Phoria and Tropia: When the Eyes Drift but Nothing Looks Double
Two eyes, one direction
Both eyes have to point at the same thing for binocular single vision to work. That alignment is not a fixed mechanical property; it is actively maintained by a control system that compares the two images and drives the eyes until the mismatch disappears. When the system is intact, the eyes stay aligned. When the demand exceeds what the system can hold, the eyes drift, and what happens next depends entirely on whether the brain is still receiving two images or has learned to ignore one of them.
The distinction that matters: latent versus manifest
A heterophoria is a misalignment that appears only when the two eyes are prevented from working together. Cover one eye and the covered eye drifts to its resting posture; uncover it and fusion pulls it back. Nothing is visible to the person, nothing looks double, and the deviation is described as latent. Its direction is named: esophoria when the eye tends to turn inward, exophoria when it tends to turn outward, hyperphoria when it tends to sit high.
A heterotropia is the same deviation present while both eyes are open. The eye is visibly turned; the person is described as having a manifest squint. Here the interesting part is why many people with a tropia do not complain of double vision at all. In a developed tropia the brain suppresses the image from the deviating eye, or develops an abnormal retinal correspondence, and the second image simply never reaches awareness. Suppression is an adaptation, and a costly one: it removes the confusion but also removes the binocular cues that support stereopsis.
How the distinction is actually measured
The cover test is the whole method in miniature. With the patient fixing a target, one eye is covered and the uncovered eye is watched for a movement to take up fixation: if it moves inward, the eye had been drifting outward. Then the cover is moved across so that the previously covered eye is exposed and the other is covered, which reveals the posture of the first eye. If no movement occurs with either eye covered and both open, there is no manifest deviation. To expose a latent one, the eyes must be dissociated: the cover is held in place long enough for the covered eye to drift, then removed, and the recovery movement is observed.
Because a phoria is a balance rather than a lesion, the useful question is not "is there a deviation" but "how much reserve is left". That is measured as fusional reserves: prisms are introduced until the patient reports blur, then break, then recovery. The blur point is the limit of the accommodation-linked response; the break point is the limit of fusion; the recovery point is how quickly the system re-establishes itself. A near exophoria with generous reserves is unremarkable. A small exophoria with reserves that break almost immediately is a different clinical situation entirely.
What counts as normal
Widely used clinical norms, often attributed to Morgan, put distance lateral phoria at about one prism dioptre of exophoria with a spread of roughly two dioptres either side, and near lateral phoria at about three dioptres of exophoria with a spread of about three. In other words, a small outward tendency at near is not a disease; it is the commonest finding in comfortable patients. The related ratio of convergence to accommodation, the AC/A ratio, is generally quoted at around four to six prism dioptres per dioptre.
Why a phoria can stop being silent
Compensation is expensive. It costs sustained muscular effort, and it depends on reserves that vary with fatigue, illness, sleep and concentration. A person can carry a phoria comfortably for years and then lose the margin: the deviation is unchanged, but the reserve is no longer enough. That is why the same individual reports symptoms during a deadline week and not on holiday, and why a prescription change that slightly alters the demand can push a borderline system over the edge.
Misconceptions
- "A phoria is a squint." A phoria is a latent tendency that fusion controls; a squint is a manifest deviation. The distinction changes both the prognosis and the plan.
- "If I do not see double, the alignment must be fine." Absence of diplopia is compatible with a tropia, because suppression removes the second image. It is also compatible with a decompensating phoria, because the symptom is effort rather than doubling.
- "Prism cures the deviation." Prism relieves the demand by moving the image; it does not change the posture. Whether it is the right long-term answer depends on the cause and on the fusional reserves, which is why it is prescribed after measurement rather than guessed.
- "Children grow out of a turned eye." Some deviations reduce as the face grows, but a manifest deviation in a child is a reason for prompt assessment, because the associated suppression and amblyopia can become permanent.
When to get it looked at
Seek assessment for a deviation that has become constant, for double vision that is new, for a turned eye noticed by someone else, for symptoms that appear only at near or only when tired, and for any child with a suspected squint or a persistent head turn. Sudden diplopia, especially with headache, weakness or a difference in pupil size, is not an optical problem and needs urgent medical assessment rather than a new pair of glasses.