Functional Defects of the Eye: Terminology

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  • 0:00 Visual Disturbances
  • 0:15 Color & Vision
  • 1:50 Strabismus
  • 2:40 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Cross-eyed, wall-eyed, old-eyed, double vision, night blind, color blind. All of those are pretty familiar sounding terms to you, right? But do you know the technical terms for them? If you don't, then get going on this lesson!

Visual Disturbances

How many fingers do you see on screen right now? Two, right? Nope, it's actually one and you've got double vision. Sorry. I'm joking, of course. But some people really do have double vision or other visual disturbances of the eyes. Let's define a few of them here.

Color & Vision

So what is the correct term for double vision? It's diplopia, where 'diplo' means 'double' and '-opia' means 'vision.' Everything from problems with the lens to the cornea to muscles of the eye can cause double vision.

With double vision, you have twice what you need to have. But if you are blind in half of your visual field, then this is called hemianopia, a state where half of the visual field in one or both eyes is lost. 'Hemi-' means 'half,' 'an-' means 'without,' and as I just mentioned before, '-opia' means 'vision.'

While diplopia means you have double what you need to see in your visual field and hemianopia means you are blind in one half of the visual field, nyctalopia refers to a condition where a person has difficulty seeing at night, night blindness. 'Nyct(o)' means 'night,' as in 'nocturnal.'

Although a person with night blindness has problems seeing things at night, there are those who have problems seeing the world as most people even during the day. These are the people who have monochromatism, or complete color blindness. 'Mono-' means 'one,' like 'mono'rail, 'chrom'ate- refers to color like the colors of Google 'Chrom'e, and '-ism' implies some sort of condition.

While you may not be colorblind, have night blindness, or double vision, almost everyone will inevitably develop presbyopia, the inability of the eye to focus on nearby objects as a result of advanced age. 'Presby-' means 'old age,' as in a group of elders in a presbytery. So, in a way, presbyopia more simply just means old eye.


Some of us may not have old eyes, but we may have crossed eyes or wall eyes. Those are two types of strabismus, a condition where the eyes point in different directions.

When one or both eyes deviate inward, toward the nose, we call this esotropia, or crossed eyes. 'Eso-' means 'inward,' like the inner knowledge of something 'eso'teric. 'Trop-' means 'turn.'

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