CONCEPT

Binocular vision is the supreme refinement and the purpose of the phylogenetic and embryological evolution of human neuro-ocular sensory-motor skills, as defined by Queré himself. Fusion is a fundamental part of it.

Fusion

It is the meeting of sensory messages received by both eyes with retinal correspondence or limited disparity. We can define Fusion (F) as the second degree of binocular vision according to Worth, with Simultaneous Perception (PS) and stereoscopic vision (V.E.) being the first and third degrees respectively. Although it can be called academic, this division has some practical utility. Simultaneous perception represents simple sensory fusion, fusion involves an added motor response, and stereoscopic vision involves perceptual synthesis at a higher level.

Fusion is a sensory function, but it depends on the motor apparatus. It is not exactly a reflex, but there are sensory prompts that cause the movement necessary for fusion. It involves a continuous readjustment by which we unconsciously tend to merge. The macular points are the most sensitive, this fusional sensitivity decreasing as we move away towards the periphery.

Fusion involves unitary perception despite the reception of separate and different retinal images by each eye. Thanks to the integration of these 2 discretely different single-eye images in a single perception, the vision of the form and the third spatial dimension is achieved. This capacity depends on the retinal reception of the images of an object by 2 corresponding points, which have a common spatial value within areas (small in the center and greater in the periphery) of single vision, known as areas of Panum. And not only in primary position of gaze, but in its versions and vergences.

We will not consider the concepts of cortical inscription, cortical disparity, cortical binocular actions, cell columns, simple, complex, hypercomplex and non-oriented cells, based on the works of Hubel, Wiesel, Barlow, Blakemore, Nikara, Bishop, Lund, Sokoloff … For having been the subject of comments on the topic of Retinal Correspondence.

We will limit ourselves to remembering the concept of the horopter as the common place of all the points in space that stimulate corresponding retinal elements. The horopter represents the places of intersection of visual directions common to both eyes. And it is geometrically represented by a line with a central point (fixation point, which causes stimulation of the foveas of each eye), together with a myriad of other points, each of which causes the stimulation of a retinal point in one eye. , corresponding to a retinal point in the other. The primitive notion is complicated and we go from the Vieth-Müller circle, to the geometric horopter, to the real longitudinal one, to the Hering-Hi11ebrand horopter to the modifications of Ogle, Ames and areas of Panum.

The fusional area of ​​Panum extends along this line, and the visual objects in this space appear as simple / due to the flexibility of the fusion, although they do not strictly stimulate corresponding retinal points. This fusion area is narrow in the center of the horopter, and widens towards the periphery, within the range of fixation disparity. We have gone from the concept of a point to that of a fusional area.

 

Pseudofusion

It is the false merger or anomalous merger according to Duke-Elder. By definition (requiring bifoveal fixation), the strabic subject does not have fusion, except:

- When the deviation is absent in controlled latent strabismus.

- In intermittent strabismus at certain gaze distances.

- In incomitant strabismus in certain gaze directions.

In the case of strabismus with a small angle of anomaly, a false fusion or pseudofusion is possible. Subject sees the controls for both sights, but his eyes are not moving. The exam is actually done on the subjective angle. In this case, the retinal area involved in binocular cooperation is parafoveal or paramacular. For there to be true or objective fusion, it requires fusion movements such that the objects are always fixed by the foveas. Sometimes the subject has the impression of merging images while the fovea of ​​the deviated eye does not fix: it is the pseudofusion or subjective fusion, due to variation of the angle of anomaly (the images are fused views, but the eyes do not move) or to true fusion movements with persistence of the angle of anomaly.

 

Fusion amplitude

Fusion breadth is understood to be the range in which the fusion is maintained. Likewise, it can be defined as the ocular displacement between maximum divergence and convergence, preserving fusion.

The intensity of binocular fusion is evaluated by forcing the eyes to make versions or limit vergences, which do not break the fusion. Its measurement in clinical practice is carried out with the vergences for being easier and more comfortable. For practical purposes, it can be defined as the largest prism, in the appropriate direction, that a patient's eye can tolerate while maintaining clear single vision or the same measurement obtained by the haploscope.

