Dissociated Strabismus Complex



Dissociated Strabismus Complex


M. Edward Wilson



The dissociated strabismus complex (DSC) is an intriguing ocular motor phenomenon that was first reported more than a century ago.1 The classic pattern of the complex is of a nonfixating eye slowly elevating, extorting, and abducting on the spontaneous loss of binocular function or with cover testing (Fig. 18.1). A reversal of these movements is seen with recovery and refixation. Latent nystagmus is also usually present. The most often quoted early descriptions are from Bielschowsky.2,3 Over the years, these complex deviations have been given many descriptive titles, attesting to our incomplete understanding of their varied manifestations:



  • Dissociated vertical deviation (DVD)


  • Dissociated vertical divergence


  • Alternating sursumduction


  • Occlusion hypertropia


  • Dissociated double hypertropia


  • Anatropia


  • Dissociated horizontal deviation (DHD)


  • DSC






Figure 18.1 Dissociated strabismus complex. The nonfixating left eye is elevated, abducted, and extorted.

DVD, attributed to Raab,4 has become the most commonly used clinical label for this type of strabismus. This is an oversimplified description, however, because it implies a purely vertical deviation.

Although the slow elevation of the nonfixating eye is the most recognizable feature of the complex, torsional or abducting components can predominate in one or both eyes, producing considerable diagnostic confusion. The diagnostic label DSC is preferred because it easily incorporates movements in the vertical, horizontal, and torsional axes.5,6,7,8,9,10 Latent or manifest-latent nystagmus and subnormal binocular vision should also be considered part of the complex. The DSC label avoids the awkward semantics of describing dissociated strabismus manifesting almost entirely horizontal or torsional movements as a DVD. Components of DSC include the following:



  • Dissociated elevation


  • Dissociated abduction


  • Dissociated extorsion


  • Latent nystagmus


  • Subnormal binocularity

For documentation, DSC should be subdivided into DVD, DHD, and dissociated torsional deviation (DTD) in records of patient examination findings. For instance, one eye may elevate markedly under cover with mild extorsion (4+ DVD, 1+ DTD), whereas the fellow eye may reveal marked abduction, moderate extorsion, and only mild elevation (4+ DHD, 3+ DTD, 1+ DVD) when covered. With this scheme, the familiar term of DVD is still used to describe the vertical component of the complex. However, horizontal and torsional movements are difficult to describe when DVD is used to name the entire complex. Because DHD has become a more clinically recognized entity, using DSC as an umbrella term avoids the temptation to overlook the intimate relationship between DVD and DHD, DTD, and latent nystagmus. If DVD or DHD is seen alone, it still can be labeled as such. On close observation, however, at least one other component of the complex is nearly always present. In a patient with prominent DVD, documenting subtle coexisting DTD, DHD, or latent nystagmus gives the examiner more confidence in the diagnosis of dissociated strabismus and further justifies the DSC label.


Theories of Causation

Although the exact cause of DSC remains unknown, recent investigations have more accurately documented and defined the movements that occur in both the fixating and the nonfixating eye in patients with DSC.11,12,13 In his Costenbader lecture, Guyton13 summarized the eye movements that occur in DSC. A comitant drift of both eyes occurs with the fixating eye adducting, depressing, and intorting. The nonfixating eye abducts, extorts, and elevates. The horizontal muscles produce the horizontal component of the drift, whereas the oblique muscles produce the cyclovertical drift. This produces the slow phase of the latent nystagmus. Comitant saccades then occur that compensate for the drift of the fixating eye. This produces the fast phase of latent nystagmus. In conjunction with the vertical and cyclovertical vergence movements, there is an upward vertical version that is necessary to compensate for the depression of the fixating eye caused by the vergence movement. This version is produced mostly by the inferior oblique muscle of the fixating eye and the superior rectus muscle of the nonfixating eye. Also, eye movement recording documented a horizontal version, away from the fixating eye, presumably to compensate for the collective abduction effects of both oblique muscles as they become active in the fixating eye. The combined effect of these movements is that the fixating eye stabilizes, but the nonfixating eye is driven into a variable state of elevation, extorsion, and abduction. Defining these movements may help us design newer treatments to reduce manifest DSC, but it does not explain why they exist in the first place. Guyton theorized that the movements serve to damp latent nystagmus and stabilize the fixating eye when latent nystagmus is present. This “nystagmus blockage” function is a learned response, according to Guyton, which helps prevent a decrease in visual acuity that would otherwise occur with latent nystagmus.

