Re: What is the visual 'blind spot'? What causes it? Why do Dr's check it?

Matthew:

     Well, let me start by saying that everybody has a blind spot in each 
eye.  The blind spot exists because that is where the optic nerve exits the 
eye and continues through the brain.  The retina has millions of 
photoreceptor cells that detect and transmit light.  At the optic nerve, 
there are no photoreceptors, thus no vision.  The optic nerve, also called 
the optic disc, has a densely arranged nerve bundles and any damage here 
can cause severe vision loss.  This is why it is inspected so closely by 
optometrists and ophthalmologists.  The size of the blind spot is usually 
quite similar from person to person.  However, some may have enlarged blind 
spot from congenital malformations (means you are born with it) and some 
have underlying pathological process that is causing the enlarged blind 
spot.  The color of the optic disc along with size, shape, and amount of 
the "dip" or "cup" inside the disc can tell us about pathology in the eye. 
 Pale colored discs and large "cups" can be indicative of underlying 
disease process.  Usually the size and shape are the same between two eyes 
of the same person, but again, if they are assymetric, it can indicate a 
damaged or diseased eye.  It can also be physiological, meaning that you 
are simple born with assymetric discs.   
     You can test your blind spot by having two dark targets spread about 5 
inches apart on white paper.  Start close to the paper, close or cover one 
eye, stare at one of the targets, and slowly bring your eye away from the 
paper until you cannot see the other target  out of the corner of your open 
eye.  You must cover one eye because two eyes together gets rid of the 
blind spot.  Good luck!

Kenton McWilliams