 

MEASURE

The measurement of the amplitude of fusion will be carried out at two fixing distances, normally 30cm. and 6m. The accommodative component will be eliminated using the patient his correction, and choosing a fixation point, which causes a constant level of accommodation. Its limit is reached when the patient appreciates a horizontal separation or blurring of the object ("subjective end point"), or when the observer notices an inability of the eyes to maintain the foveal fixation position ("objective end point"). Next, the decrease in prismatic power or in the movement of the synoptophore arms that enables the subject to once again achieve single binocular vision is the measure of the point of “subjective fusional recovery.” This test will be performed in divergence and convergence.

The amplitude of fusion (AF), depending on the axis studied, can be:

- horizontal

In convergence

In divergence (or convergence (-) according to authors)

- Torsional or cyclofusion.

 

The normal values ​​of fusion amplitude depend on the nature of the stimulus or test used (foveal, macular or paramacular), state of fatigue, training, attention, ...

Basically its measurement can be done in space with the help of prisms or by means of the synoptophore, other times it is practiced with transparencies or polarized projections *.

 

AF metering with prisms

It will be done from far AND near using the patient his usual correction. AF with far prisms. The subject with both eyes open will fixate a bright spot at 6m. By interposing prisms of increasing power, the divergence angle will be sought, and later the convergence angle (it is carried out in this order to avoid the convergence spasm, which would distort the measurements), which, caused by the prisms, cannot be compensated with the fusion. To study FA in divergence or convergence (-) (since in the maintenance of fusion the eyes are never divergent, even if they fix a distant object, a bar of nasal-based prisms is placed in front of the eyes (the image seen by the subject deviates outward so the eye diverges) until it causes diplopia ("subjective end point") or the eye pops out ("objective end point"). The value of this prism will indicate the value of the AF in divergence.

The same procedure is then carried out with a time-based prism bar, placing prisms of increasing value before the eyes until they trigger diplopia. The value of the prism causing diplopia will measure the AF in convergence. If you wink your eyes during the measurement, you gain a few prismatic diopters.

After each of these measures, the power of the prism can be reduced until the single binocular vision is restored. The value of this prism will mark the "subjective fusional recovery point". AF with near prisms: All these measurements will be repeated in vision close to 30cm, after a few minutes of rest to avoid the convergence spasm.

To avoid prismatic distortion, the prisms can be distributed between both eyes, the measurement is usually carried out with Berens square prisms, Berens or Pigassou prismatic rules and Risley's rotating prism. The measurements are somewhat different depending on the method used, since for example with Berens square prisms fusion is requested instantaneously and staggered while with prismatic rulers the AF is measured in the maintenance of simple binocular vision.

Hugonnier considers the following FA values ​​to be normal:

Horizontal AF

- far diverging (D): 7-8 DP

- far convergence (C): 20-25 DP

- close up in divergence (D *): 10-15 DP

- close to convergence (C '): 35-40 DP

Vertical AF

3-5 DP (as collected by Duke-Elder)

Measurement of PA with synoptophore

The synoptophore studies the fusion in distance vision. For this, two fusion sights are used with identical parts (or parts to be fused) and with foveal controls seen by a single eye. These controls would give away the deletion if it existed.

Thus the letters L and F can be merged into the letter E. The study of suppression can be carried out from far and near with other methods, such as Worth's lights, Remmy's diploscope, etc. as explained in the Suppression topic.

The stage of fusion (F) to the synoptophore usually follows that of simultaneous perception (PS), with which we already have previous data on the objective angle, which physiologically we know can have a hyperphoria of 1-2 DP and a horizontal angle -10 to +40 With the subject positioned at your target angle, you are prompted to move the synoptophore arms until you see only one image, checking that there is no blanking of the controls. And the angle indicated by the apparatus is noted as fusion angle (F).

The AF is determined by moving the arms of the synoptophore first in divergence, the eyes following this movement until diplopia appears. The new angle is noted, and the difference from the first angle measured indicates the diverging AF. Then it is returned to the original fusion position (F), and a convergence movement will be impressed on the arms until the image is blurred (by the accommodative effort prompted by the convergence effort) or diplopia appears.

Although highly variable, it is admitted that in a normal subject the PA to the synoptophore is 8 DP in divergence and 60 DP in convergence, and vision clouding often appears at 30. The vertical AF to the synoptophore does not usually exceed 2-3 or 3-6 according to authors. For torsional AF or cyclofusion measured subjectively by Hofmann and Bielchovsky with the haploscope, 12-200 arc are given as normal values. Although it should be noted that here the fusional movements play a less important role than the central fusion. Cyclo-fusion can be studied by means of a white circle crossed by a black cord, seen directly by the right eye and through a Dove prism by the left eye.

