color vision deficiency

Ishihara cupcakes

What allows us to see color?

The retina (the tissue that lines the back of the eye) contains specialized cells called photoreceptors that respond to light. There are two types of photoreceptors: rods and cones.  Rods are responsible for vision in dim light, and cones are responsible for vision in bright light as well as color vision. There are three types of cones; each type is sensitive to a different wavelength, and thus a different color of light. An S cone is sensitive to short wavelengths (blues), an L cone is sensitive to long wavelengths (reds), and an M cone is sensitive to medium wavelengths (greens). The information gathered from these cones is used by the brain to create our perception of color.  Normal trichromatic color vision involves the presence and proper functioning of all three cones.  Color vision defects arise when one of these cones is missing or altered.

What is color blindness?
The term "color blind" is misleading, as most people use the term to describe someone with a color vision deficiency. They can see colors, but have trouble distinguishing between certain colors and shades.  Color vision defects can be inherited or acquired. 

• Inherited: These color vision deficiencies are present at birth, affect both eyes equally, and are stable over a person's lifetime. They occur in about 8% of males (the highest prevalence is found in non-Hispanic whites 1 ) and 0.5% of females (2).
• Acquired: Unlike inherited deficiencies, these occur during a person's lifetime, may affect one eye or both eyes unequally, and may progress. Eye diseases or injury, the use of certain medications, or exposure to certain chemicals can cause an acquired color vision issue.

What types of color vision deficiency exist? 3

1. Achromatopsia, also known as rod monochromatism, is true, total color blindness. There is a complete absence of functional cones, so everything is seen in shades of gray. This rare and severe condition is typically associated with other signs like light-sensitivity, poor vision, and nystagmus (eye jitter). 

• Inheritance pattern: Autosomal recessive
• Prevalence: 0.00003% of males 

• Partial or incomplete achromatopsia refers to even rarer conditions in which there is only one of the three types of cones present/functioning.  Blue cone monochromacy (BCM) is the most common of these, but with a prevalence of one in 100,000, it is still very rare.  

2. Dichromacy is a category of color deficiency in which there are only two types of cones present instead of three. 

• Protanopia- missing L (red) cone. 
• Inheritance pattern: X linked recessive
• Prevalence: ~1% of males, ~0.1% of females
• Deuteranopia- missing M (green) cone. 
• Inheritance pattern: X linked recessive
• Prevalence: ~1.5% of males, ~0.01% of females
• Tritanopia- missing S (blue) cone. 
• Inheritance pattern: Autosomal recessive
• Prevalence: 0.008%, males=females

3. Anomalous Trichromacy is a category of color deficiency in which all three types of cones are present, but one type of cone's sensitivity is off.  Specifically, one cone's spectral sensitivity is shifted, resulting in reduced sensitivity to the wavelength of light (color) that the cone is intended to absorb. These are the most common and most mild color deficiencies.

• Protanomaly- malfunctioning L (red) cone.
• Inheritance pattern: X linked recessive
• Prevalence: ~1% of males, 0.01% females
• Deuteranomaly- malfunctioning M (green) cone, or "green weak." This is the most common color vision defect.
• Inheritance pattern: X linked recessive
• Prevalence: ~5% of males, ~0.4% females
• Tritanomaly- malfunctioning S (blue) cone. 
• Inheritance pattern: Autosomal dominant
• Prevalence: 0.0002%, males=females

"Protan" is used to refer to both protanopia and protanomaly, and "deutan" is used to refer to both deuteranopia and deuteranomaly. Protan and deutan defects are both considered red-green color deficiencies. "Tritan" defects (tritanopia and tritanomaly) are considered blue-yellow deficiencies.

For the nerds out there, a Punnett not-so-square depicting why males are far more likely to present with an X-linked recessive inherited trait:

The biology behind red-green color blindness

So if a boy is red-green color deficient, it is likely that the boy's maternal grandfather was also red-green color deficient, and the boy's mother/grandfather's daughter was a carrier.

