V-Pattern Esotropia and Exotropia

Duane first described V-pattern strabismus in 1897.[1] Other investigators also studied these phenomena, contributing to the understanding of them. Costenbader and colleagues recommended measuring the horizontal deviations in upgaze and downgaze as part of all routine motility examinations.[2]

Two types of V patterns can occur, as shown in the images below.

V-pattern exotropia is an exodeviation with greater exotropia in upgaze than in downgaze.

V-pattern esotropia is an esodeviation with greater esotropia in downgaze than in upgaze.

Comitant horizontal deviations are those in which the angle of deviation is the same in all gaze positions. Incomitant horizontal deviations are those in which the angle of deviation varies in different gaze positions. Horizontal deviations may have lateral incomitances and vertical incomitances. In lateral incomitances, significant differences exist in the amount of horizontal deviation in primary gaze versus side gaze, whereas, in vertical incomitances, significant differences exist in the amount of horizontal deviation in upgaze versus downgaze.

When the eyes diverge more than 10 prism diopters (PD) from upgaze to downgaze, an "A" pattern is present; when the eyes converge more than 15 PD from upgaze to downgaze, a "V" pattern is present.

Other types of vertical incomitances, which are less common, include X, Y, and lambda patterns. This discussion is limited to V-pattern strabismus.

Multiple factors may be responsible for V-pattern strabismus, and, in a given individual, one or more factors may contribute toward its cause.

Inferior oblique muscle overaction

The most widely accepted mechanism contributing to the causation of V-pattern strabismus is associated with inferior oblique muscle overaction and/or relative underaction of the superior oblique muscles, as shown in the images below.

Inferior oblique muscle overaction produces significant horizontal deviation in upgaze. In upgaze, the horizontal action of the inferior oblique muscles is abduction; therefore, inferior oblique muscle overaction produces relative divergence in elevation. A V-pattern results in greater exotropia (or less esotropia) in upgaze and greater esotropia (or less exotropia) in downgaze. Not all V-pattern strabismus is associated with oblique muscle dysfunction.

Classification of eye movement abnormalities ans strabismus (CEMAS) has suggested that superior oblique overaction be described as overdepression in adduction and inferior oblique overaction be described as over elevation in adduction

Facial and orbital configuration

An association exists between individuals with hypoplasia of the malar bones, anti-Mongoloid lid slants, and S-shaped contours of the lower lid, and overaction of the inferior oblique muscles and V-pattern strabismus. Patients with craniosynostosis have shallow orbits, and the angle between the visual axis and the insertion of the inferior oblique muscle is increased. This increased angle increases the abduction ability of the inferior oblique muscles in upgaze, resulting in a V-pattern. Contributing to this condition is the complex interplay of these factors with excyclotorsion of the globe.

Effect of the pulley systems

Upward displacement of the pulley systems around the lateral rectus muscle may cause V-pattern strabismus.

Horizontal muscle overaction

Initially, this theory was believed to be responsible for V-pattern strabismus, but no convincing evidence supports it. In V-pattern esotropia, the medial rectus muscle was believed to overact, thereby increasing esotropia in downgaze, and, in V-pattern exotropia, the lateral rectus muscle overaction was responsible for increased exotropia in upgaze.

Vertical rectus muscle imbalance

One theory states that V-pattern strabismus occurs because of underacting superior recti muscles and corresponding overacting inferior oblique muscles. This hypothesis was applied when nasal and temporal displacements were used to treat A- and V-patterns. This theory has been abandoned.

Abnormal oblique muscle insertions

Alterations of the angle of the insertion of the oblique muscle with the visual axis can reduce cyclorotation and abduction forces and increase the vertical function of the oblique muscle.

Anomalous horizontal rectus muscle insertions

Among the suggested causes of V-pattern strabismus are anomalies of the horizontal rectus scleral insertions. They may be inserted above or below the usual positions.

Ocular Torsion as a cause of Pattern Strabismus

Association of excyclotorsion with V pattern and incyclotorsion with A pattern was first reported by Piper in 1963. He reported that the apparent rotation of the insertion of the rectus muscle due to torsion of the globe is the etiologic factor for pattern strabismus.

For example in V-pattern strabismus with inferior oblique overaction the eye will be extorted. This will rotate the insertions of the rectus muscle counterclockwise in the right eye and clockwise in the left eye. An abducting force will be created by the superior rectus in upgaze and adducting force from inferior oblique in downgaze resulting in a V pattern.  

Acquired form

V patterns are acquired after superior oblique muscle palsy and are a prominent feature of bilateral superior oblique muscle palsies.

Frequency

United States

Although the exact prevalence of V-pattern strabismus in a given population is not known, 12.5-50% of patients with horizontal strabismus have an associated A or V pattern. V-pattern esotropia is not as commonly seen as V-pattern exotropia.

Mortality/Morbidity

Patients may experience diplopia that affects their everyday activities, such as driving and reading. In addition, the inability of patients to fuse in different gaze positions can interfere with other activities, including sports and recreation. The cosmetic appearance of V-pattern strabismus coupled with its social implications may be bothersome to patients. Over time, the associated anomalous head position could lead to secondary neck pain and similar problems.

