Three situations can lead to hypotropia of the nonfixating eye in a patient with DVD:

1.Hypertropia in the nondominant eye: the patient appears to have greater DVD amplitude in the nondominant eye: when he changes the fixation and fixates with that eye, despite its own DVD, hypotropia in the other one becomes evident.

2.True hypotropia of the nondominant eye. When the occlusion of this eye is performed, the magnitude of DVD will determine the position reached by the eye: it can be aligned, hypo, or hypertropic.

3.Nondissociated hypertropia in the dominant eye leading to hypotropia of the fellow eye in binocular conditions. These patients seem to have greater DVD amplitude in the dominant eye.

Most cases of DVD that show hypotropia are due to surgical overcorrection, but other causes such as asymmetric DVD associated with vertical deviation or deep unilateral amblyopia may be responsible for this clinical feature. Accurate diagnosis is essential for correct surgical management [52].

Summary for the clinician

■To choose the surgical procedure for DVD, we need to take into account: (1) VA; (2) vertical deviation incomitance; (3) oblique muscles dysfunction with A or V pattern; (4) DVD symmetry.

■Symmetric DVD with good bilateral VA, without oblique muscle dysfunction: four surgical alternatives: (1) Bilateral large SR recession. (2) Bilateral retroequatorial myopexy (posterior fixation) of the SR combined with or without recession of these muscles. (3) Four oblique muscles weakening procedure. (4) Bilateral IR resection.

■Bilateral DVD with deep unilateral amblyopia: three available procedures: (1) Unilateral SR recession. (2) Unilateral IOAT. (3) Unilateral IR resection or tucking.

■DVD with IOOA and V pattern: (1) Bilateral IOAT. (2) Bilateral SR recession added to bilateral IO recession.

■DVD with SOOA and A pattern: (1) Bilateral SR recession. (2) Bilateral SR recession + SO posterior tenectomy. (3) Four oblique weakening procedure.

■Symmetric vs. Asymmetric surgeries for DVD: Bilateral symmetric procedures are performed for caseswithbilaterallysymmetricDVD.Asymmetric DVD is more common and these cases require asymmetrical techniques.

13.3Dissociated Horizontal Deviation

DHD has become a more recognized entity in the last few years and is usually related to the horizontal deviation associated with DVD in patients with early onset strabismus history. The main diagnostic sign of DHD is the presence of a horizontal variable deviation, ET, or XT that changes with fixation of each eye, unrelated to di erent accommodation or presence of primary and secondary deviation due to weakness or restriction.

It is a slow and variable horizontal movement, similar to the intermittent hypertropia that characterized the DVD. Commonly both conditions coexist; both are variable and di cult to measure and are also more prominent during inattention.

In DHD, we cannot neutralize the horizontal deviation by the classical prism and alternating cover test. Alternate cover testing must be performed slowly allowing the nonfixating eye time for the slow drift to fully manifest. It is also necessary to make the right eye fixate first and neutralize with prism the left eye deviation, and then let the left eye fixate and neutralize the right eye deviation.

The reversed fixation test (RFT) [53] is useful to diagnose DHD. During this test, the patient is asked to fixate through the prism that neutralizes the deviation of one of his eyes and then the occluder is shifted to the uncovered eye without the prism and it is observed for any refixation movement when the cover test is performed. The test is positive when a refixation movement which can be measured placing prisms in front of this eye is observed.

Brodsky et al. [54] found that 50% of his patients with consecutive XT had DHD demonstrated by a positive RFT. Seven of the 14 patients with DHD had a greater exodeviation when fixating with the preferred eye. In our series, seven patients had greater exodeviation when fixating with the dominant eye, seven patients had greater esodeviation when fixating with the nondominant eye, and three cases had XT when fixating with the dominanteye and ET when fixating with the nonpreferred eye. Only one patient had greater ET when fixating with the dominant eye. These findings seem to support his hypothesis that the exodeviation is usually smaller with the nonpreferred eye fixating (Fig. 13.4).

DHD is often observed to be larger with visual inattention than when the prisms measurements are done, and the eye position under general anesthesia (GA) usually shows greater deviation than the measured angle in the awake state.

Examining the patient under GA [55] is extremely useful to decide the amount of surgery to be done. The

eye position under GA used to show greater exodeviation when the innervational forces are abolished. The forced duction can diagnose a restriction and the spring back test can determine a medial rectus (MR) muscle weakness when it was previously recessed.

Wilson and McClatchey, in 1991 [5], recommended graded unilateral lateral rectus (LR) recession for the treatment of DHD, and this was the most common method to treat it when surgery is indicated.

It was said that bilateral surgery is less often required for DHD than for DVD. However, DHD is almost always associated with DVD, so we consider that bilateral surgery to treat both is a good option in many patients [56].

All our patients had DHD coexisting with DVD; ten cases received bilateral surgery to treat both conditions, five underwent surgery just for the DVD because the horizontal deviation was small, and two patients received surgery for the horizontal deviation alone despite having DVD as well.