How is color deficiency diagnosed?
Color deficiency is usually detected in childhood, either when a child has difficulty with color naming or when he/she is checked during a routine eye exam.  Some tests that may be used include:

• Pseudochromatic plates: These tests involve distinguishing numbers or symbols amongst colored dots.  The Ishihara test is probably the most recognized of these tests (and the inspiration for the cupcakes seen at the top of the page).  The Ishihara test does not test for blue-yellow color deficiencies, whereas the HRR test does.   
• Arrangement tests: These tests involve arranging discs in color order, beginning with a fixed pilot disc. Two such tests are the Farnsworth-Munsel 100 (interestingly has only 85 discs, not 100) and the Farnsworth  D15. There is an online version of the D15 test here. Because color perception varies depending on the type of display and lighting you are working with, this test is not definitive or diagnostic. Based on my sample size of 2 (thanks Matt and Lee), it seems to be fairly accurate.
• Lantern: The Farnsworth Lantern (Falant) is a color-naming test typically used by US federal agencies to test color vision. 
• Anomaloscope: This is a color matching test. Half of a circle is presented as pure yellow, and the other half can be adjusted by the observer to varying proportions of red and green to achieve a perceived match.  

Fun Fact: All adult male squirrel monkeys are red-green color deficient. 

How are color vision deficiencies treated?

For the most part, those with color vision deficiencies learn to recognize color by different means (ie: brightness, location) and develop their own system of coping, whether that involves a special way of organizing things, or asking others for help when matching things. There is no cure for color vision deficiencies at this time, but there are some products out there that can potentially enhance color discrimination.

• Xchrom Lens: This is a red-tinted soft contact lens that is worn in the non-dominant eye.  The red tint is intended to enhance color perception by changing the lightness of colors (ie: makes greens look darker), increasing the number of shades a person can see. 
• Chromagen:  Chromagen lenses are available in glasses or contact lens form.  A range of 8 colored filters are available, and a series of tests are done to determine which color(s) is/are appropriate.  The filters are intended to change the wavelength of each color going into the eye, potentially enhancing color perception and discrimination. Chromagen lenses are also marketed as part of dyslexia therapy.
• EnChroma:  These lenses are designed for people with anomalous trichromacy (all 3 cones are present, but one cone's sensitivity is shifted).  EnChroma lenses use a notch filter to restore the spectral separation between cones, potentially reducing color confusion and enhancing color perception. 
• Gene therapy? This may be in our future.  In 2009, researchers from the University of Washington and the University of Florida (Go Gators!) cured color blindness in two squirrel monkeys using gene therapy 4. 

CliffsNotes: Most color vision deficiencies are inherited, not acquired. The most common form is red-green deficiency in males. 

Additional recommended resources:

• AOA: Color Deficiency
• NEI: Facts About Color Blindness
• NLM: Color Vision Deficiency
• EyeSmart: Color Blindness

Essential Eye Nutrients

Food and lifestyle choices can influence your eye health and may help to reduce the risk of some age-related eye diseases.  Check out this quick summary of the nutrients essential for good eye health.
*Disclaimer: I am not a nutritionist or a dietician.

• Zinc (red meat, poultry, oysters, beans, fortified cereal)
• The Food and Nutrition Board recommends: 11 mg/day for males, 8 mg/day for females
• Zinc is an essential mineral that is involved in cell metabolism.  It is highly concentrated in the retina and choroid of the eye.
• Vitamin E (nuts, seeds, vegetable oils)
• The Food and Nutrition Board recommends: 15 mg or 22.4 IU/day for both males and females
• Vitamin E is a powerful antioxidant that is involved in immune function.
• Vitamin C (citrus, strawberries, broccoli, red and green pepper, kiwi)
• The Food and Nutrition Board recommends: 90 mg/day for males, 75 mg/day for females
• Vitamin C is an antioxidant that helps absorb UV radiation. It is found in high concentrations in the lens, aqueous humor, and vitreous humor of the eye.  On a long term basis, increased intake of vitamin C (alone or in combination with other antioxidants) has been shown in some studies to reduce the risk of developing nuclear sclerotic cataracts (1, 2).

