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Pattern strabismus indicates significant difference in the amount of horizontal deviations measured in upgaze and downgaze, as compared to the primary gaze. It is imperative to be able to detect a pattern, while examining a case with strabismus, for successful surgical alignment. This article covers basic and practical aspects of pattern strabismus and lays the foundation of general fundamentals for managing such patients.
Pattern strabismus is recognized by the following codes as per the International Classification of Diseases (ICD) nomenclature:
ICD-10 data (https://www.icd10data.com/ICD10CM/Codes/H00-H59/H49-H52/H50- )
H50.02 Monocular esotropia with A pattern • H50.021 …… right eye • H50.022 …… left eye
H50.03 Monocular esotropia with V pattern • H50.031 …… right eye • H50.032 …… left eye
H50.06 Alternating esotropia with A pattern
H50.07 Alternating esotropia with V pattern
H50.12 Monocular exotropia with A pattern • H50.121 …… right eye • H50.122 …… left eye
H50.13 Monocular exotropia with V pattern • H50.131 …… right eye • H50.132 …… left eye
H50.16 Alternating exotropia with A pattern
H50.17 Alternating exotropia with V pattern
- Pattern strabismus (also known as vertically incomitant, comitant horizontal strabismus) was first described by Alberto Urrets-Zavalia in 1948. 
- Urist coined the term “A” and “V” patterns. 
- Pattern strabismus indicates significant difference in the amount of horizontal deviations measured in upgaze and downgaze, as compared to the primary gaze.
- Pattern strabismus is known by its alternative term “Alphabet patterns” in European literature. 
- Traditionally, it includes A and V patterns, which are considered clinically significant, when the difference between upgaze and downgaze is greater than 10 prism diopters (PD) and 15 PD for A and V patterns, respectively. Owing to the physiological necessity of convergence in downgaze, the degree of deviation is set lower for an A pattern, as compared to a V pattern . The deviation is measured approximately 25 degrees in upgaze (chin-down) and 35 degrees in downgaze (chin-up), from the primary gaze position.
- V pattern exotropia (V- XT) is an exotropia with greater divergence in upgaze than downgaze. (Figure 1)
- V pattern esotropia (V- ET) is an esotropia with greater convergence in downgaze than in upgaze. (Figure 2)
- A pattern exotropia (A- XT) is an exotropia that increases in downgaze and decreases in upgaze. (Figure 3)
- A pattern esotropia (A- ET) is an esotropia with increased convergence in upgaze and relative divergence in downgaze. (Figure 4)
Other patterns include the following:
- Y-pattern (Pseudo inferior oblique overaction): In this pattern, there is relative exodeviation in upgaze as compared to primary gaze and downgaze. They are thought to be due to anomalous innervation to lateral rectus muscles in upgaze. 
- Lambda or inverted-Y or λ pattern: Here the eyes are orthophoric in upgaze and primary gaze, and exodeviation is present in downgaze. It is seen in bilateral Superior Oblique (SO) overaction.
- X- pattern: In this pattern, exodeviation is present in primary gaze that increases significantly in upgaze and downgaze. It occurs in long-standing exotropias due to leash effect of tight lateral rectus muscles.
- Diamond or ♢ pattern: Here there is relative convergence in upgaze and downgaze as compared to the primary gaze.
- Arrow pattern: This pattern has orthophoria in up and primary gaze, with relative convergence in downgaze. Seen in bilateral SO palsy.
- Pattern strabismus coexisting with horizontal strabismus is seen in 12-50% of all cases. 
- They are frequently associated with congenital and paralytic strabismus.
- Overall, V-pattern is the commonest.
- Duane’s retraction syndrome often has an associated V- or a Y- pattern. 
- Spina bifida and hydrocephalus are commonly accompanied by an A- pattern. 
Horizontal muscle overaction
- Urist proposed that imbalance of horizontal recti muscles caused pattern strabismus.
- Overactions of medial recti (MR) in downgaze and lateral recti (LR) in upgaze lead to V-pattern esotropia.
