Medical Care
As with comitant esotropias and exotropias, nonsurgical means can be employed to alleviate the ocular deviation.
Significant refractive errors should be corrected to aid in ocular alignment.
Prisms and orthoptic training may be attempted when indicated. However, the incomitant nature of A-pattern deviations makes these modalities less effective, and, when an A-pattern is present, they are rarely beneficial.
Surgical Care
Surgically treat only A-patterns of clinical significance. When planning surgery, the surgeon should recognize that the primary and reading positions are functionally the most important positions of gaze; direct efforts at minimizing deviations in these positions should occur. When planning for strabismus surgery to minimize an A-pattern strabismus, assess superior oblique muscle overaction and inferior oblique underaction. Most patients with large A-patterns have significant oblique muscle dysfunction. [11, 12]
-
Vertical displacement of the horizontal rectus muscle insertions is recommended when the A-pattern is small (< 20 prism diopters) and there is little or no apparent overaction of the oblique muscles.
To reduce an A-pattern, the medial rectus muscles are moved toward the direction of vertical gaze where the convergence is greater (upward); this loosens the muscle in upgaze and tightens the muscle in downgaze, which results in a relative weakening of adduction in upgaze and strengthening in downgaze.
The lateral rectus muscles are moved toward the direction of vertical gaze where the divergence is greater (downward); this has the effect of tightening the muscle in upgaze and loosening the muscle in downgaze, resulting in the relative weakening of abduction in downgaze and strengthening in upgaze.
The amount of vertical displacement of the horizontal recti is one-half to a full tendon width. A one-half tendon width vertical transposition on 2 horizontal rectus muscles eliminates approximately 15-20 prism diopters of A-pattern deviation.
Some surgeons vary the amount of vertical displacement, depending on the size of the A-pattern (ie, more displacement for larger A-patterns); others perform the same amount of displacement on all patients.
The vertical transposition of the horizontal recti usually is combined with a resection or recession of the horizontal recti to correct the deviation in primary position.
Vertical displacement of the horizontal recti has little effect on primary position eye alignment or on ocular torsion. [13]
Binocular surgery for A-pattern esotropia
Bilateral medial rectus muscle recession for the amount of deviation in primary gaze with upward displacement of both medial recti
Alternatively, bilateral lateral rectus muscle resection for the amount of deviation in primary gaze with downward displacement of both lateral recti
Binocular surgery for A-pattern exotropia
Bilateral lateral rectus muscle recession for the amount of deviation in primary gaze with downward displacement of both lateral recti
Alternatively, bilateral medial rectus muscle resection for the amount of deviation in primary gaze with upward displacement of both medial rectus muscles
Monocular surgery for A-patterns
The medial rectus muscle is displaced upward, and the ipsilateral lateral rectus muscle is displaced downward. This surgery usually is combined with the appropriate recession-resection procedure.
The medial rectus is weakened (decreased adduction), and the lateral rectus muscle is strengthened (increased abduction) in upgaze. The opposite occurs on downgaze.
The transposition has no net vertical effect in primary position.
A retrospective study reviewed the response of surgery measured in prism diopters of correction per millimeter of recession after bilateral recession of the medial rectus muscle for the treatment of congenital esotropia with or without vertical displacement of the muscles for the correction of A or V patterns. Upward displacement of the medial rectus muscles was found to increase the surgical dose/response relationship in patients with A-pattern esotropia. They report, for example, that when measuring the dose/response at distance testing, a correction of 2.43 prism diopters per millimeter was noted when the muscle was displaced upward versus 1.56 prism diopters per millimeter of correction when no vertical transposition was undertaken. [14]
-
Weakening of the superior oblique muscles is indicated when overaction of the superior oblique muscles is present and the A-pattern is large. [15]
Various procedures have been used. Weakening procedures include tenotomy, tenectomy, graded recession, or lengthening with a silicone expander. [16]
When overaction of the superior oblique muscles is associated with a clinically significant horizontal deviation, horizontal muscle surgery should be performed at the same time as the oblique muscle surgery. A bilateral recession, bilateral resection, or recess-resect procedure is performed on the horizontal rectus muscles to correct the horizontal misalignment in primary position. [17]
Bilateral superior oblique tenotomies correct large A-patterns associated with superior oblique overaction. The average amount of A-pattern corrected by this surgery varies from 23-45 prism diopters of A-pattern correction in downgaze (less divergence) postoperatively.
