Binocular vision is the coordination and integration of what is received from the two eyes separately into a single binocular percept. Proper functioning of binocular vision without symptoms depends on several factors which can be considered under three broad headings:
The anatomy of the visual apparatus.
The motor system that coordinates movement of the eyes.
The sensory system through which the brain receives and integrates the two monocular signals.
Anomalies in any of these can cause difficulties in binocular vision, or even make it impossible. This is illustrated schematically in Fig. 1.1 . In considering the binocular difficulties of a patient, therefore, all three parts of the total system need to be investigated:
Anatomy. Abnormalities in the anatomy of the visual system can be either developmental, occurring in the embryological development of the bony orbit, ocular muscles, or nervous system, or acquired through accident or disease.
Motor system. Even if the motor system is anatomically normal, anomalies can occur in the functioning which can disturb binocular vision or cause it to break down. These may be due to disease, or they may be malfunctions of the physiology of the motor system. For example, excessive accommodation due to uncorrected hypermetropia can result in excessive convergence due to the accommodation–convergence relationship. This is a frequent cause of binocular vision problems. Examples of disease affecting the motor system are haemorrhages involving the nerve supply to the extraocular muscles, changes in intracranial pressure near the nerve nuclei, or pressure on the nerves or nerve centres from abnormal growths of intracranial tissue. Such conditions require urgent medical attention to the primary condition and early recognition is therefore essential. The investigation for this type of pathology is discussed in Chapter 17 .
Sensory system. Anomalies in the sensory system can be caused by such factors as a loss of clarity of the optical image in one or both eyes, an image larger in one eye than the other (aniseikonia), anomalies of the visual pathway or cortex, or central factors in the integrating mechanism. The ultimate goal of binocularity is stereopsis ( DeAngelis, 2000 ), which improves motor skills at near distances ( O’Connor, Birch, Anderson, & Draper, 2010 ), but has a minimal effect beyond about 40 m ( Bauer, Dietz, Kolling, Hart, & Schiefer, 2001 ).
Stereopsis is not the only benefit from binocularity: there is a binocular advantage in terms of visual acuity and contrast sensitivity. The benefit in terms of contrast sensitivity is underestimated in the ideal conditions of visual acuity testing. For example, when driving in snow or with a dirty windshield, binocularity markedly outperforms monocular vision ( Otto, Bach, & Kommerell, 2010 ). Binocular performance is better than monocular at a wide range of tasks ( Sheedy, Bailey, Buri, & Bass, 1986 ). Difficulties in the coordinating mechanism of the motor system can also be accompanied by adaptations and anomalies in the sensory system, such as suppression, abnormal retinal correspondence, or amblyopia. These may occur to lessen the symptoms caused by the motor anomaly, as adaptations of the sensory system.
The anatomical, motor, and sensory systems must be normal for good binocular vision. The position of the eyes relative to each other is determined first by their anatomical position. Humans have forward-looking eyes placed in the front of the skull, and this brings the visual axes of the two eyes almost parallel to each other. In most cases, they are slightly divergent when the position is determined only by anatomical factors, and this is known as the position of anatomical rest ( Fig. 1.2 , position 1). In normal circumstances, this state seldom exists, as physiological factors nearly always operate. When a person is conscious, muscle tone and postural reflexes usually make the visual axes less divergent: the position of physiological rest ( Fig. 1.2 , position 2). The fixation reflex triggers initial convergence which takes the eyes to the position of functional rest ( Fig. 1.2 , position 3). For distance vision, fusional vergence then acts to bring the eyes to the active or primary position .
For emmetropes, there is negligible accommodation during distance vision but significant accommodation for near vision. Therefore, for near vision another physiological factor affecting the position of the eyes is the accommodation–convergence relationship: the eyes will converge as accommodation is exerted, and this is accommodative convergence . An awareness of the proximity of an object induces proximal vergence and, finally, for near vision there is, as with distance vision, fusional (disparity) vergence , which positions the retinal images on corresponding points (or within corresponding Panum’s areas). In Fig. 1.2 , the angle D is the sum of accommodative convergence, proximal vergence, and fusional vergence for near vision.
