Floaters are translucent dots and squiggles that waft to and fro within vision and in front of what is being looked at.
They are very common.
They may be of any shape and can be difficult to describe.
Terms like "a spider" or "a fly" or "a cobweb" or "frog spawn" are often used and they may appear so real that the affected individual may reach out to try and catch them.
The fancy medical name for them is "Muscae Volitantes" which means flies that flit about.
What are they? The main internal cavity of the eye is filled with a jelly like material called "the vitreous".
This is mostly made up of water which is chemically linked to fine molecular stands.
This arrangement gives the vitreous some structure.
When we are children the vitreous is usually like a set, but wobbly, jelly.
As we get older its structure gradually changes.
The fine strands tend to clump together more and some of the water forms fluid pockets within the gel.
Little by little the vitreous becomes more sloshy.
The strands within it can then move around more freely like a fine net curtain in a gentle breeze.
Floaters are more common in eyes that are short sighted.
This is because the vitreous is more fluid in this condition.
The mobile strands within the vitreous are only noticed if they happen to cross near the line of vision against a bright even background.
This may be white paper, or a bright or evenly cloudy sky, or something similarly illuminated.
If the person is looking at a complex mixed scene then the floaters tend to be lost amongst the details of the scene.
What is actually seen is the shadow cast by the strand (floater) upon the retina.
The retina is the vital "seeing" layer that lines the back of the eye.
Although the actual dimensions of a floater within the vitreous may not change its appearance can.
This may happen, for example, if its distance from the retina varies.
If the floater is just in front of the retina it will cast a relatively small but sharp shadow.
If it drifts further forwards from the retina it will then cast a larger more fuzzy shadow; making it look as if the floater is bigger.
Over time the fluid pockets trapped within the vitreous grow larger.
This fluid may eventually break through to the outer surface of the gel.
This may happen suddenly and can cause a "flashes and floaters" disturbance to vision.
Free fluid now surrounds the jelly part of the vitreous which shrinks inwards and becomes much more mobile.
As this slops around within the eye it may bump the retina.
This causes an electrical signal to pass to the brain and the individual "sees" a momentary lightening like phantom flash of light, usually to the outer side of vision.
These flashes are usually perceived after dark.
They tend not to be noticed during the day when other light drowns them out.
The flashes peter out over several weeks because the gel shrinks even more and no longer bumps the retina.
The floaters tend to persist and there may be one very distinct and roughly circular floater that accompanies the flashes.
This is due to a ring like thread that used to be the bond between the vitreous and the nerve of vision at the back of the eye but is now free floating within the cavity of the eye.
This process where the vitreous becomes more fluid and collapses is called "Posterior Vitreous Detachment (PVD)".
Before a PVD happens the outer surface of the vitreous is in contact with the retina at the back of the eye.
As the vitreous separates from the retina and shrinks inwards it should do so without disturbing the retina.
However occasionally it can pull on the retina and tear it.
This can then lead to a retinal detachment.
This is why the state of the retina should be checked by an eye specialist if someone notices the sudden appearance of new floaters.
As well as the normal strands within the vitreous, floaters can be caused by bleeding or inflammation within the eye.