increase the brightness of your light.
So you think that you want the brightest light setting, when looking at the back of a horse's eye. But actually, if you haven't used tropicaide to dilate the pupil, if you shine a really bright light into the horse's eye, you're just going to induce a pupillary light reflex and the pupil's just gonna close down and make your job a lot harder. So if you just dim the light down a little bit from the maximum, you can just reduce the extent of that PLR induced and make it a little bit easier by by keeping the people slightly more enlarged.
Taking a closer look, at all of these dials, these will vary, in appearance depending on the make and model, but these are kind of your standard ones. But it's important to remember that most of these ophthalmoscopes are actually designed for humans and not horses. So sometimes there are settings on there which actually you, you just won't use.
It's, so the patient side, of this one is on the left, and that's the side that usually has most of the dials on it. This one's a really basic ophthalmoscope. It's got pretty limited options, but it's perfectly fine for the purpose of looking at the fundus in a horse in a, in a first opinion setting.
So this knob here is called the philtre dial or the philtre switch. And this changes the appearance of the light that's being admitted, so it can turn it green, white, or it can add crosshairs, for example, the crosshairs is more of a a human thing. So almost ignore the crosshairs and the green, what you want this on is this setting.
So this is a, a white circle. This down here is called the aperture dial, and this will alter the size, and sometimes the shape of the circle of light that's emitted. So in humans, obviously they've got really small pupils, so you can set this to, emit a really small white circle.
So that would reduce any light scatter and reflection. So you kind of want the smallest circle you can get away with, but in horses, we're going to want the big one. Some models will also have a blue circle setting on this dial here for your fluoresce staining and slip beam features.
So yeah, for the horse funding exam, we want the white circle and we want to use this one to change it to the biggest setting. Then we've got these dials on the side, both here and here. And these are called the diopter dials.
So rotating those will change what's called the diopter, and the value that you're changing it to will be displayed in this little window on your side down here. So if you don't need to wear glasses, or if you've got contact lenses in that are correcting your refractive error, the funders should be in focus at around 0 diopters. So this should be 0.
There should be a little number in here that says 0. If it doesn't say 0, you need to change it by spinning these little knobs on the side here. However, if you do wear glasses and you take them off to use the direct ophthalmoscope, you then have to change that little number down there to your prescription, with the green numbers being plus and the red numbers being minus.
So for example, if you're short sighted. Say your prescription is about -2, you need to make sure that the number in this little window is red, which is -2. If it was + 2, it would be green 2.
And also if you find a lesion in the fundus and it's out of focus compared to the surrounding area when you're on zero D, you can scroll through those dioptic powers and see at which point the lesion becomes in focus. So if the lesion becomes more in focus when you add on positive, so green diopter settings, that tells us that the lesion is raised, so it's coming towards you, and vice versa. And that's quite cool, but it takes a bit of practise.
This is just showing a different type of of direct ophthalmoscope, a relatively common hiney one. And this is the patient side that you can see here, and as you can see, it's a little bit different. So the philtre switch has a few more options on it.
So it's got white, green, the green's also called red free. And then above that, you have all these other options, here. So there's different aperture sizes, there's the blue light, there's a slip beam, there's crosshairs, there's a half circle.
Again, lots of things that we don't really need. As with the other one, you want this on the large white circle. So at the minute here, it's on the medium one, but we'd want it on this one here.
And then once you've got all of these knobs and dials set correctly, you can start examining the eye. How do we do that? So, you might read that.
If you're examining the right eye of a horse, you should use your right eye and vice versa. But that's not really the case. That's a pretty, kind of purist way, I guess, of seeing it.
But just do what feels comfortable. So I'm very, left eye dominant, so I'll tend to use my left eye for both sides, and that's just the way that I find I get the best images. So, with direct ophthalmoscopy, you want to start at arm's length away from the horse.
And place your non-dominant hand on the horse's head collar, so that you know where you are in space once you're kind of looking through your ophthalmoscope. Your dominant hand is holding the ophthalmoscope. You want to look through the viewing window and shine the light straight at the horse's eye until you see that bright petal reflex.
And then once you have that topial reflex in your view, looking down the ophthalmoscope. You basically use that as a guide and slowly start moving in closer to the horse, while still always looking through the ophthalmoscope of the torpial reflection. And when you're about as close as I am in this picture, possibly actually even a little bit closer, you'll hopefully start to see things come into focus.
Possibly, for example, some colours, an edge of an optic nerve head, etc. If you ever lose that torpedo reflex whilst you're moving towards the horse, reset, so come back to arm's length and start again. So once you're up close and you think you can start to see things, you need to start looking around.