The National Eye Institute's Age-Related Eye Disease Study (AREDS) found that daily intake of 80 mg zinc, 2 mg copper, 500 mg vitamin C, 15 mg beta carotene, and 400 IU vitamin E could reduce the risk of progression to advanced Age-related Macular Degeneration (AMD) by about 25% and visual acuity loss by 19% in individuals at high risk for the disease (3).  Since that study, researchers have found that substituting beta carotene with 10 mg lutein and 2 mg zeaxanthin is safer and more effective (4).

• Vitamin A (beef liver, carrot, pumpkin, sweet potato)
• The Food and Nutrition Board recommends: 900 micrograms or 3000 IU/day for males, 700 micrograms or 2300 IU/day for females
• Vitamin A is essential in the proper functioning of the retina, as well as the conjunctival membrane and cornea.  Most Americans get sufficient vitamin A from their diet, but vitamin A deficiency (VAD) is actually the leading cause of preventable blindness in children world-wide (5).
• Lutein (kale, spinach, collards, egg yolks) & Zeaxanthin (gogi berries, orange peppers, corn)
• The American Optometric Association recommends: 10mg/day of lutein and 2mg/day of zeaxanthin
• Lutein and zeaxanthin are carotenoids that are abundant in the macula of the eye as well as the lens. They are antioxidants that help lower your risk for Age-related Macular Degeneration (AMD) and cataracts.

From Review of Optometry

A follow-up to AREDS- AREDS 2- found that people with the lowest dietary levels of lutein and zeaxanthin who added supplements of the two had a 26% reduced risk of developing advanced AMD.  Those with the lowest dietary intake of lutein and zeaxanthin also saw a 32% risk reduction in progression to cataract surgery with the introduction of these nutrients (6).  Most Americans only get 1-2 mg of lutein and zeaxanthin combined on a daily basis through their diet, so many of us fall into this category.

Fun Facts: Vitamins A, D, E, and K, as well as lutein and zeaxanthin, are fat-soluble.  A small amount of dietary fat (ie: olive oil, avocado, nuts, seeds) can help maximize the body's absorption of these nutrients.  Cooking (steaming, sautéing, or puréeing) leafy greens actually allows more access to the important nutrients by breaking down plant cell walls.

• Omega-3 Fatty Acids (wild salmon, mackerel, tuna, sardines)
• The American Heart Association recommends eating at least two servings of fish (especially oily fish)/week
• The 3 types of omega-3 fatty acids the body uses are ALA, DHA, and EPA.  DHA and EPA have structural and protective functions in the retina, and they also have anti-inflammatory properties.  Beyond the cardiovascular health benefits, diets rich in DHA and EPA have been linked to a significant improvement in dry eye symptoms (8, 9). 

Eye vitamins and other supplements can help you meet your recommended daily intake of these nutrients. Be sure to discuss this with your optometrist or ophthalmologist.  I also recommend communicating with your primary care doctor before beginning any supplementation regimen- it's important to give your PCP the full picture of what you do to manage your health.

If you want to find an optometrist that specializes in ocular nutrition, look here.

CliffsNotes: What you eat can influence your eye health and may help to reduce the risk of age-related eye diseases.  

Additional Recommended Resources:

• AOA: Diet and Nutrition
• All About Vision: Nutrition
• Get EyeSmart: Diet and Nutrition
• Eyefoods: A Food Plan For Healthy Eyes
• Visionary Kitchen: A Cookbook For Eye Health

Keratoconus

A topographical map of the front surface of the eye in a patient with keratoconus.

The red indicates the steepest part of the cornea (the cone).

What is keratoconus?
Keratoconus is a degenerative disease in which the cornea (the clear tissue of the front of the eye) progressively thins and bulges. This area of bulging resembles a cone. The National Keratoconus Foundation (NKCF) has a good introductory video here.

What are the symptoms?
The cornea is normally smooth and dome-shaped, and this allows light to focus clearly on the retina. With keratoconus, the irregularity of the cornea prevents light from focusing clearly on the retina, causing blurred and distorted vision. Glare and light-sensitivity may occur as well. Symptoms usually present in the late-teens/early-twenties. The disease generally affects both eyes, though one eye can be more severely affected than the other.