The amount of weakening induced by the superior oblique tenotomy can be graded. A tenotomy closer to the origin causes more weakening than a tenotomy closer to the insertion. For mild-to-moderate A-patterns, bilateral superior oblique tenotomy with disinsertion of the posterior seven eighths of the superior oblique tendon, leaving the anterior fibers intact, is advocated. This technique selectively weakens the vertical and abduction functions of the muscle but has minimal effect on intorsion. The likelihood of a postoperative cyclovertical deviation is decreased. [18]
Most authors agree that little eso shift (less exodeviation) occurs in upgaze, but the result of bilateral superior oblique tenotomies on primary gaze is controversial. Some have found no significant alteration in primary position alignment postoperatively. Therefore, some surgeons do not alter the amount of horizontal muscle surgery performed. [19]
Others believe that the loss of abducting forces from this surgery results in an eso shift (less exodeviation) in primary position of 10-15 prism diopters. These surgeons adjust the amount of horizontal muscle surgery performed to compensate for the anticipated change in primary position alignment.
Bilateral superior oblique posterior tenectomy can be also be used to correct A-patterns associated with superior oblique overaction. It functions by partially weakening the superior oblique muscle, with the goal of selective weakening of the abduction function of the muscle. The benefit of a superior oblique posterior tenectomy (over a tenotomy) is that it lessens the risk of induced superior palsy. This may be preferred when bifoveal fixation is present preoperatively. However, there is a tendency for tenectomy to cause mild undercorrection of the superior oblique muscle overaction postoperatively. [20]
The superior oblique can be weakened using a silicone tendon expander; a synthetic material is used to lengthen (weaken) the superior oblique tendon.
A superior oblique tenotomy is performed, and a silicone implant is inserted between the cut ends of the tendon.
The amount of weakening can be altered by varying the length of the expander. Typically, a 4-7 mm piece of No. 240 silicone band is used as an expander. Longer pieces are used to correct greater amounts of overaction.
This surgery is beneficial because it allows for graded lengthening of the superior oblique tendon and is less prone to overcorrection and is reversible.
In a recent study by Sharma, bilateral weakening of the superior oblique tendon via tenotomy with insertion of a 6 mm silicone expander produced a mean correction of 23 prism diopters of A-pattern, which corrected 95% of the preexisting A-pattern. [21]
Bilateral superior oblique recession has been used in small studies by Romano and Drummond, respectively, to correct an A-pattern.
This technique allows the superior oblique to be weakened in a graduated manner without the tendency to cause overcorrections.
In a study by Sharma, bilateral recession of the superior oblique produced a mean correction of 30.7 prism diopters of A-pattern.
Although some studies report it as effective, this technique is not commonly used to eliminate an A-pattern.
Symmetrical horizontal transposition of the vertical rectus muscle insertions
The vertical rectus muscles are secondary adductors. By altering their site of insertion, the adducting forces can be modified.
To reduce an A-pattern esotropia, the superior recti are moved temporally 5-7 mm to lessen their adducting effect (less convergence) in upgaze.
To reduce an A-pattern exotropia, the inferior recti are moved nasally 5-7 mm, thereby enhancing their adducting effect (more convergence) in downgaze.
This surgery is not commonly used because most surgeons have found it to be ineffective and unpredictable.
Although not widely used, bilateral inferior oblique advancement has been advocated for the treatment of A-pattern esotropia. [22]
This technique consists of disinserting the inferior oblique muscle, passing it under the lateral rectus muscle, and reattaching it to the sclera 2 -3 mm superior to the lateral rectus and approximately 8 mm posterior to its insertion.
For esotropia, this technique is combined with horizontal muscle surgery.
In a study by Goldstein, the average correction of A-pattern was 23 prism diopters (range 4-33 prism diopters) with bilateral inferior oblique advancement.
Slanting of the muscle insertions has been used successfully to treat A-pattern esotropia in the absence of superior oblique overaction. The recessed medial recti are reattached to the sclera with the superior border of the muscle reattached 3 mm posterior to the insertion of the lower border of the muscle. This procedure preferentially weakens the superior aspect of the muscle more than the lower aspect of the muscles. Slanting the muscle insertion in this manner decreases the muscle's ability to adduct the eye in upgaze, decreasing the A-pattern.
