Convergence ability is the ability to turn both eyes inward to align the visual axes on a near object. It is an essential eye movement function for daily near tasks such as reading and smartphone use.
The following two tests are used to evaluate convergence ability.
Convergence testing should be performed as part of routine examination. Insufficient convergence is called convergence insufficiency (CI), and its prevalence in 5th to 6th graders is reported to be 3.5–5% 1). In adults, CI accounts for 15.7% of new-onset strabismus, making it a common condition 1).
There are four types of convergence:
Convergence and divergence movements begin to develop around 3 months of age, and responses to stepwise disparity changes are observed at 4 to 5 months of age.
It is found in 3.5 to 5% of children in grades 5 to 6. In adults, it accounts for about 15.7% of new-onset strabismus, making it a relatively common condition1).
When convergence is insufficient, the following symptoms may appear during near work such as reading.
When engaged in prolonged VDT (visual display terminal) work, symptoms of convergence insufficiency are often observed as technostress eye syndrome. On the other hand, some individuals with convergence insufficiency may be asymptomatic.
Convergence disorders include three main conditions: convergence insufficiency, convergence paralysis, and convergence spasm.
Convergence Insufficiency
Near phoria: Exophoria or intermittent exotropia (greater than at distance)
Fusional convergence: Positive fusional convergence <20PD (fails Sheard’s criterion)1)
Near point of convergence: NPC prolonged (>6 cm)1)
Eye movements: Adduction is normal
Convergence paralysis
Convergence spasm
Near deviation: High-degree esotropia
Pupil: Accompanied by miosis
Accommodation: Complicated by accommodative spasm
Eye movement: Both restriction and miosis disappear with monocular duction
In convergence insufficiency, symptoms can be quantitatively assessed using the Convergence Insufficiency Symptom Survey (CISS) or the Diplopia Questionnaire1).
Some patients with convergence insufficiency are asymptomatic. Therefore, objective measurement of NPC is important in examination, and convergence disorder cannot be ruled out based on symptoms alone.
In many adults, a clear preceding cause cannot be identified1). The following risk factors have been reported.
Convergence paralysis results from lesions at the midbrain dorsal level. The main causes are as follows:
A condition in which both eyes adduct paroxysmally and persist.
This is a standard test of convergence ability and is easy to perform without special equipment.
Use a fixation target (pen, pencil, or a small toy with an interesting character for children) and a ruler. Dedicated convergence and accommodation testing rulers are also available. Perform under appropriate refractive correction if refractive error is present.
Record both NPC and CRP (e.g., NPC 7 cm, CRP 12 cm).
The main reference values are shown below.
| Item | Normal value | Abnormal value |
|---|---|---|
| NPC | 5–10 cm | >10 cm (reduced convergence) |
| CRP | Approximately 15 cm | — |
| NPC in infants | Possible up to the nasal bridge (to nose) | — |
| Convergence excess | — | <5cm |
In patients suspected of convergence insufficiency, repeated testing shows a characteristic finding of gradual NPC prolongation.
This test more closely simulates typical near work conditions.
Use a dot card or Brock string. The card should be at least 20 cm long, with black dots spaced 1–2 cm apart along the midline. If there is a high refractive error, perform the test with correction.
Record as “Convergence 8 cm (using Brock string).” When convergence ability is good, similar results are obtained with NPC testing and jump convergence testing.
Measured using a major amblyoscope, rotary prism, or Bagolini striated lens. Normal range is -5 to +15 degrees. Record whether the measurement was taken at the blur point or the break point (diplopia).
The ratio of accommodative convergence (AC) to accommodation (A), quantifying the convergence response per unit of accommodative stimulus. Normal value is 4±2 (PD/D). It is important to perform the test under full refractive correction.
There are two measurement methods:
The following points are important in the differential diagnosis of convergence disorders.
In patients with convergence insufficiency, repeated testing shows a gradual increase (lengthening) of the near point of convergence. A single test may miss the condition, so multiple tests are essential to confirm reproducibility and evaluate fatigue effects.
Treatment for convergence disorders varies depending on the underlying condition.
Convergence exercises and prism glasses are considered standard treatments.
Convergence Exercises
Method: Perform convergence training for a short time each day
Mechanism: Improves motor (fusional) convergence
Prognosis: Relatively good
Prism glasses
Indications: When symptoms do not improve with training
Prescription: 2–4Δ base-in for each eye (total correction of 4–8Δ)
Method: Perform a wearing test with glasses corrected for near vision to determine the optimal prescription.
Surgical Treatment
Indications: When there is a manifest distance deviation and prism glasses are insufficient 1)
Procedure: Lateral rectus recession ± medial rectus resection 1)
Note: Explain the risk of postoperative distance diplopia 1)
In children, office-based convergence training is more effective than home-based training, with the advantage of being able to manage compliance and participation (evidence level I+, Good, Strong) 1). In young adults (19–30 years), office-based training was superior to home-based training in improving positive fusional convergence, but there was no difference in NPC and symptom improvement 1). In general, training outcomes in adults are less consistent than in children 1).
As for the natural course of convergence insufficiency, spontaneous improvement is generally not expected 1). However, convergence insufficiency after concussion may improve over time 1).
Practice bringing a fixation target such as a pen tip close to the nose and converging until double vision occurs, repeating for a short time each day. In children, office-based training performed at an eye clinic is considered more effective than home training 1). If training does not improve symptoms, prism glasses may be considered.
Convergence consists of the following four components:
| Type | Mechanism | Characteristics |
|---|---|---|
| Tonic convergence | Adjustment from anatomical resting position to physiological resting position | Strong in early childhood |
| Accommodative convergence | Occurs with accommodative effort | Quantified by AC/A ratio |
| Fusional convergence | Voluntary movement for binocular alignment | Primary in daily near vision |
| Proximal convergence | Psychogenic response to perceived proximity | Sensory component |
The convergence center is located at the dorsal midbrain level. Lesions in this area (Parinaud syndrome, aqueductal syndrome) cause convergence paralysis. Convergence-retraction nystagmus is induced by upward gaze and is a specific finding of pretectal area damage.
The essence of convergence insufficiency is incomplete fusional convergence. The relationship between convergence and accommodation is not strictly proportional but operates within a certain range. This interaction can break down due to prolonged near work in inappropriate environments, leading to persistent decline in accommodative and convergence functions.
In convergence insufficiency with accommodative dysfunction, both accommodative convergence and fusional convergence are reduced, and sustained near work leads to exophoria at near. Technostress eye syndrome from VDT work is a representative condition.
