Diplopia—seeing double—is a symptom with many potential causes, both neurological and ophthalmological. Accurate diagnosis and appropriate plans of management can be achieved with careful history taking and clinical examination. In this article we review the practical points for clinicians dealing with diplopia. The general approach is demonstrated in fig 1.

A diagnosis of functional diplopia should not be entertained based simply on the absence of gross ocular misalignment, because sometimes very subtle misalignment of the ocular axes, which are difficult to elucidate at the bedside, may require more sensitive tests. On the other hand, one should not be surprised to see gross ocular misalignment without diplopia as brain plasticity usually takes over if diplopia is longstanding and the image from one eye is suppressed.

HISTORY TAKING

Careful history taking has an important role in the management of diplopia and details of the history can be used to focus examination on areas of relevance. The following points in the history should be elucidated.

Is the diplopia monocular or binocular?

Whether the diplopia is monocular or binocular should be determined first. Should the symptom of diplopia persist with one eye occluded, the patient has monocular diplopia, the causes of which are usually ophthalmological with refractive error being the most common (table 1). Binocular diplopia resolves when one eye is occluded and it is caused by “misalignment of the visual axes”, and can be due to many different aetiologies.

It is very important to distinguish monocular from binocular diplopia because their possible causes differ significantly. Monocular diplopia is usually caused by intraocular pathology, therefore detailed ophthalmological assessment is required. The characteristics of the images may help to localise the problem. For example, a combination of glare and impaired visual acuity can be seen in monocular diplopia caused by cataract. Patients with macular disease …