Pattern Strabismus

Key Points at a Glance

Key Points of This Disease

• Pattern strabismus is a condition where the horizontal deviation differs between upgaze and downgaze, also called A-V pattern strabismus.
• V pattern (difference of 15Δ or more) is most common, followed by A pattern (difference of 10Δ or more). Y, X, and λ patterns are rare.
• It occurs in 15-25% of all horizontal strabismus cases.
• The main cause is oblique muscle dysfunction (inferior oblique overaction → V-pattern, superior oblique overaction → A-pattern), but orbital pulley abnormalities and ocular torsion also contribute.
• Compensatory head posture (chin-up or chin-down) is present, and in A-pattern exotropia especially, it hinders the acquisition of stereopsis in children.
• Clinically significant patterns or abnormal head postures are indications for strabismus surgery.
• Depending on the presence of oblique muscle overaction, the surgical procedure is either oblique muscle weakening or vertical transposition of the horizontal rectus muscles (Trick procedure).

1. What is Pattern Strabismus?

Pattern strabismus is a condition in which there is a significant difference in the amount of horizontal deviation between upgaze and downgaze. It is also called A-V pattern strabismus and is known by the alternative name “alphabet pattern.”

It is classified into the following five types.

• V-pattern: The difference in deviation between upgaze and downgaze is 15 prism diopters (Δ) or more. This is the most common type.
• A-pattern: The difference in deviation between upgaze and downgaze is 10Δ or more.
• Y-pattern: Exophoria or exotropia increases only in upgaze compared to primary position and downgaze.
• X-pattern: Exophoria or exotropia is present in primary position, and the deviation increases in both upgaze and downgaze.
• λ (lambda) pattern: Orthophoria in upgaze and primary position, but exophoria or exotropia in downgaze.

A- or V-pattern is observed in 10–25% of all horizontal strabismus cases. A recent single-center report from China (analysis of 8,738 horizontal strabismus cases) found A- or V-pattern in 10.36%, with V-pattern exotropia being the most common³.

Q What is the difference between V-pattern and A-pattern?

A

V-pattern is defined as a difference in deviation between upgaze and downgaze of 15Δ or more, while A-pattern is 10Δ or more. The direction of deviation also differs; for example, in V-pattern exotropia, the deviation increases in upgaze, whereas in A-pattern exotropia, it increases in downgaze. For details, see the section “Main Symptoms and Clinical Findings”.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

Subjective symptoms associated with pattern strabismus vary depending on the type of pattern and the accompanying horizontal strabismus.

• Diplopia: Especially noticeable in gaze directions where the deviation is larger. May complain of significant diplopia when looking downward.
• Compensatory head posture: Adopts a characteristic chin position to use the gaze direction where good eye alignment is achieved.
  • A-pattern exotropia: Good eye alignment when looking upward → Chin down
  • V-pattern exotropia: Good eye alignment when looking downward → Chin up
  • A-pattern esotropia: Chin up
  • V-pattern esotropia: Chin down (may achieve binocular vision and gross stereopsis)
• Asthenopia: Especially in adults with A-pattern exotropia, difficulty with binocular vision when looking downward often leads to eye strain.
• Impact on stereopsis in children: In A-pattern exotropia, binocular vision when looking downward becomes difficult, hindering the development of stereopsis.

Clinical Findings (Findings Confirmed by Physician Examination)

V-pattern

V-pattern exotropia: The exodeviation increases when looking upward and decreases when looking downward.

V-pattern esotropia: The esodeviation increases when looking downward and decreases when looking upward.

Inferior oblique overaction: In V-pattern, the frequency of inferior oblique overaction is high. Adduction of the eye is deviated upward and inward.

A-pattern

A-pattern exotropia: The exodeviation increases in downgaze and decreases in upgaze.

A-pattern esotropia: The esodeviation increases in upgaze and decreases in downgaze.

Superior oblique overaction: In A-pattern, the frequency of superior oblique overaction is high.

Rare types include the following:

• Y-pattern: Exodeviation increases only in upgaze compared to primary position and downgaze. It is thought to be caused by abnormal innervation of the lateral rectus muscle.
• X-pattern: Exodeviation increases in both upgaze and downgaze. It is due to the “leash effect” of the contracted lateral rectus muscle in long-standing exotropia.
• λ-pattern: Orthophoria in upgaze and primary position, with exodeviation only in downgaze.