So if you just stand there and look straight ahead, you won't see the sides. So you've really got to get in there and look right, left, down, up and move quite a lot more than you probably think you have to. And I'll say that it's not easy, especially if you're not doing this regularly, so don't expect to see, you know, a big detailed perfect view of the Funddus because you won't.
This will give you a really highly magnified view of the Funddus with this technique. So for example, you might look into the fundus and actually the optic nerve head almost completely fills your field of view, depending on how close you are. So you might not even know you're looking at the optic nerve head if you're unfamiliar with with what you're seeing.
But the good thing about direct ophthalmoscopy is the image you're seeing will be the correct way up. And also what you see will be relatively high in detail. So this is great for assessing kind of small blood vessels around the optic nerve head or any small lesions you might find.
But yeah, the downsides are you've got to be really close to the horse's head eventually. It also doesn't give you any form of what we call stereopsis, so depth perception. And it's also, yeah, a really small field of view, so you've got to go on the hunt once you've got something in focus to avoid missing things.
Moving on to indirect ophthalmoscopy, for this you need two pieces of equipment rather than one, the first being a light source, and the second being a lens, also called a condensing lens. Now your light source can be something as simple as a pen torch, your direct ophthalmoscope, or if you have this extra attachment here. You can actually put that on the handle of your direct ophthalmoscope and that's called a fin off Trailluminator.
And it gives you a really nice focal beam of light, so it lets, so yeah, just you get less scattering and unwanted reflections and things like that. And if you're using one of these 3 things over in the top left corner there, that's going to be called monocular indirect ophthalmoscopy. However, the best way to perform this is to use what we call an indirect ophthalmoscopy headset, which is this thing in the middle here.
And using this headset is, I mean it's basically a light source that's mounted to your head. So it frees up one hand to open the horse's eyelids. And because the headset is binocular, indirect, you'll also get some depth perception with this technique, which makes appreciating any raised or depressed lesions a lot easier.
But these aren't cheap, you know, they're a few 1000 pounds, but they're totally worth it if you're doing a lot of ophthalmology. Like your direct ophthalmoscope, there's a few different philtres and settings on the on the headset, but again, what we want is just a plain white circle. There's a variety of handheld condensing lenses available, and their job is to basically magnify the image.
So you can get various sizes with different diopter ratings, but basically, the smaller the lens diopter rating, the greater the magnification you'll get. So for horses, you probably either want what's called a 20 diopter condensing lens, or this one here, which is called a 2.2 pan retinal lens, and you'll see a lot of ophthalmologists reach for this one because it gives you, a kind of good trade-off between, kind of field of view and magnification.
And these glass, Volk ones are the best, you know, your money can buy. They're a few 100 pounds each, and they will last you decades. But you can buy, really cheap plastic ones on eBay for about 20 to 30 pounds to kind of get you going.
And yeah, I used one of those for a few years as an ambulatory vet and it did fine. It was only when I then moved on to the bulk ones that I realised. How, you know, they're not actually that great, but if you don't know any different, they are, they're fine.
So in terms of the technique for indirect ophthalmoscopy. It's a little bit more challenging to master, but once you've figured it out, it's actually a lot more rewarding than direct ophthalmoscopy. And it's also safer, it's a well, a safer bet in terms of you're less likely to miss something with this technique.
So I'll go over both binocular and monocular indirect, but they're, they're pretty similar. So if you're lucky enough to have a headset, get that on your head, turn it onto the white circle setting and adjust the eye pieces at the front so that they're in your, so that they're kind of in line with your eyes. So the front bit, in front of your eyes moves towards you in a way.
You want to make sure it's a relatively snug fit on your head, so it's not going to move around. Then you want to get your condensing lens and hold that in your dominant hand, so I'm right-handed. And then you want to place your non-dominant hand somewhere on the horse's head collar, or if you need to open their eyelids, you've obviously got that hand free to open them up.
And the trickiest part of indirect ophthalmoscopy is probably getting the lens in the right place, so that you can use it and drop it down in front of the eye at the right angle to get a decent image. So what you want to do is with your hand that's holding the lens. Rest your little finger next to the eye and use that as a type of hinge joint where you can comfortably rotate the lens down in front of the eye and then back up away from the eye as needed, but always keeping that little finger against the horse so you know where you are in space.
And to give you a little bit of stability as well. So you want to start with this setup here in the picture, but with the lens rotated outwards out of the way. So I've got it dropped down in front of the eye in that photo.
Then when you shine the light of your headset directly at the horse's eye, you will get the topial reflex just like you did with the direct ophthalmoscope. And then, and only then, you slowly rotate that lens down in front of the eye, as I've done in that picture. And you want to make sure that that lens is perpendicular to the plane of the light beam that's coming from your headset.
