To the Editor I read with interest the article by Fang et al1 describing the incidence and etiologies of acquired third nerve palsy among people living in Olmsted County, Minnesota. The authors correctly emphasize that although compressive lesions have a higher likelihood of pupil involvement, the lack of pupil involvement does not rule out a compressive process, including that from an intracranial aneurysm. The authors also emphasize (as do Newman and Biousse2) that, although the incidence of aneurysm-related third nerve palsy is low (6%), rapidly diagnosing this etiology is crucial to prevent permanent neurological dysfunction or death from a subsequent aneurysm rupture. The question is when one should perform an emergent imaging in a patient with an acute third nerve palsy. I have always taught that emergent imaging is required in every patient with an acute third nerve palsy except for patients with complete external third nerve dysfunction but isochoric and normally reactive pupils, in which case a causative aneurysm is extremely rare. Indeed, only 1 case caused by an aneurysm has been reported in the literature.3 The table that describes the characteristics of the 145 cases of third nerve palsy in the study by Fang et al1 indicates that among the 9 patients with an aneurysmal third nerve palsy, 6 had no pupillary involvement and 2 had complete external third nerve dysfunction. I would like to know if either of the 2 patients with complete external third nerve dysfunction had pupillary sparing. If so, this would suggest that it is best to perform an emergent imaging in every patient with an acute third nerve palsy of unknown origin regardless of the pupillary findings; if not, the practice of not performing an emergent imaging in a patient with a complete but pupil-sparing third nerve palsy would seem reasonable.
Miller NR. When Should Emergent Imaging Be Performed?. JAMA Ophthalmol. Published online May 25, 2017. doi:10.1001/jamaophthalmol.2017.1427