The Human Eye-Dissection of a Complicated Diagram (analysis questions at end)
A Quick Overview:
This is a cross section of the human eye, you will quickly notice a lot of structures with complicated names. I am here to tell you that it isn’t that bad. Many of those structures (cornea, lens, aqueous and vitreous humor) have one job and that job is to be as transparent as possible so that light can pass through them and hit the retina. The retina lines much of the inside of the eye and can be thought of as an “image detector”. It is the light sensing part of the eye and when light hits it signals are transmitted to the brain that you interpret as images. There are a few more nuances here and there but that is pretty much it. If something becomes opaque that is normally transparent then light can’t get into the retina and vision is decreased. If the retina is damaged or destroyed then parts of the eye that detect light aren’t going to work and vision will decrease as well. The optic nerve carries the information from the retina to the brain, if the optic nerve is destroyed or interfered with then loss of vision will result. The goal of this handout is to walk you through the different anatomical segments of the eye and show you through the mechanisms of disease how each one can lead to visual impairment or blindness. The medical knowledge is more or less just for fun and to aid in your understanding of the anatomical structure in question.
Story 1: Conjunctiva and Pink Eye
15 year old Dave Whitten wakes up in the morning with dull pain in his right eye and it is very itchy and watery. His whole eye hurts and he holds his hand over it as he stumbles to the bathroom mirror and sees a hideous pink eye. His little sister recently had pink eye in both of her eyes. His mother told him not play with her but he did anyway.
Explanation: Conjunctivitis is an inflammation of the conjunctiva. What is the conjunctiva? It is the tissue that covers the surface of the sclera(whites of the eyes) and the inside of the eyelids. Many things can cause conjunctivitis including infection, allergic reactions, and chemicals. In this case it is infectious, both viruses and bacteria can cause conjunctivitis (mostly viruses) but in general the infection resolves in a few days on its own and does not require or warrant any treatment.
Story 2: Extreme Conjunctivitis
Years later Dave is doing aid work in an African village. He is helping to deliver a baby and when it is born a nurse drops an antibiotic containing solution into its eyes. Dave asks the nurse why she is doing this and she points to the blind man seen below stumbling through the village with a walking stick.
Explanation: Conjunctivitis in newborn babies is a serious cause of preventable blindness globally and a major target for prevention by the World Health Organization. Mothers infected with chlamydia or gonorrhea can get these bacteria in their baby’s eyes as it is being born. Chlamydia or gonorrhea can cause severe conjunctivitis that leads to the conjunctiva in the eyelid scarring and turning the eyelashes inward toward the eye. Eye lashes feel soft and gentle on your skin but to your cornea they are as sharp as knives and repeated blinking tears the cornea apart and causes it to scar and become opaque. This is a big problem when one of the cornea’s primary jobs is to be transparent. If no light can pass through the cornea to get to the retina then it is never detected and blindness results. The bacteria can be killed with antibiotic containing eye drops which prevents this entire scenario from occurring. This is why in American hospitals all babies get antibiotic containing drops in their eyes shortly after birth and this intervention is performed in the third world to prevent this cause of blindness.
Story 3: Cornea-A matter of clarity and focus
Dan and Bart are two friends in their early 30s who have had to wear glasses their entire lives. They both hate the look of their glasses and have had terrible experiences with contact lenses. They have heard that the part of the eye responsible for focusing power is the cornea and that there is a laser based surgery called LASIK that can give you excellent vision. Dan decides to go to the local LASIK specialist, Dr. Walker, for the procedure. Bart feels that Dr. Walker’s Lasik procedure is way too expensive and he has heard about another surgeon, Dr. Malcolm Practice who performs the surgery in a small room behind a bowling alley. Dr. Practice is currently having some “licensing issues” but his prices are unbeatable and with every surgery you get a free shoe rental at the bowling alley. The combination of low prices and free shoe rentals are too much for Bart to resist. After all, what better way to test vision than a post surgery game of bowling? When Dan recovers from his procedure he is happy to realize that he has perfect vision and will never need glasses again. Unfortunately one of the first things he sees are Bart’s hideously scarred corneas, one of which is seen below.
