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Structure of the Eye, and
Common Surgery performed.
Host: Donna Newhouse
Speaker: Dr Francis, an eye surgeon in Thunder Bay
Dr Francis has a background in treating problems
with eyesight in the elderly, and has also worked
in a mobile Lasik clinic throughout Ontario.
Thanks to Jean Petree, near Boston, for reviewing
this document prior to publication and to
http://www.biols.susx.ac.uk/home/George_Mather/Linked%20Pages/Physiol/The%20Eye.html
for supplemental information and images.
Structure of the Eye
The skin of the cornea has three layers.
An oily layer, a watery layer, and a mucous
layer.
The Oily layer keeps water from evaporating.
The Watery layer, made in the lacrimal ducts,
lubricates the eyes.
The Mucous layer provides nutrients and fights
infection.
Dry eyes is a common complaint. The tears produced
by the body lessen over time.
Q: My mother got surgery recently for a new
lacrimal duct. What went wrong (with her
eyes)?
A: Sometimes we put in a plug to prevent
loss of tears.
Q: That was Not the Problem. She had excessive tearing.
A: "DCR". Dacryo-cysto-rhinostomy.
We fashion a new passageway for tears.
If you feel in the corner, there is a bump.
Lacrimal Sac. We run a tube so the tears
pass into the nose.
Q: So the tears are just carried away?
A: Tears flow constantly into nose,
and into the bloodstream.
The Canal of Schlemm drains tears from
around the eye into the blood and the nose.
Q: This gets into the immune system?
A: Correct. The immune reaction, especially from viral
contact. Look under a microscope, and you see all these
bumps. If you get allergic, you see bumps all over the
conjunctiva.
Blephritis. This is inflammation of the lids, and
the glands around the eye lashes. The eyelids produce
tear film. So the tears don't happen properly. This
is often misdiagnosed or treated incorrectly, and
causes a nagging recurring problem for the patient.
Rather than just prescribing drops for lubricating
the eye or for a surface infection, we found the
real treatment is to clean under the lid, and remove
any infections we actually find there. It is still
a difficult area to treat, and can take 3 or 4 weeks
of treatment.
The Cornea.
This is the first structure that light hits. Contact
lenses sit on the cornea.
This is also treated during refractive surgery using
lasers. By altering the shape we can fix blurred vision.
The layers of the Cornea:
- epithelium
- Bowman's layer
- stroma
- descemets membrane
- endothelium
The Epithelium is the outside skin, and is most likely
to be injured if something comes in contact with the eye.
Pencils, tree branches, fingers; these all tend to affect
the epithelium. This heals very well, often in only 48 hours.
Bowman's layer is a very thin layer underneath the
epithelium. This is *not* replaced by the body. This is
often damaged in construction environments, like with
staples or other flying bits from power equipment.
When this layer heals, a white spot remains in
the field of vision forever.
The Stroma. That's the "beef" of the cornea! This forms
the lens. It is made of fiberglass and collagen.
The cornea and aqueous humor make a lens.
The cornea has a lens power of 43 diopter.
Q: How often do you perform Cornea Transplants?
A: They are fairly uncommon. We refer one person
per year here in Thunder Bay. In Toronto we did
3 or 4 a week. So I'd say perhaps ten a week in
the Province of Ontario. Compare that to Thousands
of cataract operations done each week in the
province.
Q: I've heard that corneas are Irreparable.
A: Well, specifically when that happens, it
warrants a transplant. When people have donated
their eyes, we take a circle, the size of a dime.
Q: So what causes irreparable damage?
A: This varies, but I've seen it caused by
scarring from injury. From stitches. From
infection. Caraconis. From cold sores in the eye.
Damaged cornea will Swell up. When damaged,
they do not let water move in and out, and so
you can diagnose the problem.
When shaving a cornea, we must be very careful
not to remove too much of it.
Inflammation. The cornea can become cloudy.
