Dissociated vertical deviation (DVD) is a component of dissociated strabismus complex in which the deviating eye elevates, abducts, and excyclotorts. There is still no unanimous agreement about the mechanism and the pathophysiological basis of DVD. Brodsky1 found some similarities between dorsal light reflex in primitive animals and DVD in humans. This reflex acts as a righting reflex in primitive animals in which the eyes are on either side of the head (eg, fish) and helps them to maintain their vertical position. Brodsky believes that the lack of binocularity evolution in humans causes humans to maintain and activate this reflex, and by the effects of nonsimultaneous impulses of movement in sight, the vertical divergence movement appears as DVD. Guyton et al.2 believe that vertical fusional vergences are produced primarily by the oblique extraocular muscles and that DVD is caused by a series of vergence movements that are meant to control cyclovertical latent nystagmus. According to the studies of Cheeseman and Guyton,3,4 oblique muscles are die main cause of DVD. Many treatment modalities have been suggested, in which the main goal is to weaken the elevator muscles or to strengthen the depressor muscles. Some studies have suggested that anterior transposition of the inferior oblique muscle (ATIO) is valuable, especially if DVD is accompanied by inferior oblique overaction (IOOA).5-7 ATIO works mainly by limiting elevation. This article investigates the effect of ATIO on DVD without IOOA and DVD with concurrent IOOA, and compares the results in these two groups.
PATIENTS AND METHODS
Patients with DVD who were treated by two of the authors at Farabi Eye Centre, Tehran, Iran, were included in this study. Patients with DVD of 5 pd or more underwent surgery. Our exclusion criteria in this study were previous operation on vertical muscles, vertical transmission of horizontal muscles in previous operations, planned vertical transmission of horizontal muscles, restrictive strabismus, paralytic strabismus, cerebral palsy, and craniofacial syndromes.
Previous or planned recession or resection of the horizontal rectus muscles did not affect inclusion or exclusion. To measure the amount of DVD, we used the prism and the alternate cover test with die eyes in primary position, fixating on an accommodative target at 6 m with full refractive correction, similar to the method described by Mallette et al.8 Measurements before and after the operation were performed by an orthoptist. The function of die inferior oblique (IO) muscle was graded on a scale of -4 to + 4.
All of the patients were operated on by one of two authors (RN or FA). To reach the IO muscle, an inferior temporal limbal based conjunctival incision was used. The lateral and inferior rectus (IR) muscles were isolated with a muscle hook. After the IO muscle was isolated with a muscle hook, its fasciai attachment was lysed, and a small hemostat was applied in the center of the muscle (approximately 10 mm from the muscle insertion) to stop bleeding after separation of the muscle. The muscle was cut in its scierai connection and a 6/0, doublearm polyglactin suture (Vicryl, Ethicon) was passed through the stump with locked bites at either edge. The muscle was reattached through scierai tunnels by using the crossed-swords technique such that the new insertion was immediately lateral to and colinear with the IR muscle insertion. The new insertion was not spread out, and its width was approximately 2 to 3 mm.
Table 1 summarizes patient data and the results of surgery in both groups. In group 1, 12 patients (19 eyes) underwent surgery, and in group 2, 9 patients (15 eyes) underwent surgery. Table 2 shows the median and range of DVD and IO function before and after surgery.
Patients 4 and 5, who had IOOA without DVD in the left eye, underwent graded recession of the IO muscle. The mean follow-up periods for groups 1 and 2 were 9.4 months (range, 6 to 12 months) and 9.0 months (range, 6 to 12 months), respectively.
As shown in Table 2, the reduction of DVD in both groups was statistically significant. Patients 6 and 8 in group 1 showed DVD in the right eye after ATIO of the left eye. In patient 9 from group 1 and patients 1 and 4 from group 2, the amount of DVD in the right eye increased after surgery on the left eye. Patient 4 from group 1 and patient 3 from group 2 had a slight overaction of the superior oblique muscle after surgery. Of seven eyes with DVD of 15 pd or more (four eyes from group 1 and three eyes from group 2), five had residual DVD of 5 pd or more.
