16.1 Introduction

Dissociated vertical veviation (DVD) is a complex oculomotor disorder described more than a century ago, whose etiology remains controversial and whose surgical management is not entirely satisfactory.

It is characterized by an intermittent, variable, and slow upward drifting of either eye (DVD), combined with extorsion (dissociated torsional deviation [DTD]) and abduction (dissociated horizontal deviation [DHD]) of the nonfixing eye. ^{2 ,​ 3} The downward vertical drift of the hypertropic eye takes place together with intorsion and adduction (Video 16.1). It is usually found in patients with early onset strabismus and profound sensory anomalies. These patients often have latent nystagmus (LN), head tilt, and associated oblique muscle dysfunction.

Ever since Stevens described it in 1895, ⁴ many authors have sought to explain the nature of DVD, and various theories have been proposed. During the last few years Guyton ⁵ and later Brodsky ⁶ renewed the controversy about the pathogenesis of this anomaly.

Diverse treatments have been attempted, but the best current expectation is to minimize the deviation or to improve its control. With conventional therapy, DVD cannot be fully eradicated.

16.2 Clinical Characteristics

Patients with DVD usually have poor fusion and suppression. ⁷ When DVD is controlled it is termed “latent” (only seen when the eye is covered); when it is uncontrolled it is termed “manifest” (spontaneously visible to others). ⁸ Manifest DVD creates a cosmetic problem, particularly during periods of inattention, fatigue, stress, or lack of concentration.

The age of onset of DVD is usually early. ⁹ A mean age of 9 months was documented by Stewart et al. ¹⁰ It often becomes manifest after the achievement of a horizontal alignment, so it is important to detect it before the surgical treatment of an esotropia to treat both simultaneously.

DVD is usually bilateral. ^{11 ,​ 12 ,​ 13} It can be symmetrical or asymmetrical, and many cases are so markedly asymmetrical that they can appear to be unilateral (Video 16.2). Very asymmetrical DVD cases are usually associated with unilateral deep amblyopia. However, amblyopia is not a necessary prerequisite for asymmetry in DVD. Despite the fact that larger DVD amplitude is usually seen in the nonfixating eye, cases with larger DVD in the fixing eye do exist and may show hypotropia of the fellow eye in binocular conditions. ^{14 ,​ 15 ,​ 16 ,​ 17}

Video-oculography (VOG) is a technique that measures horizontal and vertical deviations during binocular viewing as well as after occluding each eye. The bilateralism of most of pseudomonocular DVD cases can be detected with VOG. A true vertical deviation added to the dissociated hypertropia explains the DVD asymmetry in many cases (Fig. 16‑1, Fig. 16‑2, Fig. 16‑3, Fig. 16‑4).

Oblique muscle dysfunction causes DVD incomitance in different gaze positions, and the presence of a true vertical deviation (hypo- or hypertropia) causes DVD asymmetry.

The nondissociated vertical tropia can be lesser or larger than the amplitude of the DVD. When the magnitude of the DVD is smaller than the nondissociated hypertropia, the hypotropic eye is never the higher eye. During cover testing, the hypotropic eye can become hypertropic if DVD is larger than the vertical tropia, or it can remain aligned if the DVD is of a similar magnitude to the vertical tropia. This situation may be erroneously interpreted as monocular DVD.

Unmasking bilateral DVD is very important when surgery is planned. Bilateral symmetrical procedures are performed for cases of symmetrical DVD, but asymmetrical DVD is more common and must be treated with asymmetrical surgery. Determining the difference in the DVD magnitude for each eye is critical to choosing the proper surgical approach in asymmetrical cases and avoiding overcorrection.

DVD can be comitant or incomitant. Incomitant DVD is differing hypertropia in different gaze positions. DVD incomitance is mainly due to oblique muscle dysfunction and, sometimes, vertical rectus asymmetry.

When DVD is associated with inferior oblique overaction (IOOA), the hypertropia is larger in adduction and a V pattern may be observed. In extreme adduction, a true hypertropia may be seen in addition to DVD. Cases with superior oblique overaction (SOOA) show larger hypertropia in abduction of the nonfixating eye than in primary position (PP) with A pattern. ^{18 ,​ 19}

A unique feature of DVD is the response to changes in light density: a downward movement of the occluded eye when filters of increasing density (Bagolini filter bar) are placed before the fixing eye (Bielschowsky’s phenomenon). ²⁰

The red glass test also yields unique results in these patients. Regardless of whether the red filter is placed before the right or the left eye, the patient always sees the red light below the white one. ²⁰

Another singular behavior of DVD is evidenced by Posner’s test ²¹ : when occluding one eye, the eye moves upward; when occluding the contralateral eye (keeping the other eye occluded), the hypotropic eye moves upward and the hypertropic eye moves downward, becoming aligned in the vertical plane.

Patients with DVD are usually asymptomatic, but in those cases where significant hypertropia occurs spontaneously or in those with associated horizontal misalignment, surgical treatment should be considered.

DVD neither disappears nor improves over time. In 100 patients with DVD followed by Harcourt ²² for a mean of 7.3 years, no significant decrease in the magnitude of DVD was observed.

