Ptosis, miosis, and anhidrosis. The pathological term for the disease is “oculosympathetic paresis.” The term explains some of the other findings, including ipsilateral conjunctival injection, ipsilateral nasal congestion, upside down ptosis, and harlequin sign. It also explains why the presentation is subtle.
Unlike third nerve palsy, it is not the levator palpebrae superioris that is affected. Instead, it is the sympathetically-innervated Mueller’s muscle, which causes only 1-2mm of lid droop. Do an online images search of Horner’s syndrome, and you will see how subtle this can appear. Probably the best we can do is notice slight asymmetries and investigate further.
The miosis is easily missed too. If we typically examined patients in the dark, the miosis would stand out. But we see them in bright lights, where both eyes show a relative degree of miosis. If you notice anisocoria, turn the lights out. The Horner’s pupil will remain small. Even if it does eventually dilate, a delay to dilation is characteristic.
In summary, Horner’s syndrome is subtle. The ptosis is only 1-2mm, unlike the more dramatic ptosis of third nerve palsy. Miosis is less noticeable in the bright lights of the emergency department.
Take Home Points:
-Due to the sympathetically-innervated Mueller’s muscle, the ptosis of Horner’s syndrome is 1-2mm
-Miosis of Horner’s syndrome is best appreciated in the dark