To see double, it is necessary to have vision in both eyes. The vision need not be equal or even good in the eye responsible for the second image, but in general, the symptoms of diplopia are more likely to be bothersome if the second image creates enough awareness to be competitive and, therefore, annoying. To better understand diplopia, it is necessary to understand the mechanism of single binocular vision.

Retinal Correspondence

Each eye provides the brain with a more or less complete view of what is in front of it. In normal humans, the two eyes relate to one another in such a way as to produce a single image from two "pictures," a right-hand picture and a left-hand picture. This union of images is carried out through a complex neurologic organization beginning with corresponding retinal points.

The most notable of the corresponding retinal points are the two foveas. When the fovea of each eye is stimulated independently, the brain registers that the object "seen" by each fovea is in the same place regardless of the direction the eyes are pointing. The two foveas in the normal situation can be considered the principal corresponding points. The fovea is also the retinal location responsible for the best visual acuity. Away from the fovea, the two retinas relate to that each point in the right retina has a corresponding point in the left retina. Specifically, the right nasal retina contains points that correspond to their counterpart in the left temporal retina, and vice versa. By this scheme, the two eyes produce a single cyclopian eye, which can be envisioned as being centered around the occipital cortex, with the two foveas being the only truly anatomic as well as sensorially corresponding points and with the nasal-temporal and temporal-nasal retinal relationships existing elsewhere in the right and left hemispheres. The binocular visual field, which is made up of stimuli from both eyes, is slightly smaller in its peripheral extent than either monocular visual field. The reason for this phenomenon is that the lower temporal field of each eye is seen only by the ipsilateral eye because the nose blocks the contralateral eye.

When stimulation of corresponding retinal points or areas produces single vision, normal retinal correspondence is said to be present. Conversely, when stimulation of corresponding retinal points produces diplopia or when stimulation of noncorresponding retinal points produces single vision, anomalous retinal correspondence is present. Anomalous retinal correspondence may be harmonious or nonharmonious, depending on whether the angle of anomaly is equal to (harmonious) or less than (nonharmonious) the angle of strabismus. Nonharmonious retinal correspondence, which serves no apparent purpose, may be only a testing artifact whose presence gives a clue as to the depth of the sensorial adaptation. Strabismus in early life followed by suppression and then sensory reorientation is the basis of anomalous retinal correspondence. This response can be considered an innate, binocularly functioning antidiplopia mechanism.

Suppression

The physiologic solution to the diplopia potential that occurs when the two eyes send the brain an independent picture is fusion, which is the normal binocular state. An expedient response when fusion is not possible is suppression of one image. Suppression of the image from one eye is for the most part relative; that is, even in the case of profound suppression a person can be made aware of the second image if a more intense stimulus is presented to the suppressed eye. That is, in most instances the suppression can be overcome if sufficient stimulus is given to the nonpreferred eye. Nonetheless, suppression, which occurs at the cortical level, is an effective mechanism for eliminating diplopia.

When strabismus produces the potential for double vision in infants or children, who have an immature visual system, suppression occurs rapidly, is profound, and causes a permanent breakdown in the normal binocular fusion process. This profound suppression may be alternating so as to maintain normal vision in the eye that happens to be used at the moment for fixation. On the other hand, the suppression may habitually affect just one eye; in this case, there is fixation preference for the other eye, and amblyopia occurs in the habitually suppressed eye.

The earlier the onset of strabismus and the longer it is left untreated, the more profound the suppression will be. Suppression may be reversed in most cases with timely and effective occlusion therapy. Both the establishment of suppression, with or without amblyopia, and the reversal of suppression through occlusion and other amblyopia treatments occur during the immature or plastic period beginning shortly after birth and persisting until ages 6 to 9 years. Treatment of amblyopia with timely and persistent occlusion of the preferred eye can be effective. There is no specific treatment that directly enhances fusion. At present, the only effective strategies to enhance fusion are indirect methods intended to equalize vision, such as patching and alignment of the eyes by means of surgery or hyperopic optical correction and/or the use of bifocals in a few selected cases as early in life as possible.