- Conversely, an A-pattern exotropia arises due to increased divergence in downgaze caused by underacting MR in downgaze and underacting LR in upgaze.
- Electromyographic studies, however, disproved this theory by showing similar electrical activity in horizontal muscles in patients with pattern strabismus and incomitant strabismus. 
Vertical muscle dysfunction
- Brown advocated the opinion that pattern strabismus is caused by primary anomalies in the actions of vertical recti muscles, whose tertiary action is adduction.
- Underaction of superior rectus (SR) leads to less adduction in upgaze, and overaction of its yoke inferior oblique (IO) leads to increased abduction in upgaze, resulting in a V-pattern.
- Conversely, underaction of inferior rectus (IR) and overaction of its yoke superior oblique causes an A-pattern.
- This theory was refuted due to lack of conclusive evidence.
Oblique muscle dysfunction
- This theory, proposed by Knapp in 1959, is the most accepted in the etiology of pattern strabismus. 
- The oblique muscles have abduction as their tertiary action.
- If the IO muscles are overacting, it will lead to relative divergence of eyes in upgaze, resulting in a V-pattern.
- The converse happens when SO muscles are overacting, leading to relative divergence of eyes in downgaze, resulting in an A-pattern.
1. Craniofacial anomalies
- Mongoloid type of facial development (consisting of malar hyperplasia, temporally upward slanting of the palpebral fissures, and a straight lower lid margin) has a predisposition for V-pattern XT and A-pattern ET.
- Conversely, anti-Mongoloid type of facial development has associated A-XT and V-ET.
2. Heterotopia of muscle pulleys
- Extraocular muscle (EOM) pulleys are condensations of posterior tenon’s capsule, which act as the functional origin of EOMs.
- Histologically, they are composed of collagen, elastin, and smooth muscles and are thought to stabilize eyeball during globe rotations by minimizing sideslip relative to orbit.
- Clark and colleagues have studied EOM pulley positions using high-resolution MRI and concluded that pulley heterotopia by itself can result in pattern strabismus, irrespective of oblique muscle dysfunction. 
3. Anomalies of muscle insertions and cyclotorsion
- Anomalies of insertions of horizontal, vertical or oblique muscles are also known to cause A and V patterns.
- In V patterns the insertion of MR have been reported to be higher than the normal and those of LR lower than normal.
4. Sagittalisation of oblique muscle insertions
- Gobin introduced the concept of sagittalisation of oblique muscles, which attributes pattern strabismus to changes in angle between the muscle axis and globe axis. 
- As seen in figure 5, anterior displacement of the trochlea (as seen in in hydrocephalus with frontal bossing) reduces the angle between the two axes, and makes the SO a better depressor, thereby causing an A-pattern. 
- Desagittalisation caused due to retroplacement of trochlea (as seen in plagiocephaly) leads to the converse and produces a V-pattern. 
- Eyeball torsion displaces the anterior path of EOMs, altering the vector forces to abnormal directions.
- Kushner showed that excyclotorsion displaces the SR temporally (thereby creating an abduction vector in upgaze) and IR nasally (thereby increasing the adduction vector in downgaze), which tends to produce a V-pattern. 
- Incyclotorsion leads to the opposite and results in an A-pattern.
- This is usually seen in association with congenital cranial dysinnervation disorders and rarely in isolation; it commonly causes a Y-pattern, due to anomalous nerve supply to LR during upgaze movement. 
Selective compartmental innervation of horizontal rectus muscle
- Studies have shown that superior and inferior halves of EOM have distinct innervations, hinting to a possible contribution in causation of pattern strabismus. 
- Anterior transposition of the IOs can cause anti-elevation syndrome, which produces a Y-pattern.
- Large bilateral recessions of the IRs, in patients with thyroid eye disease, weaken the adducting effect of the IRs in downgaze, thereby producing an A-pattern. 
- Iatrogenic alphabet patterns can also result from an overcorrection from prior surgical treatment of pattern strabismus. 
- Guyton et al opined that loss of fusion predisposes the oculomotor system to cyclodeviations of the eyes that, in turn, causes A and V patterns. 