Caraconis. The cornea becomes thin and
irregular in places, making vision blurry.
Anterior Chamber. This is 3mm in depth. Filled
with fluid, called Aqueous Humor. Moved from
ciliary body into the anterior chamber, fluid
drains out of the Trabecular Meshwork, into the
Schlemm Canal into the venous system.
Iritis. We see white cells in the humor. Like
arthritis, it is caused by a problem immune
system. It looks like the dust we see in the
light of a cinema.
Pupil. An aperture, or hole in the iris. This
isn't actually a tissue, but is the absence of
tissue. The pupil regulates the amount of light
getting in. People with light blue eyes and
people without pigment are not as good at blocking
light, so they become light sensitive.
Lens. Provides refractive power of 15-17 diopters.
This makes a total of 60 diopters of power in the
eye. The lens has no blood supply, so it depends
on the other chambers for nutrients.
The lens can change its shape. It loses this
ability, usually to create nearsightedness. This
happens after 40 years of age. So children have
incredible focus power, perhaps 20 diopters. At
40 years of age this will drop to 4-5 diopters.
And at 80 years perhaps 1 diopter. The lens becomes
thicker with age.
It is surrounded by a Capsule. When we remove
a lens, we are very careful to preserve this capsule
all around. We do this by making a very small
incision.
Vitreous. This weighs about 4g. It is twice as
dense as water.
As people get into their 50s and 60s the
vitreous gets watery. The pressure in the
eye changes, and bits of the wall of the eye
can get loose. When patients report flashing
lights, or floaters, this suggests such a loosening
of the retina. With early diagnosis, we use
a laser to fasten it back against the wall of
the eye. This procedure for retinal detachment
reduces vision at the points where the laser
strikes tissue, but it spares the important
vision, and is usually effective.
There are 10 layers of cells in the retina.
The rods and cones are light receptors.
There are two places in the retina we
give special names to.
The Fovea is the centre, and is where
very fine detail vision takes place. Around
that is the Macula, where fine detail, such
as reading happens.
Macular Degeneration. Sometimes abnormal blood
vessels grow underneath, causing bad vision
in the elderly.
Diabetic Retinopathy. Strokes cause venal or
arterial interruption in blood supply. Such
events must be cared for within hours as the
retina ceases to function if deprived of oxygen
for longer than that.
Q: Do you ever see rapid degeneration?
A: Sometimes it can make a hole, or a tear in
the tissue. We check blunt injuries right away.
Also with Blood Trauma. Sometimes we see bruising
in the periphery.
Q: Diabetes. How does this cause retinopathy?
A: [1] Diabetes causes failures in circulation.
[2] Diabetes causes production of an enzyme which
encourages arteries to grow in wrong places.
Sclera. That is the white part of the eye. The
solid part. This is 1mm thick at the back, and
only .5mm thick at the equator. Common cause
of eruptions. Sometimes we have to lift up and
look under the sclera for damage.
Limbus. Transition zone.
Uvea. Uveitis is the same as Iritis.
This is the coloured part of the eye. It is very
vascular, and has constrictor muscles. It has
ciliary body which functions to produce fluid
which goes into another chamber. It alters the
tension on the zonials which change the shape
of the lens.
Choroid Layer. These have blood vessels
which nourish the retina.
Zonnules of Zinn is the drainage area of the cornea.
These are a network of collagen fibers which
hold the lens like a hammock. These fibers
connect the lens to the ciliary process and
are sometimes called suspensory ligaments.
They does Not drain the Vitreous Humor, just the
Aqueous Humor.
Glaucoma is where pressure in the eye is elevated.
It is thought to be affected by the Trabecular Meshwork.
Glaucoma first affects the peripheral vision and
often goes unnoticed, even while it is causing
damage. This tends to affect people aged 50 and
over.
Q: is it treatable?
A: There are lots of treatments.
[1] There are drops which act on the ciliary body.
[2] There are drops which work on the Outflow side,
the Trabecular Meshwork.
