As people age, the lenses in their eyes can become the target of cataracts. These cataracts cloud the lenses and slowly reduce the clarity of their sight until they are targeted for replacement. The natural lens is replaced with an intraocular lens (IOL). This exchange is viewed as a near miracle because of the enormous boost to the patient’s vision. Reports of 20/20 vision are common. Colors are brighter and details that had faded years ago suddenly reappear. Everyone is happy.
Now, it is time for the bad news.
Many patients have a problem that shows up at night when viewing a light. A halo appears around or near the light. For most, this happens when seeing the headlights of oncoming cars while driving or riding in a car. This phenomenon is called dysphotopsia. It is very common and usually of short duration. For most people, the unwanted change is vision goes away within 3 to 4 weeks. Ophthalmologists estimate that only about 1 in 1,000 will need a second intraocular lens exchange to correct the problem. However, about 1 in 10 want it.
A second type of dysphotopsia happens called negative dysphotopsia.
This is a similar type of vision disorder following cataract surgery. In this instance, instead of a halo, the patient experiences a dark region on the temporal edge of the field of vision. Although the problem the creates this situation completely rings the field of vision, the brain tunes out the phenomenon on the nasal side of vision because the nose itself always blocks most of the sight in that direction. While the two types of dysphotopsia are similar, the cause is slightly different for each.
Both the dysphotopsia and the negative dysphotopsia are caused by the presence of the new lens.
The eyeball is designed so that when it works right, the light strikes the natural lens of the eye. Light is bent so that the light waves flow to a specific area of retina at the back of the eye. Cells in that area work to produce signals that are relayed to the brain by the optic nerve and interpreted as sight. The artificial lens that is placed into the eye works somewhat like a pair of very tiny glasses. The difficulty is how the light strikes the new lens.
Light that strikes the edge of the lens may be bent differently than light that passes through the lens.
Dysphotopsia is caused by the light that hits the edge of the artificial lens. The edge of the lens acts more like a mirror and reflects rather than refracts the light waves. A portion of the light that enters the eye is aimed slightly away from the proper location. This light becomes the undesired halo image around lights at night. Negative dysphotopsia does not happen only at night. The eye normally sees everything that passes through the natural lens in roughly the same way. When light passes through the very edges of the new lens or misses it completely, the images produced do not flow evenly from the central images being received by the brain from the eye. Because the brain determines that the distorted images do not belong in this picture, they are perceived as dark regions or spots.
The reality is that for nearly everyone who has either of these problems following cataract surgery, they will improve with time.
If the doctor is honest with the patient and gives the proper the warnings prior to the surgery, these effects should not be a cause for real concern. It is the unprepared patient that suddenly believes that his or her vision has been seriously compromised by a poorly performed surgery and lawyers may be called.
The brain has a tremendous ability to filter vision to remove items that are not supposed to be there.
It filters out the naturally occurring dark spot where the retina meets the optic nerve so that images seem complete and whole. The brain eliminates the shadows caused by the blood vessels that pass through the eye, and keeps the images clear and sharp. Given a little time, the brain will likewise eliminate the dark regions and halos as it is trained to ignore these phantoms.