When suppression exists, harmonious anomalous correspondence is usually present. This means that the suppressed, or actually partially suppressed, eye is being used to fill in the visual picture more or less as a "helper." The misaligned picture that the suppressed eye sends to the brain is sensorially modified by the anomalous correspondence. This phenomenon can be confirmed by noting the difference in size between the monocular and binocular visual fields. For example, in an exotropic individual who has right-eye vision of 20/20 and left-eye vision of 20/200, the suppressed eye of the exotrope, particularly in the peripheral monocular field, contributes significantly to the visual field; this contribution can be plotted with binocular visual field testing. With facultative, or alternating, suppression, either eye may be used for fixation while the other eye is suppressed momentarily and acts as a "helper." The instantaneous sensorial adaptation seems clinically to be as profound in facultative suppression as in habitual suppression with amblyopia but differs in that it occurs on an instantaneous and alternating basis. That is, the suppressed eye has the capability for reversal to become the fixating eye at any time.

A clinical situation that stresses the importance of binocularity in the presence of suppression occurs in the adult who has exotropia and who also has serviceable vision in each eye, with or without alternation. This type of strabismic individual will have an expanded binocular visual field. A person with large angle exotropia may be able to see peripheral objects out to 180° or more on the side of the deviating eye. Using noncorresponding retinal points united in harmonious anomalous retinal correspondence at least in part, these patients enjoy an expanded, and to a certain extent useful, visual field without diplopia. When these adult, large-angle exotopic patients with an enlarged binocular visual field have their eyes straightened surgically, they should be warned that they will, at least for a short time, experience what they may interpret as "tunnel vision." This sensation is experienced as the individual reverts from an expanded to a normal binocular field. Such patients can be reassured that the early postoperative, uncomfortable feeling always goes away, and therefore reverting from an expanded to a normal visual field should not be a deterrent to having their eyes straightened.

Physiologic Diplopia

Normal persons with no strabismus when provoked experience physiologic diplopia because all objects except the object of regard (what we are looking at) are seen by noncorresponding retinal points and thus produce double images. Objects closer than the object of regard are seen by the noncorresponding temporal retina of each eye; such near objects produce doubled and crossed images (crossed, or heteronymous, diplopia). Objects farther than the object of regard, provided the object of regard is inside infinity (inside 20 feet for practical purposes), are seen by the noncorresponding nasal retina of each eye; such distant objects produce uncrossed double images (direct, or homonymous, diplopia). Fortunately, normal humans have the innate capacity to ignore the panorama of potential double images and to be visually aware only of what is useful and comfortable. In the pathologic manifest strabismic state, esotropia produces uncrossed diplopia, and exotropia produces crossed diplopia.

It should be emphasized both to normal individuals and to those with strabismus from a variety of causes that a person with two seeing eyes can always experience diplopia. The normal person is the one who with proper motivation can successfully avoid diplopia and maintain comfortable, effective, single binocular vision. Avoidance of physiologic diplopia by normal individuals is aided by the fact that psychologically they know what they are looking at, and the doubled images are seen by retinal areas with poorer resolution.

Panum's Fusional Space

The horopter is defined as the sum of points in space that stimulate corresponding retinal points along the horizontal plane and tire seen singly: Panum's fusional space includes the area slightly farther from the person that stimulates binasal retinal areas and the area slightly closer to the person that stimulates bitemporal retinal areas. The summation of these stimuli, seen singly and with depth, produces stereoscopic vision. Normal binocular vision with stereopsis and binocular vision without stereopsis should be distinguished. For example, a patient with 30° or alternating exotropia has binocular vision, as evidenced by an enlarged binocular field; an esotropic patient also exhibits binocular violon, although it is less obvious and less important clinically. However, such patients, who are binocular in the broader sense, will be unable to appreciate stereopsis, which is the ultimate reward of binocular single vision.

Posttreatment and Other Types of Diplopia

In the clinical situation when adult patients with longstanding strabismus are being considered for surgery to straighten the eyes, they can be tested for the likelihood of postoperative diplopia by placing fully correcting prisms in front of the eyes while a distant object is fixated. They are then asked whether they see singly or not. Although this may be a useful technique to anticipate transient postoperative diplopia and to show patients what diplopia will look like if they do not already know, it is seldom a reliable predictor of persistent bothersome postoperative diplopia. Thus, preoperative diplopia testing is definitely a limited clinical tool.