How is it treated?
Early in the disease process, eyeglasses or soft contact lenses may do the job. But as the cone progresses, the cornea becomes more irregular, and vision cannot be adequately corrected with glasses or soft contacts. Thus, you need something that will help mask the irregularity of the front surface of the eye. That's where rigid gas permeable (RGP or GP) contact lenses come in.

Image from NKCF

The type of contact lens that is best depends on the severity of the disease and the location of the cone. Some of the options available include:

• Custom soft contact lenses- custom-made soft lenses specifically designed for keratoconus.
• Corneal GP lenses- small rigid lenses that sit on the clear part of the eye (cornea).
• Scleral GP lenses- large-diameter rigid lenses that sit on the white part of the eye (sclera). 
• Hybrid lenses- lenses with a GP center and a soft periphery, or skirt.
• "Piggybacking"- when a GP lens is placed on top of a soft lens for increased comfort.

Fitting a keratoconic patient in any of these contact lenses is typically considered a specialty fitting, so you can expect to spend a little more time and money on the process and the lenses compared to a standard soft contact lens fitting. Be sure to discuss the options with your optometrist to figure out what is best for your individual case. If he/she is not able to perform the specialty fitting, he/she can certainly refer you to an optometrist in your area that can.

What are the surgical treatment options?
For various reasons, some cases of keratoconus cannot be treated with the above methods. In such cases, surgery may be indicated.

• Intacs® are small, semi-circular inserts that are implanted into the middle layer of the cornea to flatten the cone-like area. Intacs are FDA approved for the treatment of keratoconus.

Intacs®

• Collagen cross-linking (CXL) is a treatment procedure that involves administering riboflavin eyedrops and then exposing the cornea to UV-A light. This promotes cross-linking of the collagen fibers of the cornea, which stiffens the cornea and prevents further bulging. The outer layer of the cornea is removed in epithelium-off CXL, and left intact in epithelium-on CXL. CXL gained FDA approval in the US in April 2016, and we are starting to see some insurance coverage for the procedure.

• Conductive keratoplasty (CK) involves using heat in the form of radiofrequency energy to change the curvature of the cornea. Using a thin probe, radiofrequency energy is applied to small areas of the cornea in a circular pattern, creating constriction of the corneal tissue and reducing astigmatism. This procedure is being done by some doctors (off-label) in conjunction with CXL and/or Intacs.

• Corneal transplants may be necessary in 10-20% of keratoconus cases. In a full-thickness corneal transplant, or penetrating keratoplasty (PK), the diseased cornea is removed and replaced with a healthy donor cornea.  Deep anterior lamellar keratoplasty (DALK) is a partial-thickness corneal transplant that leaves the bottom layer of the patient's cornea intact, only transplanting a portion of the donor cornea. This potentially allows for a faster healing time and less risk for graft rejection (1).

Photo of a patient post-PK

CliffsNotes: Keratoconus is an eye disease that causes irregularity in the front surface of the eye. The best treatment is determined on a case-by-case basis, so talk to your eye doctor about what your options are.

Additional Recommended Resources:

• National Keratoconus Foundation
• AOA: Keratoconus
• All About Vision: Keratoconus

Glaucoma Awareness Month

January is Glaucoma Awareness Month, so I think we all know what this month's post is about. According to the World Health Organization (WHO), glaucoma is the second leading cause of blindness world-wide (1), and Open Angle Glaucoma affects more than 2 million people in the US alone (2).

A cupcake rendition of the optic nerve

What is glaucoma?
Glaucoma is a group of diseases that damage the optic nerve. The optic nerve is like a cable, made of about 1 million nerve fibers, that sends signals from the retina (the tissue that lines the back of the eye) to the brain. When the optic nerve is damaged, permanent vision loss results.

There are many types of glaucoma, the most common of which is Primary Open Angle Glaucoma (POAG). In most cases of glaucoma (but not all!), the pressure within the eye is higher than normal. The eye has a clear fluid, called aqueous humor, that circulates in the front part of the eye and flows out through a structure called the angle (here is a good visual). Through various mechanisms, this fluid may not flow out of the front of the eye properly, leading to elevated eye pressure, or intraocular pressure (IOP). High IOP can slowly damage the optic nerve over time.


