About Eye Lesion due to laser - an Anatomical Physiological discussion.

Hello there.
When we think of Laser damage to eyes, the first thing that comes to our head is: RETINAL DAMAGE.
i was wondering one of these days, and i thought that our concepts may be a bit divergent to the facts.
Let's try to understand the anatomy of the eye so that i can make my point clear.

ANATOMY OF THE EYE - in short!

The light comes in thru the CORNEA, passes the anterior and posterior chambers, which are divided by the iris (that layer of circular muscles that gives the color of your eyes). the light then passes thru the cristalin, and further on, thru the vitreous humor, reaching finally the Retina.
the retina is nourished with blood from the coroid membrane, a rich capilar network that constantly supplies the retina. The cornea on the other hand, receives a very little nutried support via diffusion from the arterial network of the esclera (the white stuff that makes out the eye globe). the celular turnover of the cornea is very low - the cells takes a looong time to reproduce. the celular turnover of the retina is fast - when we look at the supporting cells - of course that rods and cones from the retina are neuron cells that barely reproduce at all at adult's age. but the other cells do reproduce.

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Now comes my point:
IF the retina is a much more metabolic active cell, it is much more proned to sustain defences against exterior aggression than the cornea cells, which in turn have a much slower metabolism.
In fact - diffractive eye surgery, such as PRK or lasik, uses a perpendiculary incidence of IR laser thru the eye - remodelating the cornea, but not damaging the retina, can you get this point?

SO in conclusion:
i think the laser eye damage is much more proned to affect the CORNEA than to affect the RETINA.
and this has practical issues - specially in cases of accidents:
Retinal damage = Blindness.
Corneal damage may be repaired - thru diffrective surgery, or even corneal transplantation.

Ok. i could keep talking about it the whole night thru - but i preffer to make it a short post so we can discuss further ok?

It would greatly help my post if someone could tell me the specifications (wavelengh and potence) of the lasik/PRK eye laser surgery machine, and also if there is any written papper in a medical magazine that deals with these topics, it would be quite handy here, ok?

Best regards, and i am waiting for a nice discussion!
Heleno Paiva
by the way: i am a 5th year Medical Student - graduation should be in July 2011.

PS: this is my first thread opened in this forum - i mean, not a reply, u see? =)

Google search says lasik uses 193nm wavelength but it can remove 1/4 of a micrometer of corneal tissue at a time. UV does not damage nearby cells, which is why they use UV lasers.

I think laser damage "burns out" cones and rods in the retina, since they are neuron cells they don't reproduce.

I have heard of laser damage "going away" after a couple of days though.

C'mon stem cell research, regrow my retina!!

Hey Pontiaccg5!
this is my point!
Retina cells are neuron cells indeed, but ALSO - they receive a lot of blood and nutrients, which means that unless they are killed, they will be able to regenerato to almost 100% of their functions.
what i mean is:
there can be some damage, but retina can heal itself.
on the other hand - any kind of corneal damage won't heal properly - for cornea receives a lot less nutrients and is a much more stable cell.
Any sort of corneal damage - even if not lethal, will lead to permanent decrease in eye sight - such as the LASIK surgery does - except that here there is damage, not healing.
how do you think?
OH - it is true! i am mistaken. Lasik uses UV not IR;
just got to the wrong side of the chart! heheh

I would think the cornea transmits all visible wavelengths back to the retina, so the cells don't absorb the heat, just pass it back. Somewhat like a glass of water, the light goes through but doesn't heat the water.

I would think if it was a brief hit with a weaker laser that the cells could revive themselves. But with a high power laser focusing through the cornea down to a couple of cells, well that's a lot of power over a couple of little cells. I could see them doing as well as plastic or leather under a high power laser. They probably are completely killed.

I'm not exactly sure, but I don't think that the cornea is relevant unless your playing with a CO2 laser. I'd be more worried about a CO2 laser boiling my eye fluids before it got to my cornea :crackup:

The shorter wavelength range up to about 315 nanometer is not transmitted through the eye and is absorbed by the cornea. In a laser eye treatment, the cornea is being shot on by UV lasers, usually excimer lasers. This removes a bit of the tissue and shapes you cornea th right way to improve your eyesight. After the surgery, it takes a while for the corena to recover, depending on the method used this may be a few hours or days (not completey sure, but it's in this time scale), but it does heal!

This is actually an example of how lasers can damage your eyes, the surgical method is controlled way. UV lasers are dangerous because of cumulative effects of the exposure, but getting hit by a high power UV laser won't damage the retina, but only the cornea, which might be fixable if your eye isn't shot to pieces. The UV is completey absorbed in a very short distance, so your retina is safe for UV lasers below 315nm (but not the rest of your eye).

