Marshall M. Parks
The vergences are reflexes, an integral factor in strabismus. They are the principal cause of concomitant deviations as well as the principal compensator, masking the deviation contained in a phoria rather than allowing it to become an overt tropia.
The vergence eye movements are produced by a group of compound reflexes, only two of which are identifiable because they are measurable; these are fusional vergence and accommodative convergence. The fusional vergences are optomotor reflexes designed to improve and maintain the alignment of the eyes so that similar retinal images project on corresponding retinal areas, a requirement for single binocular vision that utilizes normal retinal correspondence. The accommodative convergence is a reflex linking convergence automatically to accommodation and supplying the most economical innervating method for achieving proper alignment simultaneously with a change in the dioptric power of the lens as attention shifts rapidly between a remote point and a proximal point. Another vergence reflex, or perhaps other vergence reflexes, influences the alignment but remains ill-defined and not measurable; this unidentifiable entity which exerts vergence influence on the alignment makes up the tonic vergence.
The ciliary musculature contracts on the arrival of impulses at its myoneural junctions. The accommodative response is graded according to the quantity of impulses arriving. The degree of innervation dispatched to the ciliary muscles is associated with comparable gradations of innervation to the medial rectus and the sphincter pupillae. The combination of these three separate motor innervations producing the response of accommodation, convergence, and miosis is called the synkinetic near response. Normally the innervation proceeds to the respective muscles supplying these functions according to a ratio that permits relatively clear vision and approximate bifixation. Because convergence and miosis are also responses within other reflex systems, their particular responses to a near stimulus are designated as accommodative convergence and pupillary constriction to near stimulus. Accommodation differs from convergence and miosis in that it occurs only within the framework of the synkinetic near reaction.
The following four stimuli produce the synkinetic near response:
A blurred retinal image focused posterior to the plane of the retina. Regardless of whether the object projecting as a blurred image is far away or near, accommodation is applied. Accompanying the accommodation are the other two components of the synkinetic near response.
Bitemporal disparate retinal images. Accommodative convergence is used to reduce the bitemporal disparity in some patients similar to the way fusional convergence is used. Bitem-poral retinal image disparity alone is enough to initiate the near response in some patients with exodeviations. Often a synkinetic near response to the bitemporal sharply focused retinal images occurs in the young exodeviating emmetrope even while fixating at a distance.
Awareness of near. A psycho-optic influence reflexly causes the subject to put forth the synkinetic response while attentive to proximal points in space. The opposite, awareness of far, causes relaxation of the synkinetic near response while the patient is attentive to remote points in space. The subject induced to respond counter to these psycho-optic drives (i.e., to relax the synkinetic near activity while looking at a near point through plus lenses or to stimulate this activity while looking at a far point through minus lenses) gives only a fraction of what the total response would be were he allowed to respond naturally in conformity with the awareness of near and far influences. Recognizing their inability to prevent all convergence during measurement of alignment at near positions while the subject looks through plus lenses, many investigators have labeled this inevitable convergence response “proximal convergence.” Clinically, a similar situation has been recognized while doing simulated distant alignment measurements on instruments requiring near fixation through plus lenses. In variably, such simulated distant measurements are more convergent than true distant measurements. However, such terms as “proximal convergence” and “instrument convergence” create a concept that these are separate entities within the spectrum of vergences rather than mere findings produced by the unnatural stimulation of the synkinetic near response.
Voluntary convergence. The synkinetic near activities usually occur at a subconscious level, as do reflexes. However, there is one outstanding exception. The synkinetic near activities that occur in response to a conscious desire to maximally converge the eyes are not reflex; this is voluntary convergence and is produced by fixating an imaginary near point. Associated with the convergence response are accommodation and miosis. Thus, voluntary convergence is a willed, forceful thrust of the entire spectrum of synkinetic near activities.
The accommodation and accommodative convergence components of the synkinetic near response are in perfect balance when each produces the satisfactory adjustments needed for clear vision and fusion over a wide range of fixation distances. If these activities are in proper relation, then the ratio of accommodative convergence to accommodation (AC/A) is normal. An abnormal AC/A is characterized by either a deficiency or an excess of accommodative convergence associated with each unit of accommodation; thus, the resulting abnormal ratio is either a low AC/A or a high AC/A.
The normal ratio is influenced by certain drugs that alter the AC/A as long as they are present; however, when the drugs are withdrawn the AC/A reverts to the premedication level. The normal ratio is altered by parasympathomimetic and para-sympatholytic drugs instilled into the eyes. Miotics potentiate the transmission of acetylcholine across the myoneural junction, allowing innervation to the ciliary muscle to produce a contraction greater than normal. This in turn results in a greater dioptric power change in the molding of the lens. However, associated with the innervation to the ciliary muscles is the same amount of innervation to the medial recti. Because the miotic has no effect on these muscles, there is no change in the quantity of convergence this amount of innervation produces. Consequently, the miotic alters the AC/A so that the response is more accommodation associated with an unchanged amount of accommodative convergence (i.e., the miotic lowers the AC/A).
