Convergence insufficiency (CI) is a syndrome in which the ability to turn both eyes inward (converge) when focusing on a near target is reduced, making it difficult to maintain binocular fusion. It is characterized by a remote near point of convergence (NPC), decreased convergence amplitude, and exophoria at near (usually greater than 10 prism diopters [∆]).
It occurs in nearly all age groups but is most common in young adults. Prevalence varies widely between studies, from 1.7% to 33%, and the incidence in the general population is estimated at 0.1–0.2%. CI is found in 11–19% of children with exophoria. There is no gender difference. CI accounts for approximately 15.7% of adult-onset strabismus.
In general, CI does not improve spontaneously. However, the severity of symptoms varies with the amount of near work. Post-concussion CI may improve over time.
It is most common in young adults but occurs across a wide age range from children to the elderly. Among children with exophoria, 11–19% have CI, and CI accounts for about 15.7% of adult-onset strabismus.
Subjective symptoms of CI worsen with near work. They become noticeable with prolonged use of reading, computers, or smartphones.
The CISS (Convergence Insufficiency Symptom Survey) developed by the CITT group consists of 15 questions answered on a Likert scale, with scores ranging from 0 (best) to 60 (worst) to quantify symptom severity. A score of 16 or higher is considered significant. Its reliability has been validated in children aged 9 to 18 and in adults.
The Convergence Insufficiency Symptom Survey (CISS) is a 15-item Likert scale questionnaire that quantifies the severity of CI symptoms on a scale of 0 to 60. A score of 16 or higher is considered the threshold for suspecting CI, and it is also used to evaluate treatment effectiveness.
Primary CI
Congenital imbalance of convergence and divergence: Due to differences in innervation, the ability to converge for near vision is limited.
Insufficient fusional convergence: CI mainly occurs because fusional convergence (the convergence that occurs to align the retinal images of both eyes) is incomplete.
Acquired CI
Fatigue and excessive near work: It tends to occur in people who engage in prolonged VDT work (technostress eye strain) or near work.
Medications and systemic diseases: Anticholinergic drugs, uveitis, post-concussion, and CNS diseases such as Parkinson’s disease can be causes.
Trauma and others: Head trauma, glasses that induce a base-out prism effect, and encephalitis can also be causes.
Convergence consists of four components (Maddox classification).
In CI, fusional convergence is mainly insufficient, but it may also be accompanied by decreased accommodative convergence (CI with accommodative insufficiency). Prolonged near work in an inappropriate environment leads to persistent decline in accommodative and convergence functions.
Although a direct causal relationship has not been established, prolonged VDT work can reduce convergence and accommodation functions, worsening symptoms of CI. It is recognized as technostress eye syndrome and tends to occur in near workers.
Diagnosis of CI is based on subjective symptoms and the following clinical examination findings. Comprehensive sensorimotor examination, refractive status assessment, and dilated fundus examination are recommended.
The main examination methods are shown below.
| Examination | Method Overview | Abnormal Value Criteria |
|---|---|---|
| Near Point of Convergence (NPC) | Move fixation target from 40-50 cm toward nose | ≥6 cm (pre-presbyopic) / ≥10 cm (presbyopic) |
| Positive Fusional Vergence (PFV) | Measured with base-out prism | Near <15-20∆ |
| AC/A ratio | Heterophoria method or gradient method | <2:1 |
Move a target such as a finger or toy from a position 40–50 cm in front of the face, slightly below eye level, slowly toward the bridge of the nose. Measure the distance from the bridge of the nose to the point where the target begins to appear double or one eye deviates outward (break point). Normal value is approximately 6–8 cm. Since the test requires concentration, it is advisable to perform it multiple times with encouraging verbal prompts rather than judging after a single attempt.
It is important to perform this test under full refractive correction. In the heterophoria method, the ratio is calculated from the difference in strabismus angle between distance (5 m) and near (33 cm). Normal value is approximately 4±2, and it is low in convergence insufficiency.