Race

No racial predilection exists.

Sex

No sexual predilection exists.

Age

No age predilection exists.

The most common presenting signs of V-pattern strabismus are as follows:

• Inward or outward deviation of the eyes

• Abnormal eye movements in different gazes

• Asthenopia (eyestrain)

• Abnormal head posture, chin down in V-pattern exotropia and chin up in V-pattern esotropia

• Diplopia in older children and adults

The physical examination includes a general eye examination combined with a detailed ocular motility examination. The ocular motility examination, performed with the refractive error corrected, consists of the following:

• Note any abnormal head position.

• Perform cover tests with prisms in the primary position, 30° upgaze and 30° downgaze at near and at distance.

• With ocular rotations, look for associated underaction or overaction of oblique muscles.

• Perform a head tilt test if V-pattern strabismus is noted in patients with suspected bilateral superior oblique palsy.

• Measure torsion in patients with suspected inferior oblique muscle overaction to check for excyclotorsion.

• When required, perform sensory testing, including distance and near stereoacuity, and other tests.

See Pathophysiology.

Laboratory tests are not required, except for those routinely performed prior to surgery.

Imaging studies are not required routinely, although they may be beneficial in patients with certain conditions (eg, craniosynostosis) or in patients in whom abnormalities of the pulley systems are suspected.

There is a definite role for imaging of the orbit, which may be considered in select cases in which the pattern cannot be attributed to any of the ocular motility findings, and also in unusual cases. 

In mild cases, no treatment may be required.

In intermittent deviations, which are exotropic in nature, overcorrection with minus lenses (-2 to –4 D) can be tried. The rationale is that the minus lenses stimulate accommodative convergence and reduce the exodeviation angle; however, this does not address the vertical incomitance.

For small comitant deviations, prisms can be tried to control the deviation and to relieve asthenopic symptoms. Base-in prisms are used for exotropia; base-out prisms are used for esotropia.

Indications for surgery include the following: to correct the underlying deviation and to enable fusion, to achieve and regain comfortable binocular single vision, and to correct abnormal head posture when present.

In both esotropia and exotropia, the V-pattern incomitance is considered surgically significant when the difference between horizontal deviations in upgaze and downgaze is equal to or greater than 15 PD. Prior to surgery, determine if the V-pattern is associated with oblique muscle dysfunction. The most common pattern encountered is inferior oblique muscle overaction and relative superior oblique muscle underaction or overelevation and underdepression in adduction.

Surgical treatment of V-pattern strabismus with oblique muscle dysfunction

These patients usually require appropriate horizontal rectus muscle surgery that is combined with inferior oblique muscle weakening procedures. For V-pattern esotropia, medial rectus muscle recessions (and/or lateral rectus muscle resections) along with inferior oblique muscle weakening (or superior oblique muscle tucks) are performed. For V-pattern exotropia, lateral rectus muscle recessions (and/or medial rectus muscle resections) and inferior oblique muscle weakening are performed. Inferior oblique muscle weakening procedures should be performed equally on both eyes.

Several studies have confirmed the role of and reported surgical success with, inferior oblique muscle weakening (recessions) in both V-pattern esotropia and V-pattern exotropia. In one study, the stereoscopic function improved in 33.3% of patients, and, in another study, a significant improvement in fusional status was noted postoperatively after a combination of horizontal rectus muscle surgery and inferior oblique muscle weakening.[3, 4]

Inferior oblique muscle weakening procedures include inferior oblique muscle recession, anteriorization, myotomy, and myectomy. The degree of inferior oblique muscle overaction determines the procedure of choice. For mild-to-moderate inferior oblique muscle overactions, recessions are preferred. Depending on the severity of the inferior oblique muscle overaction, graded recessions can be performed.

Quantification (grading) of the inferior oblique muscle recession for V-pattern strabismus was studied by a group of investigators.[5] These investigators recessed the inferior oblique muscle from 8 mm for a V-pattern of 12 PD with +1 inferior oblique muscle overaction to 12 mm for a V-pattern of 30 PD with +3 inferior oblique muscle overaction. A satisfactory outcome was observed in 75% of patients with a preoperative V-pattern of less than 20 PD. A success rate of 57% was noted in patients with a preoperative V-pattern of more than 29 PD. Undercorrections were common, hence their recommendation that increased amounts of surgery be done.

For severe inferior oblique muscle overaction, the anteriorization procedure is sometimes preferred. A study compared graded recession to anterior transposition of the inferior oblique muscle for V-pattern strabismus and concluded that anterior transposition is as effective as graded recession.[6]

Inferior oblique muscle myotomy or myectomy also can be performed. A study by Lee et al showed inferior oblique Z-myotomy was effective in correcting a V-pattern strabismus in patients with inferior oblique overaction under the degree of + 2.[7]

If a reoperation is required after inferior oblique muscle myectomy, localizing the muscle may be difficult.