- Guyton and Weingarten showed that patients with intermittent exotropia (formerly fusing) who lost fusion after surgical overcorrection, may develop A or V patterns. 
- Clinical presentation depends on the amount of deviation of the underlying strabismus in primary gaze.
- If the angle of deviation is very large to prevent possibility of fusion in any head position, then the pattern may not affect the presentation at all.
- If the angle of strabismus is small, so as to allow fusion in upgaze or downgaze, there might be an abnormal head posture (AHP) like chin-up or chin-down posture as the manifestation of the pattern.
- In general, chin-up posture is seen in A-ET and V-XT, and chin-down posture is present in A-XT and V-ET. 
- Occasionally pattern strabismus may manifest as late as the age of 40s, when the presbyopic patients have to turn their eyes downwards to read through the bifocal segment.
Aims of investigation
The aims of investigation are:
- To detect and quantify the alphabet pattern.
- To determine the etiology for the pattern, if possible.
- To assess its significance in the management of the strabismus.
- Urrets-Zavalia was the first to emphasize the importance of measuring deviations in upgaze and downgaze. 
- Alternate Prism Cover Testing (APCT) in all the diagnostic positions of gazes is imperative to document the deviation and type of pattern.
- The deviation is measured approximately 25 degrees in upgaze (chin-down) and 35 degrees in downgaze (chin-up), from the primary gaze position. 
- Distance testing should be done at 6 metres to avoid interplay of accommodation with the deviation.
- Care should be taken to measure the deviation with full optical correction of the patient. An accurate refraction is a must; otherwise a pseudo A- or V- pattern can be encountered as in cases with accommodative esotropia. 
- Ocular motility with special emphasis to observe elevation/depression in adduction.
- IO overaction (IOOA) as seen clinically is depicted in figure 6. The grades (1+ to 4+) of IOOA are shown in figure 7. 
- Thorough slit-lamp examination should be done to look for any anomalous muscle insertion.
- Binocular vision is usually subnormal in patients with pattern strabismus.
- In general, patients with incomplete pattern such as a Y- or λ pattern have better stereopsis. 
- Worth 4-dot test often reveals fusion at near, but rarely shows any fusion response for distance.
- Patients with AHP often exhibit sensory fusion at their preferred head posture, but not in their forced primary gaze. 
- Fundus photography is recommended for objective documentation of the amount of torsion.
- Imaging of the orbit, such as dynamic MRI, should be considered in selected cases where the etiology of pattern could not be attributed to any of the ocular motility findings on clinical examination. 
- Small horizontal deviations accompanied by a minimal A or V pattern may not require any treatment, apart from correction of the refractive error and treatment of amblyopia.
- Most of the cases have a significant deviation and require surgical correction.
- Treat horizontal and vertical deviation in useful positions of gaze.
- Maintain, improve or restore a comfortable field of binocular single vision (BSV).
- Correct or eliminate any AHP.
- Primary gaze deviation > 20 PD.
- Extreme elevation in adduction associated with bilateral IOOA.
- Cosmetically unacceptable AHP.
Timing of surgery
- Surgery should be done only after repeated clinical examinations reveal a significant, consistent, reproducible, and reliable measurement of deviation in the diagnostic positions of gazes. 
- Early surgery (before the age of 8 years) has been advocated to promote development of BSV. 
- In patients with significant AHP, early surgery has been recommended to preserve BSV.
Surgery on the Oblique Muscles
- Most cases with pattern strabismus have associated oblique muscle dysfunction.
- Bilaterally symmetric IO or SO weakening procedure is performed to collapse the pattern. 
- If there is an associated dissociated vertical deviation (DVD), IO Anterior transpositioning is preferred over myectomy.
- In cases with no oblique muscle dysfunction, transposition surgeries are performed to negate the pattern.
- Horizontal muscle recession-resection (to treat the primary gaze deviation) with transposition of muscle by one-half tendon to full tendon width to collapse the pattern. 