Surgical management strategies available for DVD cannot cause DVD to disappear. Achieving a latent deviation is therefore our goal.

16.3 Treatment

When DVD is controlled or only appears occasionally, we should merely strengthen fusional mechanisms and provide spectacle correction to achieve the clearest possible image in both eyes.

Surgical treatment should be considered when a significant hypertropia manifests spontaneously, when an anomalous head posture is evident, or when there is an associated horizontal misalignment.

The fixation preference, the bilateral visual acuity, the presence of anomalous head posture, the incomitance pattern of the deviation, and the symmetry of the DVD must be considered in designing a surgical plan.

16.3.5 Comitant DVD, with Good Bilateral Visual Acuity, without Oblique Muscle Dysfunction

Large SR recession with or without hang-back technique is one of the most-used procedures in these cases ^{23 ,​ 53} (Video 16.3). The advantages of this procedure are that it can solve asymmetrical cases, that it is technically easy to perform, and that it has few complications. Fig. 16‑5, Fig. 16‑6, and Fig. 16‑7 show a patient with symmetrical DVD receiving symmetrical SR recession.

Posterior dissection is required to clean attachments between the SR and the levator muscle to avoid eyelid retraction and lid fissure asymmetry.

The SO tendon attaches to the undersurface of the SR muscle via the “frenulum.” When the SR muscle is recessed, an intact frenulum could cause the SO tendon to shift posteriorly with the SR muscle. If the frenulum is divided as the SR is recessed, the SO tendon will not retract and can scar to the SR muscle insertion. Kushner ⁵⁴ warned that separating this connection between the SR muscle and the SO tendon could predispose the patient to SO tendon incarceration syndrome, particularly because a recession of approximately 10 mm would place the new insertion directly over the SO tendon.

The amount of SR recession depends on the size of the hypertropia, and the recession can be done asymmetrically when the magnitude of the hypertropia is different in each eye (Table 16‑1).

Table 16.1 Superior rectus recession in dissociated vertical deviation

Hypertropia	SRR magnitude in both eyes
10 Δ ± 2.5	8 mm
15 Δ ± 2.5	10 mm
20 Δ ± 2.5	12 mm
23 Δ –25 Δ	14 mm
Abbreviation: SRR, superior rectus recession. Source: Adapted from Prieto-Diaz J, Souza-Dias C. Estrabismo. 5th ed. Buenos Aires: Ediciones Científicas Argentinas; 2005:232.

Weakening of the SR muscle modifies the horizontal deviation in PP, causing a mean 6 Δ exodeviation, which should be taken into account when planning surgery. This procedure increases the exodeviation in patients with exotropia, especially those with V pattern, because it weakens one of the adductors in upgaze. To avoid this, the SR should be recessed and transposed nasally to minimize upgaze exodeviation.

Improvement of a direct head tilt (toward the shoulder of the fixing eye) is an added benefit of SR recession due to weakening of a PP intorter.

Large SR recession may cause limitation of elevation, but this improves over several months.

Lorenz et al ⁵³ compared the immediate as well as the long-term effect of three different surgical procedures on DVD. The procedures were faden operation of Cüppers of the SR muscle 12 to 14 mm posterior to its insertion, faden operation of the SR muscle combined with a 3-mm recession of the muscle, and 10-mm hang-back recession of the SR muscle. The initial effect on the DVD was similar between the faden and 10-mm recession groups. However, they found the long-term effect of the faden combined with a 3-mm recession of the SR muscle to be superior.

16.4 Anomalous Head Posture

Children with DVD frequently have anomalous head posture. ^{55 ,​ 56} They can have head turn, as they usually fixate in adduction, but they can also have head tilts. The head tilt can be toward the shoulder of the fixating eye (direct tilt) or toward the contralateral side (inverse tilt).

Although the association between DVD and head tilt is common, there is no evidence confirming causality.

Jampolsky reported an increased hyperdeviation of the contralateral eye on head tilt to either side with the Bielschowsky head tilt test (BHTT), ^{57 ,​ 58} which is exactly the inverse of the behavior seen with SO palsy or SR overaction/contracture syndrome.

Direct tilt is observed in younger patients with horizontal strabismus who also adopt a head turn demanding more vestibular innervation to increase adduction, therefore facilitating monocular fixation (Fig. 16‑8). Direct tilt improves the vertical alignment when a contracture of the SR of the non fixing eye exists or in asymmetrical DVD cases in which the fixing eye has the larger DVD.

Brodsky et al ⁵⁶ claimed that direct tilt is not compensatory for binocular vision, while a head tilt toward the higher eye (inverse tilt) serves to neutralize the hyperdeviation and stabilize binocular vision. Most patients who adopt an inverse tilt can maintain better vertical alignment in that position (Fig. 16‑9).

Guyton ⁵⁹ proposed that adopting an anomalous head posture can influence LN. The head tilt would damp the LN pattern associated with the fixing eye, and therefore, surgery on the fixing eye is always necessary to abolish head tilts.

A great number of patients with DVD do not have head tilt, suggesting that other factors are playing a role.

The head tilt should be taken into account when a patient with DVD needs surgery to attempt to also improve head position.