How a Cataract Affects Vision

Looking through a cataract is similar to looking through a dirty window. Unlike a dirty window, however, a cataract cannot be cleaned, and the process by which they develop is irreversible. It is helpful to understand how a cataract can affect your vision once it has developed. Below is a list of symptoms and visual effects caused by cataracts, as well as some pictures to help you visualize how these changes may appear to you.

There are multiple ways in which a cataract affects vision. These include:

Decreased vision (often described as cloudy, hazy, foggy or blurry vision)

glare and light sensitivity

halos around lights

loss of contrast sensitivity

changes in color perception

shifts toward nearsightedness (myopic shift or increased myopia)

difficulty performing tasks in low light conditions

ghosting or monocular double vision (double vision that occurs in only one eye)

frequent glasses prescription changes

difficulty driving at night, twilight, or in poor weather conditions

Any combination of these symptoms may affect a patient with a cataract. However, some of these symptoms may go unnoticed, due to the fact that cataracts have a tendency to develop over years, or even decades. In other words, your changes in vision may be so subtle and slow that you do not immediately notice them.

Decreased or Blurry Vision

Decreased or blurry vision results from the eye’s inability to focus light precisely on the retina. In the early stages of cataract formation, particularly nuclear cataract formation, distance vision may become more blurry, whereas near vision may become clearer. This is due to the fact that the cataract initially causes the lens to bend light more, leading to an increased myopic refractive error, or better near vision. In later stages, there is increased scattering of light, and the lens is no longer able to bring light rays to focus at a single point. This is why glasses can no longer correct your vision when a cataract progresses beyond a certain point. Glasses can only shift the natural point of focus; they cannot bring scattered light rays to a single focal point.

Photographic simulation of clear versus blurry vision as it might appear in a patient with a cataract.

Glare and Light Sensitivity

Glare is very common in the setting of cataracts. It results from the scattering of light that occurs because of the irregularities in the cataractous lens. Not only does glare affect the clarity of your vision, but some patients also find it painful. Many will identify this symptom as light sensitivity. You can minimize your discomfort from glare by controlling the light around you whenever possible.

Photographic simulation of how glare might appear due to bright sunlight in the presence of a cataract.

Halos around Lights

Halos appear as a ring of light around a light source, such as oncoming headlights or street lamps. This tends to affect patients at night, particularly if they are driving.

Glare and halos can cause significant drops in vision that occur temporarily while viewing an oncoming light source. For this reason, ophthalmologists frequently perform “glare testing” during a cataract evaluation to determine visual function in certain lighting conditions. Patients may test well in perfect conditions, but in certain situations find their vision drops below that which is legal for driving and/or below a level with which they are comfortable. Most patients are aware of this phenomenon when it occurs; if this happens to you, you should be sure to report it to your eye care provider.

Photographic simulation of how halos might appear around light sources during night driving in the presence of a cataract.

Loss of Contrast Sensitivity

Loss of contrast sensitivity occurs because there is less light, or visual information, reaching the retina. Contrast sensitivity allows you to distinguish subtle differences in colors and between objects. It also allows you to determine where one object ends and another begins, or to distinguish an object clearly from its background. In other words, objects that have small differences in contrast become difficult to distinguish.

Changes in Perception of Color

As the cataract forms, colors start to look dull and “washed out”. Whites may take on a yellow or gray hue, and because this happens over many years, you may not notice it occurring. However, patients will almost always notice the brightness of color following cataract surgery and see the “yellowing” or dullness of color when comparing an eye that still has a cataract to an eye that has had a cataract removed.

Photographic simulation of how changes in color perception may appear in the presence of a cataract.

Monocular Double Vision or Ghosting

Monocular double vision is a term that refers to double vision that occurs when viewing with only one eye. This is in contrast to binocular double vision, which occurs when viewing with both eyes, and results from eye misalignment. Monocular double vision, on the other hand, is caused by abnormalities in the visual axis. A clear visual axis is essential to the production of normal vision. With monocular double vision, multiple images may be formed because light rays never come together on the retina. This is commonly referred to this as “ghosting,” and some patients will describe this phenomenon as a shadow image.

Ghosting is also seen in the setting of dry eye, corneal disease and uncorrected refractive errors, among other things. If you have ghosting that results from dry eye, it may improve or resolve when you blink, whereas if your ghost image results from a cataract, it will not improve with blinking.

Shift Toward Nearsightedness

This is termed a “myopic shift,” because the change in refractive error is toward nearsightedness, or myopia. A myopic shift occurs in the majority of patients who develop cataracts. Less commonly, patients may experience a hyperopic shift, or a shift toward farsightedness. In addition, cataracts can lead to astigmatism formation within the lens, a condition referred to as “lenticular astigmatism.”

If you develop increased nearsightedness, you may find that you can read better than you did previously without your glasses, but your distance vision will now be more blurry.

Frequent Prescription Changes in Your Glasses

As your cataract develops, it leads to changes in the way your lens bends light. This means your prescription changes, and such changes may occur frequently as your cataract progresses. Unfortunately, this is not a predictable process and occurs at a different rate in all patients.

Difficulty Driving at Night, Twilightand in Poor Weather Conditions

This results from a variety of the above listed symptoms. Blurry vision, light sensitivity, glare, halos, ghosting and loss of contrast sensitivity all lead to difficulty with vision at night, at dusk, and in poor weather conditions.

The symptoms listed above cause varying degrees of disability in patients. Some symptoms can also be caused by other diseases in the eye. It is important to understand the symptoms you are experiencing, so you can report them to your eye doctor. Your doctor can then determine the cause of your symptoms and help you decide whether cataract removal would benefit you.