Double vision (diplopia) can be monocular or binocular. Monocular diplopia, that is, diplopia that is still present when one eye is closed, is quite common and is usually due to a cataract or corneal disease. Some patients say that they can see double when they mean that the vision is blurred. A clear distinction must, therefore, be made. Binocular double vision of recent onset should always be treated as a serious symptom. It is usually disabling, preventing the patient from working or even walking about. Some patients discover that the symptoms are relieved by placing a patch over one eye. Slight degrees of double vision can be compensated by a head tilt or turn and the nature of the adopted head posture can help to identify the cause of the double vision. In the same way, if the history is elucidated carefully, noting, for example, whether the diplopia is worse for near or distance vision or whether there is horizontal or vertical displacement of the second image, then a possible cause can be suspected even before examining the patient.
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