What condition can cause diplopia that persists even with one eye covered?

Diplopia, or double vision, that persists even when one eye is covered typically indicates a problem that is not due to muscle weakness or nerve palsy, such as those associated with cranial nerves III or IV.

When diplopia occurs due to weakness of cranial nerves, it is usually positional; that is, if one eye is covered, the visual system can compensate for the weakness, and double vision often resolves. However, if the cause is related to an irregularity in the cornea or lens, such as astigmatism or cataracts, the visual distortion affecting both eyes can persist regardless of whether one eye is covered. This means that the underlying optical issue affects the quality of vision in both eyes simultaneously, leading to continued diplopia.

Understanding this context helps in recognizing that structural issues in the eye, rather than nerve or muscle function, lead to persistent diplopia regardless of eye position or occlusion.