Using a major amblyoscope, rotary prism, Bagolini striated lens, etc., measure the range of convergence over which binocular single vision can be maintained while keeping accommodation constant. Normal range is −5 degrees to +15 degrees.
Treatment of CI is performed stepwise according to severity. It is based on refractive correction and combines convergence training, prism glasses, and surgery.
In symptomatic CI, treatment begins with correction of refractive errors. This includes slight undercorrection of hyperopia and full correction of myopia. Good lighting and breaks during near work are recommended.
Convergence training improves fusional convergence. It is important to perform it daily, even for short periods.
Home Training
Pencil push-ups: Focus on a small target and slowly bring it toward the nose while maintaining single binocular vision.
Convergence cards: Hold the card at the bridge of the nose and shift gaze from the farthest point gradually to nearer targets.
Stereograms: Cross-view two horizontally separated images to produce a third fused image in the center.
In-Office Training
In-office vision therapy: Intentionally and controllably manipulate target blur, disparity, and proximity to eliminate suppression and normalize convergence and accommodation.
Computer vergence training (CVS): A program using random-dot stereograms that gradually increases the required convergence amount. Progress can be monitored.
A Cochrane systematic review by Scheiman et al. (2020) of 12 RCTs (1289 participants) showed high-certainty evidence that in-office therapy with home reinforcement in children leads to better convergence ability compared to pencil push-ups alone or computer therapy. Children treated with base-in prism reading glasses did not show significant improvement. In adults, base-in prism glasses improved symptoms but did not improve convergence ability1).
Prescribed when convergence training does not improve symptoms. Use the minimum prism power needed for comfortable single binocular vision at near. Test wearing 2 to 4∆ base-in in each eye (total 4 to 8∆ correction) incorporated into near refractive glasses to determine the optimal prism power. Wear constantly during near work.
Surgery is indicated for refractory CI or CI with intermittent exotropia.
Indications for surgery:
Main surgical procedures include bilateral lateral rectus recession, bilateral medial rectus resection, and unilateral lateral rectus recession with medial rectus resection. Botulinum toxin injection is also an option for refractory cases.
Reported success rates for convergence training are 70–80%, and most patients remain asymptomatic one year after discontinuing treatment. However, maintenance of effect varies individually, so regular follow-up is recommended.
The exact disease mechanism of CI is not fully understood, but neural centers controlling convergence movements have been identified.
This refers to a condition in which both accommodative convergence and fusional convergence are insufficient due to underlying accommodative dysfunction, preventing adequate convergence movements. The relationship between convergence and accommodation normally has some flexibility, but prolonged near work in an inappropriate environment can disrupt this relationship and lead to persistent functional decline. Technostress eye strain from VDT work is a typical example.
CI can occur in association with several neurological disorders.
The Convergence Insufficiency Treatment Trial – Attention and Reading Trial (CITT-ART) was a randomized multicenter clinical trial that examined whether treatment of symptomatic CI improves reading performance in children aged 9 to 14 years.
Participants were randomly assigned to office-based vergence/accommodative therapy or office-based placebo therapy. After 16 weeks, CISS scores were not significantly different between the two groups, indicating that office-based vergence/accommodative therapy was not more effective than placebo therapy in improving reading comprehension in symptomatic children with CI.
This result suggests that even if CI treatment improves convergence ability and symptoms, it may not directly lead to improved reading comprehension.
Several ophthalmologists have pointed out methodological limitations in the 2005 CITT. The office-based treatment group was prescribed significantly longer treatment time than the other groups (inequality of treatment dose). Additionally, “pencil push-ups” have been criticized for not accurately representing conventional vision therapy that includes diverse exercises using accommodative targets.
Spontaneous remission of symptoms has been reported in CI patients. Therefore, it is considered important to include a placebo group when evaluating treatment efficacy.
