bubble_chart Overview

Concomitant strabismus refers to the misalignment of the visual axes of both eyes, where the extraocular muscles and their nerve innervation show no qualitative changes. The deviation angle remains equal in all directions, regardless of which eye is fixating. In non-concomitant strabismus, the deviation angle varies depending on the fixating eye. For instance, when the affected eye is fixating, the deviation increases. The primary causes of non-concomitant strabismus include underaction or paralysis of the extraocular muscles. Other causes include mechanical restrictions of the extraocular muscles (e.g., restriction), abnormal nerve innervation (e.g., Duane retraction syndrome), and conditions like A-V patterns. Primary concomitant esotropia is divided into two main categories: accommodative and non-accommodative. Accommodative esotropia further includes refractive and high AC/A types.

bubble_chart Pathogenesis

1. Refractive accommodative esotropia: (refractive accommodative esotropia) is the mechanism of disease, caused by uncorrected farsightedness combined with abnormal fusional divergence amplitude. If a patient has farsighted eyes, accommodation is required to achieve a clear retinal image. Changes in accommodation lead to changes in convergence, known as accommodative convergence. In other words, each diopter of accommodation is accompanied by a certain amount of prism diopter accommodative convergence. This ratio is called the accommodative convergence (AC)/accommodation (A) ratio, or the AC/A ratio. Whether a patient develops esotropia depends not only on the degree of farsightedness but also on the fusional divergence reserve. If the fusion reserve exceeds the accommodative convergence, the eyes remain aligned; otherwise, esotropia occurs.

For example: A patient has 5D farsightedness, an AC/A ratio of 4 (normal is 3–5), and a fusional divergence amplitude of 14^△ at distance. At distance, 5D of accommodation is required. Since the AC/A ratio is 4, the convergence is 20^△ (5×4), which clearly exceeds the patient's fusional divergence amplitude at distance, resulting in 6^△ esotropia. If the patient has only 3D farsightedness, 12^△ of convergence is required at distance, still severely depleting the patient's fusion reserve and causing intermittent esotropia. Fusion reserve is influenced by many factors, such as illness, mood, fatigue, etc. If the patient's farsightedness exceeds 6D, another scenario arises: the required accommodation is so large that it leads to accommodative relaxation, blurred vision, and maintained eye alignment but bilateral amblyopia, known as refractive amblyopia.

2. High AC/A accommodative esotropia:

In accommodative esotropia, due to a high AC/A ratio, excessive accommodative convergence occurs with each diopter of accommodation. This excessive accommodative convergence affects the fusion reserve, causing the patient to exhibit esotropia.

High AC/A accommodative esotropia shares similarities with refractive accommodative esotropia. The onset age is 2–3 years, with initially small and intermittent deviation angles, possibly accompanied by temporary diplopia. The child may experience dysphoria, irritability, and close one eye, later developing suppression and abnormal retinal correspondence, returning to a previous state.

Additionally, high AC/A accommodative esotropia may also involve vertical deviation and A-V patterns. These additional deviations require surgical correction in addition to refractive correction for the high AC/A ratio.

There are also differences between refractive accommodative esotropia and high AC/A accommodative esotropia. High AC/A accommodative esotropia is associated with grade I farsightedness, averaging +2.25D, and may not involve farsightedness. In contrast, refractive accommodative esotropia averages +4.75D. High AC/A accommodative esotropia shows a larger deviation at near due to the high AC/A ratio. An example is as follows:

The patient has +0.50D hyperopia, with an AC/A ratio of 15 and a fusional reserve of 12^△. The pupillary distance is 60mm. For distance vision, the patient only requires 0.5D of accommodation. Given the AC/A ratio of 15, the accommodative convergence is 0.5×15=7.5^△. For near vision, the fusional divergence ability is 12^△. The patient has 2^△ of esophoria, which can be fully overcome to achieve fusion.

When viewing at near, the situation is quite different. The fixation point must shift from far to near, as a certain amount of convergence is required to maintain binocular imaging on the fovea centralis. This is called symmetrical convergence, calculated by multiplying the interpupillary distance (in cm) by the meter angle, where the meter angle is inversely proportional to the viewing distance (in meters). If there is esophoria or esotropia at distance, this value should be subtracted; if there is exophoria or exotropia at distance, this value should be added.

