Background: Giant cell arteritis (GCA) is a systemic inflammatory vasculitis that affects medium- and large-sized arteries and can result in permanent vision loss. In rare instances, Horner syndrome has been noticed at the time of GCA diagnosis, although the mechanism of both diagnoses occurring at the same time is not entirely understood. We reviewed 53 charts of all patients diagnosed with biopsy-proven GCA in tertiary neuro-ophthalmology practice to find patients who presented with new onset of Horner syndrome at the time of GCA diagnosis. Methods: Two patients with biopsy-confirmed GCA who presented with concurrent Horner syndrome were found. Data on age, sex, and ophthalmic and neuroradiologic examination findings were collected. Results: Patient 1 was a 67-year-old man who presented with new onset of vertical binocular diplopia and was diagnosed with right fourth cranial nerve palsy. He then developed left ptosis and miosis, and was diagnosed with Horner syndrome by pharmacologic testing. He also had persistently elevated inflammatory markers. Patient 2 was a 71-year-old man who presented with new onset of binocular vertical diplopia, bitemporal headaches, and jaw ache. Both of his inflammatory markers were elevated. On examination, he had left ptosis and myosis, and small comitant left hypertropia. The diagnosis of left Horner syndrome was confirmed on pharmacologic testing and left hypertropia was attributed to skew deviation. Both patients underwent temporal artery biopsy, which confirmed the diagnosis of GCA. Treatment with high dose of oral corticosteroids commenced, and vertical diplopia has completely resolved in both patients. Horner syndrome persisted in Patient 1 and resolved in Patient 2. MRI and MR angiography of the brain and neck were unrevealing in both patients. Conclusions: This case series describes 2 patients with new diagnosis of GCA and concurrent Horner syndrome, with new diagnosis of likely nuclear/fascicular fourth nerve palsy in one patient and skew deviation in the other. In both patients, vasculitis presumptively affected vertebral arteries and their branches supplying the first-order sympathetic neurons in the brainstem. Considering the severe complication of permanent vision loss in GCA, this diagnosis should be considered in older patients presenting with concurrent new onset of Horner syndrome