How do I know if I have glaucoma?
Most cases of glaucoma do not have symptoms early on. Peripheral vision loss occurs, but by the time you notice this, the disease is substantially progressed and significant, permanent vision loss has occurred. Since there really aren't symptoms to watch out for, early detection during yearly eye exams is key.

There are some factors that increase risk for glaucoma:

• a family history of glaucoma
• being of African-American or Hispanic descent
• older age 
• high eye pressure
• thin corneas

*Note: People of any age, race, or eye pressure level can have glaucoma.

Glaucoma can also result from another disease or condition, like eye trauma or inflammation. Additionally, some studies have shown a relationship between glaucoma and conditions of altered blood flow (ie: sleep apnea, diabetes, migraines, high or low blood pressure).

How is glaucoma diagnosed?
Glaucoma is a complex disease, so many areas need to be looked at to obtain an appropriate diagnosis and management plan. Some procedures that may be involved in the diagnostic/management process:

• Tonometry: A tonometer probe is used to gently measure intraocular pressure (IOP) in millimeters of mercury. "Normal" IOP is typically under 21mmHg, though you can have glaucoma with normal IOPs. You can also have higher than normal IOPs without having damage to the optic nerve.
• Perimetry/Visual Field Testing: A visual field test assesses the function of the optic nerve. When enough nerve fibers are damaged, you develop missing spots in your field of vision. The visual field test picks up these missing spots early on (far, far sooner than you would pick them up on your own). 
• Pachymetry: This test measures the thickness of the central part of your cornea. Thinner corneas not only underestimate eye pressure, but they have also been shown to be a risk factor for glaucoma progression (3,4).
• Gonioscopy: A lens with mirrors is placed on the eye, allowing your eye doctor to get a closer look at the drainage angle of the eye. 
• Dilated fundus exam: Your eye doctor uses a high-powered lens and a slit lamp to evaluate the health of the optic nerve, best viewed through a dilated pupil. Your eye doctor may choose to take photos as well, similar to the one seen below.
• GDx, HRT, OCT: In addition to directly evaluating the optic nerve, there are scans that can be used to image the optic nerve and further assess its structure. These scans give information on the amount of optic nerve tissue loss and the rate of nerve fiber thinning. The structural defects on the optic nerve should correlate with the functional defects in the visual field.

End-stage glaucoma

How is glaucoma treated and managed?
There is no cure for glaucoma, but the damage it causes can be slowed by the use of medications and/or surgery. Of the risk factors mentioned, the only one we can control is eye pressure, so that is the focus of treatment. The goal is to increase outflow of the fluid in the eye, or decrease production of that fluid, or both. This can be achieved using medications (typically eye drops), laser procedures, or glaucoma surgery. The course of treatment depends on both the type and severity of glaucoma.

What about marijuana?  I've been asked this question more than once. Marijuana is not a legitimate form of glaucoma therapy. Yes, a study from 1971 showed that smoking marijuana reduces eye pressure, but only for about 3-4 hours after smoking (5). Glaucoma management requires around-the-clock IOP control. Drops and/or surgery are much more effective at lowering IOP over the long-run and are less detrimental to your health, so I do not consider marijuana an appropriate glaucoma treatment.


What is the prognosis?
Left untreated or uncontrolled, glaucoma can lead to blindness. But the good news is it can be diagnosed on routine eye exams, and it is treatable. See your optometrist at least every 1-2 years, or more frequently if warranted. This is especially important for those with several risk factors. If you are diagnosed with glaucoma, it is important that you are compliant with the treatment and follow-up regimens outlined by your eye doctor. Effective management requires a team effort between you and your eye doctor.


CliffsNotes: Glaucoma is a leading cause of blindness. Know the risk factors and see your eye doctor regularly!

Additional Recommended Resources:

• Glaucoma Research Foundation
• AOA: Glaucoma
• NEI: Glaucoma
• All About Vision: Glaucoma

Merry Christmas!