Continuing with the previous example: the patient's interpupillary distance is 6 cm, with 2^△ of distance esophoria. Now, when viewing at near (1/3 m, i.e., 3 meter angles), the required convergence is (3 × 6)^△ − 2^△, resulting in 16^△ of convergence to maintain fusion. In reality, the patient uses significantly more convergence than this. For near vision (1/3 m), 3.5D of accommodation is required (with an original hyperopia of 0.5D). Due to a high AC/A ratio, this induces 3.5 × 15 = 52.5^△

of accommodative convergence. However, the patient only needs 16^△, leaving an excess accommodative convergence of 36.5^△ (52.5 − 16^△). Since the patient's fusional divergence amplitude at near is 12^△, the net excess convergence at near is 24.5^△. Thus, due to the high AC/A ratio, the patient maintains fusion at distance but exhibits 25–25^△ of esotropia at near—this is referred to as high AC/A ratio accommodative esotropia.

bubble_chart Clinical Manifestations

1. Refractive accommodative esotropia:

Refractive accommodative esotropia mostly occurs between the ages of 2 and 3. Initially, the esotropia is intermittent, and temporary diplopia may occur. The child may experience dysphoria, irritability, and close one eye. After 2–3 days, suppression begins in the deviated eye, and the child no longer closes the eye. Over a longer period, abnormal retinal correspondence may develop, at which point the patient no longer experiences sensory symptoms. When the child is orthophoric, normal retinal correspondence is present, whereas intermittent esotropia is associated with abnormal retinal correspondence. Thus, abnormal and normal retinal correspondence alternate until the intermittent esotropia eventually progresses to constant esotropia with monocular fixation and amblyopia in the deviated eye. Refractive accommodative esotropia may also coexist with other ocular muscle abnormalities, such as vertical deviations and A-V patterns.

A comprehensive examination should be performed on the child, including measurement of the deviation and a cycloplegic refraction. For cycloplegia, 0.5–1% atropine ointment or solution should be used. Parents should be instructed to apply pressure to the lacrimal sac area when using atropine solution to minimize systemic absorption and avoid toxicity. Ointment is preferable, applied three times daily for three days.

After cycloplegia, a fundus examination should be performed. It should be noted that children with leukocoria may have secondary esotropia; for example, early manifestations of retinoblastoma may include esotropia.

The refractive error in accommodative esotropia can range from +2 to +6D, with most cases exhibiting significant hyperopia. One study reported an average hyperopia of +4.75D.

The fusional divergence amplitude should be measured for both distance and near vision. The normal fusional divergence amplitude should be greater than 3 for distance and 8 for near.

2. High AC/A accommodative esotropia: A comprehensive examination should be conducted, including measurement of the deviation at distance and near using the alternate cover test with prism. Cycloplegic refraction should be performed to determine the AC/A ratio, and a fundus examination should be done to rule out retinal disease.

Additionally, high AC/A accommodative esotropia should not be confused with V-pattern esotropia, where the esodeviation increases in downgaze, regardless of distance or near fixation.

3. Partially accommodative esotropia: In a sense, all esotropias are partially accommodative because prescribing plus lenses can reduce the deviation. However, traditionally, partially accommodative esotropia refers to: ① Infantile esotropia with an additional accommodative component; ② Decompensated accommodative esotropia, where a non-accommodative component is superimposed on accommodative esotropia.

In infantile esotropia, the accommodative component is most pronounced between ages 2 and 3, which is the peak period for developing accommodative esotropia. Decompensated accommodative esotropia is caused by hypertrophy or contracture of the medial rectus muscles. Regardless of the cause, partially accommodative esotropia is usually constant and often accompanied by suppression, abnormal retinal correspondence, and amblyopia.

bubble_chart Treatment Measures

1. Refractive Accommodative Esotropia:

Appropriate correction of refractive errors should be made to restore binocular single vision. The method of correction varies depending on age.

