It is important to differentiate binocular double vision from monocular vision. Binocular double vision disappears when one eye is covered and is usually caused by an imbalance of the extraocular muscles. There may be associated systemic diseases such as hypertension, diabetes mellitus or intracranial lesions.

Presentation:

• Double vision 
• Some may present with blurred vision or headache

Examination:

• Determine if the double vision is binocular by getting the patient to cover one eye and observe if the double vision resolves
• Determine if the double vision is vertical or horizontal, an imbalance of horizontal muscles causes horizontal double vision whereas an imbalance of vertical muscles causes vertical double vision.
• Examine the ocular movement for any underaction of the extraocular muscles.
• Look for associated signs especially the presence of ptosis and dilated pupil (third nerve palsy)

Management:

• If the double vision was binocular refers the patient within 24 hours. The patient will be evaluated by the orthoptic department and may be prescribed prism to fuse the images.
• If the double vision was monocular, advise the patient to consult an optician as the problem may be refractive. If the double vision can not be abolished with glasses refers the patient  to the clinic.

Figure 1 A patient with a right abducent nerve palsy. Note the failure of the right eye to  abduct on right gaze. In the elderly patient it is important to exclude hypertension  or diabetes mellitus as vascular causes of the palsy. In younger patient, trauma or  intracranial malignancy should be considered.

 
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