Case Series: Neuroplasticity and Vision Therapy in Adults with Unilateral Small-Angle Esotropia

INTRODUCTION

Neuroplasticity is the ability of the nervous system to adapt and change in response to different stimuli and experiences, which involves strengthening some synaptic connections while eliminating others.¹ This process is most robust during the critical period, which varies for different functions.¹ It was previously thought that neuroplastic changes were restricted to that period² and skills would be permanently affected if they failed to develop during that time.³ The critical period for development of stereopsis is defined to be 3 months to at least 3 years of age.⁴ Disruption of visual input can affect the development of binocularity, specifically stereopsis, in individuals with strabismus. For esotropia, the period in which stereopsis is susceptible to anomalous binocular vision was found to be soon after birth to at least 4.6 years of age.⁴

The misconception of limited improvement with treatment beyond the critical period may prevent adults from seeking treatment for their strabismus or deter health care professionals from recommending treatment, including vision therapy and strabismus surgery.⁵ There is growing evidence that neuroplasticity is possible in adults past the critical period of visual development. This is evident by improved binocularity in adults with strabismus after surgery,^5–10 improved amblyopic eye visual acuity in adults with patching and binocular therapy,¹¹ and improved oculomotor skills in adults with vision therapy.^12,13 However, there is limited research on the use of vision therapy to restore binocularity in adults with strabismus, as most reports are restricted to case studies.^14–19 Although most adult patients with long-standing strabismus do not experience symptoms, these patients may have cosmetic concerns, occupational goals, or personal goals that require alignment or binocular vision. In these cases, treatment of the strabismus may improve these patients’ quality of life^5–7 by improving visual function.

The prognosis for successful treatment of strabismus depends on several factors, including, but not limited to, the presence of a sensory fusion anomaly, direction, size, and time of onset of the strabismus.^20–22 Amblyopia can also add another layer of treatment complexity. Adult patients with strabismus desiring binocular vision may be offered various treatment options, including vision therapy. The goal of vision therapy is to eliminate sensory anomalies and improve sensory and motor fusion with the hope that this leads to improved binocularity. This case series describes adults with constant unilateral small-angle esotropia who achieved their goals of binocular vision through vision therapy. No identifiable health information was included in this case series.

CASE REPORTS

Case 1

A 33-year-old man presented with a complaint of long-standing reduced vision in his left eye and an inability to appreciate virtual reality. His goal was to improve vision and to appreciate virtual reality. Ocular history included amblyopia with poor adherence to patching therapy during childhood.

Distance visual acuities (electronic Early Treatment Diabetic Retinopathy Study [E-ETDRS]) through his presenting spectacles were right eye -1.25 sphere (20/20, letter score 88), left eye Plano sphere (20/50, letter score 65). Interferometry using a Heine Retinometer showed identification of 20/30 grating targets (left eye). Cycloplegic autorefraction, using 2 drops of 1% cyclopentolate, was right eye +0.37 - 0.50 × 170, left eye +6.12 - 1.50 × 009. Cycloplegic manifest refraction and visual acuities (Snellen) were right eye Plano sphere (20/20), left eye +6.50 - 0.50 × 180 (20/125). Because the patient had not worn a full anisometropic prescription, he was recommended to return 1 week later to finalize his spectacle prescription. Through cycloplegic manifest refraction, he experienced dizziness and diplopia. Anisometropic prescription, through trial framing, was consequently reduced. In a trial frame, the following prescription was most comfortable and recommended for full-time wear: right eye Plano sphere, left eye +5.75 - 0.50 × 015 (Snellen 20/100).

After the patient adapted to the spectacle prescription for 1 month, a strabismus evaluation (Table 1) was performed. The patient reported diagonal diplopia on Worth four-dot and synoptophore testing. Second-degree fusion was not achieved on Worth four-dot with neutralizing prism, but it was achieved during synoptophore testing at his subjective angle. Vertical prism was trial framed based on his subjective angle, and 5∆ was most comfortable, so his spectacles were remade to include right eye 2.5∆ base-down, left eye 2.5∆ base-up. Contact lenses were considered to reduce potential aniseikonia, but the patient did not want contact lenses and the amount of vertical prism was too high for contact lenses.

