Esotropia is a type of strabismus, or ocular misalignment, characterized by nasal deviation of the eye. High AC/A ratio accommodative esotropia is also called non-refractive accommodative esotropia. The accommodative convergence/accommodation (AC/A) ratio gives the relationship between the amount of convergence, or inward turning of the eyes, that is generated by a given amount of accommodation, or focusing of the lens. This ratio is numerically represented by the amount of convergence in prism diopters per unit change in accommodation in diopters. Amblyopia can occur in untreated patients, which is decreased vision due to a failure of normal neural development of the eye not in normal alignment.
Esotropia with high AC/A ratio generally presents by the age of 2 years.[1] Esodeviations are intermittent initially and usually decompensate to become constant. Sometimes there is trauma or illness related to onset. There is no sex predilection for esotropia with high AC/A ratio.
In esotropia with high AC/A ratio, patients have an excessive convergence response to accommodation while wearing their full cycloplegic correction. These patients most commonly have low hyperopia, while some have high hyperopia, myopia or emmetropia (no refractive error). Consecutive esotropia with high AC/A ratio has also been identified in patients after surgical correction for intermittent exotropia.[2]
Parents of the patient may notice a nasal deviation of one eye relative to the other eye. The patient may see either a single blurred image or a double image in which one image is clear and one image is blurred. The age of onset of strabismus should be noted. Family history of strabismus is common. The frequency of the esotropia should also be noted. Parents may note that the child looks more esotropic when focusing on objects at near compared to distance.
Stereoacuity, or binocular depth perception, using polarized glasses may show reduced stereopsis. Visual acuity should be examined in a manner appropriate for the patient's age, which may show normal vision or amblyopia in one or both eyes. Extraocular movements should be evaluated with attention to overelevation in adduction or dissociated strabismus complex. The angle of deviation should be measured at distance and near with, ideally, the prism alternate cover test. In esotropia with high AC/A ratio, the angle of deviation is greater with near fixation than at distance, generally measuring 20-40 prism diopters. The AC/A ratio can be derived through a few techniques including the gradient method and the heterophoria method.[1] The gradient method utilizes plus or minus lenses to vary the accommodative requirement to measure the stimulus AC/A ratio, or the change in deviation in prism diopters over the change in lens power in diopters. The heterophoria method utilizes differences in distance and near deviations as well as the interpupillary distance to calculate the AC/A ratio. Different AC/A ratios can be calculated among the same patients depending on the method that is used; therefore, the methods should not be considered as interchangeable.[3]
Patients with nonrefractive accommodative esotropia have a high AC/A ratio, which results in a deviation measured at near as generally 10 prism diopters or greater than that at distance. In contrast, the AC/A ratio is normal (ie, distance and near measurements are similar) in pure accommodative esotropia.
Cycloplegic refraction with retinoscopy in patients with esotropia with high AC/A ratio generally shows hyperopia around +2.25 diopters. Patients less commonly have emmetropia, high hyperopia, or myopia.
The options for treatment include observation, bifocal correction, and/or surgery. Most patients should have full correction of their refractive error. Patients with accommodative esotropia with high AC/A commonly have resolution of esotropia at distance with full hyperopic correction, with residual esotropia at near. Most providers use bifocal glasses to reduce amplitude of esotropia at near. These bifocals are positioned higher than those utilized in adults with presbyopia; the aim is for the upper border of the bifocal lens to bisect the pupil so patients will be more likely to use the increased plus lens at near. The power of the bifocal lens generally starts at +3.00 diopters with power weaned starting at age 6-8 years before ideally eliminating the need for a bifocal by adolescence. The plus power at near reduces the need for accommodation and therefore accommodative convergence and esotropia. The goal is to preserve fusion and stereopsis at distance and near.
Some clinicians prefer to observe their patients, as many show a decrease in near deviation with time with along with the development of stereopsis at near and distance.[1] While using bifocals over observation has been debated, the research is ongoing with preliminary studies failing identify a long-term sensory benefit with bifocal use.[4, 5]
Miotics, like echothiophate iodide, are occasionally used, which suppress convergence and decrease AC/A ratio.
In cases of amblyopia, early treatment with optical penalization of the dominant eye is the mainstay of treatment, with the use of patching or occasionally atropine penalization.
Surgery may be considered, generally if the esodeviation is persistent at distance, or becomes refractory to optical treatment.[5] Surgical treatment typically entails recession, or weakening, of the inward-pulling medial rectus muscle in each eye, with or without posterior pulley fixation. The target angle for surgery is surgeon-dependent and may vary from near to distance fixation, with and without refractive and bifocal correction. Some surgeons use prism adaptation to determine the maximum deviation before deciding the appropriate surgical measurements.[1]
Patients who are treated for amblyopia should be seen at 1- to 4-month intervals depending on the extent of their treatment. Stable patients are typically seen every 6-12 months. Cycloplegic refraction is repeated at least annually and anytime esotropia worsens.
Permanent vision loss can occur if strabismus and concurrent amblyopia are not treated before patients are 4-6 years old. Early treatment of amblyopia may result in improved vision, leading to a better prognosis for development of binocular vision and a more stable alignment if surgery is required. Most patients can be weaned from their bifocals by the time they are teenagers. Some patients may have issues with fusion and stereopsis even with consistent spectacle use. Accommodative esotropia continues for most patients into early adulthood.[6] With time, some patients may have an increase in fusional divergence as well as a decrease in hyperopia and AC/A ratio, allowing for straight eyes even without correction.