Mammals, and that of course includes humans, have eyes that effectively contain a blind spot.
The blind spot occurs because where the optic nerve connects to the eye there are few if any retinal cells to transmit light messages to the brain.
So everyone has a blind spot in each eye and there is a simple demonstration that can be carried out to prove it:
• On a piece of paper write the letters R and L boldly about 5cm apart – but don’t put R on the right and L on the left, but rather the other way round:
• Next hold the paper about 15cm away from your eyes, and close say the left eye. Then focus the RIGHT eye on the letter R.
• Then slowly move the paper away or maybe towards you until the letter just suddenly seems to disappear.
• You can repeat the demonstration with the right eye, this time of course focussing the LEFT eye on the letter L
The blind spot you get in this instance is a naturally occurring blind spot.
The natural blind spot also can be found using the red square and blue circle above. Cover your left eye and look at the red square. Slowly come closer until the blue circle disappears! Move further in and it reappears again.
On the other hand if a blind spot occurs in the field of vision under ordinary conditions because of some disorder or lack of eye function, it is given the medical term “scotoma” from the Greek word used to describe darkness.
So then – answering the question set in the title of this article – a scotoma is NOT simply a natural blind spot, the term really being used in the clinical medical sense.
What does a scotoma appear like?
A scotoma may indeed be a spot-like dark area located usually in the centre of the field of vision, but it may also be a wider dark area on the periphery of vision.
Generally a central scotoma presents a serious loss of vision, whilst a peripheral scotoma may in fact go unnoticed by a sufferer because the eye doesn’t focus the peripheral area of vision information quite so accurately.
If around half of the central area of vision is affected then this is designated as another type of scotoma – a hemianopic scotoma.
Detection of a scotoma is readily carried out using a “field of vision” test that uses a computerised instrument called a Field Analyser. When you have your vision tested we are all familiar with with reading smaller and smaller letters off a standardised letter chart. Many scotomas cannot be found using this technique, as it is only testing the central vision. Many scotomas occur in the periphery of your vision.
At The Eye Practice we routinely perform a computerised visual field screening. What is frustrating is that most eye care practices only perform a computerised visual field eye test on “indication”. This means that they need to suspect something. The problem with this protocol is that a scotoma can be symptomless. A comprehensive eye test should be more than just reading a letter chart. Find out what a real eye test should involve.
It adds another few minutes to the eye exam, but there is peace of mind knowing that more of the vision has been tested.
What causes a scotoma?
There are several causes:
• Suffering a stroke or sustaining a brain injury might lead to development of a type of scotoma
• Side effects from multiple sclerosis and high blood pressure
• Toxic substances are also thought to play their part, particularly methyl alcohol and quinine
• Poor nutrition and serious vitamin deficiencies
• Blockages in veins in the retina and in particular the optic nerve
Treating a scotoma
In the past there has been little that could be done by way of treatment for a scotoma; however research into the plasticity connections of the brain and how they can be renewed is showing great promise.
New research includes drug trials and both radiation therapy and radiosurgery.