What is a Cataract?

As you get older, you will probably find that your eyesight isn’t as good as it once was.  While there are a number of issues associated with the ageing eye, cataract is one of the most common.  Cataracts are part of the normal ageing process, and they affect thousands of Australians every year. Because they are so common, it is not surprising to learn that cataract surgery is one of the most common surgical procedures performed in Australia.

Simply put, a cataract is a clouding of the eye’s lens. The lens is located just behind the iris (the coloured part of the eye). It is approximately the size of a Smartie®, and is responsible for focusing the incoming light onto the retina, which is the ‘camera film’ at the back of the eye. The lens is not visible to the naked eye, and is examined with specialist equipment.

As the lens becomes cloudy, it obscures the incoming light from reaching the retina. Cataract therefore reduces our quality of vision.  Cataract formation is accelerated by UV light, diabetes, short-sightedness, eye trauma, iritis, certain eye surgeries, certain medications and certain medical conditions.

What are the different types of cataract?

Cataracts are classified into different subtypes according to their appearance, as well as the location of the clouding within the lens.

The three most common forms of cataracts are the following, although there are many other forms of cataract as well.

  • Nuclear sclerosis cataract
  • Cortical cataract
  • Posterior subcapsular cataract

Each form of cataract has slightly different symptoms, because of the differences in the density and location of the lens opacity. Most patients have a combination of multiple types of cataract.

nuclear sclerosis cataract

Nuclear sclerosis cataract

Nuclear sclerosis cataracts appear as a yellow discolouration of the lens. This yellowing is not visible to the naked eye, but can be seen with a special microscope.

The yellow discolouration is caused by accumulation of urochrome within the lens.  Urochrome is a yellow pigment (the same one that colours your urine).

Nuclear sclerosis cataracts are predominately caused by UV light exposure, which causes free radical damage to the lens.  As a result, the delicate proteins of the lens clump together. The lens is naturally transparent, but loses this transparency when this happens.

If you have nuclear sclerosis cataracts, you might notice blurred vision, halos around lights, dullness of colours, and that you are unable to see clearly in dim lighting.

Cortical cataract

These cataracts are spoke-like opacities (or spokes of darkness) radiating from the periphery, or edge, of the lens towards the centre of the lens. We don’t really know what causes them.

It could be that they are a result of constant, lifelong stretching of the lens that occurs every time we attempt to focus our eyesight from the distance to near. This process is called “accommodation”. It involves a muscle inside the eye, called the ciliary body, pulling on our lens. This changes the shape of the lens, to bring what we are looking at into focus.

The constant stretching and relaxation of the lens shape stresses protein fibres within the lens. This trauma causes clefts (splits) of water to form within the lens, and the degeneration of lens proteins. Eventually, spoke-like calcifications of the lens occur.

If you have cortical cataracts, you will notice blurred vision, as well as  seeing halos and streaks around lights.

Posterior subcasular cataracts

Posterior subcapsular cataracts appear like a plaque on the back surface of the lens. These cataracts develop rapidly, and can profoundly reduce vision. It is not uncommon for a patient with posterior subcapsular cataract to lose all functional vision in the affected eye within just six months.

Posterior subcapsular cataract is caused by cells inside the lens suddenly growing in a disorganized fashion. These cells migrate to the back surface of the lens, where they are known as ‘Bladder cells’ or ‘Wedl cells’. Instead of growing as translucent, thin strands, these cells grow as dense, round clumps. This gives the appearance of an opaque plaque on the back surface of the lens.

What are the symptoms of cataract?

Even though cataract is a simple problem to understand, cataracts cause a myriad of different eye symptoms. This is because cataract doesn’t just make the lens cloudy: the cataract will also change the colour of the lens, as well as the way it focuses light.

Here are some of the common symptoms of cataract.  If you experience these, you should see an eye specialist.

Reduced clarity of vision.

All forms of cataract absorb and scramble incoming light. This causes the image on the retina to be dimmed and blurred.

Patients with cataract may notice that their vision is not as sharp as it used to be, even with updated glasses. Some patients can still read the vision chart, but cataract causes a softness to the vision. Each letter will no longer be as crisp and defined.