Diabetes and Your Eyes - Part 1

It is now universally accepted that having diabetes can cause or contribute to a number of eye diseases. Most commonly cited, is 'diabetic retinopathy', in which damage to the small blood vessels at the back of the eye may lead to significant loss of vision. This diabetes-specific condition is considered in detail in "Diabetes and Your Eyes - Part 2".

In addition to retinopathy, diabetes can cause changes in vision requiring new prescription glasses, and it can also lead to other problems, including glaucoma and cataracts.



What's covered on this page:

How The Eyes Normally Work
Eye Examinations and Screening
Blurred Vision - Refractive Errors - Prescription Glasses
Cataracts

Detecting Cataracts
Cataract Surgery
Glaucoma
Detecting Glaucoma
Treating Glaucoma




 

How The Eyes Normally Work

In order to understand what can go wrong with the eyes as a result of having diabetes, it is necessary to learn how the eyes normally function.

Figure 1 shows a diagram or 'schematic' of the eye.

<<<< FIGURE 1 >>>>

 

The eye is shaped just like a ball. A tough outer covering called the sclera protects the eye and 'holds it together'; it is often referred to as the 'white' of the eye. The thin membane covering the sclera is the conjunctiva, and this contains blood vessels.

The curved area at the very front of the eye is the cornea, which is clear. The cornea is very sensitive, and has an intricate nerve supply. The angle between the cornea and the iris is the drainage angle, where fluid leaves the eye in order to maintain a balanced pressure within the eye.

Light enters the eye through the cornea and then through the hole in the iris called the pupil. The pupil controls the amount of light entering the eye and is open wide in dim light, but is much smaller in bright sunlight. The light is then focused onto the back of the eye (retina) by the lens. If the light focuses in front or behind the retina, then this is called a refractive error, which needs correction with the right prescription lens as glasses or contact lenses. A cloudy area in the lens is a cataract and can obscure vision.

Light landing on 'photoreceptor' cells of the retina is converted into electrical messages that are then sent to the brian via the optic nerve. (The retina does not contain pain- or touch-sensitive nerve fibres). An efficient network of blood vessels supplies the cells of the retina, supplying oxygen and nutrients essential for it's proper functioning.

There are two types of vision - central (straight-ahead) vision and peripheral (side) vision. The section of the retina concerned with central vision is known as the macula.

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Eye Examinations and Screening

Regular eye examinations are a crucial part of your diabetes health care, even if you have perfect "20/20" vision and are not aware of any problems with your eyes.

You should ideally have your eyes checked out at least every year by a qualified ophthalmologist with experience of screening for diabetic retinopathy. Your health care team should be able to advise you on screening procedures operating in your locality. Screening should include a dilated eye examination for a thorough investigation of the retina, and ideally you should also have the pressure in your eyes checked too. (See the sub-section of "Your Diabetes Care", "Eye Examinations", for more information.)

Guidance for Health Professionals:

Ministry of Health publications: http://www.health.govt.nz/publication/diabetic-retinal-screening-grading-monitoring-and-referral-guidance

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Blurred Vision - Refractive Errors - Prescription Glasses

Normally, light enters the eye, passing through the cornea and lens, and is then focused on the retina. The lens inside the eye adjusts to allow your eyes to focus, and lets you 'see' things clearly, either close up or far away.

'Refractive error' is said to occur if the light does not focus on the retina; if the light focuses in front of the retina then the person is said to be 'short-sighted' and if the light focuses beyond the retina then the person is said to be 'long-sighted' (See Figure 2).

 

<<<< FIGURE 2 >>>>

 

Short-sightedness or 'myopia' is common in many people and occurs as a result of a change in the shape of the eye. This frequently occurs during childhood/adolescence, and this 'refractive error' remains for the life of the individual.

People with diabetes frequently suffer from temporary changes in vision; these tend to be as a result of changes in blood glucose level, rather than being due to a permanent change in the shape of the lens. When the blood glucose level rises, the glucose concentration in the fluid of the eye also rises and the lens swells.