And it's not too close and not too far away from the horse's eye. And you'll have to just play with that distance a little bit to find out what works. Eventually it'll just become muscle memory, but it's usually about 3 to 5 centimetres away.
Just make sure you don't go so close that you but the lens up against the horse's cornea, cos the cornea is super, super sensitive. And if you do that, the horse is gonna feel it. It's not gonna feel nice and it might make them a bit tricky for the rest of the examination.
If you lose the tapeto reflection at any point or your view goes and you just see darkness, you're probably just not shining your light at the eye anymore. So again, just reset, rotate the lens out of the way and start again. So use that torpedo reflection as your guide to know that you're looking in the right place.
And if you do all of that, you should see a lovely view of the fundus. And notice, at no point do you kind of get really up close to the horse with this. You always stay at arm's length.
It's really tempting, when you start doing this. If you, if you can't see much, you want to get in closer, but actually that will make it harder. So always stay at arm's length.
So that's our binocular indirect. With monocular, I don't have a photo, sorry, but all that's different is that your light source, so you know, your pen torch, you direct your almoscope. Is being held in your non-dominant hand at your temple and being shone into the horse's eye at arm's length.
And as a result of that, because you've got your condensing lens in the other hand, you don't have a hand free. So if the horse is sedated and its lids are probably a bit droopy and half closed, you just have to ask the owner or an assistant to gently open the horse's lids for you whilst you're performing this. And the lens technique, in terms of dropping it down in front of the eye is exactly the same as the binocular technique.
And again, always stay at arm's length the whole time. So within direct ophthalmoscopy, with both the monocular and binocular, the view that you'll get will be very different to that which you get with the direct technique. So you will get a much wider field of view, giving you a really nice overview of a much larger area compared to the direct ophthalmoscope, which is great to have a kind of quick scan for anything really obvious.
But the trade-off is that the detail you see won't be as good. So we would often kind of have a quick scan scan around with this technique, so with our indirect, and then if we saw a lesion or if we wanted to look at something in more detail, we'd then pick up the direct ophthalmoscope and look with that. So, they're kind of complimentary techniques rather than two separate options and I think that's the best way to to think of them.
One slightly tricky thing to get your head around with the indirect ophthalmoscopy is that the image you see through the lens is going to be inverted and flipped. So it's going to be upside down and back to front. Which sounds kind of complicated, but so long as you know that's the case, you can figure out where exactly any lesions are that you're seeing.
I always help it kind of, it's easier if you draw them, on a piece of paper as you see them, and then at least you've got that to take home and you can figure it all out in your head later. But for example, if you see a lesion, through the condensing lens, that's the that's a kind of dorsolateral aspect of your view, in real life, it's going to be opposite. So it's going to be ventromedial, if that makes sense.
Yeah, and the other difference with it is that you don't have to get your head right up against the horse. So I think that kind of longing, longer working distance within direct ophthalmoscopy makes a little bit safer too. So I'm going to show you a video now which is actually of a dog because I couldn't find one of a horse.
And this is showing someone performing indirect ophthalmoscopy. And This is the, I think they're doing monocular. You can't actually see the light source, but yeah, it's most probably monocular.
And I think what it shows really nicely is how much of the fundus you can see with this technique, but also how easy it is to lose the image, and then you have to kind of reset and and move it a little bit. So I'll just play this for you now. So as you can see, we've got the optic nerve head coming in here.
And you can see the whole optic nerve head, but also lots of things around it as well. So the blood vessels, our tapetum, and our non-petal fundus as well. And then the other eye.
It's always a bit trickier, and I think they struggled a little bit more with this. Everyone has a has an easier side. There's optic nerve coming in, you can see the vessel going down from it, but actually in real life it's going up from it, if you remember.
So This is a horse, and just to illustrate the difference in terms of what you will see. So with the direct ophthalmoscope. On the left, you're going to get this really, really zoomed in view.
With the optic nerve head almost completely filling your view potentially, but the detail is really good. So you can almost count the blood vessels around the optic nerve head on that picture on the left. Whereas with the indirect, you'll be able to get the whole of the optic nerve head like you saw in the dog, plus the surrounding fundus all in the same image.
So you can see here that there's actually a lesion visible in this fundus on the right, and it's much easier to see that with the wider field of view, with the indirect. Whereas to see that with the direct, you would have to go and have a look around for that. But I don't know if you can appreciate that the trade-off is that the detail isn't as great.