The wavelength range from 315nm to 1400nm requires stronger eye protection. Mainly because the cornea is transparent to these wavelengths. The light then gets focussed (!) on the retina. The retina absorbs a lot of this light. Laser light is usually coherent and monochromatic, making a nice small spot size with a high peak intensity. This will irreversibly damage the retina if the power is too high. That's why we wear laser safety eyewear.

your tekst says the "the celular turnover of the retina is fast". True, but read a bit futher: "of course that rods and cones from the retina are neuron cells that barely reproduce at all at adult's age." Barely as in practically NOTHING!!! This means although the retina is active and has a high metabolism, once cells die, they're permanently dead. And this is what happens when you get hit by a high power laser.

Above 1400nm, the light is either out of focus at the back of the eye or the eye isn't even transparent to it. This is futher in the IR than most common lasers (except CO2). The danger from these wavelength is thus a bit lower. But if you take a CO2 laser as example, they will do a lot of damage because of the high power, so don't think you're safe just because of the wavelength. CO2 lasers are also used in surgery to cut tissue, but that's an example where this happens controlled and not in your eye.

(I got the wavelength ranges from the EN207 standard)

Yes, you guys are right.
I thought in mistake, because at first I was thinking as if the surgery used IR lasers. but it uses UV, and doesn't reach retina, of course.
it would be a lot of damage to do such procedure using IR after all.
Ok - so be it.
and a propos: I browsed pubmed.com for some scientific papers regarding eye injury, and all that there is is retinal damage anyway. So i guess i might accept that corneal damage is probably not happening.
thx for your opinions!
regards,
Heleno
think this ends this topic now, eh?

The IR range is large. Around 700nm the visible stops (although around 800 you can still see a faint light), and this IR is dangerous. Yag lasers at 1064nm are thus very dangerous. CO2 lasers operate at 10600nm, and this is so far in the IR that the cornea absorb this (and thus can be damaged by it). But because nobody has a far IR or VUV pointer (and most lasers fall in the 315nm-1400nm range anyway), all we hear about is retina damage, which is also the worst of all damages that can occur in the eye.

Some people have voiced concerns about cataracs being caused by exposure to near-uv (405 nm bluray) as well.

I think the reason people are mostly concerned with retinal damage is because this is often impossible to repair. The cells can heal to some degree on their own as they have blood supply and metabolism, but there is little to do beyond that.

Damage to the cornea, lens or even vitreous humor leaves as least some possibility for surgical repair, as these things a more 'mechanical' than the retina.

I think the reason people are mostly concerned with retinal damage is because this is often impossible to repair. The cells can heal to some degree on their own as they have blood supply and metabolism, but there is little to do beyond that.

END QUOTE

The problems in diagnosing eye damage are multi fold...

one:

The brain "remaps" damaged areas out of the visual field and compensates for damaged areas to a extent, so unless you have a really bad burn or a burn on a fringe area with fewer adjacent "pixels" to compensate with, many burns appear to "heal" that do not.

Two: The means of diagnostics are A. Pt reports of black areas or blurred areas. B. Physical lesions visible to the doctor or scientist. Once the brain "remaps", it is difficult to force the eye/brain combination to reuse a area. Systems such as carefully printed grids, huge spheres with scanning lasers, or spherical led matrices must be used, to aid the patient in detection. A patient can report a permanent black spot and yet the doctor will find no lesion.

Three: Eye surgeons will tell you, not all patients have the same retinal tissue damage thresholds. Blue/Green CW Retinal surgery argons and yags can vary their output power from a few mW to three or more watts. Spot sizes usually adjust from a few tens of microns to 1000 microns or more, depending on who made the slit lamp. According to my Biomedical Engineer friend, Photo-coagulation with visible lesions starts anywhere from 75 mW to 250 mW, but some times lesions are seen at lower power, and some patients require significantly higher power. Your aiming at tissue, and its characteristics widely vary. Damage however has been reported at FAR FAR less power. In a few cases damage has been documented at roughly 15 mW or less. Granted, one of those cases was with a group of young kids, daring each other, to see who could hold the pointer in the eye the longest...

Four, The safety standards were based on 1970s testing, testing done on animals, and later, human patients, and interpolating. Diagnostic Optics were not very good back then, and so errors could have occurred in both directions, more or less irradiance. The safety levels are based on statistics, ie a probability that damage will occur. if 5 mW is one incident of damage in 10,000 or 100,000 exposures, then 5 mW might be safe enough to a government regulator and its advisory board to be allowed in public circulation. Woe betide you, if you are approaching 100,000 exposures. Until lately, when small, cheap, lasers were not available, the lack of publicly available systems kept injuries very rare. That might need to be readdressed.