Wishing you all a very merry Christmas and a happy New Year!

contact lens DOs and DON'Ts

Cutest contact lens case in history? Obviously.

While many people wear contact lenses without ever having any issues or complications, thousands of Americans are affected annually by contact lens-related eye infections.  In my own experience treating such cases, many, if not all, are due to poor contact lens hygiene and/or contact lens abuse.  A recent American Eye-Q® survey (1) found that, among contact lens wearers:

• 57% admitted they wore disposable contacts longer than directed
• 54% reported they waited 4 to 6 months or longer to change contact lens case
• 39% stated they did not clean lenses with a multipurpose solution daily
• 35% said they did not wash their hands prior to handling lenses
• 26% stated they wore lenses while swimming
• 21% reported they slept in contacts

Those are dangerously high percentages! So this blog post is my attempt to help spread the word on contact lens hygiene.  Here are my "DOs and DON'Ts" of contact lens wear.   

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DOs

1. DO wash and dry your hands thoroughly before touching your contact lenses.  Your fingers have a ton of germs on them, and your contacts can act as a vector, transmitting bacteria to your eye (2).

2. DO remember to care for your case.  Rub and rinse your contact lens case with contact lens solution (not water), and store it upside down on a clean tissue.  Change your case AT LEAST every 3 months- I would recommend monthly.  A study done in 2010 showed that, on average, well over half (as many as 81%) of the contact lens cases tested were contaminated (3).  

3. DO clean your contact lenses regularly.  When removing your contact lenses, place the lens in the palm of your hand and pour multi-purpose solution onto the lens.  Gently rub the lens with your finger and rinse the lens with solution.  THEN you can put the lens in a clean case, fill with fresh solution, and store.  Researchers have demonstrated that the most effective cleaning regimen involves rubbing AND rinsing contact lenses before storage (4).

4. DO remove lenses and see your optometrist immediately if you experience eye redness, pain, discharge, sensitivity to light, or blurred vision.  These are often symptoms of an eye infection, and should be addressed quickly to avoid further complications.  

5. DO have a back-up pair of glasses. In the event that you lose a contact or have to be out of your contacts due to an infection, having a pair of glasses is a must.

6. DO see your eye doctor every year.  Even though you may feel your prescription hasn't changed (and it may not have), a yearly exam and contact lens evaluation allows your eye doctor to check the health of your eyes as well as your vision.

DON'Ts

1. DON'T swim or shower or go in a hot tub with contact lenses in. The goal is to keep water away from your contact lenses, because water contains all sorts of nasty microbes. When those microbes come in contact with your lenses, they can adhere to them and infect your cornea (the clear part of the front of the eye). A certain parasite found in water and soil called Acanthamoeba can cause a rare but very serious infection that is difficult to treat (Google "Acanthamoeba keratitis"- the images will scare you straight).  Some studies have concluded that one-third of Acanthamoeba keratitis cases are associated with swimming, and the risk of infection with this parasite is six times greater when swimming in contact lenses (5). For safe swimming and water sports, consider talking with your eye doctor about the option of prescription goggles. If contact lenses must be worn, reduce your risk of complications by wearing daily disposable contact lenses with air-tight goggles, trashing the contacts once you're out of the water. 

2. DON'T wear your lenses for longer than your eye doctor recommends.  I come across many patients that do this in order to save money.  I can pinch a penny with the best of them, so I can certainly understand the motivation behind this.  But trust me when I tell you that the cost of treating an eye infection (multiple office visits, prescription eye drops, etc) could cost you far more than what you could save by stretching out your lenses.  Not to mention the fact that you only get one pair of eyes, so the risk really isn't worth it.

3. DON'T "top off" the existing solution that remains in the case.  The used solution has been exposed to microbes from your contact lenses and from the case. Dump out all of the old solution in the case, clean the case, and use fresh multi-purpose solution every time you store your lenses.