Weak cycloplegics partially prevent innervation arriving at the myoneural junction of the ciliary musculature from producing the expected dioptric increase in the refractive power of the lens. This is due to the reduction in the motor units firing in the ciliary musculature because the cycloplegia competes with the motor end-plates for the acetylcho-line. Yet, the innervation simultaneously dispatched to the medial recti produces the normal convergence. The result is that the cycloplegic diminishes the accommodation response without changing the accommodative convergence (i.e., the cycloplegic raises the AC/A).
The normal AC/A in children is pliable, an attribute that allows adjustments in uncorrected myopia and exodeviation.
In uncorrected myopia, the patient tends to adjust the AC/A to a high ratio in order to obtain the maximal accommodative convergence and the deficient accommodation required for near vision. This compensation assists the myope to enjoy near binocularity. After the patient wears glasses for near vision for a few weeks, the high AC/A reverts to normal because it is no longer needed.
The young child with exodeviation tends to have a high AC/A which permits the patient to compensate maximally for the deviation and the bitemporal retinal image disparity it causes. After the exodeviation is corrected with surgery the ratio reverts toward normal because the high AC/A is no longer needed.
An abnormal AC/A may be either a high or a low ratio. A high AC/A causes more convergence for near fixation than for distance fixation, with the actual difference between them being determined by the severity of the AC/A abnormality, which may vary from slight to marked. A high AC/A may occur in a patient with orthophoric eyes for distance fixation as well as in the patient with an esodeviation or an exodeviation. The patient with a high AC/A who is either orthophoric or exodeviated for distance was described by Duane1 as having a convergence excess. The patient with a high AC/A with exodeviation at distance has a divergence excess.1
The high AC/A is invariably the etiologic factor causing the convergence excess, and in this setting the high AC/A is primary (i.e., it is primarily defective and not a secondary change in a normal AC/A). Depending on the refraction of the patient and the severity of this primarily abnormal AC/A, the eyes are straight, esophoric, or esotropic at distance, but regardless of the distant alignment, the esodeviation is invariably greater at near fixation. However, the high AC/A in the patient with exotropia with divergence excess usually is an altered normal ratio that exercised its attribute of pliability, becoming a high ratio to help offset the exodeviation. Yet, the primarily defective high AC/A can occur in any type of patient, even one with exotropia. It is possible that the patient with divergence excess may have either a high ratio that evolved from a normal one or a primarily abnormal high ratio, although the latter occurs infrequently. Most importantly, the primarily abnormal high AC/A is unpliable, unlike the normal AC/A. Surgically aligning the exodeviated eyes in a patient with divergence excess whose high AC/A is primarily defective results in a persistent esodeviation at near fixation, whereas the preoperative high AC/A adapted from a normal one promptly returns to normal postoperatively, nullifying any trend to near esotropia.
The primarily defective high AC/A that causes convergence excess tends to improve after the patient reaches 8 years of age. Improvement may gradually continue over the next several years, but usually a trace of high AC/A persists throughout life. Approximately 50% of the patients with the primarily high AC/A show this improvement trend while the remaining 50% retain their same high AC/A into adulthood. Miotics normalize the high AC/A but only while the drug is used; the miotic produces no permanent change in the AC/A. Surgery on the horizontal rectus muscles may improve the high AC/A somewhat; the greater the severity of the high AC/A, the greater is the effect of surgery on the ratio. There is no orthoptic technique that can improve the high AC/A.
A low AC/A causes less convergence for near fixation than for distance fixation, with the actual difference between them being determined by the severity of the AC/A abnormality, which may vary from slight to marked. A low AC/A may occur in a patient with orthophoric eyes for distance fixation as well as in a patient with esodeviation or exodeviation. The patient with a low AC/A who is either orthophoric or exodeviated for distance was described by Duane as having convergence insufficiency.1 The patient with a low AC/A and esodeviation at distance has a divergence insufficiency.
The low AC/A in convergence insufficiency is a primarily defective ratio that never improves with age. Although weak cycloplegics temporarily improve the low AC/A, this is of no practical value because the blurred vision and asthenopia produce symptoms more disturbing than those caused by the low AC/A. Neither surgery of the horizontal recti nor orthoptics improve the low AC/A in convergence insufficiency.
The low AC/A in divergence insufficiency is a relatively rare clinical entity. A very few esodeviated children who fuse at near fixation but who are either esotropic or have a large esophoria at distance have been studied; however, surgery has not been performed in a sufficient number to determine if there is a trend for the low AC/A to improve after the distance esodeviation is eliminated. Divergence insufficiency is found as a gradually appearing phenomenon in a few elderly patients, but advancing divergence palsy in a child or young to middle-aged adult accompanied by recent onset of distance homonymous diplopia is the dangerous history that demands neurological work-up. Consequently, it is not known whether the low AC/A is the adjusted normal pliable AC/A that evolved to help offset the esotropia or whether it is a primarily defective, unpliable ratio that just happens to occur in small-angle esodeviation.