Understanding Vision Impairment

2.1: When things go wrong with the front of the eye

Now that we have a good understanding of the structure of the eye, and of how vision works, let’s look at what effect eye problems have on vision. I’ll introduce this by considering which part of the eye is affected, rather than what disease is causing it. We’ll look at specific eye conditions later, but for now it’s enough to consider that most diseases that affect (for instance) the cornea cause similar types of functional impairments — which are quite different from the functional impairments caused by diseases affecting the photoreceptors, which are in turn different from diseases affecting the optic nerve, etc.

Let’s work from front to back.

Front of the eye — cornea and lens

The job of the front of the eye is all about getting a nice focused image on to the retina at the back of the eye. So the problems that cause vision impairment are generally:

  • The shape of the cornea is wrong but it’s still clear, so it’s just that the image isn’t focused. This may include some cases of keratoconus and corneal transplants. Visual acuity is usually a pretty good reflection of how good or bad the vision is. The functional problem is generally about not being able to see fine detail, rather than any problem with low contrast or low light. Note: In some cases this kind of problem is fixable by using a contact lens to bridge the distorted surface so that a clear image can be formed. Those types of lens can be exceedingly complicated to fit, so you really need to find someone with real skill in that area.
  • Some part of the eye that’s supposed to be perfectly clear is hazy or has opacities. Examples include corneal dystrophies, corneal scarring, cataracts, and vitreous haze. People with these conditions may have trouble seeing finer detail, but their bigger issues often revolve around glare (all types) and trouble seeing things that are lower contrast. Many have very significant functional problems, but still have good visual acuity.
  • Or both.
Key point: Problems with parts in the front of the eye tend to leave your overall visual field unaffected. The total field of view should be normal size, and any blur, haze, glare or low contrast problems affect vision fairly evenly across the entire visual field.