“All great truths begin as blasphemies.”
-- George Bernard Shaw
To understand the issues of driving with a homonymous hemianopsia, we have to better define the question. Too often the question is presented as, “Can an individual with homonymous hemianopsia drive safely?” This is the wrong question! The question today should be “Which homonymous hemianopsia patients are safe to drive?” Many research studies have found that even without the kind of clinical patient selection criterion, adaptive devices, therapy and driver's training that a potential hemianopsia driver should undergo, a significant portion of hemianopsia patients in these studies demonstrated that they may have potential to drive safely.
If we look at the group of all hemianopsia patients, those who are safe to drive will be a very small group. This is owing to the great variability of associated problems of cognition, visual neglect, visual perception, alertness and ability to compensate. No clinician or researcher would ever argue that all hemianopsia patients are safe to drive.
Let us look instead at a limited group of hemianopsia patients for whom the higher order deficits have been screened to rule out cognitive deficits, visual neglect, and poor processing speed. In this group visual field expanders have been prescribed where indicated and the patients trained with these devices and given scanning training. Then these patients have been screened with a behind-the-wheel driving evaluation, we would see a much smaller group. But within that group, would emerge a patients that could have the potential to return to driving.
It is less about the visual field
Another question I see that demonstrates a failure for some to understand where the problem resides is “How much visual field is required to drive safely?” As clinicians that have worked for many decades with hemianopsia patients, we have learned that the visual field defect is only a small part of the driving safety issue. It is usually about the constellation of problems from the brain injury and each individual's ability to compensate.
While the type and size of visual fields are factors, the higher order cognitive functions are far more important to safe driving than the size of the visual field. These higher cognitive and perceptual functions determine if the patient can safely compensate. The real question should be expanded to, “On a case-by-case basis does this patient with an acquired brain injury from stroke, tumor, trauma or other cause, have the higher-level cognitive skills, compensatory skills, optical devices, experience, stamina driving skills and discipline to drive with a reduced visual field?"
All hemianopsia are not created equal!
Let us look at two patients with identical measurable visual field, both presenting with left homonymous hemianopsias. The first has an isolated stroke in the right occipital lobe no deficits other than the visual field loss. This patient has no visual neglect and no deficits in saccadic eye movements that would impair compensatory scanning and searching into the area of loss. With training and appropriate devices, this patient may have potential to return to safe driving. The second patient has an identical appearing left homonymous hemianopsia but from a stroke in a different location, the right parietal lobe. Thus this patient also has severe left visual neglect, impairments in saccadic eye movements and thus will never return to driving. If we only look at the visual field results, these patients look identical, but they are totally different cases.
If a state law looks only at the visual field loss to determine if driving is possible, they would treat both patients the same, denying them both the option of a driver’s license. While the second patient should not drive, this can needlessly devastate the first patient's life, robbing the patient of independence, ability to get to work, and to lead an otherwise normal life.
How do we predict safety?
The other question we must ask is, “What tests and evaluations best predict safe driving and what are the potential weaknesses that must be addressed in training?” Various neuropsychological tests can give us information on who may have potential to drive safely. More research to establish which tests give us the most effective data is needed. Additionally, behind-the-wheel research studies continue to expand our information on the unique driving behaviors of the hemianopsia driver.
Driving, however, is a complex function. Prior experience, stamina, motivation, and discipline combined with visual status and mental functioning all can shape the impact on safety. After all the testing and treatments are completed to help select those who show potential to drive, a behind-the-wheel driving evaluation with a driving rehabilitator experienced with acquired brain injury and hemianopsia is needed. Only during the behind-the-wheel examination and training can the full complexity of driving be evaluated and training performed to improve specific skills like lane position, use of optical devices and mirrors.
The most important question is, “Have we learned to treat each person as a unique individual, understanding that impairment, disability and handicap are not one in the same?”
Should state laws prevent all Hemianopsia driving?
Setting an arbitrary visual field width to discriminate against all hemianopsia patients is now seen by many current researchers as a needless burden on the portion of hemianopsia patients that have the ability to return to safe driving. Below is what a number of researchers have observed:
As Dr. Eli Peli, Senior Scientist from Harvard’s Schepens Eye Research Institute stated in Driving With Confidence, A Practical Guide to Driving with Low Vision:
“It is clear that not all people with hemianopia function at the same level and many probably could not drive safely. However, a fair percentage of these patients may compensate for their visual loss to such an extent that they can drive as safely as any driver.”
