Alamo Optometry Blog

March 11, 2013

Visual Fields

Filed under: Uncategorized — gkblog @ 8:59 pm

(As appeared in Alamo Today, February 2013)         

Since we now have a new automated visual field machine in the office, I thought it would be a good time to discuss this important topic.  I think most if not all patients have had their visual fields tested when they have visited the optometrist.  Most patients just assume it is something done during the visit, but few are probably aware of the many indications to assess visual fields and what possible visual field deficits can tell about your eyes and overall health.

            To understand visual fields, we must first discuss how peripheral vision is created and processed by the brain.  It is important to note that each area of the retina corresponds to the opposite area in the visual field, i.e. the superior area of the retina corresponds to your inferior visual field, and the temporal retina (area closer to your ears) corresponds to the nasal visual field.   The photoreceptors throughout the retina all send their signals to one area in the back part of the eye that comes together and then exits the retina as the optic nerve.  Part of each optic nerve crosses to the other side of the brain at a place called the optic chiasm located very close to the pituitary gland.  Then these fibers travel through different parts of the brain until they reach the occipital lobe, which is in the back of the head.  By knowing the type and location of the peripheral vision defect, your optometrist can have a good idea where the potential issue is.  For example, a pituitary gland tumor will cause a temporal field loss in both eyes.  Since the nerve fibers of the nasal optic nerve cross over by the pituitary gland, those fibers are therefore damaged and will cause a visual field deficit. 

            As a visual field defect can occur anywhere along the optic nerve trail, a condition of the optic nerve itself will cause a specific kind of field loss.  As most people are aware, the location of your optic nerve is your blind spot because there are no photoreceptors on the nerve itself, just the fibers from the photoreceptors.  Therefore an enlarged blind spot will show up when there is an infection, inflammation, or trauma of the nerve; the area immediately surrounding the nerve head will be swollen and therefore will not function properly.  Most types of optic nerve diseases will also show specific patterns of field loss like glaucoma.  Glaucoma is one of the more well-known diseases that are monitored by visual fields.  Since glaucoma slowly kills the optic nerve, there will be a corresponding field loss associated with the dead and dying section of the nerve.

            One of the other main reasons to assess visual fields is to make sure there are no effects on the macula from certain medications.  Early detection can very often help to prevent vision loss.  It is common that visual field loss will precede actual vision loss, so it is not sufficient to follow these patients by routine vision assessment alone.  Some common medications that warrant this are tamoxifen (for breast cancer), and plaquenil (a drug commonly used to treat some autoimmune diseases such as lupus).  There are certain types of visual tests that should be done on these patients to monitor them.  Communication with the prescribing doctor is also paramount to inform him/her of any visual effects so any modifications in their treatment plan can be made. 

            Unfortunately, doing this test does require a patient to have relatively good vision.  So for conditions such as advanced macular degeneration or other conditions with severe vision loss, a visual field assessment is not very useful because the patient does not have the acuity necessary to do the test.  A visual field test is a valuable tool in assessing the eyes, visual system, and the body as a whole as certain medications and diseases such as diabetes and hypertension can cause conditions in the eye and brain that would show up in an eye exam and/or a visual field test.

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