22 - Vision impairments - CCMTA Medical Standards

Last updated on March 5, 2021

Vision impairment and medical fitness to drive.

22.6.1 Impaired visual acuity – Non-commercial drivers
22.6.2 Impaired visual acuity – Commercial drivers
22.6.3 Visual field loss – Non-commercial drivers
22.6.4 Visual field loss – Commercial drivers
22.6.5 Loss of stereoscopic depth perception or monocularity – All drivers
22.6.6 Diplopia
22.6.7 Impaired colour vision
22.7.1 Visual acuity
22.7.2 Visual field
22.7.3 Contrast sensitivity
22.7.4 Examination of visual functions (EVF)
22.7.5 Visual field test (VFT)

22.1 About vision impairment

Vision impairment is defined as a functional limitation of the visual system and can be manifested as:

Reduced visual acuity
Reduced contrast sensitivity
Visual field loss
Loss of depth perception
Diplopia (double-vision)
Visual perceptual difficulties, or
Any combination of the above

This chapter focuses on common vision impairments and medical conditions and treatments that can cause vision impairments.

Common vision impairments

Impaired visual acuity

Visual acuity is the ability of the eye to perceive details. It can be described as either static or dynamic. Static visual acuity, the common measure of visual acuity, is defined as the smallest detail that can be distinguished in a stationary, high contrast target (e.g. an eye chart with black letters on a white background). When tested, it is reported as the ratio between the test subject’s visual acuity and standard “normal” visual acuity.

Normal visual acuity is described as 20/20 or 6/6 in metric. A person with 20/40 vision (6/12 metric) needs to be 20 feet (6 metres) away to distinguish detail that a person with normal vision can distinguish at 40 feet (12 metres). The standard Snellen chart for measuring visual acuity and a table of standard ratings is included in 22.7.1

Dynamic visual acuity is the ability to distinguish detail when there is relative motion between the object and the observer. Given the nature of driving, dynamic visual acuity would seem to be more relevant to licensing decisions than static visual acuity. However, barriers to the use of dynamic visual acuity for decision-making include:

The absence of a practicable method of testing dynamic visual acuity
Limited research on its relevancy for driving, and
The lack of established levels of dynamic visual acuity required for driving safely

Myopia, hyperopia, presbyopia and astigmatism (refractive errors)

Myopia, hyperopia, presbyopia and astigmatism are conditions associated with reduced visual acuity. They are known as refractive errors and are the result of errors in the focusing of light by the eye.

Myopia (nearsightedness) is a condition in which near objects are seen clearly but distant objects do not come into proper focus. Individuals with normal daytime vision may experience “night myopia.” Night myopia is believed to be caused by pupils dilating to let more light in, which adds aberrations that result in nearsightedness. It is more common in younger individuals and people who are myopic.

Hyperopia (farsightedness) is a condition in which distant objects are seen clearly but close objects do not come into focus. Age-related farsightedness is called presbyopia. It is not a disease, but occurs as a natural part of the aging process of the eye and usually becomes noticeable as an individual enters their early to mid 40s.

Astigmatism is a visual condition that results in blurred vision. It commonly occurs with other conditions such as myopia and hyperopia.

Visual field loss

The visual field is the extent of the area that a person can see with their eyes held in a fixed position, usually measured in degrees. The normal binocular (using both eyes) visual field is 135 degrees vertically and 180 degrees horizontally from the fixed point.

The visual field can be divided into central and peripheral portions. Central vision refers to vision within 30 degrees of the point of fixation or gaze. The macula, a small area in the center of the retina, is responsible for fine, sharp, straight-ahead central vision.

Peripheral vision allows for the detection of objects and movement outside the scope of central vision.

Visual field impairment refers to a loss of part of the normal visual field. The table and diagram on the following two pages provide further information on various types of visual field defects. The term “scotoma” refers to any area where the area of lost visual field is surrounded by normal vision.

Hemianopia, vision loss in one half of the visual field, or quadrantanopia, vision loss in one quarter of the visual field, can occur as a result of a stroke, trauma or tumour. They are not usually caused by a problem with the eye itself.