Q Why do patients with pattern strabismus have a habit of tilting their head?

A

This is a phenomenon called compensatory head posture. In pattern strabismus, the deviation is smaller in a specific gaze direction, so the patient adjusts by raising or lowering the chin to use that direction as the primary gaze. For example, in V-pattern exotropia, the eye position is better in downgaze, so the patient tends to adopt a chin-up posture.

3. Causes and Risk Factors

Multiple mechanisms are involved in the development of pattern strabismus.

• Oblique muscle dysfunction: The most frequent cause.
  • Inferior oblique overaction: Produces V-pattern. V-pattern exotropia is often accompanied by inferior oblique overaction.
  • Superior oblique overaction: Produces an A-pattern. A-pattern exotropia is often accompanied by superior oblique overaction.
  • Superior/inferior oblique overaction can be primary or secondary to superior/inferior oblique palsy ¹.
• Orbital pulley system abnormalities: Can produce A- or V-patterns mimicking oblique overaction ^1,2. In craniofacial anomalies, V-pattern is more common. In craniosynostosis such as Crouzon syndrome, orbital imaging shows all extraocular muscles rotated outward, with the superior rectus located temporally, resulting in V-pattern strabismus. Crouzon syndrome is also often associated with congenital absence of the superior oblique muscle ⁵.
• Ocular torsion:
  • Excyclotorsion: The superior rectus is displaced temporally and the inferior rectus nasally, producing a V-pattern.
  • Incyclotorsion: The superior rectus is displaced nasally and the lateral rectus temporally, producing an A-pattern.
• Restriction of horizontal recti: Contracture of the lateral rectus in large-angle exotropia causes an X-pattern.
• Abnormal innervation: Produces a Y-pattern, either alone or associated with congenital cranial dysinnervation disorders (CCDDs).
• Abnormal vertical insertion of horizontal muscles or orbital bone deformity: Reported as contributing factors to A- or V-patterns.

Prevention and Daily Care

There is no established method to prevent pattern strabismus itself. If your child has a habit of tilting the head or turning the face, there may be an underlying eye misalignment, so please consult an ophthalmologist early.

4. Diagnosis and Examination Methods

Evaluation of pattern strabismus requires measurement of ocular alignment in multiple gaze directions.

• Three-position measurement: Measure ocular alignment in primary position (straight ahead), approximately 25 degrees upward (chin-down position), and approximately 35 degrees downward (chin-up position), and quantify the difference in deviation. This is the basis for diagnosing pattern strabismus.
• Measurement under refractive correction: To minimize accommodative factors, measurements should be taken while wearing appropriate refractive correction (e.g., glasses).
• Observation of compensatory head posture: Check for the presence of compensatory head postures such as chin-up or chin-down. In A-type esotropia, a chin-up posture may be observed.
• Cover test: A basic test to evaluate the presence and degree of horizontal and vertical strabismus.
• Alternate cover test: Used to measure the amount of deviation including phoria. Also check for the presence of dissociated vertical deviation (DVD).
• Evaluation of oblique muscle overaction: Observe the vertical deviation of the adducting eye in horizontal gaze (lateral gaze) to assess the degree of inferior oblique overaction and superior oblique overaction. Also note any asymmetry between the left and right eyes.

For differential diagnosis, it is important to distinguish from dissociated vertical deviation (DVD). DVD frequently coexists with inferior oblique overaction and may present findings similar to V-pattern.

5. Standard Treatment

If the pattern is clinically significant or if there is a compensatory head posture, strabismus surgery is indicated. Surgery also corrects any coexisting horizontal deviation. In esotropia, the prognosis for stereopsis is generally poor, and if the period for acquiring stereopsis has passed, surgery is for cosmetic purposes. Therefore, whether to actively treat the pattern depends on the cosmetic prominence.

Surgical Procedures for V-pattern

With oblique muscle overaction: Inferior oblique weakening (recession) is the first choice. Bilateral oblique muscle surgery can address a difference of 20–25 PD between upgaze and downgaze.

Without oblique muscle overaction: Vertical transposition of horizontal rectus muscles (Trick procedure). The medial rectus is moved downward and the lateral rectus upward. Vertical transposition of half the tendon width has been reported as an effective procedure for a long time⁴.

When DVD is present: Anterior transposition of the inferior oblique, suturing it to the insertion of the inferior rectus, is effective. It is often combined with horizontal strabismus surgery.