For infants aged 4–5 months or younger with esotropia, including intermittent esotropia, if the farsightedness exceeds +2.00D, glasses should be prescribed, incorporating the full cycloplegic refraction results with an additional +1.50D. Since infants primarily focus on near objects, pupillary distance must be measured, and sometimes an elastic headband is required.

There was once a misconception that most infants are farsighted and that the degree of farsightedness decreases with age. In reality, only 10% of infants under 1 year have bilateral farsightedness exceeding +2.00D, and the degree of farsightedness tends to increase rather than decrease with age until after 7 years old, when it gradually diminishes.

For infants aged 4 months to 4 years with esotropia and a refractive error greater than +1.50D, the full cycloplegic refraction should be corrected without additional power.

For children aged 4 and older with esotropia, the minimal lens power that achieves binocular single vision, esophoria, and optimal visual acuity should be prescribed. The treatment goal is esophoria rather than complete orthophoria, as maintaining esophoria allows the child to preserve, train, and increase fusional divergence amplitude. In 4-year-olds, full correction of the cycloplegic refraction may not yield the best vision. Slightly reducing the power may improve vision and binocularity, but excessive reduction to enhance vision can lead to intermittent esotropia and permanent damage to stereopsis.

If the child’s accommodative esotropia has progressed to constant esotropia with a tendency for monocular fixation and amblyopia, the full cycloplegic refraction should be corrected, and the sound eye should be patched to treat amblyopia. Once vision is restored, anti-suppression and fusional divergence training should be conducted to restore binocular single vision.

If the patient also has vertical deviation or A-V pattern, surgical intervention is necessary in addition to correcting refractive errors and treating accommodative esotropia.

High AC/A Accommodative Esotropia

2. High AC/A Accommodative Esotropia (high AC/A accommodative esotropia):

For infants aged 4–5 months or younger, glasses should be prescribed, correcting the full cycloplegic refraction with an additional +1.50D. Bifocal lenses are unnecessary as infants primarily use near vision.

For 5-year-olds, the full cycloplegic refraction should be corrected, supplemented with +3.00D bifocal lenses. The added power must be accurately positioned at the upper edge of the lower eyelid, unlike presbyopic lenses. If the bifocal segment is too low, it will be ineffective.

If residual esotropia persists after wearing glasses, a re-examination is needed. If the refractive error differs by more than +1.0D, new glasses should be prescribed.

Follow-up intervals: every 3 months for older infants, every 6 months for children aged 2–5, and annually after age 5. Cycloplegic refraction should be performed during follow-ups. For children aged 4–5, glasses should be replaced if the refractive error differs by more than +1.0D.

Similar to refractive accommodative esotropia, a major concern in high AC/A accommodative esotropia is amblyopia. After refractive correction, regular follow-ups are essential to prevent developmental amblyopia. If the patient misses follow-ups and developmental amblyopia occurs, aggressive treatment is required. Occlusion therapy is the first choice, but penalization therapy may be used if the child resists patching.

If the child is over 5 years old, the power of the bifocal lenses can be appropriately reduced by 0.75 to 1.0D to maintain fusion and keep the child in a state of esophoria when viewing near objects. This helps train fusional divergence ability. When the child reaches 9 to 10 years old, the bifocal lenses can be discontinued, but orthoptic training should still be performed to eliminate suppression and further train fusional divergence ability.

Topical anticholinesterase agents, which assist in eliminating bifocal lenses for high AC/A accommodative esotropia, can gradually increase fusional divergence ability by progressively reducing the concentration of the preparation.

3. Partially accommodative esotropia: Regardless of the form, the treatment first involves fully correcting the accommodative component. If amblyopia persists, occlusion therapy should be administered. If amblyopia has been cured, surgical correction of the residual esotropia is necessary. The deviation should be measured for both distance and near vision, and the surgical dosage is based on the residual deviation at distance after correction. This measurement serves only as a general reference during surgery. For example, in cases with a high AC/A ratio, empirical evidence suggests performing slightly more surgery than the measured amount. For instance, if the esotropia at distance is 18^△, the original plan of bilateral medial rectus recession of 3mm can be increased to 3.5mm, or bilateral medial rectus posterior fixation surgery may be performed.