bubble_chart Overview

Intermittent exotropia is a type of strabismus that falls between exophoria and concomitant exotropia. It refers to a condition where the visual axes often diverge, initially occurring during distance viewing. When looking at distant objects, the amplitude of fusional divergence exceeds that of fusional convergence, leading to exotropia. Before intermittent exotropia develops, exophoria typically occurs first.

bubble_chart Clinical Manifestations

Intermittent exotropia often occurs in early childhood, initially only manifesting when looking at distant objects. As the condition progresses, the frequency and duration of intermittent exotropia increase, and eventually, exotropia may also occur when viewing near objects. The manifest phase of intermittent exotropia often appears during fatigue, illness, drowsiness, or lack of concentration. In children with immature vision, intermittent exotropia may cause temporary diplopia, which is quickly suppressed, accompanied by abnormal retinal correspondence.

A common symptom is photophobia, where the patient frequently closes one eye in bright outdoor sunlight. The reason for this is unclear, but it is speculated that when viewing distant targets outdoors, the absence of nearby objects to stimulate convergence, combined with the bright sunlight dazzling the retina and disrupting fusion, causes the patient to shift from latent exotropia to manifest exotropia. However, it is not certain that closing one eye is solely to avoid diplopia; it is also possible that bright light affects the fusional convergence amplitude in patients with intermittent exotropia, leading to the closure of one eye.

Intermittent exotropia may be associated with A-V patterns and can also coexist with other vertical deviations, such as dissociated vertical deviation.

bubble_chart Auxiliary Examination

All strabismus items should be examined, with special attention to the measurement of diagnostically significant deviation angles, such as the deviation angle at distance. It is best to have the patient fixate on a target >6m away to fully assess the degree of exotropia and determine the type of exotropia, as the timing and method of treatment differ. During measurement, refractive errors should be corrected to control accommodation. If intermittent exotropia occurs only at distance, and the distance deviation angle is at least 15^△ greater than the near deviation angle, a cover test should be performed. Cover one eye for 30–45 minutes, and when the cover is removed, the two eyes must remain separated—that is, when one eye is uncovered, the other must still be occluded with an eye patch. Quickly measure the near deviation angle using the alternate prism cover test upon uncovering, then check the distance deviation angle, preventing the patient from having any opportunity for fusion. Compare the results with the pre-cover deviation angle. Burian and Franceschetti observed a group of 237 surgical cases, finding only 10 to be of the divergence excess type. Thus, most exotropia patients with a larger distance deviation angle than near deviation angle should be classified as resembling the divergence excess type.

Measure the deviation angle in upgaze and downgaze to determine the presence of A-V patterns.

Measure the deviation angle when looking to the left and right to assess any lateral incomitance issues. By definition, lateral incomitance refers to exotropia patients whose deviation angle when looking to either side is 20% smaller than that in primary position. Clinical practice has shown that patients with lateral incomitance are prone to surgical overcorrection, resulting in an A-V pattern.

Assess stereopsis: Even during the phoria phase, the patient’s stereopsis must be measured. Abnormal stereopsis indicates a decline caused by intermittent manifest deviations. A continued decline in stereopsis over several months strongly suggests surgical intervention as an indication for correcting intermittent exotropia.

bubble_chart Treatment Measures

1. Cycloplegic refraction examination: For patients with significant refractive errors, especially astigmatism and anisometropia, full correction should be provided to ensure a clear retinal image. For exotropia accompanied by myopia, full correction is necessary. For exotropia accompanied by hyperopia, correcting hyperopia will reduce accommodative convergence, thereby increasing exotropia. Whether full or partial correction is required depends entirely on the degree of hyperopia, the patient's age, and the AC/A ratio. Typically, hyperopia less than +2.00D in infants may not require correction. For older patients, correcting hyperopia is usually necessary to avoid refractive fatigue. Elderly patients with exotropia and presbyopia, whose accommodation is weakened, may need minimal correction for near vision if hyperopia is present.

2. Minus lenses: Using minus lenses to correct intermittent exotropia can serve as a temporary measure. They may be placed in the upper part of bifocal lenses to treat excessive divergence or in the lower part to treat convergence insufficiency, stimulating accommodative convergence to control exotropia. However, this treatment method is not recommended, as it often causes visual fatigue in children.

3. Prism and occlusion therapy: Base-in prisms can enhance central foveal stimulation in both eyes. Approximately 1/2 to 1/3 of the deviation can be corrected by prism-induced fusion. Recent studies suggest that occlusion is an effective non-surgical treatment in the initial stage of intermittent exotropia. With this method, about 40% of patients can convert from manifest exotropia (distance vision) to latent exotropia. Early-stage intermittent exotropia, where the deviation is mostly latent with infrequent and minor manifest deviations, does not typically warrant surgical intervention.

4. Surgical treatment: The optimal age for surgery in congenital exotropia remains debated. Some advocate for early surgery to prevent progression to constant exotropia. Lyle suggests that since most intermittent exotropia patients maintain good distance fusion and binocular vision, surgical outcomes at ages 2–3 or after 10 are nearly identical, allowing for observation over several years. Jampolsky recommends delaying surgery for infants with immature vision to avoid overcorrection, using minus lenses to enhance fusion and alternating occlusion to prevent suppression. Surgery should be considered if fusion deteriorates rapidly or the deviation stabilizes.

Surgical indications depend on fusion control, deviation magnitude, and patient age. Early surgery is advised for congenital exotropia without intermittent features. He Yushi recommends surgery for deviations exceeding 20^△; Jampolsky suggests 15^△ or more; Hiles advocates for deviations over 20^△ with significant manifest components and visual decompensation. From the perspective of retinal correspondence, intermittent exotropia may develop abnormal retinal correspondence and suppression to eliminate diplopia and confusion. The ideal surgical timing is before these complications arise.

