Horizontal deviations can be divided into 2 broad categories, as follows: esotropias and exotropias. Esotropia designates a convergent horizontal strabismus; exotropia designates a divergent horizontal strabismus.[1] Horizontal deviations are subdivided further into comitant and incomitant deviations (also referred to as concomitant and noncomitant, respectively). Comitant refers to an ocular deviation that does not vary with the direction of gaze; incomitant describes an ocular deviation that varies with the direction of gaze.
Vertically incomitant describes a horizontal misalignment of the eyes in which the magnitude of the horizontal deviation differs in upgaze when compared to downgaze. The following are common patterns seen in vertically incomitant horizontal deviations: A-patterns, V-patterns, and, less commonly, Y-patterns; lambda-patterns; and X-patterns. These patterns are named using letters of the alphabet whose shapes have visual similarities to the ocular motility patterns that they describe. Vertical incomitance may be seen with both esotropias and exotropias.[2]
The term A-pattern designates a vertically incomitant horizontal deviation in which there is more convergence in midline upgaze and less convergence (increased divergence) in midline downgaze. By convention, an A-pattern is not considered to be clinically significant unless the distance measurements of the ocular deviation in midline upgaze (25° above primary gaze) and midline downgaze (25° below primary gaze) differ by at least 10 prism diopters. The term A-pattern is used because the vertical lines that comprise the letter A converge near the apex of the letter and diverge at the bottom of the letter. The appearance of the letter A reflects the clinical situation.
An A-pattern esotropia is an inward deviation of the visual axes in which there is more inward deviation of the eyes in midline upgaze than in midline downgaze. An A-pattern exotropia is an outward deviation of the visual axes in which there is more divergence of the eyes in midline downgaze than in midline upgaze. Lambda-pattern is used to describe a subtype of A-pattern strabismus. In this situation, little change occurs in the amount of ocular deviation from midline upgaze to primary position, but increased divergence occurs between primary position and downgaze.[3]
Various conditions may cause A-pattern incomitance of horizontal deviations in vertical gaze. Some individuals may have more than one factor underlying their A-pattern strabismus. Etiologies are outlined below.
Oblique muscle dysfunction
With significant A-patterns, version testing usually reveals superior oblique muscle overaction. The tertiary abduction effect of the superior oblique muscle is believed to produce the A-pattern. The abducting force is greatest in downgaze, the superior oblique's primary field of action, causing an increased relative divergence of the eyes in downgaze.
Generally, 2 types of oblique muscle dysfunction are associated with an A-pattern, primary superior oblique muscle overaction and secondary superior oblique muscle overaction. Primary superior oblique muscle overaction refers to overaction of the muscle with no identifiable etiology. The exact cause of the overaction remains unclear. Several hypotheses of the overaction exist. Why some individuals manifest oblique muscle dysfunction is unknown. Neurologic and mechanical hypotheses have been proposed. Inferior oblique muscle paresis is a rare entity that can cause secondary overaction of the ipsilateral superior oblique muscle.[4]
Horizontal rectus muscle dysfunction
As a proposed cause or contributing factor in the development of an A-pattern strabismus, horizontal rectus muscle dysfunction could explain why A-pattern strabismus may occur without apparent superior oblique muscle overaction.
According to this theory, an A-pattern esotropia would be due to underaction of the lateral rectus muscles; an A-pattern exotropia would be due to underaction of the medial rectus muscles. Electromyographic studies of patients with A-pattern strabismus support this theory.[5]
Vertical rectus muscle dysfunction
Abnormally functioning vertical rectus muscles have been proposed as a cause or contributing factor in the development of A-patterns. This theory is based on the tertiary adduction action produced by the vertical rectus muscles in their field of action to produce the A-pattern.
According to this theory, overaction of the superior rectus muscles would result in increased adducting effect (increased convergence) in upgaze. Underaction of the inferior rectus muscles would result in decreased adduction (decreased convergence) in downgaze, thereby producing an A-pattern.
Abnormalities of extraocular muscles or globe torsion
A-pattern strabismus may be related to the complex interplay of the ocular muscles and orbital soft tissues. Abnormalities in the location of the orbital connective tissue sleeves (which act as mechanical muscle pulleys) can cause incomitant deviations, simulating overaction of the superior oblique muscle. The heterotopic muscle pulleys, probably a superiorly displaced lateral rectus muscle pulley, may be the etiology of A-patterns, and the oblique muscles may be normal.[6]
Torsion of the globe may be the etiology of horizontally incomitant strabismus. Globe torsion may be due to abnormal oblique muscle function or loss of fusion.[7] Globe rotation alters the relationship of the extraocular muscles and/or extraocular muscle pulleys. By altering the vertical alignment of the horizontal rectus muscle insertions, these muscles can act as partial elevators or depressors. Conversely, by altering the horizontal alignment of the vertical rectus muscle insertions, these muscles can act as partial abductors or adductors. The vector forces would be changed in both magnitude and direction. According to this theory, intorsion of the globe alters the vector forces of the rectus muscles, causing an A-pattern.
