A 60-year-old man is found to have unequal pupils, more obvious in dim illumination. Why does this patient need a chest X-ray?

When there is a difference in pupil size, I always find it difficult to work out if the eye with the larger pupil is the affected eye or if the eye with the smaller pupil is the affected eye. A good way to work this out is to estimate the pupil diameter for each eye under normal room lighting conditions and then to see what happens when the lights in the room are dimmed. When light levels are reduced pupil diameter should increase. In this case, the difference in pupil size is more obvious in dim light. This means that the pupil diameter of the larger pupil has increased but the pupil diameter of the other eye has not increased by the same amount or at all. This means that the muscle responsible for widening the pupil is not working and this is a sign of Horner’s syndrome.

Horner syndrome usually affects only one side of the face. Common signs and symptoms include:

• A persistently small pupil (miosis).
• A notable difference in pupil size between the two eyes (anisocoria).
• Little or delayed opening (dilation) of the affected pupil in dim light.
• Drooping of the upper eyelid (ptosis).
• Slight elevation of the lower lid sometimes called upside-down ptosis.
• Sunken appearance to the eye.
• Little or no sweating (anhydrosis) either on the entire side of the face or an isolated patch of skin on the affected side.

The most obvious sign is the difference in pupil size. The other signs can be very difficult to detect. One cause of Horner’s syndrome is a tumour (Pancoast) on the top (apex) of the lung. The nerves controlling pupil dilation surprisingly pass near the apex of the lung and a tumour here can interfere with the way the nerve works leading to a small (miosed pupil).

Unless another cause is obvious, a person presenting with Horner’s syndrome deserves a chest X-ray to rule in or rule out Pancoast tumour.