Emmetropia and Ametropia – Anatomy and Physiology Review

by Richard Mitchell, MD

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    00:01 Ok, let's look at the eye itself, so the eye, it turns out, is not entirely, completely global.

    00:08 I mean, it is global, but it's not entirely spherical.

    00:12 So the transverse diameter is there and there is some variability.

    00:16 You can't have people who have little eyes or people who have larger eyes.

    00:20 But somewhere in that 21 to 27 millimeters is kind of the lateral dimension.

    00:25 The anterior-posterior dimension is actually a little bit more squished.

    00:29 So this is why it's not completely spherical but close.

    00:33 And so the actual dimension is a little bit less than the transverse dimension, always OK. The important point about all the structures of the eye is actually to focus light appropriately on the retina.

    00:47 That's what we're trying to do.

    00:48 So there is a certain focal length that extends from the lens into the retina, and that's that green triangle there in the middle.

    00:57 And when everything is perfect, we have emmetropia and bringing the light in is a combination.

    01:05 Focusing it appropriately is a combination not only of the lens, but also of the cornea. And so that's going to be an important element.

    01:14 It turns out that some of the water, some of the tear fluid and other fluids released by conjunctiva are also going to be important for kind of the important refract tile focusing of light.

    01:26 But predominantly it's going to be lens and the cornea and it focuses light appropriately on the retina.

    01:35 Now, it doesn't always work perfectly.

    01:38 This is an exaggeration.

    01:39 It's not ever quite this bad.

    01:41 But this is an ametropia as opposed to emmetropia, A-M-E-tropia or refractive error.

    01:47 And it means that the light entering the eye does not focus appropriately on the retina. So what the retina sees will be fuzzy.

    01:56 This can be a combination of effects related to the cornea and the corneal curvature or the lens.

    02:05 And it really the ultimate result is that we don't have the capacity to focus the light so that it's a nice pinpoint thing back on the retina and you can see where its focus.

    02:18 It's kind of in the middle of the posterior segment of the eye.

    02:25 So Amaterasu is divided into two kind of broad categories, nearsightedness, or myopia, or farsightedness, hyperopia. So we're going to talk about myopia first and then we'll talk about hyperopia.

    02:38 What happens in myopia is that the lens is either too curved or the cornea is too curved or the actual length of the eye is too long. And as a result, light coming in is not focus back on the retina, which is that yellow structure, and that will make you nearsighted. This is frequently so the actual length of the eye being too long is a frequent reason why young adults as they're growing may develop nearsightedness that the eye actually orbit grows a little bit too quickly relative to the rest of the structures that may catch up over time.

    03:20 Also, as the lens curvature changes with time.

    03:22 And we'll talk about that.

    03:24 OK, so that's what happens with nearsightedness.

    03:28 So how do we fix that? The light is focused in front of the retina.

    03:32 We can only focus on objects that are near, far away.

    03:35 Objects tend to be out of focus.

    03:38 So what we do is we call it we put a lens in front of the cornea and that is glasses as such as you see on me.

    03:47 And we cause the light to diverge ever so slightly.

    03:50 So we have concave lenses that caused light to diverge.

    03:54 And now when it hits the cornea in the lens, we get the exact appropriate refraction to have it land just on the retina.

    04:07 So thank goodness for that, because otherwise I'd be looking at you through, I could, you could only look fuzzy to me.

    04:14 Okay, hyperopia, they kind of flip side of the coin, farsightedness, the focal length is too short.

    04:22 So the retina is too short relative to where the lens and the cornea are focusing the light. This can also happen because the cornea is too flat or the lens is too flat. And as a result we focus basically beyond where the retina is not good but fixable.

    04:41 Absolutely. So we can only usually focus on things that are far away, near objects will tend to be out of focus.

    04:47 How do we fix that? Well, we use convex lenses that cause convergence of the light, so we allow it to focus now appropriately on the retina. Pretty simple, which keeps that keeps optometrist's in business.

    05:05 Astigmatism is a little bit more complex, but kind of the same process if there is a symmetry of the eye so you can see that the eye is really not round or there's uneven curvature of the lens or of the cornea.

    05:19 All these things will cause light to come in and be refracted in ways that are not clean and pure. And so you have uneven focus in astigmatism, pretty much whether you're looking at things far away or things that are near.

    05:32 So astigmatism is just asymmetry of the lot, the eye, asymmetry the lens, asymmetry of the cornea you have end up with multiple focal points and things look fuzzy everywhere.

    05:44 Is that the end of the game? Can we fix this? Well, absolutely, because what we can do is we can put in an appropriate corrective lens that's asymmetric and allows the light to be focused, takes a little bit more creativity, a little bit more work on the part of the optometrist.

    06:01 But fortunately, because most of us don't have a purely spherical eye, this is something that can be done for everybody.

    About the Lecture

    The lecture Emmetropia and Ametropia – Anatomy and Physiology Review by Richard Mitchell, MD is from the course Introduction to Ophthalmology.

    Included Quiz Questions

    1. In front of the retina
    2. On the retina
    3. Behind the retina
    4. On the optic nerve
    5. In the visual cortex
    1. Convex
    2. Concave
    3. Flat
    4. Cavo-convex
    5. Asymmetric
    1. Multiple
    2. One
    3. Two
    4. Zero
    5. Three

    Author of lecture Emmetropia and Ametropia – Anatomy and Physiology Review

     Richard Mitchell, MD

    Richard Mitchell, MD

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