4. DON'T sleep in your contact lenses.  Some contact lenses are FDA approved for extended wear, but I personally do not recommend sleeping in any soft lenses.  Sleeping in contact lenses increases your risk of infection- studies show anywhere from a four to ten times greater risk (6,7).  If you are using extended wear contact lenses, make sure you take them out at least once a week to clean and disinfect them overnight.  If you are considering extended wear contact lenses, I encourage you to talk to your optometrist and discuss the risks/benefits. 

5. DON'T wear any type of contact lens- even cosmetic or costume lenses that don't provide visual correction- without a prescription.  Contact lenses are FDA classified as medical devices, and require a valid prescription from an eye care professional.

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After reading this list, it's probably not hard to see why I'm a big fan of daily disposable contact lenses.  You eliminate most of these areas of risk- no case, no solution, no cleaning regimen.  Convenient and safe.  We'll talk in more detail about daily disposable lenses in a future post.  

CliffsNotes- DO: wash your hands, rub and rinse CLs, rub and rinse case, take CLs out and see optometrist if experiencing problems, have a pair of back-up glasses.  DON'T: let water come in contact with CLs, stretch the life of CLs, re-use existing solution, sleep in CLs, buy/wear CLs without a valid Rx.  

Additional recommended resources:

• Contact Lens Safety
• CDC: Contact Lenses
• AOA: Contact Lenses
• All About Vision: Contacts

diabetes and the eye

Today is World Diabetes Day, and this month is American Diabetes Month, so it seems like a great time to talk about diabetes!  I would venture to guess that almost everyone reading this post either knows a person with diabetes or has diabetes themselves.  According to the 2014 Center for Disease Control (CDC) National Diabetes Statistics Report,  9.3% of the US population has diabetes, and 27.8% of those with diabetes are undiagnosed.  That means nearly 1 in 10 Americans have diabetes, and more than 1/4th of those with it don't know it.  The increasing prevalence of diabetes has been referred to by many as a global epidemic.  In this post, we'll briefly go over what diabetes is, and then we'll go into detail about how diabetes affects the eye.

What is diabetes?  Diabetes is a group of diseases characterized by high blood glucose levels as a result of either a problem with how insulin is produced or how insulin works, or a combination of both.  Glucose is the body's main source of energy, derived from the food we eat.  Insulin is a hormone that allows glucose to be absorbed from the blood and converted to energy.    

• In Type 1 diabetes, the body attacks the beta cells of the pancreas, which are responsible for producing insulin.  As a result, the pancreas produces too little or no insulin.  So a person with Type 1 diabetes depends on outside sources of insulin, such as insulin injections, insulin pumps, or even inhaled insulin, to be able to metabolize glucose.  This type of diabetes is typically diagnosed early in life, and it accounts for about 5% (CDC) of diabetes cases.  
• In Type 2 diabetes, the body either doesn't produce enough insulin or the cells of the body don't use the insulin produced properly.  Most people with this type of diabetes are able to achieve blood glucose control with diet, physical activity, blood glucose monitoring, and oral medication as needed, although some require insulin therapy.  Type 2 diabetes is usually diagnosed in adulthood, and accounts for 90-95% of all cases.  Risk factors for Type 2 diabetes include a positive family history of the disease, being overweight, physical inactivity, age, ethnicity (African Americans, Hispanics/Latinos, Native Americans, Asian Americans and Pacific Islanders are at particularly high risk), a history of gestational diabetes, pre-diabetes, high blood pressure, and high cholesterol (AOA).
• Gestational diabetes is glucose intolerance experienced during pregnancy.  It is typically diagnosed during the 2nd or 3rd trimester.
• Pre-diabetes is a term used to refer to above-normal blood glucose levels that are below the threshold for diagnosing diabetes.  Intervention (ie: healthy diet, physical activity, weight loss) can reduce the rate of conversion from pre-diabetes to diabetes.  In 2012, the CDC found that 37% of Americans 20 years of age and older fell into the pre-diabetes category in 2009-2012 (CDC).

How does diabetes affect the eye?   