Inferior oblique muscle weakening procedures have very little, if any, effect on the horizontal deviation in the primary gaze.

Surgical treatment of V-pattern strabismus without oblique muscle dysfunction

Vertical displacement of horizontal rectus muscle insertions: Introduced by Knapp,[8] this technique shows favorable results in patients with mild-to-moderate V patterns with no apparent oblique muscle dysfunction.

In determining the direction of displacement, the authors follow a general rule wherein the medial rectus muscle insertion is always moved toward the closed end of the V (downward) and the lateral rectus muscle insertion is always moved toward the open end of the V (upward), as outlined in the image below.

Note the following:

• V-pattern esotropia - Bilateral medial rectus muscle recessions with downshift

• V-pattern esotropia - Bilateral lateral rectus muscle resections with upshift

• V-pattern exotropia - Bilateral lateral rectus muscle recessions with upshift

• V-pattern exotropia - Bilateral medial rectus muscle resections with downshift

The amount of displacement depends on the difference in the amount of deviation in upgaze and downgaze. A half tendon width displacement corresponds to a 5-mm vertical shift of the insertion, and a full tendon width displacement corresponds to a 10-mm vertical shift of the insertion. While performing the vertical displacement, maintaining the relationship between the original insertion of the muscles and the sclerocorneal limbus is important. The image below illustrates half tendon width vertical shift of muscle insertion as well as full tendon width vertical shift of muscle insertion.

One study showed that vertical displacement of horizontal muscle in V-pattern exotropia without oblique muscle dysfunction corrected an average V of 20 pd.[9]

Oya et al studied the effects of vertical offsets of horizontal rectus muscles in V-pattern exotropia without oblique muscle dysfunction. They conclude that this technique effectively corrects the pattern deviation and that postoperatively no patient had an A pattern or any torsional disturbance.[9]

Slanting the horizontal rectus muscle insertions with or without recessions: For a V pattern, selective recession of the superior fibers of the medial rectus muscle or the inferior fibers of the lateral rectus muscle can be performed. The amount of correction from this type of surgery is minimal. An observational case study evaluated the therapeutic effects of surgically slanting the muscle insertions on the effect of the V-pattern in patients with exotropia.[10] A slanting surgical reinsertion line with a 3- to 4-mm difference between the upper and lower corners of the muscles was created. The study showed that the mean amount of reduction in the V-pattern was 10.3 PD in the primary position and 29.8 PD in upgaze. This procedure can be combined with recessions and/or vertical rectus muscle displacement of the horizontal rectus muscle insertions. The image below depicts slanting of the rectus muscle insertion with and without recession.

One study showed that both medial rectus slanting recession and medial rectus recession with downward transposition corrected V-pattern esotropia, with no clinically or statistically significant differences in success rate.[11]

Horizontal transpositions of the vertical recti muscles: The vertical recti muscles are secondary adductors. By altering their site of insertion, the adducting forces can be modified. In V-pattern esotropia, the inferior recti muscles can be moved temporally. In V-pattern exotropia, the superior recti muscles can be moved nasally.

This procedure is not commonly preferred because of the following:

• The results are difficult to predict.

• If fusion is present, the horizontal, vertical, and torsional component change that results from the transposition may change the sensory status.

• Usually, a horizontal deviation occurs that requires surgery. In this case, additional surgery on the vertical recti muscles may increase the risk for anterior segment ischemia.

This procedure may be considered as an option in patients in whom the inferior oblique muscles have been maximally weakened and are not overacting, and a residual V pattern persists.

When considering monocular surgery with an associated V pattern and planning a recess-resect procedure, the medial rectus muscle is displaced downward and the lateral rectus muscle is displaced upward in the same eye. Goldstein reported that an 8-mm displacement of the horizontal recti muscles monocularly corrects V-patterns as large as 30 PD.[12] The image below illustrates a recess-resect procedure in the same eye.

No effective medical treatments exist.

Although regimens vary among surgeons, regular follow-up visits are necessary to determine if the eyes are well aligned and if fusional abilities are present and stable.

Surgery for the patient with strabismus usually is performed as an outpatient procedure.

If left untreated, loss of fusion and stereopsis could occur. If the patient freely alternates, the chance of amblyopia is reduced.

Complications associated with surgery include undercorrection and overcorrection and/or residual or new oblique muscle dysfunction.

Postoperative complications include residual A- or V-pattern strabismus, diplopia, and undercorrection or overcorrection of the horizontal deviation.

The prognosis for patients with this condition is good. Only one surgery is required to correct the deviation in 80% to 85% of patients; 10% to 15% of patients may need a second or third procedure for good alignment.

Success for binocularity depends on the patient's age, the time of presentation, and any previous amblyopia. Failure to detect and appropriately treat this condition could result in loss of binocularity and possibly amblyopia

Postoperative drift in patients with comitant exotropia alone and patients with intermittent exotropia with coexistent A or V pattern has been studied, and it has been seen that patients with a coexistent V or A pattern have a reduced risk of postoperative recurrence.[13]