An important consideration related to hemianopia is the potential for anosognosia. Anosognosia is a condition in which a person with an impairment caused by a brain injury is unaware of the impairment. Research indicates that hemianopic anosognosia is relatively frequent, occurring in approximately two-thirds of those with hemianopia.

Unawareness of visual field deficits has an obvious negative impact on safe driving performance.

Types of visual field defects*
Type	Description	Causes
Altitudinal field defect	Loss of all or part of the superior or inferior half of the visual field, but in no case does the defect cross the horizontal median	More common: Ischemic optic neuropathy, hemibranch retinal artery occlusion, retinal detachment Less common: Glaucoma, optic nerve or chiasmal lesion, optic nerve coloboma
Arcuate scotoma	A small, arcuate-shaped field loss due to damage to the ganglion cells that feed into a particular part of the optic nerve head, which follows the arcuate shape of the nerve fibre pattern; the defect does not cross the horizontal median	More common: Glaucoma Less common: Ischemic optic neuropathy (especially nonarteritic), optic disk drusen, high myopia
Binasal field defect (uncommon)	Loss of all or part of the medial half of both visual fields; the defect does not cross the vertical median	More common: Glaucoma, bitemporal retinal disease (e.g. retinitis pigmentosa) Rare: Bilateral occipital disease, tumour or aneurysm compressing both optic nerves
Bitemporal hemianopia	Loss of all or part of the lateral half of both visual fields; the defect does not cross the vertical median	More common: Chiasmal lesion (e.g. pituitary adenoma, meningioma, craniopharyngioma, aneurysm, glioma) Less common: Tilted optic disks Rare: Nasal retinitis pigmentosa
Blind-spot enlargement	Enlargement of the normal blind spot at the optic nerve head	Papilledema, optic nerve drusen, optic nerve coloboma, myelinated nerve fibres at the optic disk, drugs, myopic disk with a crescent
Central scotoma	A loss of visual function in the middle of the visual field, typically affecting the fovea centralis	Macular disease, optic neuropathy (e.g. ischemic, Leber's hereditary, optic neuritis), optic atrophy (e.g. from tumour compressing the nerve, toxic/metabolic disease) Rare: Occipital cortex lesion
Homonymous hemianopia	Loss of part or all of the left half or right half of both visual fields; the defect does not cross the vertical median	Optic tract or lateral geniculate body lesion; temporal, parietal, or occipital lobe lesion of the brain (stroke and tumour more common; aneurysm and trauma less common). Migraine may cause a transient homonymous hemianopia
Constriction of the peripheral fields leaving only a small residual central field	Loss of the outer part of the entire visual field in one or both eyes	Glaucoma, retinitis pigmentosa or some other peripheral retinal disorder, chronic papilledema after panretinal photocoagulation, central retinal artery occlusion with cilioretinal artery sparing, bilateral occipital lobe infarction with macular sparing, nonphysiologic vision loss, carcinoma-associated retinopathy Rare: drugs

(*From https://www.merckmanuals.com/professional/eye-disorders/approach-to-the-ophthalmologic-patient/evaluation-of-the-ophthalmologic-patient - Adapted from The Wills Eye Manual, Douglas J. Rhee, M.D. and Mark F. Pyfer, M.D.© 1999 by Lippincott Williams & Wilkins.)

visual field defects diagram

(* Source National Eye Institute .)

Blindness/low vision

Total blindness is the complete lack of vision and is often described as no light perception. A person may be considered “blind” even though they have some vision. There is no universally accepted level of visual acuity to define blindness. In North America and most of Europe a person is considered to be legally blind if their visual acuity is 20/200 (6/60) or less in the better eye with the best correction possible, or if their visual field is less than 20 degrees in diameter. The World Health Organization (WHO) defines “low vision” as visual acuity between 20/60 (6/18) and 20/400 (6/120) or a visual field between 10 and 20 degrees in diameter. The WHO definition of “blindness” is visual acuity less than 20/400 (3/60) or a visual field less than 10 degrees.