• The most common eye disorder associated with diabetes is diabetic retinopathy.  Diabetic retinopathy is progressive damage to the small blood vessels that supply the tissue lining the inner surface of the eye (the retina).  The AOA has some great videos to help illustrate the disease here.  Diabetic retinopathy is the leading cause of new cases of blindness and low vision in Americans age 20 to 74 (AOA).   In 2005–2008, of adults diabetics 40 years of age or older in the US, 28.5% had diabetic retinopathy, and 4.4% had advanced diabetic retinopathy—with conditions such as clinically significant macular edema and proliferative diabetic retinopathy—that could lead to severe vision loss (CDC).  

Proliferative diabetic retinopathy

• Non-proliferative diabetic retinopathy (NPDR) occurs when the capillaries of the retina balloon (microaneurysms) due to weakening of the vessel walls.  They may leak blood (hemorrhages) and/or fat deposits (exudates) into the retina.  NPDR is classified in 3 stages- mild, moderate or severe.  No treatment is typically indicated for milder cases, unless there is clinically significant macular edema present.  Frequent monitoring is imperative, as more severe cases may warrant treatment. 
• Proliferative diabetic retinopathy (PDR) occurs when blood vessels that nourish the retina become blocked, shutting down the blood supply to parts of the retina.  The retina then sends out signals to grow new blood vessels (neovascularization).  These new vessels grow on the surface of the retina and/or into the gel-like substance that fills the back of the eye (the vitreous).  These new vessels are bad news because they have weak, thin walls that can leak blood or cause scar tissue to grow.  PDR has a high risk of vision loss if it is left untreated. Typical treatments may include scatter laser therapy (PRP), injecting medications into the eye (intravitreal injections), and/or removing and replacing the vitreous (vitrectomy).   
• Macular edema results when the capillary walls weaken and allow fluid to leak into the area of the retina that is responsible for your central, sharpest vision (the macula).  Macular edema can occur at any stage of diabetic retinopathy, and can even be present with 20/20 vision.  Like PDR, macular edema has a high risk of vision loss if it is left untreated.  Treatment for macular edema is typically intravitreal injections, and/or focal laser therapy if needed.  

              Some tools your eye doctor may use to diagnose and evaluate diabetic retinopathy: 

• Retinal cameras are used to take photographs of the retina (like the one seen above) in order to monitor retinopathy.
• Optical coherence tomography (OCT) can be used to evaluate how much swelling or edema is present.  An OCT is a scanning laser that can assess the thickness of the retina. 
• Fluorescein angiography (FA) may be ordered to evaluate leakage and guide treatment.  A dye is injected into a vein in the arm and photos of the retina are taken as the dye reaches the retinal vessels.

• Other eye disorders associated with diabetes: 

• A cataract is a clouding of the lens of the eye.  It often develops earlier and progresses more rapidly in diabetics compared to non-diabetics (AOA).  
• Eye movement disorders can occur secondary to diabetic neuropathy, resulting in double vision.  
• The optic nerve can also be affected.  There may be swelling (diabetic papillopathy) or damage due to insufficient blood supply (anterior ischemic optic neuropathy) (ADA).
• Diabetes can also cause blurred vision in the absence of all of the above.  It is believed that high blood glucose levels cause increased fluid absorption in the lens of the eye, thus changing its shape and causing fluctuations in vision.  

How often should a diabetic see their optometrist?  Type 1 diabetics should have a dilated eye exam within 5 years of diagnosis, and at least yearly thereafter.  Type 2 diabetics should have a dilated eye exam upon diagnosis, and at least yearly thereafter.  Your optometrist may suggest more frequent examinations based on the presence and severity of diabetic eye disease, or they may refer you to a retinal specialist for treatment if indicated.  The strongest predictor of diabetic retinopathy is the duration of diabetes, so it becomes increasingly important for diabetics to have regular eye exams.  Do not wait for symptoms to see your eye doctor!  Early intervention is key.

CliffsNotes: Diabetes is a multi-organ disease, and avoiding its complications takes a life-long commitment.  Yearly dilated eye exams are an important part of that commitment.  

Additional recommended resources:

• NEI: Diabetic Eye Disease
• AOA: Diabetic Retinopathy
• American Diabetes Association (ADA) 
• dLife
• World Diabetes Day