Permanent refractive errors are treated by prescription glasses or contact lenses. It is important to be able to distinguish the permanent problem from the temporary effect caused by high or variable blood glucose levels. A pair of glasses prescribed on a "bad day" may not be appropriate for your daily needs. It makes sense then, to get your eyes tested when your blood glucose levels are stable and near-normal! So when you go to your opticians for a regular sight check, be sure that your blood glucose levels have been relatively stable for at least a couple of weeks.

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Cataracts

When a portion of the lens becomes cloudy, this is known as a cataract. It may block incoming light and can lead to loss of vision.

As we age we naturally tend to lose some of the transparency of the lenses in our eyes, and cataracts are quite common in older people.

Sometimes cataracts do not obscure vision to any noticeable extent, in which case, you may be surprised to hear that you have a cataract.

Prolonged high blood glucose levels can also lead to cloudy areas developing in the lens. Proteins in the lens may clump together, forming cloudy areas that distort or block vision. Sometimes double vision occurs.

Detecting cataracts

Some people with cataracts say that their vision is blurred, or it seems as if they are looking through fog. Sometimes the light is scattered by the cloudy areas in the lens and this leads to a 'glare' effect, which may be particularly noticeable on bright sunny days or when driving at night. Halos may appear around lights. Sometimes cataracts cause diminished contrast, and colours appear dull. Cataracts can also cause double vision.

Cataract surgery

Cataracts may not cause serious problems with vision, and may go untreated for many years. However, if you are suffering from decreased vision which is affecting daily activities - such as reading, or driving - then surgery may be necessary.

The cloudy lens is removed, and replaced with new clear plastic one. Antibiotic eye drops may be needed for a few days afterwards, in order to prevent infection. Inflammation may be reduced with eye drops containing a steroid mixture. The surgery shouldn't take longer than about an hour, and is usually done under a local anaesthetic (the eye is made numb by an injection, and the person stays awake throughout the procedure).

Cataract surgery is generally very successful in restoring good vision - in people who don't have diabetes, that is. In people with diabetes, the impact of cataract surgery on vision is reduced. This is probably due to other complicating factors, such as retinopathy.

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Glaucoma

Glaucoma is another common eye problem that is common in many people as they get older, including those with diabetes. The pressure of the fluid in the eye rises, causing damage to the optic nerve - the nerve that relays vision information from the eye to the brain. This can lead to loss of vision and, potentially, blindness.

The increased pressure that is characteristic of glaucoma is caused by partial or complete blockage of the system that usually allows drainage of fluid from the eye. There are two main types of glaucoma that are both more common in people with diabetes:

  • Open angle glaucoma - the angle where the cornea meets the iris (see Figure 1) isn't sufficient for drainage of the fluid from the eye;
  • Neovascular glaucoma - new blood vessels grow into the iris (rubeosis) and block drainage. This is sometimes the case in proliferative retinopathy.


What's your risk of glaucoma?

  • People over 60 years of age are more likely to develop glaucoma
  • People with diabetes are, in general, twice as likely to develop glaucoma than people who do not have diabetes; this risk is even greater in those with advanced diabetic retinopathy
  • African Americans additionally have a higher risk of developing glaucoma than people of other ethnic origins

 

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Detecting glaucoma

Initially, glaucoma starts off as a silent disease - although you have no symptoms, damage is occurring to the eye without your knowledge. As the disease progresses though, 'peripheral' (side) vision starts to become affected. The loss of peripheral vision continues, and eventually central vision may become affected too.

Glaucoma is diagnosed using three procedures:

  • Measurement of pressure in eye(s) (tonometry)
  • Examination of optic nerve
  • Peripheral vision assessment (visual field test)

The drainage angle in the front of the eye an be examined using a procedure called gonioscopy, in which a special magnifying contact lens is placed on the eye.

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Treating glaucoma

There are no magic cures for glaucoma, but some treatments can slow the disease progression, providing it is detected early on.

Medication is usually the first line of treatment, and this may be in the form of eye drops and/or tablets:

  • Eye drops work to reduce the pressure in the eye either by reducing the amount of fluid that it is produced, or by increasing the efficiency of the drainage network in th eye.
  • Oral medications (i.e. tablets) also work by reducing the production of fluid.

Laser surgery may be necessary to increase the outflow of fluid. It has a relatively high initial success rate (80 %) but more surgery may be required later. Other surgical techniques for improving drainage of the eye are sometimes used.

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