Double vision (diplopia): why it happens, and why children rarely report it

The brain normally merges the images from both eyes into a single scene. When that merging is impossible, the result is double vision: a single object perceived as two separate images.

This article covers the types of double vision, why children usually do not show it, and when it warrants seeing a doctor without delay.

Binocular diplopia is present with both eyes open and disappears when one eye is covered. It comes from the eyes being unable to point at the same spot, and it points to alignment problems.

Monocular diplopia persists even with the other eye covered; it is described as a ghosted or doubled image in one eye. The cause usually lies in that eye's own optics, such as astigmatism or changes involving the lens.

A simple test separates them: cover one eye. If the doubling disappears it is binocular; if it persists it is monocular. That information is valuable to your doctor too.

A child's brain does not tolerate the discomfort for long; it suppresses one of the conflicting images. As a result the child does not see double and voices no complaint.

On the surface the problem has vanished, but in reality two-eyed vision has been switched off. The suppressed eye's development falls behind, and amblyopia settles in by this route.

So the absence of a double-vision complaint in a child does not mean alignment is fine; it means the brain has solved the problem quietly.

The adult brain does not have the same flexibility and cannot readily deploy suppression. So when alignment problems develop later in life, double vision is experienced as an obvious and troubling complaint.

That difference explains why the same underlying problem looks so different in the two age groups.

Double vision of sudden onset needs prompt assessment at any age, and especially so when it comes with headache, a drooping eyelid, restricted eye movement or balance problems.

In children, behavior matters as much as any complaint: frequent squinting, covering one eye, or always tilting the head to the same side can all be the brain's way of coping with double vision.