The patient was diagnosed with microtropia and mixed mechanism (strabismic and anisometropic) amblyopia in his left eye. Prognosis for strabismus treatment was guarded, in which a favorable prognostic factor includes normal correspondence and unfavorable prognostic factors include esotropia and amblyopia. The patient was motivated to achieve his visual goals, and in-office 45-minute weekly vision therapy was recommended. Home therapy (15-20 minutes daily) with 2 hours of patching (right eye) during near activities was prescribed. Vision therapy focused on developing binocularity by reducing the frequency of his esotropia and improving vision in his left eye. Base-out prism was discussed, but he wanted to try vision therapy first. Strabismus surgery was not recommended owing to the esotropia being a small angle, and the risk outweighed the benefits.

The patient’s vision therapy program progressed from improving monocular visual function, to establishing normal peripheral then central fusion, to—lastly—establishing efficient binocular vision in open space. Vision therapy activities (Table 2) were categorized into monocular visual skills, vergence, and antisuppression training. The beginning of the program aimed at normalizing accommodative and oculomotor skills in the left eye. Accommodative techniques were first performed monocularly, starting with increasing accommodative amplitude and awareness and then moving on to facility. Monocular gross saccades were trained first, then progressing to medium and fine saccades. Form discrimination therapy was also included. These visual skills were later integrated with vergence training.

Given the constant nature of the patient’s strabismus but normal sensory fusion at his objective angle, divergence training was performed from his objective angle²² in the synoptophore and Wheatstone stereoscope. Antisuppression therapy was incorporated to reduce suppression and develop pathological diplopia awareness when he was strabismic. When he achieved fusion at orthoposition, out-of-instrument divergence training at near using vectograms and Tranaglyphs was initiated. Computer vergence therapy (Vision Therapy System 4) with random dot stereopsis targets of 600" was used to help him appreciate stereopsis. Convergence training was gradually incorporated. During the last stage, vergence training advanced from near to intermediate and then far distances.

After 30 sessions and daily patching, the patient’s amblyopic eye visual acuity improved to 20/25 and he could appreciate random dot stereopsis of 250". In addition, the frequency of his esotropia changed from constant to intermittent. Although base-out prism could be beneficial in stabilizing his binocularity, he elected not to proceed with this treatment because he already attained his goal. Patching was tapered (3 days per week for the first month, 2 days per week for the second month, and 1 day per week for the third month) before discontinuing, and home maintenance therapy was prescribed.

At the 3-month post–vision therapy progress evaluation, the patient still retained random dot stereopsis. Refraction remained unchanged, but left eye visual acuity regressed to 20/40 (letter score 73). To the authors’ knowledge, there are no studies regarding regression in amblyopic eye visual acuity in adults, but research in children shows that regression can occur after discontinuing treatment when amblyopic eye visual acuity significantly improves or treatment is initiated at an older age.²³ Contact lenses were discussed, but he preferred spectacles. It was recommended that he restart patching 1 hour a day and return in 2 months for a follow-up, but he was lost to follow-up.

Case 2

A 25-year-old woman presented with a complaint of blurred vision while performing slit lamp examination as a result of her eyes “alternating fixation.” Ocular history included amblyopia in her left eye with good adherence to patching therapy during childhood. Her goals were to resolve her amblyopia and develop stereopsis.

Distance visual acuities (Snellen) through her spectacle correction (+4.00 sphere both eyes) were 20/20 right eye, 20/30 left eye. A previous cycloplegic refraction revealed +5.50 sphere both eyes, but the patient reported discomfort with that prescription. Noncycloplegic manifest refraction of +4.75 sphere both eyes was found, trial framed, and finalized for full-time wear; it was well tolerated and distance visual acuity was unaffected.