Surgical Procedures for A-pattern

With oblique muscle overaction: Superior oblique weakening (recession) is the first choice.

Without oblique muscle overaction: Vertical transposition of horizontal rectus muscles (Trick procedure). The medial rectus is moved upward and the lateral rectus downward.

In the Trick procedure (vertical transposition of the horizontal rectus insertions), the muscle is moved by half to full tendon width. A mnemonic is “MALE” (Medial to Apex, Lateral to Empty space).

Other types of surgical procedures are as follows.

• Y pattern: Superior transposition of the lateral rectus muscle
• X pattern: Recession of the lateral rectus muscle
• λ pattern: Weakening of the superior oblique muscle

The usefulness of the Slanting method, a technique involving oblique suturing of the horizontal rectus muscle insertion, has also been reported.

Precautions and side effects in treatment

Strabismus surgery is often performed under general anesthesia, and especially in children, attention must be paid to the risks associated with anesthesia. After surgery, vertical strabismus may become newly noticeable, or alternating hypertropia may recur like a seesaw, sometimes requiring reoperation. Please consult a strabismus specialist regarding the timing of surgery and the choice of surgical method.

Q What methods are chosen for surgery?

A

The surgical method differs depending on the presence or absence of oblique muscle overaction. If overaction is present, weakening of the corresponding oblique muscle is performed; if not, the Trick method, which involves vertical transposition of the horizontal rectus muscle insertion, is combined. For details, refer to the “Standard treatment” section.

6. Pathophysiology and detailed mechanisms

The mechanism of pattern strabismus is understood primarily through changes in the course and direction of action of the extraocular muscles.

Mechanism due to oblique muscle abnormalities

The inferior oblique and superior oblique muscles are involved in torsion and vertical movement of the eyeball. The inferior oblique has extorsion, elevation, and abduction actions; when overactive, it increases exodeviation in upgaze, producing a V pattern. The superior oblique has intorsion, depression, and adduction actions; when overactive, it produces an A pattern. Oblique muscle overaction can be primary or secondary to contralateral muscle palsy^1,2.

Mechanism due to ocular torsion

When the eyeball is extorted (outward rotation), the insertion of the superior rectus shifts temporally and that of the inferior rectus shifts nasally. As a result, the abduction effect increases in upgaze, forming a V pattern. Conversely, with intorsion, the superior rectus shifts nasally and the lateral rectus temporally, producing an A pattern.

Involvement of the Orbital Pulley System

Malposition of the orbital pulley, which acts as the functional origin of the extraocular muscles, can alter the direction of muscle action and cause A or V patterns even without oblique muscle overaction ^1,2. In craniofacial abnormalities, the pulley position changes due to orbital morphological abnormalities, leading to a high frequency of V patterns ⁵.

Leash Effect (Mechanism of X Pattern)

When the lateral rectus muscle contracts due to long-standing exotropia, it pulls the eyeball in the abduction direction like a string being pulled, both in upgaze and downgaze. This is the cause of the X pattern.

References

1. Ghasia FF, Shaikh AG. Pattern Strabismus: Where Does the Brain’s Role End and the Muscle’s Begin? J Ophthalmol. 2013;2013:301256. doi:10.1155/2013/301256. PMID: 23864934; PMCID: PMC3707271.
2. Kekunnaya R, Mendonca T, Sachdeva V. Pattern strabismus and torsion needs special surgical attention. Eye (Lond). 2015;29(2):184-190. doi:10.1038/eye.2014.270. PMID: 25412718; PMCID: PMC4330283.
3. Zhu B, Wang X, Fu L, Yan J. Pattern Strabismus in a Tertiary Hospital in Southern China: A Retrospective Review. Medicina (Kaunas). 2022;58(8):1018. doi:10.3390/medicina58081018. PMID: 36013485; PMCID: PMC9414984.
4. Scott WE, Drummond GT, Keech RV. Vertical offsets of horizontal recti muscles in the management of A and V pattern strabismus. Aust N Z J Ophthalmol. 1989;17(3):281-288. PMID: 2679813.
5. Coats DK, Paysse EA, Stager DR. Surgical management of V-pattern strabismus and oblique dysfunction in craniofacial dysostosis. J AAPOS. 2000;4(6):338-342. doi:10.1067/mpa.2000.110337. PMID: 11124667.