Anatomic abnormalities of facial structure
Although their exact significance remains unclear, certain facial characteristics have been noted in some individuals with vertically incomitant strabismus.
A-pattern esotropia has been associated with both flat lid margins and eyes in which lateral canthi are higher than medial canthi, producing a mongoloid appearance.
A-pattern exotropia has not been associated with the appearance of lid fissures.
The frequency of A-pattern deviations among the general population is unknown. Among individuals with strabismus, the prevalence of an A-pattern varies among different studies, ranging from as low as 4.5% to as high as 36%. The ratio of A-pattern esotropia to A-pattern exotropia is approximately 2.2:1.
Mortality/Morbidity
In some individuals, the presence of an A-pattern strabismus may be insignificant and only cosmetically bothersome to the patient and the patient's family. Conversely, the misalignment of the visual axes of the 2 eyes may interfere with the patient's ability to fuse and develop normal binocular vision. It also may cause diplopia in children and adults. Abnormal vertical head postures may develop to place the eyes in a position of minimal deviation to restore single binocular vision.
A child with an A-pattern esotropia may be orthotropic in primary gaze, and even able to fuse in downgaze, but manifest a significant esotropia in upgaze. Although this situation may be functionally satisfactory, the cosmetic appearance of the inward deviation in upgaze may be quite disconcerting to the parents.
Conversely, a child with an A-pattern exotropia may appear aligned in upgaze but not be able to fuse in downgaze. Parents may be unaware of the eye misalignment because the eyes may appear straight when they look at their child. Although this may be cosmetically acceptable, this is functionally undesirable.
Inward or outward deviation of the eyes is the most common presenting problem.
The vertical variation in the magnitude of the horizontal deviation may not be obvious to the parents or the patient.
A head tilt (chin up or down) may be present, as the individual adopts a head posture that allows the eyes to remain in the position of minimal deviation. This compensatory maneuver minimizes diplopia and allows binocular viewing.
Measurements of the amplitude of horizontal deviation are obtained by prism and alternate cover testing in primary position and, then, with the eyes 25° in upgaze and 25° in downgaze. Measurements should be made while the patient wears proper refractive correction and fixates on an accommodative target at distance. Note any underaction and overaction of the oblique muscles on versions and any compensatory abnormal head posture. Examine the palpebral fissure configuration (presence of a mongoloid appearance).
Clinical findings of A-pattern esotropia are as follows:
Esotropia increases in midline upgaze and decreases in midline downgaze.
Eyes may be straight in downgaze and primary gaze.
Patients may demonstrate a chin-up posture, a compensatory maneuver that requires the eyes to be in downgaze for straight-ahead viewing. This posture places the eyes in the position of gaze where less inward deviation (more divergence) of the eyes occurs, possibly allowing single binocular vision.
Clinical findings of A-pattern exotropia are as follows:
Exotropia increases in midline downgaze and decreases in midline upgaze.
Eyes may be straight in upgaze and primary gaze.
Patients may demonstrate a chin-down posture, a compensatory maneuver that requires the eyes to be in upgaze for straight-ahead viewing. This posture places the eyes in the position of gaze where less outward deviation (more convergence) of the eyes occurs, possibly allowing single binocular vision.
Patients with A-patterns may manifest signs of superior oblique overaction, including overdepression in adduction, incyclotorsion of the involved eye(s), and/or associated vertical strabismus.[8]
Patients may demonstrate a tight superior oblique muscle on forced duction testing.
Most patients with congenital overaction of the superior oblique muscles do not manifest subjective complaints of torsion; instead, they manifest objective evidence of intorsion by indirect ophthalmoscopy. This may be most noticeable in downgaze.
Refer to those articles on esotropia and exotropia listed in the Differentials section for the indications for neuroimaging of patients with strabismus.
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.
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.[9, 10]
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.[11]
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.[12]
Weakening of the superior oblique muscles is indicated when overaction of the superior oblique muscles is present and the A-pattern is large.[13]
Various procedures have been used. Weakening procedures include tenotomy, tenectomy, graded recession, or lengthening with a silicone expander.[14]
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.[15]
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.[16]
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.[17]
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.[18]
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.[19]
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.[20]
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.
James L Plotnik, MD, FACS, Consulting Staff, Department of Ophthalmology, Division of Pediatric Ophthalmology, Arizona Pediatric Eye Specialists
Disclosure: Nothing to disclose.
Specialty Editors
Simon K Law, MD, PharmD, Clinical Professor of Health Sciences, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Disclosure: Nothing to disclose.
J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute
Disclosure: Nothing to disclose.
Chief Editor
Hampton Roy, Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Disclosure: Nothing to disclose.
Additional Contributors
Michael J Bartiss, OD, MD, Medical Director, Ophthalmology, Family Eye Care of the Carolinas and Surgery Center of Pinehurst
