In the last post we saw that the Medical Review Board (MRB) recommended the 120 degrees binocular Field of Vision (FOV) standard to replace the current 70 degrees monocular standard. I will attempt a summary of the highlights of Dr. Michelle Tregear’s presentation to the Board :
- A 2007 report showed no increased crash risk for individuals with 120 degrees FOV in the horizontal field compared to those with normal FOV.
- Binocular FOV standard is used in other countries like Australia, New Zealand and Canada. A binocular FOV standard probably better assesses function than a monocular standard since people use both eyes to drive, even if there is limited vision in the poorer eye. Binocular testing also helps in seeing how one eye compensates for another.
- Over 53 % of the drivers who were granted a vision exemption from the FMCSA, have FOV in the better eye greater than or equal to 120 degrees, and 46% had a FOV less than 120 degrees. The majority of drivers with FOV less than 120 degrees, had FOV of greater than 110 degrees, and 96% had a FOV of at least 100 degrees in the better eye. Even the poorer eye with the visual deficit had some degree of vision in most of the drivers.
Dr. Tregear went over some of the FOV testing methods: The Goldman perimetry (a trained technician maps out the visual fields), The Humphrey perimetry test (a light spot projected on an automated screen is used to map out the visual fields), and the standard confrontation testing used by most of us as clinicians and medical examiners. For a review of FOV in general: See the Scottish Sensory Centre Web site which has anatomical reviews with diagrams and some nice pictures including descriptions of the Goldman and Humphrey Perimetry testing apparatus http://www.ssc.education.ed.ac.uk/courses/vi&multi/vnov072i.html
- The confrontational method of visual testing is the least dependable. The studies cited did not find an increased crash rate when abnormal confrontational FOV is compared to normal confrontational FOV. Confrontational testing was found to be more useful for severe FOV loss. As far as the types of confrontational testing used, the red comparison test was more sensitive than finger counting and face description, which can not identify FOV loss. Confrontational testing only detects FOV loss in the periphery, and can miss even severe FOV loss in the central 30 degrees. It was thought that since we as medical examiners really have no good way of evaluating FOV loss that we should at least be trained in proper confrontational testing methods.
- Increased crash risk is seen when standard perimetry showed FOV loss, and in individuals with moderate to severe FOV loss in the central 20-30 degrees.
- Increased crash risk is seen in glaucoma, and retinitis pigmentosa when FOV is decreased. No studies showing evidence of increased crash risk in CMV drivers and many studies were based on passenger car studies.
- Medical conditions that may require further testing as part of the driver certification examination include glaucoma, diabetic neuropathy, retinitis pigmentosa, ocular hypertension The risk factors for these conditions were detailed in the report.
- It was determined that the panel lacked evidence to change the standard to also require a FOV of 20 degrees above and below fixation, although it would seem appropriate, for practical purposes.
In conclusion: The MRB decided to return to a binocular vision standard as seen in other countries and using the 120 degrees FOV. There was no evidence to support using 140 degrees instead of 120 degrees, and studies showed no increased crash risk for FOV of 120 degrees when compared to normal FOV (Normal FOV was previously described in this report as 160 degrees monocular and 180 degrees binocular). It was reported that even though there was no data regarding crash risk in the approximately 1,926 individuals in the FMCSA Vision Exemption Program, that crashes are rare any way for this population, and the data may not be of use for the MRB to consider prior to making their decision. Individual who fail to meet the 120 degrees binocular standard are to be referred to an ophthalmologist or optometrist for further testing and if the individual still can not meet the standard, he or she will have to apply to the FMCSA for an exemption under the Vision Exemption Program.
Drivers with a diagnosis of macular degeneration, glaucoma, retinitis pigmentosa, diabetic retinopathy, cataract or any other conditions that may affect the visual field should be referred to an ophthalmologist or optometrist for further testing.
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