Ask which eye, and ask what time

Sudden painless monocular vision loss in someone over 50 is a history that resolves itself if two questions are asked properly. The first is which eye, and it is not as obvious as it sounds: patients routinely present describing loss of vision on one side when what they have is a hemianopia, and covering each eye in turn during the history separates a neuro-ophthalmic field defect from a monocular problem before any examination begins.

The second is what time. Not what day - what time. Anterior ischaemic optic neuropathy characteristically declares itself on waking, and a patient who noticed the loss on getting out of bed is describing something different from one whose vision faded over an afternoon. The onset pattern, the presence of transient obscurations beforehand, and whether the second eye has ever been involved are all free and all recorded before anyone reaches for an investigation.

And in anyone over 50 with this presentation, ask the giant cell arteritis questions in the same breath: jaw claudication on chewing, scalp tenderness when brushing hair, new headache, shoulder girdle stiffness, weight loss. A negative answer to all five is worth as much in the notes as a positive answer to one.

• Cover each eye during the history - 'lost vision on the left' is often a hemianopia
• Ask the time of day of onset; waking onset is characteristic of anterior ischaemic optic neuropathy
• Ask about jaw claudication, scalp tenderness, new headache, girdle stiffness and weight loss in everyone over 50
• Ask about preceding transient visual obscurations, which patients rarely volunteer
• Record the negatives explicitly; they are what justify the decision not to treat

Why it matters

The distinction that determines whether this is an emergency is usually available before any test is ordered.

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