Its not just wavelength, its pulse length, irradiance (power vs spot size) and patient condition, plus where the beam bits the eye, as well as corneal transmission and retinal adsorption. The damage mechanisms change with power and pulse time, a 4 nanosecond pulse from a IR yag will ablate (micro explosion or instant vaporization) the retina. While a blue green argon works by slowly heating/boiling the tissue where the blood adsorbs the light. Low power Blue green damage is roughly the same mechanism as making a hard boiled egg, you end up with congealed, swollen, crosslinked tissue, just like the egg white. While this is great for MDs, giving them more selective tissue exposure, it leaves a great deal of uncertainty in where damage occurs elsewhere.

Keep in mind for retinal laser surgery a MD might fire 300 lesions, damaging a huge amount of tissue to save the eye. But he is careful where he places the shots. You have a built in blind spot in the fovea of the macula, which is where the optic nerve joins the eye. Blast the central fovea and your blind in that eye.

Also, you might consider that constant exposures to increased blue light (which is normally only 2-5% of the solar or tungsten spectrum) causes loss of response in the blue green spectrum, but this comes back 2-3 months after overexposure has stopped. This was detected in eye surgeons, whom, even with the safety shutters on the viewports of the slit lamp, get excessive exposures to blue. Why is this significant? Well.. LED light sources are band sources, blue led dies pumping a yellow phosphor, and are often excessive blue. The eye is tuned to a solar spectrum with weak blue. Stay tuned, that little problem of less then full spectrum lighting may take 5 years to be noticed... If it is a problem. A recent trip to home depot for apartment lighting showed a huge issue with weak, blue enhanced, sources in the "name of saving the planet".

The rule is, keep the laser light out of the eye. And these days, non laser sources such as LEDs are climbing in power, you might wish to no longer assume that incoherent is safe...

And why did I have to go and learn all this? I developed unspecified optical neuropathy in my 20s, and first noticed the problem while driving home from doing a laser show. It had nothing to do with laser show safety.. Its just a nasty disease that tends to hit males 20-40 years old, and little is known about why. I'm lucky, post treatment, my vision is great for my age.

AND ANOTHER DAMN REVISION OF FORUM SOFTWARE THAT NULLS PARAGRAPH INDENTING! YIKES!!!!, I wonder if the code guy really hated his English teacher that much....

Steve

Its an interesting story how suspected laser damage lead you to seek medical attention, and by that find another condition - i suppose very lucky considering the circumstances!

As for the brain compensating: i suppose thats a mechanism that often prevents people from seeking treatment. When a small area is damaged, you are likely to notice the spot, but the brain will compensate over time, filling it in with information as it sees fit. If the area is very small, its likely not to be noticeable after some time, leaving the victim with the believe that the injury has somehow healed on its own.

Also, there is the matter of getting used to things. I have -1/-2 nearsightedness, but never had any problem with it until i started driving. I never figured one should be able to read roadsigns that far away, and things at a distance used to be blurry, but i suppose i figured they just were.

I have gotten so used to the condition that i still don't wear contacts at home, i just pop them in when i go outdoors, especially when driving (which is rather dangerous without, especially on highways).

I know what you mean, I was like that at first with glasses. If I tried to drive now without my glasses I wouldn't be able to read the speedometer. I need to go get another eye exam, the jerks online wont sell me contacts unless I had an exam this year.

Another thing that I think is weird, What I call red looks red to me but what you call red might look blue to me even though it looks red to you, you know what I mean? How would we ever know? I don't know of a way to describe a color

I know what you mean with the last one, its a philosophical question essentially.

I think your actual perception doesnt really matter for pratical use - we just agree that certain objects have certain colors. This can also be applied to light sources, so we could agree that 650 nm is called 'red', 532 is called 'green' and 473 is called 'blue'. How your eyes see it, or your brain interprets it, is of little practical consequence.

Interesting experiment: ask someone who has no knowledge of physics at all which colors are 'closer': red and blue, or red and green. I suppose many people would choose 'red and blue' as long as they have no concept of wavelengths and such.

Pontiacg5 said:

I'd be more worried about a CO2 laser boiling my eye fluids before it got to my cornea

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You mean retina? All light entering the eye gets to the cornea first.

Cyparagon said:

You mean retina? All light entering the eye gets to the cornea first.

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I think i meant the fluids on top of the cornea, just the normal water that is always on your eye. It sounds kind of stupid now though

Pontiacg5 said:

I think i meant the fluids on top of the cornea, just the normal water that is always on your eye. It sounds kind of stupid now though

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There is a laser safety report that describes big lab laser injuries. One has a quote something like " I sat there and watched blood fill my eye" after a qswitched IR pulse.

If that doesnt encourage safety and respect, I don't know what does.

Steve

LSRFAQ said:

" I sat there and watched blood fill my eye"
Steve

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Makes me think of the intro to that James Bond movie.

I seriously wouldn't want to catch a hit from a CO2 laser anywhere on my body. I hit my hand once completely on accident and still have the scar.