Brodsky14 presented evidence that DSC can be attributed to an atavistic resurgence of the dorsal light reflex that emerges when bifixation and high-grade stereopsis are absent, such as in early-onset strabismus patients. In humans, the eyes have been found to retain some of their primitive function as balance organs. Unequal visual input can induce a central vestibular imbalance in which the internal sense of vertical no longer corresponds to the gravitational vertical. In further support of this theory, Brodsky15 showed that the patients with DSC report a tilt in the subjective visual vertical with monocular occlusion. This momentary finding is then annulled by a cyclovertical divergence movement of the eyes. According to Brodsky,15 the human form of the primitive dorsal light reflex produces a twofold movement. A vertical divergence movement is present to realign the interpupillary axis of the eyes relative to the altered internal representation of the vertical. Second, a cycloversional movement is present that torsionally rotates the eyes in the direction of the tilted visual world to correct the perceived visual tilt. The vertical component of this reflex is a primitive adaptation of vertical divergence seen in fish. The phylogenetically newer cycloversional component is likely an exaptation. In other words, it did not arise as a primary adaptation, but was co-opted during evolution to restore vertical orientation under monocular viewing conditions in frontal-eyed humans.

In summary, Brodsky14,15 presented evidence that, in DSC, a subjective sensation of visual tilt under monocular conditions produces two compensatory eye movements: a vertical divergence movement to realign the eyes relative to an altered internal representation of vertical and a cycloversion (torsion) movement that rotates the eyes to neutralize a perceived visual tilt.


Incidence and Associations

DVD, if researched diligently, probably represents the most common hyperdeviation seen in a strabismus practice. Helveston16 recorded DVD in 11% of 1,000 consecutive patients with strabismus or nystagmus. Wilson and Parks17 found DVD in 62% of 98 patients with congenital esotropia who were followed up to or beyond 6 years of age. Other investigators have reported this association in up to 92% of patients.18 The age of the patient at the time of surgical treatment for congenital esotropia does not affect the incidence of DVD.17,18 Incidence figures for all aspects of DSC may depend on how carefully subtle findings are sought. DHD, DTD, and latent nystagmus can be detected in most patients with DVD. Considerable variability can occur, however, because some components of the complex appear to be present during some examinations and absent during others. DHD prominent enough to require horizontal muscle surgery occurs in 5% or more of patients with congenital esotropia.19 This number is increasing as DHD becomes recognized more easily and differentiated more accurately from consecutive exotropia. DSC can also occur in association with acquired esotropia, exotropia, and hypertropia. DSC is not seen when high-grade stereopsis (bifixation) is preserved, however. Therefore, all patients with DSC who show signs of binocularity are classified as having the monofixation syndrome.

Inferior oblique muscle overaction develops in more than 60% of patients with congenital esotropia.17 Therefore, DSC often coexists with inferior oblique overaction. DVD can simulate inferior oblique muscle overaction by becoming manifest in adduction as the nose interrupts fixation. Equally confusing is the association of a true hypertropia with DVD or a true esotropia or exotropia with DHD. These relationships are discussed later in this chapter. Latent or manifest-latent nystagmus occurs so frequently in association with DSC that it should be considered part of the complex. Latent nystagmus without coexisting congenital nystagmus is rarely seen in the absence of DSC.


Ocular Manifestations

DSC is almost always a bilateral but asymmetric condition. In an analysis of 170 patients, von Noorden20 found bilaterality in 86% and asymmetry in 91%. When a unilateral dissociated deviation is detected initially, careful observation over several examinations usually reveals an asymmetric, bilateral DSC rather than a truly unilateral finding. Dissociated deviations may manifest spontaneously or may be controlled by binocular fusion mechanisms and remain latent. Even when manifest, they are nearly always intermittent, varying with the state of attention of the individual. Visual inattention often produces a larger deviation that can be measured by even prolonged alternate cover testing, and variability is the rule. Deviations can appear small and well controlled on one visit, only to be large and manifest spontaneously a brief time later. Complaints related to manifest DSC usually do not include diplopia. Although suppression is usually present, red lens testing can sometimes elicit diplopia. For DVD, the fact that the red light always appears below the white light—regardless of which eye has the red lens before it—is unique among vertical deviations. In addition, when fusional movements are measured, patients with DVD have large vertical fusional vergence amplitudes in both directions, which is another unique feature. Although dissociated horizontal and torsional movements are being recognized with greater frequency, DVD remains the predominant manifestation in most cases of DSC.

Placing base-down prisms before the higher eye or base-up prisms before the lower eye until all refixation movements are neutralized can quantitate a true hypertropia. The alternate cover test reveals these refixation movements to be upward in one eye and downward in the other, in accordance with Hering’s law of equal motor innervation. The absence of upward refixation movements in either eye on alternate cover testing usually distinguishes DVD from true vertical tropia. In addition, the upward deviation is very slow (2–40 degrees/second) in DVD compared with true hypertropia (200–400 degrees/second).16 In addition, one or more of the other components of DSC (DTD, DHD, or latent nystagmus) can almost invariably be detected in patients with prominent DVD.

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Jul 10, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Dissociated Strabismus Complex

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