Strabismus and amblyopia evaluation findings are listed in Table 3. Cover test revealed a comitant constant left esotropia of 4∆ at distance and 2∆ at near. She was found to have harmonious anomalous correspondence, 1∆ of nasal eccentric fixation in the left eye, and no random dot stereopsis. The patient was diagnosed with microtropia and strabismic amblyopia in her left eye. She was offered patching therapy alone to treat her amblyopia or patching with vision therapy to treat both amblyopia and strabismus. Her prognosis for strabismus treatment was guarded, where esotropia, anomalous correspondence, and amblyopia with eccentric fixation are unfavorable prognostic factors. Strabismus surgery was not recommended owing to the small size of her angle. Prism was contraindicated because of the presence of anomalous correspondence.

The patient was motivated to achieve her goals and elected weekly 45-minute in-office vision therapy, including 10-15 minutes of daily home therapy. The main objective of therapy was to improve vision in her left eye, as well as eliminate anomalous correspondence and develop binocularity. The vision therapy activities (Table 4) were categorized into monocular visual skills and vergence training. The beginning sessions focused on eliminating eccentric fixation in the left eye and improving accommodative skills. After achieving 20/20^-1 left eye in three visits, motor stimulation using Flom’s swing technique in the synoptophore was performed to diverge to an ortho eye alignment. Computer vergence therapy with random dot stereopsis targets was used to help her appreciate stereopsis. Vectograms were also incorporated during divergence training in open space.

The patient had to discontinue vision therapy because she relocated from the area. However, after 10 sessions, her visual acuity normalized to 20/20^-1 in the left eye and she was able to appreciate random dot stereopsis of 400". The patient has not returned to the area, so some examination findings could not be reevaluated.

Case 3

A 25-year-old woman presented with a complaint of pain over her right eye that worsened with extended near work or driving for a long time. It started 2 years ago and did not improve with pain medication. Previous medical records revealed a history of partially accommodative esotropia since age 2 and no stereopsis. Systemic history included type 1 Chiari malformation without ocular manifestations. Her goals were to reduce headache symptoms and establish binocularity.

Distance visual acuities (Snellen) were 20/15 in each eye through contact lenses (right eye +1.75 sphere; left eye +1.50 sphere). Cover test revealed a comitant, high-frequency intermittent left esotropia of 4∆ with asymmetric dissociated vertical deviation and variable left hypotropia of 4∆ at far and near. Her accommodative amplitude and facility were normal in each eye. On the prism adaptation test, she adapted to both horizontal and vertical prism. Additional examination findings from the strabismus evaluation are listed in Table 5.

She was diagnosed with microtropia with left hypotropia and harmonious anomalous correspondence. Prognosis was guarded for improvement of headache symptoms and poor for binocularity. Her unfavorable prognostic factors for strabismus treatment include esotropia, vertical deviation, and anomalous correspondence. Strabismus surgery was not recommended owing to the size of her angle. Prism was contraindicated because of anomalous correspondence and a positive prism adaptation test. After a discussion on the guarded prognosis, she elected to start vision therapy, weekly 45-minute in-office sessions, to see whether her symptoms would improve.

The main objective of vision therapy was to eliminate anomalous correspondence and establish normal correspondence. The vision therapy activities (Table 6) were categorized into monocular visual skills and vergence training. The initial therapy sessions focused on motor stimulation using Flom’s Swing technique in the synoptophore, along with enhancement of monocular accommodative skills. Once she began displaying covariation, in- and out-of-instrument vergence (convergence, divergence, and vertical vergence at near and distance) training were introduced. After-images were used to monitor correspondence. On computer therapy, large random dot stereopsis targets were initially used, and target size was decreased over time. She would occasionally use vertical prism (4-5∆ base-down right eye) in-office to help with fusion but did not desire a prism prescription for full-time wear.

After 26 weeks of vision therapy, the patient had to discontinue because she was moving out of the area but was given home maintenance therapy. She could appreciate random dot stereopsis of 250" and noted a significant improvement in headaches following therapy and a difference in her binocular depth perception. The patient has not returned for follow-up, so some examination findings could not be reevaluated.

DISCUSSION

This case series offers some support that neuroplasticity continues through adulthood, and optometric vision therapy can facilitate this process. With vision therapy, all three patients gained the ability to appreciate stereopsis. These cases show that the visual system can be trained and improved after the critical period in adult patients with unilateral small-angle esotropia who never appreciated stereopsis.

