Monocular Precautions

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Article initiated by:

Lama Assi, MD MPH

All contributors:

Alaa Malik, Lama Assi, MD MPH

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Background

1. Monocular vision is defined as a condition in which one eye is unable to register images in coordination with the other eye.
2. Patients with monocular vision are those with only one eye, or those with good vision in only one eye.
3. Specific definitions of monocular vision impairment or blindness vary in terms of the visual acuity required for both the worse and better-seeing eyes. For example, the Federal Aviation Administration defines monocular vision as complete or legal blindness in one eye only.

Epidemiology

1. The majority of studies on the epidemiology of blindness and vision impairment describe bilateral causes of vision loss without making a distinction for those with unilateral vision loss. The leading causes of blindness worldwide, leading to blindness in one or both eyes, are cataracts, age-related macular degeneration, glaucoma, diabetic retinopathy, and trachoma. In developed countries, the major causes of blindness are trauma, glaucoma, diabetic retinopathy, vascular occlusions, cataracts, and age-related macular degeneration. In children, the leading causes are xerophthalmia, congenital glaucoma, congenital cataracts, optic atrophy, trauma, amblyopia, refractive errors, and trachoma. 
2. Trauma is the most common cause of non-congenital unilateral blindness in children, and a major cause of unilateral blindness in adults. The major types of ocular trauma are chemical exposure, thermal and UV exposure, mechanical globe trauma, corneal abrasions and lacerations, and an intraocular foreign body.
3. In the Salisbury Eye Evaluation Study, the leading causes of monocular blindness in people with visual acuity 20/40 or better in the fellow eye were retinal pathological diseases other than macular degeneration, diabetic retinopathy, and trauma (18.2%), trauma (16.9%), macular degeneration (15.6%), amblyopia (14.3%), and cataract (11.7%). Among those with visual acuity worse than 20/40 in the better-seeing eye, the leading causes of monocular blindness were macular degeneration (33.3%), glaucoma (18.2%), retinal pathological diseases (15.2%), and diabetic retinopathy (9.1%).  
4. In a study conducted in rural Indonesia in 2001, cataract was the leading cause of unilateral low vision (48.0%). Only two participants had unilateral blindness, one was secondary to amblyopia, and the other one was secondary to trauma.
5. In the National Eye Health Survey (NEHS) conducted between March 2015 and April 2016 in Australia, the primary causes of unilateral vision impairment among indigenous populations were uncorrected refractive error (64.5%), cataract (10.7%), and diabetic retinopathy (4.2%). The primary causes of unilateral blindness were retinal disease other than diabetic retinopathy, age-related macular degeneration, or vein occlusion (19.4%), corneal disease (16.7%), and cataract (13.9%). Among the nonindigenous populations, uncorrected retractive error (56.7%) and cataract (13.7%) were also the leading causes of unilateral vision impairment, followed by amblyopia (6.4%). Amblyopia (18.8%) and enucleation (14.6%) were primary causes of unilateral blindness among this group.

Challenges of Monocular Vision

1. Patients with monocular vision have loss of depth perception and visual field reduction, which may lead to worse visual-motor coordination and spatial orientation.
2. Patients with monocular vision therefore rely on weaker depth perception cues, including accommodation, linear perspective, interposition, texture gradient, relative size, light and shadow, relative brightness, aerial perspective, and motion parallax.
3. As a result of these changes to visual function, patients with monocular vision may be more likely to have motor vehicle accidents and falls.
4. Patients with monocular vision also engage in less physical activity compared to those with binocular vision. Reduced activity levels may be a result of impaired depth perception and reduced visual fields, resulting in safety concerns about mobility, falls, or the fear of falling.