In Automobile Driving Performance of Brain-Injured with Visual Field Defects , T Schulte, H Strasburger, E Muller-Oehring, E Kasten and B Sabel 1999, American Journal of Physical Medicine & Rehabilitation, researchers performed a driving simulator-based study of six hemianopsia patients and a similar size group of normally sighted. They summarized:
“Contrary to our expectations, the findings showed no reliable difference in the performance of visually impaired and the normally sighted subjects on a driving simulator. …Thus on a practical level our results indicate that the suspension of driving privileges for persons having visual field impairments may be unwarranted on the basis of visual field loss alone.”
In a study by Racette & Casson (1999), Visual field loss and driving performance: a retrospective study Abstracts of the Eighth International Conference Vision in Vehicles, they studied 13 homonymous hemianopsia patients and 7 homonymous quadranopsia patients. They determined those who were unsafe, those who need additional assessment, and those who were safe. Only 23% of the hemianopsia patients were found unsafe and none of the quadranopsia patients were deemed unsafe.
“Clearly, the evidence provided by these reports indicate that homonymous visual field defect and homonymous hemianopia by itself can not be an absolute and inevitable contra-indication for practical fitness to drive.”
A 2009 study, On-road driving performance by persons with hemianopia and quadrantanopia, Investigative Ophthalmology Vis Sci 50 (2) 2009, J. Wood, G. McGwin, J. Elgin, M. Vaphiades, R. Braswell, D. DeCarlo, L Kline, G Meek, K Searcy and C. Owsley studied 22 hemianopsia and 8 quadranopsia patients and a normal control group driving over a 14.1 mile course of city and interstate driving. Two back seat evaluators, who were masked to the status of the patient, evaluated the drivers. They found 100% of normal drivers were safe to drive and 73% of hemianopsia and 88% of quadranopsia patients were safe to drive.
The study concluded that:
“Some drivers with hemianopia or quadrantanopia are fit to drive compared with age-matched control drivers. Results call into question the fairness of governmental policies that categorically deny licensure to persons with hemianopia or quadrantanopia without the opportunity for on-road evaluation.”
Continued research is crucial to define all of the parameters of hemianoptic driving. Information from these studies helps us define the best candidate, the areas of weakness and will guide driving rehabilitation specialists in training these patients.
A study by Bower et al, from The Schepens Eye Research Institute, Harvard Medical School, Boston, Massachusetts, Driving with Hemianopia, I: Detection Performance in a Driving Simulator, published November 2009 in Investigative Ophthalmology and Visual Science, tested twelve hemianopsia patients without visual neglect or cognitive loss and twelve matched normals on a simulator test over a two hour period. The hemianopsia patients were tested without visual field expanding systems and they demonstrated significantly more difficulty in detection of suddenly appearing pedestrians on their impaired side inside the simulator.
There was great variability in pedestrian detection among the small group of 12 hemianopsia patients with older driver's demonstrating lower rates. The authors of this study warned that simulator studies may not match results in real world driving and they further suggested that this also means we must look at each driving candidate individually. They stated:
“In determining fitness to drive for people with HH, the results underscore the importance of individualized assessments including evaluations of blind-side hazard detection.”
The same scientists now plan to do similar tests with patients using visual field expanders. Our years of work would support that the visual field expanders and training can help fill in detection of pedestrians in many patients, but more research is needed.
How could states regulate hemianopsia licensing?
It is clear from the research that we cannot make generalizations about the driving safety of all hemianoptic drivers. Thus simply removing visual field requirements could lead to hemianopsia drivers being licensed who have other cognitive or perceptual problems at make them unsafe.
States that still contain absolute prohibitions against driving with homonymous hemianopsias should consider removing these, and replacing them with a process to judge each patient individually based on current science. The process should include mandatory evaluation with a low vision specialist experienced in hemianopsia for evaluation and treatment followed by additional therapy/training as needed including occupational therapy if indicated. Then a behind-the-wheel driving evaluation and training as appropriate to each case with a certified driving rehabilitation specialist should be completed.
Then, the doctor with the report of the driving rehabilitation specialist would file a special application with the state. The states medical advisory committee would review each case individually. If the application is approved, the patient would have to demonstrate adequate driving skills on an extended state behind-the-wheel test by the state driver's license bureau. Restrictions on type of driving and time of day could be considered in each case cases.
Please contact us if you have any questions:
The Low Vision Centers of Indiana
Richard L. Windsor, O.D., F.A.A.O., D.P.N.A.P.
Craig Allen Ford, O.D., F.A.A.O.
Laura K. Windsor, O.D., F.A.A.O.
Indianapolis (317) 844-0919
Fort Wayne (260) 432-0575
Hartford City (765) 348-2020