Focusing and aiming, working as a team
Looking at something close takes two linked actions. When the link is out of balance, or the reserve that holds the eyes straight runs low, close work gets hard.
Two controls, one link
Part 02 · 01When we look at something close, two things happen together: the eyes focus (accommodation) and they turn inward (convergence). The two are linked, so focusing automatically pulls the eyes inward a certain amount.
Comfortable reading
Reading feels comfortable and the eyes stay aligned.
Strain, blur or doubling
The eyes struggle to stay aligned during close work, causing strain, blur or double vision.
Convergence insufficiency
Part 02 · 02The most common binocular vision problem we see. The eyes can’t turn in comfortably, or can’t hold that position for long at reading distance. After a few minutes words may drift, double or blur, and reading becomes tiring.
It’s common in children and in adults who do a lot of close work, and it usually responds well to vision therapy.
How it’s treatedWhen the eyes drift out
Part 02 · 03When the eyes turn in
Part 02 · 04Focusing harder pulls the eyes inward. In some children that pull is enough to turn an eye in. There are two common types, and they’re treated differently.
Uncorrected long-sightedness
The child has to focus hard to see clearly, and that extra focusing turns the eyes in. Glasses with the full prescription usually straighten them.
Focusing pulls in too much
Even with the right glasses, focusing up close drags the eyes in more than normal, so the turn is bigger when reading. Bifocal lenses take over some of the near focusing.
AC/A and how phorias decompensate
For optometristsAccommodative convergence
Convergence produced per dioptre of accommodation. Fairly linear through the normal range, so a useful predictor.
The reserve
Fine-tunes alignment and holds a phoria straight. This is the reserve that runs down.
Decompensating exophoria
Sustained near work, fatigue or illness run down positive fusional vergence; the exophoria decompensates and may break into intermittent exotropia.
Refractive accommodative ET
Uncorrected hyperopia with a normal AC/A. Deviation similar at distance and near. Full cycloplegic correction.
High AC/A accommodative ET
Eso larger at near than distance despite full correction. Near addition (bifocal or executive). Mixed forms are common.
The common near-work culprit
Exo greater at near, remote near point of convergence, reduced positive fusional vergence at near. Office-based vergence and accommodative therapy with home reinforcement has the strongest evidence.
Convergence driving accommodation (the CA/C ratio) is a separate concept, left out here for simplicity.
Key takeaways
Part 02- 01Focusing and turning in are linked; the link can tip the eyes out of alignment.
- 02Convergence insufficiency is the most common cause of near-work strain, and it responds well to therapy.
- 03An inward turn in a child is often driven by long-sightedness: glasses come first.
Book a binocular vision assessment
Reading that tires quickly, or words that drift or double? We measure both controls and how they work together.
General health information only, not personal advice. Everyone’s eyes are different. An optometrist or doctor should examine you before any treatment is decided.