I have read that patients suffering from astigmatism sometimes experience monocular diplopia but I don't understand why this is.
Do we have an idea of why this occurs?
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Diplopia is not one of the more common symptoms of astigmatism, however it is certainly one of the possible manifestations. In general diplopia (double vision) is categorized as either binocular (you only have double vision when both eyes are open) or monocular (you have double vision even if only the affected eye is open). Binocular diplopia is typically caused by misalignment of the eyes (most commonly a problem with the muscles that control the eyes), while monocular diplopia is typically caused by a problem within an eye itself.
Astigmatism most commonly presents as blurry vision due to irregularities in the cornea. However if the irregularity causes the lens to refract the same image to two different areas of the retina, one would see two copies of that object and thus would have monocular diplopia.