Yu Gang's follow-up of 77 intermittent exotropia cases post-surgery showed that younger patients have a higher chance of restoring normal binocular vision, whereas most adults, even with corrected alignment, do not regain normal binocular function. Clinically, determining the best surgical timing is challenging. If the child is too young, uncooperative during exams, or surgical dosages are hard to estimate, reoperation rates increase. Around age 4.5, intellectually normal intermittent exotropia children can cooperate with basic alignment tests after repeated training, making this an appropriate surgical window. Jampolsky’s study on congenital exotropia across ages concluded: earlier surgery leads to more procedures, higher reoperation rates, and greater risks of amblyopia and fusion loss.

We believe that for children with progressive intermittent exotropia, those with a deviation greater than 20^△, and those whose manifest deviation exceeds 50% of the time, the surgical plan can be designed based on the degree of deviation and the results of the cover test. If, after 30 to 45 minutes of occlusion, the deviation for distance is at least 15^△ greater than for near, bilateral lateral rectus recession can be performed, with the surgical dosage determined by each surgeon's experience and methods. If the cover test shows that the near deviation is at least 15^△ greater than the distance deviation and less than 55^△, bilateral lateral rectus recession or recession-resection surgery on the non-dominant eye can be performed. For exotropia greater than 55^△, surgery on three muscles may be considered: recession of the lateral rectus on the dominant eye and recession-resection on the non-dominant eye. If the exotropia exceeds 70^△, bilateral recession-resection surgery should be performed.

Special consideration should be given to the so-called lateral incomitance issue. If the angle of strabismus is at least 20% smaller during left and right gaze compared to the primary position, there is a significant risk of overcorrection, especially for patients whose vision is not yet fully developed. Therefore, for patients with lateral incomitance, bilateral lateral rectus recession should be avoided. If recession-resection is performed on the non-dominant eye, 1 mm less should be done on each side.

For children whose vision is not yet mature, undercorrection is recommended. Grade I overcorrection, resulting in esotropia, is more likely to lead to monofixation syndrome compared to Grade I undercorrection, which results in exotropia. Additionally, it may cause suppression scotoma and developmental amblyopia. If the patient’s vision is mature, a Grade I overcorrection of 10–20 ^△ is ideal, as it will ultimately yield stable results. Overcorrection exceeding 25 ^△ should be avoided even in visually mature patients, as it may lead to blind spot syndrome and disrupt postoperative fusion.

If the patient has an A-pattern or V-pattern strabismus caused by overaction of the superior or inferior oblique muscles, weakening procedures for the overacting muscles can be performed concurrently with horizontal strabismus surgery. However, if both the superior and inferior oblique muscles are overacting, weakening either muscle is contraindicated.

5. Management of Overcorrection The reported prevalence of overcorrection after exotropia surgery ranges from 6% to 20%. If significant overcorrection occurs immediately after surgery, the patient should undergo reoperation within 24 hours, as muscle loss or slippage may have occurred. The lateral rectus muscle is less prone to loss compared to the medial rectus. Excessive resection of the medial rectus due to mechanical factors may also cause noticeable overcorrection, though the degree is usually less severe than in the former case.

If the esotropia is comitant, observation may be warranted. Postoperative esotropia of 10–15 ^△ may resolve completely. For small overcorrections after exotropia surgery, management depends on the patient’s age. In children with immature vision, small overcorrections should be closely monitored for the development of suppression scotoma and developmental amblyopia. If the patient shows no fixation preference, alternating occlusion can be applied. For Grade II fixation preference, occlusion therapy may be used. Additionally, a repeat refraction should be performed. Full correction should be prescribed for hyperopia, and if the deviation is greater at near, miotics or bifocal lenses may be used. If no significant improvement is observed after 4 months of treatment, the patient should be reassessed as a new case rather than simply reverting to the previous exotropia surgery.

For visually mature patients, an overcorrection of 20 ^△ is ideal. If 20 ^△ esotropia persists 6 weeks postoperatively, a second surgery may be considered, but it should be performed no earlier than 6 months after the initial surgery. Preoperative forced duction testing is crucial. If restriction is detected, appropriate recession of the muscle, conjunctiva, and Tenon’s capsule should be performed.

6. Management of Undercorrection If significant residual exotropia (>15–20 ^△ ) remains after surgery, a second procedure may be performed within 6–8 weeks of the initial surgery. Such cases should be treated as new conditions. If the residual deviation is equal for distance and near, and the initial surgery was recession-resection, the same procedure can be performed on the other eye. If the initial surgery was bilateral lateral rectus recession, marginal myotomy of one lateral rectus combined with medial rectus resection on the same side may be considered. If the residual exotropia is greater at distance than near, and the initial surgery was bilateral lateral rectus recession, further recession or marginal myotomy of the lateral rectus may be performed. If the initial surgery was recession-resection, recession of the lateral rectus in the other eye can be done. During secondary lateral rectus surgery, temporal conjunctival recession is recommended to prevent scar advancement, which could compromise surgical outcomes.

For patients with undercorrection of grade I, if the residual deviation is less than 15-18^△, suppression elimination and fusional convergence training can be used to achieve a heterophoric state. If the patient has myopia, full correction should be applied. For emmetropia or farsightedness, a cycloplegic agent can be used to stimulate accommodative convergence and achieve binocular alignment. After fusion is obtained using the above methods, the frequency of eye drops can be reduced to once every 3 days and continued for 2 months. Simultaneously, base-in prisms with a power matching the undercorrection degree can be used, which is effective for visually mature patients.