Explanation: The cornea overlies the iris and pupil and plays a large role in the quality of your vision. The two parts of your eye that are responsible for the ability to focus vision are the lens and the cornea. The lens only gives you about 1/3 of your focusing ability while the cornea actually contributes the rest. The shape of the cornea determines how clear your vision is so it is possible to reshape it in order to perfect visual acuity. The nonsurgical options are glasses and contact lenses. The cornea is on the outside of the eye while the lens is on the inside so the shape of the cornea is more easily altered as well. LASIK is an acronym that stands for “laser-assisted in situ keratomileusis” which basically means using a laser to change the shape of the cornea so that your focusing ability is perfect. LASIK surgery has changed many lives and offers the prospect of a life free of glasses or contact lenses. Unfortunately it also means carving out pieces of your cornea with a laser beam which is not without its risk of complications. The cornea can easily be scarred by a poor practitioner of this procedure. Scars as you can see are opaque and one of the most important jobs of the cornea is to be transparent so that light can pass through it and hit the retina. Corneal scarring can certainly decrease vision and is a possible complication of the LASIK procedure. The corneal scars seen in the image above are more severe than you would typically see from LASIK and not really in the right areas of the eye for this procedure but anything is possible when you go to an unlicensed surgeon who practices in a bowling alley.
Story 4: Requiem for a Cornea
Bart’s corneas are so horribly scarred after his botched LASIK procedure that his vision is worse than ever. Fortunately Dr. Walker is able to perform a corneal transplant procedure using corneas from the eyes of a deceased organ donor. Once the transplant is performed Bart’s vision is better than it ever was. His eye looks like the one to the left. When he looks closely in the mirror he can see the lines where his own cornea was cut out and the new one was grafted on.
Explanation: Corneal transplants are among the most effective organ transplant procedures. They have been performed successfully since the early 1900s and as a result the eyes are one of the most commonly harvested organs from organ donors. The secret behind the success of these procedures is hidden within this picture. If you will notice the cornea is completely transparent which means that there are no blood vessels that pass through it. The white blood cells are what drive organ rejection and no blood supply means that these cells have no access to the foreign tissue and hence cannot reject it. The cells of the cornea get their oxygen from the air that surrounds them and their nutrients from the aqueous humor just beneath them.
Story 5: Cataracts and the Lens
While working at a local nursing home you meet an 84 year old man with pupils that look blue and milky like the one seen in the image to the left. It is surprising that he is able to see but he can make out images fairly well. He has some trouble interpreting color and if you hold fingers up close to him he can count how many you have up but if you are far away he can’t count them. He says that the biggest trouble his eyes have is that the whole world seems blurry but he is not significantly impaired and certainly not blind.
Explanation: The lens is a structure that along with the cornea provides focusing ability. However unlike the cornea the lens can change shape to focus on objects at different distances. This is called accommodation and functions like the “tight” and “wide” options on a camera. One of the other jobs of the lens is to be transparent and allow as much light to pass through as possible. Cataracts are a clouding of the lens that increase its opacity and prevents light from making its way into the retina. This can interfere with the ability to tell colors apart and in general causes vision to become increasingly blurry. The images below are how someone with cataracts might see the world. Cataracts generally get worse with time and can opacify the lens enough to cause blindness unless surgically corrected. Cataracts can result from general aging and are quite common among the elderly, they are also linked to long term ultraviolet light exposure, poorly managed Diabetes mellitus(high blood sugar), hypertension(high blood pressure), and diseases like Rubella that cause you to be born with them. What is Rubella you ask? You have probably never heard of it but unfortunately if current public attitudes that oppose vaccination persist tomorrow’s children may not be so fortunate.
Story 6: Congenital Cataracts and Rubella
Mary Jensen is a 26 year old woman who recently gave birth to a baby that is shown at the left. Mary never received an MMR vaccine as a child which would have protected her from getting Measles, Mumps, and Rubella for her entire life. Mary’s family did not believe in vaccinations because they were afraid that she might develop autism. Vaccinations are required to attend public school but she was home schooled throughout her childhood. In her first few weeks of being pregnant she developed a fever, cough, rashes on her face arms and chest, and a general feeling of illness. All of this lasted about 3 days and she completely recovered. Unfortunately when her baby was born it had multiple birth defects including cataracts in both of its eyes, an extremely small head, deafness, problems with its heart, and a high probability of mental retardation.