Monocular vision/Loss of stereoscopic depth perception

Monocular vision refers to having vision in one eye only and is associated with the loss of stereoscopic vision. Stereoscopic vision, in which the brain processes information from each eye to create a single visual image, is integral to depth perception in those with binocular vision.

Impaired colour vision

Individuals with impaired colour vision (colour blindness) lack a perceptual sensitivity to some or all colours. These impairments are usually congenital and, in general, drivers learn to compensate for the inability to distinguish colours when driving.

Impaired contrast sensitivity

Visual contrast sensitivity refers to the ability to perceive differences between an object and its background. Depending on the cause, a loss of contrast sensitivity may or may not be associated with a corresponding loss of visual acuity. Declines in contrast sensitivity are associated with normal aging, and can also result from conditions such as:

Cataracts
Age-related macular degeneration
Glaucoma, and
Diabetic retinopathy

Impaired dark adaptation and glare recovery

Dark adaptation refers to the process in which the visual system adjusts to a change from a well-lit environment to a dark environment. Glare recovery refers to the process in which the eyes recover visual sensitivity following exposure to a source of glare, such as oncoming headlights when driving at night.

Prolonged dark adaptation is associated with normal aging and results in decreased visual acuity at night. It may also be the result of a medical condition, and where severe, may be referred to as “night blindness.” Night blindness may be caused by a number of medical conditions including:

Retinitis pigmentosa
Vitamin A deficiency
Diabetes
Cataracts, or
Macular degeneration

As with dark adaptation, individuals require a longer time to recover from glare as they age. Cataracts and corneal edema are also associated with prolonged glare recovery.

Individuals may also experience prolonged glare recovery following laser assisted in situ keratomileusis (LASIK) or panretinal laser photocoagulation (PRP) surgery.

A number of illnesses can affect glare recovery time, with prolonged recovery times reported in individuals with diabetes, vascular disease and hypertension. Retinal conditions with demonstrated relationships to prolonged glare recovery include age- related maculopathy, “cured” retinal detachment and central serous retinopathy.

Diplopia

Diplopia (double vision) is the simultaneous perception of two images of a single object. These images may be displaced horizontally, vertically or diagonally in relation to each other.

Diplopia can be binocular or monocular. Binocular diplopia is present only when both eyes are open, with the double vision disappearing if either eye is closed or covered.

Monocular diplopia is also present with both eyes open, but unlike binocular diplopia, the diplopia persists when the problematic eye is open and the other eye is closed or covered.

Binocular diplopia, or true diplopia, is an inability of the visual system to properly fuse the images viewed by each eye into a single image. It may be caused by the physical misalignment of the eyes (strabismus) or diseases such as Parkinson’s disease or multiple sclerosis. Two of the most common causes of binocular diplopia in people over 50 are thyroid conditions, such as Grave’s disease, and cranial nerve damage.

Monocular diplopia is not caused by misalignment, but rather by problems in the eye itself. Astigmatism, dry eye, corneal distortion or scarring, vitreous abnormalities, cataracts and other conditions can cause monocular diplopia.

Nystagmus

Nystagmus is an involuntary, rapid, rhythmic movement of the eyeball. The movements may be horizontal, vertical, rotary or mixed. Nystagmus which occurs before 6 months of age is called congenital or early onset, whereas that occurring after 6 months is labelled acquired nystagmus. Early onset nystagmus may be inherited, or the result of eye or visual pathway defects. In many cases, the cause is unknown. Causes of acquired nystagmus are many and it may be a symptom of another condition such as stroke, multiple sclerosis, or even a blow to the head.

Many individuals with nystagmus have significant impairments in their vision, with some having low vision or legal blindness.

Medical conditions causing vision impairments

Cataracts

A cataract is an opacification or clouding of the crystalline lens of the eye, which blocks light from reaching the retina. Cataracts may be due to a variety of causes. Some are congenital, but few occur during the early years of life. The majority of cataracts are the result of the aging process. The presence of a cataract can interfere with visual functioning by decreasing acuity, contrast sensitivity and visual field.

