10.2.2 Visual fields
For the purposes of this publication, visual fields are defined as a measure of the extent of peripheral (side) vision. Normal visual field is: 60 degrees nasally, 100 degrees temporally, 75 degrees inferiorly and 60 degrees superiorly. The binocular field extends the horizontal extent from 160 to 200 degrees, with the central 120 degrees over lapping and providing the potential for stereopsis. Visual fields may be reduced as a result of many neurological or ocular diseases or injuries resulting in hemionopia, quadrantanopia or monocularity.5–16
Peripheral vision assists the driver to be aware of the total driving environment. Once alerted, the central fovea area is moved to identify the importance of the information. Therefore peripheral vision loss that is incomplete will still allow awareness; this includes small areas of loss and patchy loss. Additionally, affected drivers can adapt to the defect by scanning regularly and effectively and can have good awareness. Patients with visual field defects who have full intellectual/cognitive capacity are more able to adapt, but those with such impairments will have decreased awareness and are therefore not safe to drive.
A longstanding defect, such as from childhood, may lead to visual adaptation. Such defects need to be assessed by an ophthalmologist/ optometrist for a conditional licence to be considered. They should be managed as an exceptional case to the standard.
If there is no clinical indication of a visual field impairment or a progressive eye condition then it is satisfactory to screen for defect by confrontation. Confrontation is an inexact test. Any person who has, or is suspected of having, a visual field defect should have a formal perimetry-based assessment.
Monocular automated static perimetry is the minimum baseline standard for visual field assessments. If monocular automated static perimetry shows no visual field defect, this information is sufficient to confirm that the standard is met.
Subjects with any significant field defect or a progressive eye condition require a binocular Esterman visual field for assessment. This is classically done on a Humphrey visual field analyser, but any machine that can be shown to be equivalent is accepted. This must be performed with fixation monitoring. Alternative devices must have the ability to monitor fixation and to stimulate the same spots as the standard binocular Esterman. For an Esterman binocular chart to be considered reliable for licensing, the false positive score must be no more than 20 per cent.
Horizontal extent of the visual field
In the case of a private vehicle driver, if the horizontal extension of a person’s visual fields are less than 110 degrees but greater or equal to 90 degrees, an optometrist/ophthalmologist may support the granting of a conditional licence by the driver licensing authority. The extent is measured on the Esterman from the last seen point to the next seen point. There is no flexibility in this regard for commercial vehicle drivers.
A single cluster of up to three adjoining missed points, unattached to any other area of defect, lying on or across the horizontal meridian will be disregarded when assessing the horizontal extension of the visual field. A vertical defect of only a single point width but of any length, unattached to any other area of defect, which touches or cuts through the horizontal meridian may be disregarded. There should be no significant defect in the binocular field which encroaches within 20 degrees of fixation above or below the horizontal meridian. This means that homonymous or bitemporal defects that come close to fixation, whether hemianopic or quadrantanopic, are not normally accepted as safe for driving.
Central field loss
Scattered single missed points or a single cluster of up to three adjoining points is acceptable central field loss for a person to hold an unconditional licence. A significant or unacceptable central field loss is defined as any of the following:
- a cluster of four or more adjoining points that is either completely or partly within the central 20-degree area
- loss consisting of both a single cluster of three adjoining missed points up to and including 20 degrees from fixation, and any additional separate missed point(s) within the central 20-degree area
- any central loss that is an extension of a hemianopia or quadrantanopia of size greater than three missed points.
Methods of measurement of visual fields are limited in their ability to resemble the demands of the real-world driving environment where drivers are free to move their eyes as required and must sustain their visual function in variable conditions. Thus additional factors to be considered by the driver licensing authority in assessing patients with defects in visual fields include, but are not limited to, the following:
- kinetic fields conducted on a Goldman
- binocular Esterman visual fields conducted without fixation monitoring, often referred to as a roving Esterman (two consecutive tests must be performed with no more than one false positive allowed) – the test should be in the numeric field format when it is printed out or sent for an opinion
- contrast sensitivity and glare susceptibility
- medical history; duration and prognosis; if the condition is progressive; rate of progression/deterioration; effectiveness of treatment/ management
- driving record prior to and since the occurrence of the defect
- the nature of the driving task, for example, type of vehicle (truck, bus, etc.), roads and distances to be travelled
- concomitant medical conditions such as cognitive impairment or impaired rotation of the neck.
There is no flexibility in this regard for commercial vehicle drivers.
Monocular vision (one-eyed driver)
Monocular drivers have a reduction of visual fields due to the nose obstructing the medial visual field. They also have no stereoscopic vision and may have other deficits in visual functions.
For private vehicle drivers, a conditional licence may be considered by the driver licensing authority if the horizontal visual field is 110 degrees and the visual acuity is satisfactory in the better eye. The health of the better eye must be reviewed every two years. People with monocular vision are generally not fit to drive a commercial vehicle. However, if an ophthalmologist/optometrist assesses that the person may be safe to drive after consideration of the above listed factors a conditional licence may be considered by the driver licensing authority, subject to at least two-yearly review of the better eye.
If monocular automated static perimetry is undertaken on patients without symptoms, family history or risk factors for visual field loss, and shows no indication of any visual field concerns, this information may be sufficient to confirm that the standard is met. If monocular testing suggests a field defect, or if the patient has a progressive eye condition, and/or the patient has any other symptoms or signs that indicate a field defect, then binocular testing should be conducted using the Esterman binocular field test or an Esterman-equivalent test. Alternative devices must have the ability to monitor fixation and to stimulate the same spots as the standard binocular Esterman.
Sudden loss of unilateral vision
A person who has lost an eye or most of the vision in an eye on a long-term basis has to adapt to their new visual circumstances and re-establish depth perception. They should therefore be advised not to drive for an appropriate period after the onset of their sudden loss of vision (usually three months). They should notify the driver licensing authority and be assessed according to the relevant visual field standard.