Binocular vision refers to how the eyes work together to produce a three-dimensional perception of the world. Depth perception helps orient the body in space. The visual system must converge (turn eyes inward) and diverge (turn eyes outward) to maintain a clear, single, three-dimensional image. Symptoms of binocular vision dysfunction include eyestrain, double vision, blurred vision, visual fatigue, and headaches. When the binocular visual system and vestibular system are not properly integrated, dizziness and sensitivity to visual motion may result. The presence of binocular vision disorders may limit the effectiveness of vestibular therapy (Pavlau et al, 2015).
This suggests treating binocular vision disorders may improve quality of life in patients with vestibular dysfunction.
There are several types of binocular vision disorders, including amblyopia, constant or intermittent strabismus, disorders of maintaining horizontal eye alignment (convergence insufficiency, convergence excess, divergence insufficiency, divergence excess) and vertical heterophoria. It is also possible to have a torsional misalignment between the two eyes. We will discuss them below.
Amblyopia is the most common visual impairment affecting one eye, with a prevalance of about 1-5% of the world population (Bonaccorsi et al, 2014). It is a developmental disorder of the brain visual cortex, the hallmark of which is reduced visual acuity in one eye, arising from abnormal visual experience early in life (Levi et al, 2015) . Amblyopia is typically caused by a constant strabismus (eye turn), anisometropia (high refractive error in one eye), or form deprivation (typically a congenital cataract) which develops before the age of 6 years. (AOA).
Patients with amblyopia also have inaccurate accommodation (focusing), reduced contrast sensitivity, unsteady fixation, reduced oculomotor skills, spatial uncertainty, interocular suppression (see below), and reduced binocularity. Historically, amblyopia has been treated with patching therapy in children. Recent research has shown that a binocular approach to treating amblyopia may be more effective, and a better treatment option for adults. A binocular treatment approach focuses on reducing interocular suppression.
Suppression can be defined as an inhibitory force the dominant eye exerts on the amblyopic eye, which reduces the amblyopic eye’s contribution to the brain’s image.
Researchers have discovered that suppression is caused by the development of a GABA neurotransmitter inhibitory network in the visual cortex (e.g. Duffy et al, 1978). If the inhibition is removed, the visual cortex is able respond to signals from both eyes. This means that suppression need not be permanent.
Optometric vision therapy may improve visual acuity in the amblyopic eye, and improve binocularity. Patients who have dizziness and balance disorders and also have longstanding amblyopia may benefit from attempting to improve vision in their amblyopic eye and improving their binocularity if treatment has not been attempted in the past. With adults, it can difficult to judge how much improvement can be made, but the adult brain has significant neuroplasticity and improvement to the visual system is possible. (Bonaccorsi et al, 2014).
Strabismus is an inward eye turn (esotropia) or outward eye turn (exotropia). The prevalence of strabismus is estimated at 2-5% of the population, similar to the prevalence of amblyopia. (AOA 2011)
Strabismus can be constant or intermittent. A constant unilateral strabismus may or may not have associated amblyopia. Patients with constant strabismus do not have stereopsis, or proper development of binocularity, and often develop sensory adaptations to maintain single vision. Intermittent strabismics tend to have more symptoms of double vision, eye fatigue, and words moving on a page while reading. This is because their eyes are struggling to maintain fusion. Strabismus is typically treated with eye muscle surgery or vision therapy.
A vision therapy based treatment approach is usually better for patients with intermittent strabismus.(AOA, 2011). Vision therapy improves three dimensional vision, and teaches the patient to improve the coordination of their eyes to maintain it. Cases of constant strabismus require a much longer treatment course with therapy because there are often significant sensory adaptations to break down, and binocularity has to be developed.
Some cases of constant strabismus may be better managed with surgery, particularly if the eye turn is large and cosmetically bothersome to the patient. However, surgery does not guarantee development of binocular vision.
Convergence Excess (CE)
Convergence insufficiency (CI)is the most common horizontal binocular vision disorder. When shifting focus from far to near, the visual system must focus, and the eyes must turn inward to maintain single vision. Convergence insufficiency is the inability of the eyes to turn inward and maintain single vision at near. The prevalence of convergence insufficiency in children and adults is between 2.25-8.30%. (AOA, 2011) Convergence insufficiency is diagnosed by a high exophoria at near, a receded near point of convergence, and reduced positive fusional vergence.(Scheiman et al, 2005).
Scheiman et al (2005) reported on treatments of CI. Convergence insufficiency may cause symptoms of double vision, discomfort with prolonged near work, headaches, and words moving on a page while reading. The CITT treatment study found that in office optometric vision therapy is the best treatment option for CI.
In the first author’s clinical practice at Chicago Dizziness and Hearing Clinic, convergence insufficiency is the most common binocular vision condition diagnosed among patients with significant visual sensitivity. Vision therapy to treat CI may improve quality of life for many patients, and may help reduce the frequency and severity of visually triggered vertigo symptoms.
A discussion of convergence disorders in head injury is found here.
Divergence Insufficiency (DI)
Convergence excess is a condition in which the eyes turn in too much when looking at near. Convergence excess is characterized by a higher esophoria at near than distance, a high AC/A ratio, and reduced negative fusional vergence ranges (divergence ability). (AOA 2011). Symptoms of convergence excess include blurry vision at near, double vision, headaches, and difficulty with prolonged near work. Many patients with convergence excess benefit from glasses for near work. This diminishes their need to accomodate and therefore the linked convergence.
Divergence Excess (DE)
The visual system must relax the eyes when looking from near to far. Divergence insufficiency is the inability to relax the eyes to maintain single vision at distance. Divergence insufficiency is characterized by a higher esophoria at distance than near, low divergence ability at distance, and a low AC/A ratio. (AOA 2011)
Divergence insufficiency is muchless common that convergence insufficiency or convergence excess. Symptoms of divergence insufficiency include double vision at distance and blurred vision at distance.
Divergence excess, or DE, is a condition in which the eyes turn outward too much when looking at distance. Divergence excess is characterized by a higher exophoric deviation at distance than near. (AOA, 2011) Divergence excess may cause symptoms of double vision and blurred vision at distance. In clinical practice, DE is rarely seen without an exotropia at distance.
A vertical heterophoria is a vertical misalignment of the eyes. The presence of a vertical phoria has been found to be associated with symptoms of motion sickness. Vertical phorias may also cause symptoms of double vision, head tilt, and eyestrain. (AOA 2011) A very small recent study found that correcting for the vertical heterophoria with spectacle prisms reduced symptoms of motion sickness in 2 out of 4 patients. (Jackson and Bedell, 2012) Perons with vertical heterophoria are less stable than persons with no phoria (Matheron and Kapoula, 2008). Larger studies are needed here.
The visual system and the vestibular (balance system) are linked together by the vestibulo-ocular reflex. This means that dysfunction of either system affects how the systems work together. The VOR is dependent on stable visual input, which means any binocular vision disorder affecting the visual system can exacerbate dizziness and disequilibrium symptoms, particularly in visually stimulating environments and situations involving motion.(Cohen, 2013)