Diabetic retinopathy

Diabetic retinopathy is the most common eye disease in those with diabetes, results in significant impairments in vision (blurred vision, vision loss) and is a leading cause of blindness in adults. It is caused by changes in the blood vessels of the retina (microvascular retinal changes) as a result of the disease.

There are two types of diabetic retinopathy: background (non-proliferative) and proliferative. Background retinopathy reflects early changes in the retina and often is asymptomatic. However, it may result in decreased visual acuity. Background diabetic retinopathy can progress into a more advanced or proliferative stage.

Proliferative retinopathy is the result of retinal hypoxia (lack of oxygen to the retina) and carries a much graver prognosis. The lack of oxygen to the retina results in a proliferation of new vessels in the retina or on the optic disc (neovascularization).

Without treatment, the new vessels can leak blood into the centre of the eye, resulting in blurred vision. Fluid (exudate) also can leak into the centre of the macula (that part of the eye where sharp, straight-ahead vision occurs), a condition called macular edema. The leakage causes swelling of the macula, resulting in blurred vision. Macular edema can occur at any stage of diabetic retinopathy, but is more likely to occur as the disease progresses. Research indicates that approximately half of those with proliferative retinopathy also have macular edema.

An example of the effects of diabetic retinopathy on vision is shown below*.

(*Source National Eye Institute - https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/diabetic-retinopathy)

effect of diabetic retinopathy on vision

Glaucoma

Glaucoma is a group of diseases characterized by increased intraocular pressure. The increased pressure can lead to optic nerve damage, resulting in blindness. Types of glaucoma include adult primary, secondary, congenital and absolute glaucoma. Open angle glaucoma, a type of adult primary glaucoma, is the most common. It is often referred to as the “silent blinder” because extensive damage may occur before the patient is aware of the disease. Early diagnosis and treatment are important for the prevention of optic nerve damage and visual field loss (primarily peripheral vision) due to glaucoma.

An example of the effects of glaucoma on vision is shown below*.

(*Source National Eye Institute - https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/glaucoma)

effects of glaucoma on vision

Age-related macular degeneration (ARMD)

Age-related macular degeneration (ARMD) is associated with the advanced stages of age-related maculopathy, or disease of the macula. The macula is the central portion of the retina and is responsible for central vision in the eye. Most individuals with maculopathy have impairments in their central vision. Those with ARMD, however, experience a progressive destruction of the photoreceptors in the macula, resulting in profound central vision loss.

ARMD has two forms, dry and wet. The dry form is the result of atrophy to the retinal pigment, resulting in vision loss due to the loss of photoreceptors (rods and cones) in the central portion of the eye. High doses of certain vitamins and minerals have been shown to slow the progression of the disease and reduce associated vision loss.

Wet ARMD (neovascular or exudative) is due to abnormal blood vessel growth in the eye, leading to blood and protein leakage in the macula. The bleeding, leaking and scarring from these blood vessels eventually result in damage to the photoreceptors, with a rapid loss of vision if left untreated. Treatment for wet ARMD has improved. Recent pharmaceutical advancements have resulted in compounds that, when injected directly into the vitreous humor, can cause regression of the abnormal blood vessels, leading to an improvement in vision.

An example of the effects of ARMD on vision is shown below*.

(*Source National Eye Institute - https://www.nei.nih.gov/learn-about-eye-health/resources-for-health-educators/eye-health-data-and-statistics/age-related-macular-degeneration-amd-data-and-statistics)

effects of ARMD on vision

Retinitis pigmentosa

Retinitis pigmentosa is the term given to a group of hereditary retinal diseases that result in the degeneration of rod and cone photoreceptors. The diseases cause progressive visual loss, ending in blindness. Night blindness is an early symptom of retinitis pigmentosa, followed by a constriction of the peripheral visual field. Loss of central vision typically occurs late in the course of the illness.

Typically, symptoms are not prominent in childhood, but with progressive degeneration of the photoreceptor cells, vision is gradually lost during adolescence and adulthood.