For the study of AF with the synoptophore in near vision, -3 diopter spherical lenses are used in front of both eyes.

 

AF measurement with polarized projections

Although less widespread in daily clinical practice, two transparencies or two polarized projections (at 450 and 1350) can also be used, each of which is only seen by one eye thanks to the use of polarized glasses- The two tests are positioned so that are merged and then move back and forth until the merger is broken.

The technique is basically the same used in sections 1 and 2, so we do not stop at its explanation.

This is the system used in Renée Pigassou-Albuy's polarized light stereo projector, which is described in the stereopsis section.

 

As Hugonnier states, data from the Retinal Correspondence Exam (R.C.) guides this study:

1. CRN.

If the CR is normal and the deviation constant, the fusion study will be done with prisms in space (after having corrected the target angle with prismatic treatment) or with synoptophore

If the CR is normal and the intermittent deviation with close orthophoria, the fusion will be studied with a Remy diploscope, reading with a control bar or Javal grating. If with orthophoria from afar, we will use the lights of Worth. The prisms and synoptophore can be used from far and near.

2. Duality of correspondence

If the duality of correspondence is due to the tests used in the case of the synoptophore, it will be necessary to distinguish the objective fusion from the subjective one. In many cases, in entropies, convergence takes place in objective fusion and divergence in subjective fusion.

At other times duality depends on visual conditions such as gaze position. In this case we will study the fusion under the conditions that cause the C.R.N. both in space and in the synoptophore, for example: in esotropia with syndrome in A we will study fusion in the downward gaze.

3. CRA

In this case there will be no fusion, and therefore no need to study it. Unless the angle of anomaly is minimal and with a considerable subjective fusion. Its study can be carried out with synoptophore or with Bagolini's striated crystals.

 

In summary, we will say that in patients with oculomotor imbalance, fusion will be studied only if they present CRN, a tendency to CRN or duality of instrumental and spatial correspondence ... but how would we perform this test?

- If we decide to do it in the synoptophore, we would use fusion tests such as the clown with ring and cane as controls. The arms of the synoptophore are left loose and separated from the target angle. Subject sees 2 clowns. Then it positions the images at their angle and merges them into one with both controls. At that point he achieves fusion. Now the arms are locked. A divergence movement is imprinted on them, a movement that the eyes continue to maintain fusion to a point where they can no longer and diplopia appears. The angle of rupture is noted. The same will be done in convergence, the arms of the synoptophore move in convergence and the eyes follow this movement until the image appears smaller, cloudy and distant. It follows a few more diopters and then the fusion breaks. The angle of rupture at toe is noted. And finally it goes backwards until the fusion is recovered (fusional recovery point).

- It is advisable to use tests with large central controls for the accommodative ones and with greater light intensity in case of neutralization.

- The sights will be placed more divergent for convergent and vice versa.

- Depending on the age of the subject, it will be left free or placed close to its target angle, if it is small.

-First, it is assessed in divergence and convergence without stimulation, which gives us an idea of ​​his situation in daily life. And then with fast stimulations in convergence and slow in divergence, which tells us about their possible response to re-education. Likewise, the movements of the arms of the synoptophore will be fast in convergence and slow in divergence.

- To control fusional movements

They will be questioned for the presence of the controls.

The position of the corneal reflexes will be monitored.

Miosis, caused by the effort of accommodation-convergence, will be observed.

- The fusion will break not only when reaching the limits of convergence-divergence, but by other mechanical causes such as release of a hyperphoria or sensory causes such as neutralization.

NOTATION

Once the fusion (F) and fusion amplitude (AF) measurement has been carried out, the method followed, the technique practiced, the measurement obtained and any circumstance that is considered of value in the evolutionary study of the patient will be recorded in the clinical history. patient

AF, Away, CC, Synoptophore: -30, 00, +30

(Fusion width from afar, using the patient its correction, using the synoptophore for the measurement, with fusion at OS and fusion width in divergence up to -30 and in convergence up to +300)

AF, close, cc, prisms: -5 / -3, +1, +60 / + 54

(The unit of measurement is prismatic diopters and in broken ones the numerator indicates the fusional break point and the denominator the fusional recovery point)