- A useful mnemonic to remember the direction of transposition of horizontal recti muscles is MALE (Medial rectus muscle is moved to the Apex of the pattern, Lateral rectus muscle is moved to the Empty space of the pattern). For example, in V-ET, MR is shifted downwards, where as LR is moved upwards. This rule is applicable whether the horizontal rectus muscle is strengthened or weakened. Figures 8 and 9 illustrate this rule.
- Horizontal transposition of vertical recti muscle also has been employed to treat cases with pattern strabismus. 
- In general, nasal displacement strengthens the adduction vector force and temporal displacement weakens the adduction vector force.
- For example, temporal displacement of SR in A-ET and temporal displacement of IR in V-ET.
- Vertical recti transposition is not routinely performed because of higher risk of anterior segment ischemia, and also due to the fact that same surgical success can be achieved by combination of horizontal recti and oblique muscle (that carry no ciliary vessels) surgery.
Slanting of horizontal rectus muscle
- Selectively slanting the superior or inferior pole of the recessed muscles has been used to treat pattern strabismus by simulating transposition. 
- For example, for V-ET the superior poles of the MRs are recessed more than the inferior poles, which simulate the effect of an infraplacement.
|Pattern||Associated features||Surgical procedure|
|V- pattern XT||Without IO overaction||Horizontal muscle surgery (Bilateral LR Recession with upshift)|
|With IO overaction||Horizontal muscle surgery with bilateral symmetric IO weakening procedure.|
|V- pattern ET||Without IO overaction||Horizontal muscle surgery (Bilateral MR Recession with downshift)|
|With IO overaction||Horizontal muscle surgery with bilateral symmetric IO weakening procedure.
If there is associated DVD, then IO anterior transpositioning is the preferred IO weakening procedure.
|A- pattern XT||Without SO overaction||Horizontal muscle surgery (Bilateral LR Recession with downshift)|
|With SO overaction||Horizontal muscle surgery with bilateral symmetric SO weakening procedure.|
|A- pattern ET||Without SO overaction||Horizontal muscle surgery (Bilateral MR Recession with upshift)|
|With SO overaction||Horizontal muscle surgery with bilateral symmetric SO weakening procedure.|
|Y- pattern||With IO overaction||Y- pattern ET combined with IO overaction is corrected by horizontal muscle surgery with bilateral symmetric IO weakening procedure.
Y- pattern XT without IO overaction needs no treatment if fusion is present in primary and down gazes.
|Lambda (inverted Y or λ) pattern||With SO overaction||Bilateral SO weakening procedure.|
|X-pattern||Bilateral LR Recession.|
- Undercorrection of horizontal deviation with residual pattern is a known complication. It is usually well tolerated if residual pattern deviation is < 15 PD.
- Overcorrection of horizontal deviation can sometimes occur. This may lead to pattern reversal.
- Torsional diplopia, seen in cases with asymmetric IO or SO weakening procedures, is a rare, but extremely bothersome complication, which may need reoperation.
- Anterior segment ischemia, if >2 recti muscles are operated in an eye. Risk is more in reoperation cases.
- Creation of new AHP
- These patients usually have a fair prognosis in terms of motor correction.
- Moreover, the sensory outcome also improves if surgical correction is done at an early age.
- The average correction obtained by different surgeries is summarized in the table below.
in pattern (in prism diopters, PD)
|Bilateral IO weakening||15-25|
|Vertical transposition of horizontal recti||15-20|
|Bilateral IO disinsertions plus Vertical transposition of
|Bilateral SO tuck||15-25|
|Bilateral SO posterior tenotomy||20-25|
|Bilateral SO complete tenotomy||35-45|
Readers are referred to a brief lecture on pattern strabismus in the video below:
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- American Academy of Ophthalmology. V-pattern esotropia. https://www.aao.org/image/v-pattern-esotropia Accessed June 1, 2020
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- American Academy of Ophthalmology. A-pattern esotropia. https://www.aao.org/image/pattern-esotropia Accessed June 1, 2020
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- American Academy of Ophthalmology. Bilateral inferior oblique muscle overaction. https://www.aao.org/image/bilateral-inferior-oblique-muscle-overaction Accessed June 1, 2020
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