Just to mention, in case any of you have seen or heard of these, so this is called a panoptic ophthalmoscope. And although it's an indirect technique, technically speaking, it's a kind of a halfway house between the more traditional indirect and the direct ophthalmoscope in terms of the view that you'll get. So it gives less magnification in the direct ophthalmoscope, but has a much wider field of view, and the image you see is also upright and in the correct orientation.
And they're also pretty good for getting a view through quite a small pupil because the light beam that comes from this is quite small. And some vets really swear by these, and don't tend to use anything else. And, yeah, I've tried it a couple of times.
I think they're nice. I just found it really fiddly because it's pretty unforgiving, in terms of movement. So you have to be exactly the right distance to get your picture in focus.
You know, a tiny millimetre here or there, and it'll be out of focus. So, not great if you've got a wriggly animal, but definitely an option. And they're cheaper than the indirect headsets too.
OK, so we'll now go on to having a look at some photos of the fundus. So this is an area of a normal horse fundus. So as you can see, we've got our optic nerve head, which should be a really nice kind of healthy, peachy pinky colour.
And then remember that horses have what we call a a por angiotic fundus, which means that the vessels you can see are pretty limited to the area around the optic disc. And you can see that quite nicely in this photo. And in most horse horses, what we can do is split the fundus into what's called the non-terpeal and the topetal fundus, which I briefly spoke about with that picture of the the dog or cat eye that was dissected open.
So the topetal fundus is the kind of greeny area in this one, dorsal to the optic nerve head where we have the tapetum visible. And then ventrally, we have the dark non-terpeal fundus. And the dark non-terpeal fundus is always down the bottom.
But remember, if you're looking on your indirect ophthalmoscope, it will appear at the top of your image. And then in the non-typeal fundus, it's normal to see lots of these little small dots, these little small darker dots up here. And these are called stars of Winslow.
And they're just tiny little capillaries that are coming from that deeper vascular layer called the choroid, and that you're just seeing kind of end on, so they're completely normal. This is another normal fundus, but more of a zoomed in view, so you can just see the optic nerve head peeking in down the bottom there. And then we have that kind of bluey coloured tapetum, at the top.
And then some people could be forgiven to think that this horse, has a kind of massive retinal haemorrhage or something with this big red area here. But actually, that's just an area where there's no tapetum. And if you remember from the schematic, under the tapetum is that vascular layer with lots of blood vessels called a choroid.
So if you don't have any topiat them. In the dorsal area of the fundus or in a certain part of the fundus, you're going to be able to see through to the choroidal blood vessels underneath, which is what these are. And this is quite common in kind of colour dilute horses or horses with a, a white face or a big blaze, for example.
So again, completely normal, . But yeah, we often have horses referred for retal haemorrhages and actually it's just choroidal vasculature coming through. Another example here, topeal fundus at the top, non-tapeal fundus ventrally, and then where the tapetum is absent or thin, just above, the optic nerve head in that line, we've got, the choroidal vasculature showing through.
And this was a horse, just a bay horse, but it had a big blaze, and it's funny because you can sometimes, yeah, you often see these kind of stripes in the fundus when they've got a stripe on their face and they're they're really pretty. And in some horses that are at least kind of partially subalbinotic, for example, horses with kind of wall eyes, blue eyes, you can get almost a complete lack of tapetum. And also a lack of pigment in the RPA on the in the RPE.
And in those horses, the choroidal vasculature is visible both dorsally and ventrally, and you just get this kind of very pink red look at the back of the eye. And again, this is very normal. Now in some older horses, you might see, I don't know if you can appreciate this kind of greyish haze over here around the optic nerve head and a bit here as well.
And this is just myelin, which basically increases with age, and it tends to kind of radiate out from the optic nerve head because it's associated with the axons, in the nerve fibre layer of the retina. So they kind of, yeah, like finger light projections out from the optic nerve head. And again, that's very normal, usually seen in older horses.
And again, this is another old horse with just more obvious myelination. Again, completely normal. Moving on to a few of the more common abnormal but kind of incidental findings.
So these are things that you might find that, for example, a pre-purchase exam in a non-painful eye, and you have to decide kind of what to tell the owner about it. So if you perform funduscopy on a relatively regular basis, you'll probably eventually come across these lesions here called butterfly lesions or peripapillary chorioretinopathy is another name for them. And they're called butterfly lesions obviously because they have these kind of wing shaped areas of pigment loss around the optic nerve.
And sometimes you can get just kind of one wing, or two, as is in this case. And these lesions have been shown to be associated with equine recurrent uveitis in some cases, which is the leading cause of blindness in horses. But they can also be found as completely incidental findings in horses that have never had a history of uveitis and, you know, there's no other sign of of past inflammation in the eye whatsoever, so their clinical significance is a little bit unclear.