The most used technique for DHD was unilateral LR recession. Retroequatorial myopexy (posterior fixation) of the LR with a recession of this muscle is recommended by certain authors [12]. Bilateral LR recession is indicated when XT is bilateral, unilateral, or bilateral MR recession when the patient exhibits ET instead of XT. Performing a LR recession added to MR advancement is a valid alternative in cases with previous surgery on the medials.

DVD and DHD usually coexist. When the vertical or the horizontal deviation manifests frequently, a surgical plan to fix the drift of the eyes is needed. Bilateral surgery is proposed to address both conditions simultaneously [57].

Summary for the Clinician

■The main diagnostic sign of DHD is the presence of a horizontal variable deviation, ET, or XT that changes with fixation of each eye, unrelated to di erent accommodation or presence of primary and secondary deviation due to weakness or restriction.

■The technique most used for DHD was unilateral LR recession. Bilateral LR recession is indicated when XT is bilateral; unilateral or bilateral MR recession when the patient exhibits ET instead of XT. Performing a LR recession added to an MR advancement is a valid alternative in cases with previous surgery on the medials.

13.4Dissociated Torsional Deviation. Head tilts in patients with Dissociated Strabismus

There is very little information on DTD in literature. Torsional movements are involved in the genesis of this form of strabismus and oblique muscles are the main oculomotor muscles with torsional action [2, 58, 59]. DVD mechanism has been elucidated recently by means of ocular movement recording techniques. DVD would be mediated primarily by the SO in the fixating eye and the IO in the fellow eye, added to a bilateral supraversion required for the maintenance of fixation with the fixating eye. In the latter eye, only an intorsional movement is observed, because the vertical components of SO and SR are annulled. A movement of elevation, abduction and

extorsion characteristic of DVD produced by SR and OI is observed in the fellow eye. In this case, the vertical vectors would be added while the extorsion and abduction produced by the IO in upgaze would prevail on intorsion and adduction of the SR.

Children with DD frequently have head turn; they usually fixate in adduction but they also have head tilts. T he head tilt can be toward the shoulder of the fixating eye (direct tilt) or toward the contralateral side (inverse tilt) [60, 61].

This head tilt has been thought to be related to the presence of DVD, but there is no evidence confirming the relationship between these two findings.

Guyton [58] claims that adopting an anomalous head posture can influence latent and manifest LN in some cases. The head tilt would damp the pattern of LN associated with the fixing eye, and therefore, surgery on the fixing eye is practically always necessary to abolish head tilts.

Brodsky et al. [62] proposed that direct tilt is not compensatory for binocular vision, while a head tilt toward the hyperdeviated eye (inverse tilt) serves to neutralize the hyperdeviation and stabilizes binocular vision.

According to Jampolsky’s description of Bielschowsky head tilt test (BHTT) response in DVD [63], there is an increased hyperdeviation of the contralateral eye on tilting to either side, the exactly inverse behavior to that of SO palsy or SR overaction/contracture syndrome (Fig. 13.5).

Direct tilt is observed in patients without horizontal alignment and with a head turn and fixation in adduction. On tilting the head toward the fixating eye side, they are demanding more vestibular innervation to increase adduction and therefore, they could improve their monocular fixation.

The most patients who adopt inverse tilt can obtain better vertical alignment in that position.

Out of 50 consecutive patients in our series who underwent surgical treatment for DVD, only 54% (27/50) had head tilt. Of 27 cases, 14 had direct tilt (51%); the head tilt did not improve vertical alignment. They usually obtain improvement of the head position by means of the bilateral SR recession surgery.

Direct tilt improves the vertical alignment in two situations: when a contracture of the SR of the nonfixating eye exists or in asymmetric DVD cases, larger in the fixating eye.

We found inverse head tilt, which improved the vertical alignment, in 13/27 (49%) cases. Many of these patients had vertical deviation in PP and it was not rare to find SR contracture of the fixating eye. When fixing with either eye, the head tilt improved the vertical alignment.

When we have a patient with DD who needs surgery, the head tilt should be taken into account to attempt to improve the head position.

Finally, we want to point out that a great number of patients with DD do not have head tilt. This fact makes evident that there are other nonelucidated factors that

determine such a particular clinical sign.

13

Summary for the Clinician

■When we have a patient with DD who need surgery, we have to take into account the presence of head tilt to attempt to improve the head position.

■Direct tilt (toward the fixing eye) is not compensatory for binocular vision, while a head tilt toward the hyperdeviated eye (inverse tilt) serves to improve the vertical alignment.

13.5Conclusions

Obtaining long-term control of the deviation in patient with dissociated strabismus is di cult; a successful outcome in the postoperative period does not guarantee the final alignment. In treated patients with DD, we will always see some kind of movement when performing the cover test. DVD never disappears completely and the dissociated behavior in DHD also persists when testing under slow cover test.

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