and 50% of people who have it don't even know they do.

That's because it often has few symptoms

and these symptoms can appear after it's too late.

So why is this such a stealth disease?

There are two main types of Glaucoma:

open angle and closed angle.

Open angle is the most common and tends to be less severe.

It can develop slowly, over time, and go largely undetected unless it's routinely screened for.

Closed angle, while less common - can appear suddenly and be quite painful.

Both can eventually can lead to vision loss.

- My name's John Flanagan, I'm a professor

at the School of Optometry at the University of California, Berkeley.

I'm also dean of the School of Optometry there.

And for the last 40 years I've spent a lot of time researching into the problem of glaucoma.

So glaucoma is a neurodegenerative disease,

probably the world's most common neurodegenerative disease,

but it's really very specific to the eye.

To understand Glaucoma, it helps to know the basics of how the eye works.

When light reflects an object, it enters your eye, and its first stop is the cornea.

Then it's on through the pupil and ultimately the retina where the image is communicated

with your brain by way of the optic nerve.

It's then interpreted as what you see.

And all of this action is aided by the presence of clear fluids and a complex network of nerves.

If you just think for a moment about the eye, and we have this layer called the

cornea where you would put a contact lens, for example, that's clear.

And it has to be clear, otherwise light can't get in and we can't see.

Therefore it can't have blood and blood vessels throughout it feeding that tissue.

So the way that the eye feeds that tissue is by producing this fluid with all the nutrients;

the oxygen and different nutrients bathe the back to help feed it.

This all takes place in a network of fibers that surround the wall of the eye

and funnel into the optic nerve.

When the fluid at the front of the eye stops draining efficiently, pressure increases.

This can cause problems with blood flow to the optic nerve, which

kills off nerve fibres.

The eye, to stay in the correct shape, needs fluid to keep it inflated.

We need pressure in the eye because otherwise it would collapse.

So it's always this balance between the pressure inside the eye and the pressure in our capillaries.

And for most of our lives, that balance is fine.

But as we get older, the blood supply may become obstructed, and the fluid drainage not as efficient.

This extra fluid increases the pressure on your eye, which in turn damages the cells

and fibers that make up your optic nerve which then leads to vision loss.

This is glaucoma.

The most prone area, the areas that we expect damage first are actually fibers that come

in from peripheral vision, the damage happens more with these big fibers as they loop into the top and bottom.

So 12 o'clock and 6 o'clock on the optic nerve,

they're the ones that seem to be most prone

to this damage in glaucoma and die off first,

and give us very specific patterns of loss in the eye.

You'd think with drainage problems and eye pressure, you would feel it,

but that's not always the case.

That's what makes the symptoms of Glaucoma so difficult to recognize.

If your disease is more advanced, in primary open angle glaucoma you would just start to

notice areas of vision missing.

Or you know, you miss that person walking on the sidewalk or across the street initially.

So they can be quite subtle.

You really don't feel pain other than in the much rarer acute angle glaucoma attacks.

And it's not that you just feel no pain, your brain is actually overcompensating, adding

to the illusion that nothing is wrong.

We often have this idea that if somebody is going blind or losing vision that they

have sort of dark or gray areas in their vision.

And it just doesn't happen like that.

Most of the time these sort of diseases, particularly glaucoma, will slowly affect small areas of

vision, and affect them unequally in the two eyes.

So your brain is very good at merging what we receive, the view of the outside world

from each eye, and merges them together to give our percept of the world.

Just as an example, if you're looking at a house,

and the house has five windows and two doors,

if you have glaucoma, your brain would make you see that house

just as well as anybody else.

But two of the windows might be missing.

Your brain would fill in the brick work or the siding

and you would just misinterpret

the actual scene that you're looking at.

So the brain just has this amazing ability to help you feel as though nothing's wrong

as you develop these defects.

So since there are very few symptoms with Glaucoma,

you can't identify it on your own.

And your brain is filling in the gaps.

So, how do you know if you have it?

Routine screenings and knowing your risk factors, such as family history and high eye pressure, are a start.

And treatment, if caught early, can include drops, medication, laser and other surgeries.

One treatment however, doesn't appear to pass the smell test, just yet.

We know that marijuana will actually reduce the pressure in the eye.

The problem is, it's very quick and very short-lived.

So the therapeutic value of marijuana is a complete failure for glaucoma.

You would literally just have to be smoking continuously

to have an effect that might be therapeutic.

So that's a very long-winded way of saying, don't bother with marijuana with your glaucoma.

But there is some hope on the horizon.

Especially when it comes to neurological disorders, and Glaucoma is in that category.

We're seeing all sorts of really amazing discoveries about neurodegenerative disease and glaucoma.

that opens up the potential for new therapeutic

treatments that will arrest the damage.

I think there's genuine reason for optimism that we'll start having some treatments for

neurodegenerative disease as a whole.

And specifically for glaucoma.