But because they are in the area of those retinal vessels, which are really important around the optic nerve head, if you do see these lesions in, for example, a, a pre-purchase or something, it's really important to assess those blood vessels around the optic nerve head. So remember, we'll use your direct ophthalmoscope to do that because it's really good at looking at things in high detail. And If there are normal retinal vessels, so you know, you can see plenty of vessels relatively clearly, they're relatively robust, overlying these depigmented lesions, then you can probably assume that the lesions aren't going to be affecting the horse's vision at all.
However, every now and again you get these lesions, and when you look with your direct ophthalmoscope, the vessels around the optic nerve had a really, really skinny and weedy or even absent. And that basically suggests that the retina is atrophied. And that is more likely to affect vision, so yeah.
If in doubt, you know, refer ophthalmologists would be more than happy to to look at these and advise. Or advise the owner of the horse is re-examined in 6 months, you know, to check for any kind of progression. Another thing you'll start seeing, if you look hard enough, are these lesions called bullet hole lesions, also known as bullet hole chorioretinopathy or focal chorioretinopathy.
And they're these tiny little deep pigmented rings, with a dark centre. And you'll find them in the non torpedo fundus next to or below the optic nerve head. And yeah, there can be 1, there can be 2, or there can be absolutely loads of them.
And there was one study that suggested that actually you might well find these in up to 20% of horses if you actually go looking for them. And again, their exact cause is unknown. There's a few theories out there, some .
That some theories, such as, that there's been a, a vasculitis in the choroid layer. Others have suggested an association with EHV one. So yeah.
We don't know what causes them. It used to be thought that if there were a lot of them, they must affect vision to a degree because, you know, it kind of makes sense, but an article was published in 2014 that looked at horses with extensive bullet hole lesions. And they performed what's called an ERG on them, so an electroretinogram where they assessed the the retinal function basically.
And they found that even in these horses with extensive bullet hole lesions, their retinal function didn't seem to be compromised. So. Yeah, jewelry's out with this one as well.
This is another picture of a very extensive bullet hole lesions in a a horse which apparently was showing signs of visual impairment over kind of low lying obstacles, so tripping over things. And then finally RPE cola boomers, so these are also known as window defects. So a coloboma is basically just a word for absence of normal tissue, and they can occur in various different parts of the eye.
But these RPE coloboma are relatively common, and they tend to occur in the non-terpeal fundus, kind of next to the optic disc usually. And they appear as these kind of very sharply demarcated focal areas of decreased or loss of of pigment. And again, they don't seem to affect vision, and yeah, their cause is unknown.
And yeah, they're quite cute, they tend to be static. So yeah, nothing really to worry about. And this is another one here, this one was in a young foe.
So to summarise, so a fundic exam is essential in cases of poor performance or spooky behaviour in the absence of an obvious cause. And yeah, just don't mess about with these. This isn't an easy thing to do, even at the best of times.
So make your life easy by dilating. The people, sedating a horse, if at all possible, and make sure you're somewhere nice and dark. Also get familiar with the kit that you have.
So there's, there's lots of different ophthalmoscopes out there. And I think if you, if you don't really know how to use it, you're kind of often a little bit scared about using it in case you look, you know, silly in front of owners. So just get it out one day, have a play with it, you know, try it out on one of your pets, because if you know, you know, what all of the buttons and dials do, you're much more likely to reach for it and give it a go.
As I said, it's not easy. So don't give up if the first few times you try it, you struggle to get a decent image. One day you will have a light bulb moment where you get a lovely image, and from there on you just develop this muscle memory, and it just becomes so much easier.
And make sure you consider the direct and indirect ophthalmoscopy complementary to each other rather than two separate options to do the same thing. You know, they both have their pros and cons, but used together, they can give you a lot of information. And if in doubt, you know, if you're seeing something weird and you don't know what it is, either email an ophthalmologist familiar with horses or refer the horse.
You know, don't be afraid to refer it in case it comes back as normal. That's fine. In fact, that's great.
That's a really easy consult for us. And then this is the book that I was mentioning that's really good, for anyone who's interested in equine ophthalmology. It's by Brian Gilger, who's based at NC State in America, and it's just got loads of really nice Fundus pictures in it.
So if you kind of do get to the point where you do manage to get a good view of the fundus and you find something that you think might be abnormal, probably about 90% of the time if you go and look in this book. And probably more than 90% of the time actually, there will be a picture of a similar thing in this book and you'll be able to kind of say, yeah, this is what it is. So yeah, I would highly recommend getting this this book if you're interested in in a cry ophthalmology.
So that's all folks. Thank you for listening.