[3] We can use Laser on the Trabecular Meshwork.
It is primitive but effective! We make a 50 micron
hole, and do fifty of them. This damages the meshwork.
This is because the TM is working too hard at pumping,
so we just weaken it.
[4] Surgery, to put a drain in the eye. We do this
through the Cornea, under the Conjunctiva. That is
because if fluid can flow out, then it can also flow
back in, and that would be a serious source of infection.
Q: So there is no pain in Glaucoma?
A: No pain. We usually detect it when the people
need glasses, as part of a routine examination.
Fortunately, we have public awareness. Also, we
have time on our side. If the eye has a pressure
of 25 (normal pressure is 20) it takes ten years
to cause damage. So most people have eye examinations
often enough to catch this.
Q: I've heard that Marijuana reduces eye pressure.
A: You know, people will do *anything* to get a
doctor to prescribe a joint ;-) While what you say is
true, it has a lot of sedetive and analgesic
properties, and it increases appetite. So marijuana
is a better choice for different medical problems,
such as terminal illness and extreme pain.
Optic disk, also called the Blind Spot
has 1,000,000 axons. This is the optic
nerve from the retina. This is where we
see axons dying off due to high pressure.
The rim of the optic nerve is the tissue, not the cup.
We use this to help assess nerve damage. We look at
the Cup to Disk Ratio. Normal is .3 cup, where .6
or .8 is suggestive of damage.
MS, Multiple Sclerosis. This often causes Optic
Neuritis. Young people 25-40 years old experience
loss of vision in one eye, with pain. That is
because the nerve behind the eye has become
inflamed, and when they look around, say left
or right, they move it, and it hurts.
Conjunctiva. This is what gets inflamed in
Pink Eye infections.
There are four rectus muscles. The Oblique
muscles are involved in rotational movement.
When someone is cross eyed, or wall eyed (Dr
Francis used the correct terms) we can detach
the muscles (surgically) and re-attach them,
closer or further as needed to correct the
rest position of the eye.
Video Presentation I - Lasik Procedure
This doesn't take very long.
We make a flap in the cornea
by cutting the epithelium and
and flipping it back, carefully
leaving it attached.
Using the laser, we vaporize just
a little of the Corneal Tissue.
As needed, we keep it lubricated and
moist.
When we have finished, we flip the
epithelium back over again. It seals
almost instantly.
So we quickly remove any wrinkles, and
remove any debris.
To assist the instant healing, we keep
the middle of the cornea moist, but let
the cut edges dry.
Q: The patient, he looks like he is awake!
A: Yes, he is. He may be sedated with Ativan,
but we need him to look forward and concentrate
during the procedure.
We used to do laser surgery differently.
The PRK, PhotoRefractive Keratectomy involved
vapourizing the Stoma right through the
Epithelial tissue. This was a simpler procedure
for our clinics, and had a good success rate.
The problems involved healing and vision.
The Lasik procedure heals almost instantly,
where the PRK procedure took two weeks, during
which the patient had much discomfort. To protect
the cornea, we sent the patient home with a
contact lens. It wasn't an effective way to
handle all vision problems, such as astigmatism
and far sightedness. Also, we are getting some
reports that some of the early patients are
getting some clouding where we vapourized the
tissue.
Lasik procedures are improving. We can now treat
farsightedness by trimming the edges of the cornea.
Video Presentation II - Cornea Transplant Surgery
We cut a circular opening. We inject the eye
with gel to keep the pressure high during the
procedure.
We spin the nuclear material, and use
ultrasound to break it up.
We slide a pocket under the capsules so we can
just slide the old lens out, and the new lens in.
This is a difficult procedure, as we must control
the rip we have made in the eye.
The cornea is easy to see and remove.
We use a clear self sealing film over the eye
like Saran Wrap to close the openings we have
made.
Slide Presentation
A series of pictures of disorders and
abnormalities we see in the clinic.
That's all I know.