If fusional vergence is suspended, for example by covering one eye, the eyes will adopt a dissociated position at the position of functional rest. This is typically somewhat deviated from the active position. This deviation from the active position when the eyes are dissociated is known as heterophoria , sometimes abbreviated to phoria. It occurs in most people. The rare situation where a heterophoria is not present and the dissociated position is the same as the active position is known as orthophoria . It is stressed that the term ‘heterophoria’ applies only to the deviation of the eyes that occurs when the fusional factor is prevented by covering one eye or dissociated by other methods such as distorting one eye’s image so that it cannot be fused with the other. Heterophoria is sometimes described as a latent deviation; it is only detected on dissociation of the two eyes. Sometimes the eyes can be deviated even when no dissociation is introduced. This more permanent deviation is called heterotropia or strabismus . Other, less favoured terms include squint (a confusing term because it is often used by patients to refer to half closed eyes) or cast . Ocular deviations can, therefore, be classified as either heterophoria or strabismus, but there are other important practical classifications that need to be considered in investigating the binocular vision of a patient.
The symptoms and clinical features of most binocular vision anomalies fit into recognisable patterns. The recognition of these patterns is the process of diagnosis and this is an obvious preliminary to treatment. The classifications adopted here are intended to assist diagnosis ( Fig. 1.2 ). The term deviation is used generically to describe strabismus and heterophoria. Cyclotorsional and vertical deviations often occur together when they may be described as cyclovertical deviations .
Accommodation and convergence are closely linked, and this interaction needs to be considered when investigating binocular vision anomalies. For example, a patient with a problematic exophoria may use accommodative convergence to reduce the deviation, causing the accommodative lag to be lower under binocular than monocular conditions ( Momeni-Moghaddam, Goss, & Sobhani, 2014b ). Conversely, a patient with a problematic esophoria requires divergence to achieve binocular vision and exhibits decreased convergence accommodation. This may explain why esophoric cases typically have greater accommodative lag under binocular conditions than monocular ( Momeni-Moghaddam et al., 2014b ). A study found accommodative errors greater than 1 D in 11% of patients with decompensated exophoria and 22% of patients with decompensated esophoria ( Hasebe, Nonaka, & Ohtsuki, 2005 ). This is one reason why blurred vision can be a symptom of binocular vision anomalies: patients may manipulate accommodation to avoid diplopia, but at the expense of blur.
Prevalence of Binocular Vision Anomalies
Strabismus affects between 2% ( Hashemi et al., 2019 ) and 3% ( Robaei et al., 2006 ) and amblyopia 3% ( Adler, 2001 ) of the population. The prevalence of strabismus varies in different countries and ethnicities ( Hashemi et al., 2019 ).
Between 18% ( Pickwell, Kaye, & Jenkins, 1991 ) and 20% ( Karania & Evans, 2006a, Karania & Evans, 2006b ) of patients consulting a primary care optometrist have a near heterophoria which has the signs and symptoms suggestive of decompensated heterophoria. Some authors give higher prevalence figures ( Montes-Mico, 2001 ), although prevalence estimates are strongly influenced by diagnostic criteria and referral bias. There is a need to guard against ‘pathologising’ large proportions of the nonclinical population. For example, one study with the laudable aim of evaluating screening tools for nonstrabismic binocular/accommodative anomalies suggested nearly one-third of all children may have such anomalies, and yet included no assessment of symptoms ( Hussaindeen et al., 2018 ). A thorough review concluded 7% of the population are stereoblind ( Chopin, Bavelier, & Levi, 2019 ).
A large North American clinical study found that, other than refractive error, the most prevalent ophthalmic conditions in the clinical paediatric population are binocular and accommodative disorders ( Scheiman et al., 1996 ). Binocular and accommodative conditions were 10 times more prevalent than eye diseases.
Before undertaking a surgical procedure that will significantly change the refractive error, it is important to check the binocular status and consider the likely effect of surgery on any binocular vision anomaly ( Finlay, 2007 ). Binocular vision anomalies are an uncommon cause of nontolerance to spectacles ( Freeman & Evans, 2010 ).