Visual Fields Assessment

We depend more on our peripheral visual fields in driving an automobile much more than our central vision. The central 5 degrees of vision gives us sharp vision for fine detail but remaining 175 degrees of peripheral field gives us the "big picture" like the lay of the land and the movement of vehicles and people around us.

The image above shows the total potential combined binocular visual field of 180 degrees in a patient with two functions eyes. Because the visual field of our two eyes overlap,  even with total loss of visual field in one eye a patient can potentially demonstrate 135 or more degrees of visual field in the remaining eye. Note that the central macular central vision that is often impaired in bioptic drivers only accounts for 5 degrees or less of the total visual field.

There are no well established scientific guidelines for the width of visual field required to drive safely for normal or visually impaired drivers. Someone with a small field who scans and compensates for the loss of field may be safer than a person with a wide visual field, but poor attention.  Since bioptic drivers have mild to moderate impairment in central visual acuities, the concensus has been to require a reasonably wide peripheral visual field. Many states have adopted guidelines for the amount of visual fields required. There are no studies that show how much visual field is required for safe bioptic driving.

In Indiana to become a bioptic driver, 120 degrees of peripheral visual fields are required. It is understood that patients may have small central scotomas thus the 120 degrees is not 120 degrees of continous visual field but rather represents the outer boundary of the visual field.

Some conditions such as retinitis pigmentosa cause progressive loss of the peripheral visual field thus these patients are not candidates for bioptic driving.  Diseases that limit the central visual acuity, but  do not cause significant loss of the peripheral visual fields make better candidates for bioptic driving. These include Albinism, Congenital Nystagmus, macular holes, some mild optic atrophies and mild macular degeneration.

Visual fields may be assessed in several different manners. Computerized arc perimeters have become common place today in eyecare practices and are particularly sensitive in detecting glaucoma. However in some patients  such as  diabetic retinopathy patients and patients with glare problems computerized perimeters may not give a valid representation of the patient’s real world functional visual field for driving.  

Combining more than one method may provide a better understanding of the real functional field.  In our practice if the results of the computerized perimeter are not clear, we also test with a manual perimeter with a 3mm white target and do careful confrontations.