A special type of posttreatment diplopia is worth mentioning. This is the intractable diplopia that can occur after prolonged patching of one eye for amblyopia in the presence of small-angle strabismus, even microtropia. We have seen several young patients with minimal amblyopia of 20/20 in one eye and 20/40± in the other eye who underwent extensive amblyopia treatment with both patching and active stimulation. Such patients may improve a line or so in the amblyopic eye, but in the process the suppression is broken down, and diplopia occurs because nonsuppressed, noncorresponding retinal points now are competing on a more-or-less equal footing in the absence of suppression or effective anomalous correspondence. Because these patients lack fusion potential, they cannot be successfully treated with a prism or glasses. This same phenomenon can also occur spontaneously in adults with amblyopia who change the way they use their eyes. For example, such an individual may be given a new job that requires prolonged attention to fine detail such as reading blueprints or reading fine calipers. This form of "autopleoptics" can cause adult-onset diplopia in susceptible patients in the same way that prolonged patching causes it in a child. There is no adequate treatment for this type of diplopia except time and possibly altered use of the eyes, so that prolonged scrutiny of small objects is avoided.

In paradoxical diplopia, a patient with an esodeviation reports crossed diplopia, or a patient with an exodeviation reports direct diplopia. This rare and transient phenomenon occurs when there is a change in the strabismus angle that is not accompanied by an appropriate and compensating suppression and/or alteration in retinal correspondence. Monocular diplopia can result from a variety of abnormalities of the media, including corneal opacities and irregularities, polycoria, cataract, subluxated lenses, and vitreous opacities. Maculopathy can also cause monocular diplopia. In differentiating a media cause from a retinal cause for monocular diplopia the patient is asked to look through a pinhole. If the monocular diplopia disappears while looking through a pinhole, a media cause for the monocular diplopia can be inferred. If the monocular diplopia persists when the patient looks through a pinhole, a maculopathy is the most likely cause for the monocular diplopia. Receptor problems have occasionally been reported with pituitary tumors that bisect fixation and with hemorrhages in the occipital cortex that spread the receptor elements of the visual cortex. When monocular diplopia is present, binocular triplopia can occur if the visual axes are misaligned.

Motor Fusion

The two eyes move about in the frontal plane under the influence of six pairs of extraocular muscles that function in a yoked manner. The ultimate role of the extraocular muscles is to move the eyes together toward, and then to enable the eyes to maintain fixation upon, the object of regard. These two functioning units, right eye plus extraocular muscles and left eye plus extraocular muscles, initially meet at the occipital cortex. Their binocular vision capability is consummated through integration in the brainstem and, finally, is expressed through the cranial nerves, which supply the extraocular muscles. The purpose of this visual motor feedback mechanism is to maintain alignment of the visual axes on the object of regard while the person viewing and/or the object of regard is either changing positions or still. Fixation is maintained through a series of conjugate movements of the eyes whereby they move "together" in the various directions—right, left, up, and down, or a combination of these movements. Binocular yoked eye movements are called versions.

Yoke muscles receive equal, distributed innervation according to Hering's law. Antagonist muscles receive reciprocal innervation according to Sherrington's law. When an object is brought from far to near, the eyes undergo convergence with an emphasis on medial rectus contraction and lateral rectus relaxation; when an object is moved from near to far, the opposite occurs.

During accommodation, the central stimulus for fusion occurs in the occipital cortex where retinal image disparity is recognized and where a "correction" stimulus to effect alignment of the object of regard is initiated. This stimulus to bring the images together by motor fusion is mediated through the brainstem. Damage to this axis from trauma, surgery, or other causes can lead to central disruption of fusion. Patients with this problem can fuse momentarily, but they have no capacity to maintain this fusion and, therefore, have intractable diplopia. Two other conditions are somewhat like this: horror fusionis and persistent secondary deviation. In horror fusionis, the two foveas act like similar poles of a magnet and actually repel one another, producing a constant diplopia. In persistent secondary deviation, such as occurs with bilateral sixth or fourth nerve palsy or with third nerve palsy especially with aberrant regeneration, a secondary deviation drives the visual axes apart in nearly any field of gaze. These conditions may be slight variations on the constant theme of damage to the CNS regulating mechanisms.