Explanation: Congenital cataracts(cataracts you are born with) can be a result of a mother getting infected by the virus that causes Rubella early in pregnancy. Rubella is a fairly mild disease in both children and adults but if a pregnant woman is infected then the virus will severely damage the developing baby causing defects like those listed in the story. You don’t have to know much about medicine to know that blindness, deafness, heart defects, skull deformities and mental retardation are very bad things that you would avoid if you could. Unfortunately all of this could have been prevented by Mary getting the MMR vaccine when she was a child. The vaccine to prevent measles, mumps, and rubella is given in early childhood and is mandatory for those attending public school. The vaccine has been given since the 1970’s and has been phenomenally effective at eradicating these serious diseases. Why would someone not get this vaccine as a child? There was a study published by a physician in the late 1990’s that linked the MMR vaccine to the development of autism which caused many people to not get their children vaccinated. The study was poorly designed and even more poorly evidenced and is a current example of medical fraud. It is widely reviled throughout the scientific and medical communities and all of the authors have become pariahs. The belief that vaccines cause autism is the equivalent of witchcraft in the medical community. Vaccination remains one of the most successful strategies in eliminating serious disease in modern times but people that oppose vaccination are very vocal, thoroughly misguided, and extremely harmful. The reason that people have never seen these diseases is because vaccinations have significantly decreased their frequency but they did not go away. These diseases are always waiting until we let our guard down to come back. The following link is just one example of the dangers of internet intellectualism; it gives a view of what people think despite evidence to the contrary and it is absolutely terrifying, view it at your own risk: http://answers.yahoo.com/question/index?qid=20090206192507AAKHjAf
Story 7: Red eye and the Retina
There is not much of a story to tell here. Some unfortunate 10 year old girl had her picture taken by an amateur photographer and this was the horrific result. Notice the giant red pupils that make this girl look like she should be in a Twilight novel. There is an old saying that “the eyes are the windows to the soul”. I don’t know if that is true but they are certainly the windows to the retina. The red eye effect is actually light from a very close camera flash illuminating the fundus of the eye and it is also called the “red reflex”. The fundus is the rear lining of the eyeball and it contains the retina- the light detecting portion of the eye which is most of the “red”, the macula-a small round pit in the retina that is responsible for high visual acuity (the deepest point in the pit is the fovea) , and the optic disk-the place where the optic nerve plugs into the eye. It is medically important to examine the rear of the eye and there is a little tool that physicians use called an ophthalmoscope to look through the pupil and examine the fundus to see if it is diseased like in the lower left image. A healthy fundus would look like the image on the right. The ultra red dot in the center is the macula, the yellow circular portion is the optic disk and the rest is nice, red, healthy retina. This is a pleasant and healthy exam. Not everyone is so lucky, and you are about to meet them.
Story 8: The baby with the tumor in its eye
Dave is a 35 year old pediatrician who is seeing a 2 month old infant in his office for a routine check up. The infant has been very healthy and its mother states that she hasn’t noticed anything unusual. As part of the check up it is Dave’s job to examine the infant’s eyes and when he shines light into them instead of the normal red reflex he sees what is shown in the image on the left. The “white eye” effect is known as leukocoria and it is always a bad thing. The most concerning potential cause for the abnormal eye exam is a rapidly growing tumor in the inside of the eye known as retinoblastoma. Dave quickly grabs his ophthalmoscope and looks into the “white pupil eye” and sees the image on the left. Inside of the infant’s eye is a big white tumor and that is indicative of retinoblastoma. Dave is glad that he was able to detect the tumor and hopefully it has not invaded the infant’s brain. There is a chance that it would be able to be treated without the infant having to lose its eye, but a lost eye is better than a lost life.
Explanation: Retinoblastoma is a rapidly progressing cancer that is one of the more common cancers in children. It can be detected by eye exam which is why the red reflex is such an important thing. The red reflex is annoying in family pictures but can be used in this case to save lives. Retinoblastoma has a very high survivability if detected early with 90% of those afflicted surviving into adult hood. The cancer starts out as normal retinal cells that go bad and become a cancer that tries to invade the brain. You don’t have to know much about medicine to know that “cancer rapidly invading the brain” is a bad thing. Treatment may or may not result in the loss of the affected eye, but it is better to lose an eye than to die. These patients are cared for by pediatric ophthalmologists and pediatric oncologists so if this interests you, a career in one of these fields may be a possibility.
Story 9: Degeneration of the Macula
A 75 year old woman is finding that she is having a harder and harder time carrying out her daily activities. Physically she is in excellent shape for her age but she has been having progressive problems with her vision, she can’t read, sew, or drive her car. It is extremely frustrating to her because she had been independent her whole life and was a lawyer for many years. Her vision loss is in the center of her visual field but her peripheral vision is relatively normal. To her the world looks like the image to the left.
Explanation: Age-related macular degeneration (AMD) is a leading cause of vision loss in Americans aged 60 and older, affecting an estimated 10 million people. AMD blurs the sharp, central vision needed for activities such as reading, sewing and driving. It is a painless disease that slowly destroys the macula which is responsible for central high resolution vision. It leads to a central blindness that spares the periphery like in the picture above. If you looked into one of her eyes with an ophthalmoscope you would see something like this image on the right. Look at the yellow spots that are where the macula should be, these are areas where tissue has been destroyed and can no longer detect light.
