Name: NCLE ASTIGMATISM MADE EASY #1
Uploaded: 2024-11-16T07:05:36.000Z
Duration: 12 min 28 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Today we're going to start a short video series called Astigmatism Made Easy.

Adverbs of degree

I recently received a comment on one of the videos I did covering the 10 types of astigmatism in that they wish there were more examples on it and to be honest it was a really short video for a lot of information that was just kind of condensed into it.

So this will be a time for us to kind of evaluate some different points in that video but on a slower level and kind of breaking it down a little better.

Astigmatism is when light does not come to a singular point focus on the retina.

We want light to generally focus on the macula and the fovea centralis of the retina.

That's the point on the light sensitive tissue of the eye that receives the light image.

So it has all the cone photoreceptor on it that have our clear central vision, our color vision, right?

So with astigmatism, light doesn't come to a singular point focus.

It's not to that singular sharp point and we illustrate this with a lot of times the oversimplification of a football image and I want to show that because it is a great illustration.

We'll put the laces on there just for for the fun of it and you know we're going to compare that to a soccer ball or basketball or whatever.

So with a round spherical shape like a soccer ball, if you can imagine this radius is going to be the same curve as this radius.

They're going to be the same steepness, right?

So from there to there it's going to be the same pattern and you could do that all throughout, right?

That line is going to be the same steepness as this line here.

That's if your eye was perfectly spherical and you did not have astigmatism.

It doesn't mean you don't have a refractive error, right?

Because your eye could be too long like a myope or too short like a hyperope but it could still mean that light's coming to one singular point.

Astigmatism means more like a football that this meridian, right?

Our vertical 90 you know might be steeper and if you can imagine this curve on a football it's much less drastic.

So a football laying like this is a good example of with the rule of astigmatism.

Just something to remember your vertical meridian is steeper so it'd be like the cornea is steeper in the vertical meridian but this is astigmatism.

Light is entering at different steepnesses you know a flattest point and a steepest point and it's causing that light to enter and and fall upon the retina in a linear fashion instead of that point that we really want.

So if that's what astigmatism is then we also need to define you know the anatomy of the eye that can cause astigmatism or yeah so like what are the refractive medium of the eye?

What are the parts of the eye that are creating astigmatism?

So we're talking about bending light so if the cornea was completely spherical like our soccer ball right no matter how light entered it it would enter at the same radius of curvature and it would provide the same refraction same refractive properties but then we have the if this is the anterior view of the eye coming through then you have the crystalline lens which is behind it kind of suspended in the anterior chamber of the eye.

That's another lens of the eye and it too can have torricity which is you know and create astigmatism.

Much more common on the cornea but you can find it on the crystalline lens.

So the cornea supplies roughly 43 diopters of strength that's plus 43 it's a plus power lens you know and just to take a step back of how amazing it is that the cornea is like half a millimeter thick in the center it's supplying 43 diopters of strength and the crystalline lens supplies on average about plus 17 diopters and that gives us our 60 diopters of strength but either one of those surfaces can be torric right can have a cylindrical component on it which is going to make light enter it differently.

So the big question is you know how do we measure astigmatism?

Well the main instrument that's used to measure astigmatism is the keratometer.

The keratometer is what we get our k readings from and interestingly it just measures the central three millimeters three to four millimeters of the cornea right so we know our cornea horizontally measures about 12 millimeters right and then vertically maybe 10 and a half millimeters.

So we're talking about you know the corneal cap it's measuring about three millimeters just this zone of it and it's measuring the steepness it's finding out what the flattest meridian is and what the steepest meridian of the cornea is.

The average keratometer is going to measure from 36 to 52 diopters and and what that's measuring is the power of the cornea but also the steepness of the cornea.

Also remember just helpful on your test you can add a negative one auxiliary lens it's going to bring your case down to 30 diopters or you could add in the in the positive direction a plus one and a quarter lens and that's going to bring you up up nine more diopters up to 61 diopters from 52 to 61.

That's just a side note neither here nor there but we measure corneal astigmatism with k readings and I'm just going to give you an example of that.

So let's say our k readings are 42 at 180, 43.50 at 90.

What this is telling us and I'll do another drawing here across our vertical meridian our 90 the power of the cornea is 43.5 diopters at our 180 it's 42 diopters.

So the difference between those two our steepest and our flattest meridian is going to be our corneal astigmatism.

So 43.50 from you know 42 from a 43.50 that's one and a half diopters of those two values that's the amount of toricity that's the amount of cylinder value on ascribed to the cornea because remember a keratometer or an ophthalmometer you might refer to as only measures corneal astigmatism.

I'm going to do one more example of that so if we have um I'll read this out so it's actually clear okay 44.50 at 70 and then 45 at 160.

So our flattest meridian our 44.50 is at 70 and we have 45 diopters of strength at the 160.

So there is a difference of what a half a diopter between those two uh meridians the steepest and the flattest meridian so there's a half a diopter of corneal astigmatism and this brings up a great point now um so if defining astigmatism there is regular astigmatism and irregular astigmatism.

Regular astigmatism means that the principal meridians are 90 degrees apart from one another so this is measured by keratometry um you could have with the rule against the rule oblique astigmatism these are all regular astigmatism but then you have irregular astigmatism.

Irregular astigmatism cannot be we can't use a cylindrical eyeglass lens we can't use a cylindrical contact lens to correct irregular astigmatism.

 Keratoconus is commonly associated with irregular astigmatism you know that's a thinning of the cornea and you have a an oblate like a protrusion of the cornea um pelucid marginal degeneration is kind of like keratoconus it's a thinning of the anterior um inferior part of the cornea the lower part of the cornea um you could have if trauma something you know punctures the cornea um different disease can cause um corneal thinning that gives you an irregular corneal shape so another word to remember now um i'll write on the board it's a fun word this is like bonus stuff right here ectasia so it's even fun to ectasia so you'll hear corneal ectasia that is thinning so this word means thinning so it's a thinning of the cornea um thinning of the cornea can create an irregular surface so we have irregular astigmatism which is like an ectasia of the cornea you know and for these for these patients you need to use some sort of a gas permeable lens you know a lens that's rigid enough that it's going to create a tear film behind the lens and it's going to provide a brand new optical surface um because we're it's not going to be able to you can't use like a soft contact lens that drapes the cornea to correct for irregular astigmatism so that's kind of the the foundation of astigmatism here shortly in the next video series i'm going to go over with the rule against the rule and oblique astigmatism and just do some examples and some example prescriptions with that um and then you know i'm going to do another uh video after that on like compound myopic simple myopic you know mixed astigmatism those types of classifications and you know that way we could just bring it all together and you could really have a solid foundation of what is astigmatism if this video has been helpful i definitely encourage you to subscribe to the channel share it with your friends and really just to keep watching it because you know i'm truly you know a firm believer in especially as opticians contact lenses it's you know it's really just um the heart of the medical side of what we do and just uh the more that we are subject matter experts on this the more that we'll be able to help our patients and provide good service to them because you know with just a surface uh level knowledge of contact lenses we're really not doing our patients any service so just keep watching if you're studying for your ncle um trust me just just watch the videos and study your material uh find a good comprehensive guide and just read through it um there's a number of good resources out there i know carrie wilson has a great book for ncle but if you're pairing the the question the answers up with these videos i know you'll be a success on the test you know these these videos truly are just geared towards the theory behind the material and helping you to understand it better if you're not necessarily like a textbook person and you weren't fortunate enough to go to opticianary school or be able to be lectured to these are kind of hopefully supplementing and taking the place of that so anyway stay tuned and i'm going to have another video in this series coming up shortly thank you