only minimal dilation but in some clinical patients with a postganglionic lesion, the Horner’s pupil was found to respond sooner and more vigorously than the unaffected pupil.64 With a preganglionic lesion, the Horner’s pupil would be expected to only dilate minimally although validation with published findings has yet to occur.

In evaluating a patient with anisocoria, if the difference between the pupils is greater in dim light, the smaller pupil is the defective one and the clinician must differentiate between benign anisocoria and Horner’s pupil. With benign anisocoria, the pupil will react well to all stimuli and will redilate well in the dark. The Horner’s pupil reacts well to all stimuli but redilates poorly in the dark.

If the anisocoria is more evident in bright light, it occurs because the pupil constricts poorly and the larger pupil is the pathologic one. This may be caused by drugs, it may be a tonic pupil, or it may be caused by an efferent defect. Associated symptoms will assist in the diagnosis, and such a pupil abnormality, if accompanied by headache, requires a workup for intracranial involvement.

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