Story 10: Diabetes Destroys the Retina
Hector is a 70 year old man who developed Diabetes Mellitus (high blood glucose) when he was 55. His doctor told him to maintain his blood sugar within certain limits but blood tests showed for many years that he had been allowing his blood glucose to get excessively high. Ideally he would have tested his blood glucose several times a day by pricking his finger and testing the blood but he did not really notice a lot of changes in his daily life no matter what his blood glucose was therefore he saw no reason to monitor it regularly. His doctor advised him to stay away from refined sugars in his diet but Hector feels that life just isn’t worth living without several donuts a day and pie at night. What the heck does that doctor know? Refined sugars are the best kind of sugar! Over the course of the 15 years Hector has had diabetes he noticed that his vision deteriorating, it has spots in numerous areas and the way he sees the world is captured in the photograph on the left. Multiple ruptured retinal blood vessels are seen when you look into his eye with an ophthalmoscope as well as multiple yellow areas which are damaged parts of the retinal tissue.
Explanation: Diabetes is very bad for the retina and can eventually lead to blindness. If you destroy the portion of the eye that detects light then blindness is the result. Portions of Hector’s retinas have been destroyed in different locations which is why he has blurry black spots throughout his visual field. High blood glucose is horrible for blood vessels, much, much, much worse than high cholesterol. The smaller the blood vessel is the more damage high blood sugar can do to it and there are a lot of small blood vessels in the retina. Cutting off a tissue’s blood supply is the same as strangling it, the tissue will die and cease to function. Hector has type II diabetes which is related to obesity and often responsive to lifestyle modification. The best treatment for type II diabetes is to lose weight and avoid refined sugars in your diet like pastries, cakes, and pies at all costs. Treatment for diabetes involves maintaining the blood glucose in a certain range with drugs and or insulin, if a person can do this then the diabetes is considered well controlled, if they can’t it is considered poorly controlled. Even if Diabetes is well controlled blood vessels will be damaged and changes in vision will occur but there will be a longer time before it happens than if the diabetes was poorly controlled. Checking blood sugar regularly gives a patient a measure of how well the blood glucose is being controlled and physicians know if changes in the treatment plan have to be enacted to get the blood glucose under control. If blood sugars aren’t measured regularly then no one knows what is going on. Hector’s story is extremely common and many diabetics who have poorly controlled diabetes eventually lose most of their vision and suffer damage to almost every other organ in their body due to destruction of blood supplies. Diabetic damage to the retina is a little more complex than what is presented here but the net result is the same.
Author: Bill Brucker, Copyright Providence Alliance of Clinical Educators 2012
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Eye Case Studies Questions
1. Which part of the eye acts as an image detector and covers the majority of the eye's inner lining?
2. The conjunctiva is:
a. Tissue that receives and processes light
b. Tissue that covers the surface of the whites of the eyes and inside of eyelids
c. Anatomical landmark for the identification of cataracts
d. A chemical applied to the eyes to produce tears
3. What is the primary part of a healthy eye that is responsible for 20/20 vision?
a. Shape of cornea
b. thickness of conjunctiva
c. viscosity of vitreous humor
d. thinness of lens
4. What is a rare but well known serious side effect of LASIK surgery?
a. Scarring of cornea leading to a decrease of vision
b. Cancer from the laser
c. Poor response to steroids
d. Damage to the retina
5. Which is false regarding the cornea?
a. Cells of the cornea get their oxygen from the air
b. Cells of the cornea get their nutrients from the aqueous humor just beneath them
c. The cornea has no blood vessels
d. Cornea transplantation often results in rejection due to the presence of foreign cells
5. Which of the following conditions is not associated with cataracts?
a. Rubella infection
d. retinal damage
6. What is the most common cause of central blindness in older individuals?
a. Macular degeneration
b. Peripheral vision loss
c. Psychiatric retinopathy
d. Urinary infection
7. A 14 year old wakes up with dull pain in his right eye and pink conjunctiva, what is the typical expected course of his condition?
a. Usually resolves by itself within a few days
b. Requires a visit to the physician
c. Usually requires surgery
d. Usually leads to antibiotic treatment
8. What makes up most of the red in the “red eye” effect seen in photographs?
a. The retina
b. The macula
c. The lens
d. The cornea
9. A newborn is infected with gonorrhea, which causes severe conjunctivitis, what is the likely outcome if left untreated?
a. Death of the infant
b. Resolution of conjunctivitis
d. Severe long term digestive issues
10. What is the most effective way to prevent Rubella associated cataracts?
a. MMR vaccine
c. Online support groups
11. A 3 month old has what seems to be a "white eye" and no red reflex on his left, what is a detrimental effect if left untreated?
a. Cancer that invades the brain
b. Effects on sleep
c. Effects on appetite
d. Increase in rates of autism
12. A 60 year old diabetic man with uncontrolled diabetes would like to know how to prevent a decrease in vision as a result of his disease. What is the best way?
a. Eat more refined sugars
b. Don’t monitor blood glucose regularly
c. Adequate control of blood sugars
d. Frequently Miss doctor’s appointments