Monocular Precautions

The importance of monocular precautions

1. Preventing eye injury is an essential component of care provision for patients with monocular vision.
2. In addition to the potential for bilateral blindness with injury to their better-seeing eye, monocular patients may be at an increased risk of eye injury due to the changes in visual function noted above. Consistent with this hypothesis, epidemiological studies have shown that patients with amblyopia have a higher incidence of vision loss in their non-amblyopic eye compared to the general population.
3. Eye injuries can occur anywhere and at anytime. In a study using the United States Eye Injury Registry (USEIR) database, only 21% of eye injuries were due to assault (including 1% from inflicted injuries). The vast majority (79%) were unintentional.
4. To help prevent such injuries, the Occupational Safety and Health Administration (OSHA) requires eye and face protection in the workplace. However, injuries in the workplace represent only 20% of eye injuries; around 43% of them occur at home, 15% on the streets or highways, 13% in recreational sites or during sports, and 3% each in public buildings, on farms, and at schools.
5. Similarly, the AAO reports that only 700,000 (28%) of the 2.5 million eye injuries that occur annually in the United States are work-related. 125,000 (5%) of these injuries involve household products. Importantly, 90% of these injuries can be prevented through protective eyewear.
6. Therefore, it is important to educate monocular patients about the importance of protecting their good eye at all times.

Monocular precautions counseling

1. Monocular precautions should be provided to all patients with monocular vision. Patients should be counseled on these precautions at each visit with their ophthalmologist. Any additional injury to the better-seeing eye may result in bilateral vision impairment, thus impacting visual function and quality of life.
2. Patients should be counseled to wear polycarbonate glasses at all times, even when at home. 
3. If a patient requires prescription glasses, a comment should be added to the prescription to ensure they are made of polycarbonate material. For patients who do not need prescription glasses, they can obtain non-prescription clear polycarbonate glasses. Options with a clear top and reading power on the bottom are available as bifocals or progressives for patients who require reading glasses. Patients may also opt for sun protection glasses when outdoors, also made from polycarbonate material.
4. Safety glasses should be worn when engaging in household chores, as there is a risk of chemical injury from cleaning products. Safety glasses should also be worn for outdoor activities, garden and lawn maintenance, or sports.
5. Contact lenses are not protective against eye trauma and may lead to increased risk of ocular infection. Contact lens wearers should be advised to wear polycarbonate glasses in place of lenses. If patients insist to wear contact lenses despite the risk of infection, they should be instructed to wear safety glasses as well at least when engaging in household chores, outdoor activities, garden or lawn maintenance, and sports.
6. Patients should keep a spare pair of glasses with them in case their glasses break or get lost. This is to limit the amount of potential time without protection.  

Compliance with monocular precautions

1. Most patients do not receive, or recall receiving, counseling on monocular precautions. In a survey study conducted at Eye Plastic and Facial Cosmetic Surgery Center in Grand Rapids, 57% of patients with vision 20/400 or worse in only one eye did not recall being educated about protecting their good-seeing eye with polycarbonate glasses.
2. In a retrospective chart review study conducted at the University of Michigan Health System, 26% of monocular patients (defined as having visual acuity equal to or worse than 20/70 in the worse-seeing eye and better than 20/70 in the better-seeing eye) never received counseling about monocular precautions (based on documentation in patient charts).
3. A study conducted in Saint Louis among patients with unilateral anopthalmia and those undergoing a unilateral eye removal procedure revealed that only 55.3% of patients reported wearing protective lenses regularly (defined as more than 90% of waking hours). The proportion of patients wearing glasses regularly was higher among those who wore glasses prior to the removal of their eye (79.7% vs 35.6%, p<0.001). Moreover, the percentage was higher among those who had a larger number of visits over the study period. Common reasons for patient noncompliance to given precautions included good uncorrected vision, aesthetics, convenience, and lack of concern.
4. Aesthetics and convenience can be addressed through providing patients with information on local stores or online options with polycarbonate glasses. Many stores, both online and in- person, offer a variety of frames in terms of colors and styles. Lack of concern can be addressed through counseling patients at each encounter. Patients should be asked about their perceptions and understanding of the condition. Ophthalmologists should then tailor their counseling to the patient’s understanding.