Amblyopia and Strabismus

by Richard Mitchell, MD

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    00:02 Welcome back. We're going to talk about kind of eye movement disorders, that strabismus, but we're also going to talk about an entity where the brain quits processing the information coming from another eye because it's malaligned or because of other processes.

    00:20 That's amblyopia.

    00:22 And again, I want to acknowledge the really wonderful work of Dr.

    00:25 Jose to made all this possible.

    00:29 We'll deal first with amblyopia, so normally for most of us, when we see an image here of a lovely dog, it's the same in both eyes and the brain processes it equivalently from both eyes.

    00:43 But in amblyopia, as you can see in the bottom, it's out of focus. In fact, can get quite faint and out of focus with progressive disease.

    00:55 Why does this happen? It is due to an abnormal development that can be acquired due to visual impairment that causes the central nervous system to have defective processing, basically the brain learns to ignore the images coming from one eye because it's it has been blocked or otherwise obscured.

    01:21 So there are some various causes for that.

    01:24 So what happens kind of in the pathophysiology, broad strokes, there's abnormal visual stimulation, usually unilaterally in one eye, the lateral geniculate nucleus and the striate cortex of the visual center that is normally going to be activated by that eye is attenuated.

    01:41 It may, we may even have damage to that.

    01:44 But it's basically it learns to ignore kind of the defective information coming from the bad eye.

    01:52 The brains ultimately suppresses the stimuli from the affected eye.

    01:55 And remember that although photons are hitting the retina and that's providing the images we only see with her optic cortex.

    02:04 And if that doesn't get processed appropriately, it's as if nothing is coming into that eye. So the brain suppresses the stimuli from the affected eye.

    02:13 With time, the eye can be become disused and you may actually have changes in not so much the structure, but in the way that it is performing or the way that it even looks.

    02:28 And you end up with cortical blindness basically in that eye.

    02:33 OK, so the types of amblyopia, refractive amblyopia is the most common type.

    02:40 That means that you have a very abnormal refractive power that is giving you just very bad scattered light hitting the retina on that side. And that is refractive amblyopia.

    02:56 The eyes may look perfectly acceptable from the front, but it is the asymmetry that is happening either in, again, the cornea, the lens or the back of the eye that is responsible for this form of amblyopia.

    03:11 You can also have deprivation amblyopia, so if you have unilateral cataracts, this would be a severe form of the disease, but basically now you have a completely clouted lens, can't see in there.

    03:23 And the brain says, you know what, that information's not very useful and eventually will shut off and you'll have cortical blindness and occurs when the vision is obstructed. One of the forms of amblyopia, one of the causes for amblyopia is strabismus, amblyopia and strabismus simply means malalignment of the eye.

    03:44 So for looking at this child here, the visual axes appear quite different. So the information going from the deviated eye that isn't staring forward, that has a medial deviation will tend to be going to a different area on the retina.

    04:01 So what will happen in some cases of strabismus such as this? We will get cortical suppression of the image from the deviating eye because it's imperfect information or it's not enough information and the brain will turn that off. It's an important point to emphasize is that strabismus doesn't always cause amblyopia.

    04:22 Sometimes the strabismus is occurring, in fact, because the eye is trying to get the light focused exactly on the macular phobia.

    04:32 So if this is happening as a kind of a physiologic correction for some other maldevelopment, then we will have completely normal visual processing.

    04:45 But that's fine.

    04:46 They have accommodated to the particular deviation of the eye in terms of the types of deviation.

    04:55 Again, these are relatively permanently deviated in various ways.

    04:59 If it is deviated inward, the term we will use is esotropia.

    05:03 OK, now if it's deviated outward and call that exotropia, if it's deviated upwards, it's hypertropia.

    05:11 And if the eyes are deviated downward, it's hypotropia.

    05:15 All right. So now we've got the vocabulary.

    05:17 How is this happening? Why is this happening? So there may be orbital asymmetry.

    05:22 So the actual bone around the eye or the orbit itself may be asymmetric.

    05:30 And in order for the eye to focus light appropriately, it may have to deviate.

    05:40 There may be abnormal interpupillary distance.

    05:42 So if you have a very wide set, eyes are very narrow eyes that may lead to some forms of strabismus. Again, trying to provide focus of the light onto the fovea in the macula.

    05:54 You can have abnormal, extraordinary muscle insertions.

    05:57 So if you think about it, if you don't exactly put the superior oblique where it needs to be and it pulls from a slightly different angle, you may have a permanent malrotation of that eye.

    06:09 There may be muscle weakness for a variety of reasons, including developmental abnormalities in the way that the muscles are actually put there.

    06:18 You can have variation the position of the macula.

    06:21 So normally it's going to be sitting kind of adjacent in the nearby vicinity to the optic disc.

    06:29 But sometimes it may be just the way it developed a little bit offset and the eye realizing that wants to get most of the light image information to that right location. The macula will deviate the eye so that it always hits without extra effort and you can have physiologic factors.

    06:47 So you could have increased either muscle tone or increased nervous stimulation that drives accommodation at least in one eye or the other. And so that you just have a physiologic reason why you may have this.

    About the Lecture

    The lecture Amblyopia and Strabismus by Richard Mitchell, MD is from the course External Eye Diseases.

    Included Quiz Questions

    1. Lateral geniculate nucleus
    2. Optic nerve
    3. Optic disc
    4. Retina
    5. Cornea
    1. ...cortical blindness.
    2. ...strabismus.
    3. ...nystagmus.
    4. ...diplopia.
    5. ...night blindness.
    1. Cataracts
    2. Glaucoma
    3. Retinoblastoma
    4. Keratoconus
    5. Choroidal melanoma
    1. Inward deviation of the eye
    2. Upward deviation of the eye
    3. Downward deviation of the eye
    4. Forward deviation of the eye
    5. Outward deviation of the eye
    1. Position of the macula
    2. Size of the macula
    3. Density of the macula
    4. Position of the cornea
    5. Position of the iris

    Author of lecture Amblyopia and Strabismus

     Richard Mitchell, MD

    Richard Mitchell, MD

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