DVD does not respond to orthoptic9 treatments. Nonetheless, it cannot be cured by itself.10,11 Therefore, the only cure for DVD is surgery. The suggested methods for the treatment of DVD are IR resection12; superior rectus (SR) posterior fixation suture, with or without SR recession10-13,14; SR large recession15; SR recession, with the resection of IR16; and ATIO.5,6 McNeer17 observed the decrease of DVD with injection of botulinum toxin into the SR muscle. According to Cheeseman and Guyton,3·4 the oblique muscles have a significant role in vertical fusional vergence, and these authors believe that recession accompanied by anterior transposition of all four muscles can be an effective treatment for DVD.
Because DVD is mostly seen with horizontal deviation, operating on the SR and IR muscles, in addition to the horizontal muscles, can lead to anterior segment ischemia. A change in the palpebrai fissure can be another side effect of the operation.
On the other hand, if DVD occurs with IOOA, operating on the vertical muscles alone cannot resolve both problems. Elliott and Nankin18 used ATIO to correct IOOA and discovered that it reduced not only IOOA but also the amount of DVD. According to the studies of Mims and Wood, ^recession with anteriorization of the IO 2 mm away from the insertion of the IR can prevent the development of DVD. Engman et al.20 compared standard and graded ATIO for the correction of DVD and noticed no major difference statistically. Snir et al.21 reported their success in IO bilateral anteriorization with monocular graded IO recession in patients with nonsymmetrical DVD with IOOA. Quinn et al.22 mentioned that adding 7-mm resection of the IO muscle to the standard ATIO procedure offers no advantage over the standard method. Stager23 showed that, after ATIO, the neurofibrovascular bundle of the IO muscle acts as an ancillary origin and will transform the IO from an elevator to an antielevator.
In this research, we studied the effectiveness of ATIO in correcting DVD with and without IOOA, and compared the effect in both groups. According to our results, ATIO decreases the amount of DVD in both groups, which is statistically significant. Statistical analysis of the results in both groups shows that there is no major difference between the two groups with respect to the amount of corrected DVD. Therefore, using ATIO in DVD with and without IOOA is an effective method. Also, we observed the effect of ATIO in correcting different amounts of IOOA in group 2 and realized that there was a statistically significant difference.
According to the research done by Engman et al.20 and Burke et al.,6 in DVD of 15 pd or more, ATIO is less effective. In our study, of seven eyes with DVD of 15 pd or more (four eyes in group 1 and three in group 2), the amount of DVD left in five eyes (three in group 1 and two in group 2) was more than 5 pd. Our study did not show the effectiveness of ATIO for DVD of 1 5 pd or more. However, our sample size was too small to allow a general conclusion to be drawn.
Varn et al.24 suggested that, in recurrent or highdegree DVD, IO weakening and SR recess together were more effective than SR recess or IO weakening alone. However, minor or medium elevation deficiency was one of the side effects.
In eight patients (five in group 1 and three in group 2), ATIO was performed unilaterally, which caused an increase in DVD in the unoperated eye (three in group 1 and two in group 2). However, in two cases, the amount of DVD was greater than 5 pd. After unilateral surgery, the appearance of DVD in the fellow eye is not uncommon. A noticeable limitation of upgaze is produced in a unilateral operation, but this limitation is less likely to be noticed when the procedure is done in both eyes. Therefore, some studies recommend a bilateral operation, even in unilateral DVD,25'26 although others disagree with this approach.27 In our patients, of all 34 eyes, only 2 showed mild superior oblique overaction (+1) after ATIO.
According to Kushner,28 placing the IO 1 mm ahead of the insertion of the IR and spreading the new insertion of the IO muscle laterally can cause limitation of elevation in abduction and a Y or V pattern. To prevent this problem, we placed the new IO insertion on the edge of the IR, colinear to the IR insertion. In general, ATIO has been an effective method for correcting DVD. The results in both groups (DVD with and without IOOA) can be compared and are similar. Also, this method can correct DVD and IOOA at the same time. Even in cases of DVD without IOOA, a significant amount of superior oblique overaction does not appear after ATIO.
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