The cases detail vision therapy in three patients with microtropia. Intractable diplopia may occur with vision therapy, and this was discussed with all three patients, but they elected to try vision therapy to achieve their goals of improved depth perception. Although it has been reported that patients with microtropia may identify random dot stereopsis of 800", which is considered false positive, on the Randot Preschool Stereotest,²⁴ the patients in our case series ultimately achieved true random dot stereopsis following vision therapy (250" for cases 1 and 3 and 400" for case 2).

Similar to our case series, a few studies have documented improvement in binocular function in adults with esotropia following vision therapy. A retrospective review looked at adults with esotropia, as well as those with exotropia and vertical strabismus, and found an improvement in stereopsis after vision therapy.²⁵ Two cases found improvement in binocular function after vision therapy in adults with esotropia,^18,19 but they did not achieve stereopsis. In one case, an adult with esotropia and amblyopia presented with anomalous correspondence and no random dot stereopsis.¹⁸ Divergence training was performed under artificial conditions with a red lens, where the patient had to fuse diplopic images in the dark. He also had to maintain fusion of a target while walking away from it. Accommodative, oculomotor, eccentric fixation, eye-hand coordination, and patching therapies were incorporated. Following vision therapy, the patient’s visual acuity and ability to fuse diplopic images improved, but he was not able to appreciate stereopsis. In another case report, an adult patient with esotropia presented with headaches, eye fatigue, and intermittent diplopia.¹⁹ Vision therapy consisted of antisuppression, accommodative, oculomotor, eye-hand coordination, and vergence training, along with techniques to disrupt anomalous correspondence. Following vision therapy, she demonstrated normal correspondence, perceived stereoscopic depth at her centration point, and subjectively reported less eye fatigue.

The vision therapy programs in the aforementioned cases were similar to our case series, in that they all worked on accommodative and vergence activities. However, vergence training in these case reports did not utilize Flom swing. Flom swing, utilized in cases 2 and 3, is a useful therapy technique because it can eliminate anomalous correspondence in small-angle esotropes through expansion of vergence ranges.^22,26 By inducing fusional vergence, covariation of anomalous correspondence to normal correspondence can be achieved and normal fusion can be established. This may be why the patients in our case series were able to appreciate random dot stereopsis after treatment.

These three patients’ vision therapy programs were sequenced based on treatment guidelines for strabismus²² and amblyopia.²⁷ Monocular accommodative therapy and motor stimulation were performed at the beginning of therapy, followed by vergence therapy in open space. However, there were some differences in our patients’ profile and specific therapy activities, which contribute to the limitations of this case series. The patients in cases 1 and 2 had amblyopia, so other monocular (oculomotor and spatial perception) activities were included to improve visual acuity and monocular fixation. Vertical prism was prescribed for case 1, but vertical vergence therapy was incorporated for case 3. Not all patients presented with anomalous correspondence. Lastly, the patients in cases 2 and 3 were lost to follow-up before completing vision therapy, so some examination findings were unable to be reassessed. Despite differences and limitations, this case series show that stereopsis could be achieved with vision therapy in adults with long-standing constant strabismus.

Studies have also shown improvement in binocularity with other forms of treatment in adults.^5–10,28 The use of botulinum toxin²⁸ to treat strabismus has been reported to develop sensory fusion in adults. Sensory fusion development, with Worth four-dot or stereopsis, is possible in adults with strabismus after strabismus surgery owing to improved motor alignment.^5–10

CONCLUSION

Although treatment of adult strabismus can be challenging, adults with goals related to binocularity may be successfully treated. These cases demonstrate that vision therapy is a viable treatment option for motivated adults with strabismus who want to achieve binocularity. However, this case series is limited to retrospective case studies and highlights the need for prospective, placebo-controlled, randomized clinical trials, to evaluate the effectiveness of vision therapy as a treatment for adult strabismus. Eyecare providers should consider referring motivated adults with strabismus for treatment.

TAKE HOME POINTS

• Optometric vision therapy is a viable treatment option for motivated adults with long-standing strabismus.

• Stereopsis can be achieved with optometric vision therapy in adults with long-standing constant strabismus.

• Eye care providers should consider referring adults with strabismus for evaluation and treatment.