© 2010 Canadian Medical Association or its licensors

A61-year-old right-handed man presented with a two-day history of a new headache that had developedgradually several hours after he dived into a lake. The headache, which encompassed the entire left side of the head, had a maximal intensity of 10 out of 10 and was described as dull and constant with mild photophobia. The patient also reported blurry vision in the left eye. In addition, his speech was slurred, and he had difficulty swallowing. There were no carotid or vertebral artery bruits evident on physical examination. Cranial nerve examination demon-strated 2.5-mm ptosis of the left eye; in addition, the diameter of the left pupil was 1 mm smaller than that of the right in dim light (Figure 1). Both pupils were reactive, and there was no relative afferent pupillary defect. The tongue deviated to the left when protruded; there were no fasciculations (Figure 1). The results of the remainder of the cranial nerve examination were normal, including normal gag response, symmetric ele-vation of the palate and full power in the sternocleidomastoid and trapezius muscles. The results of motor, sensory, cerebel-lar and gait testing were unremarkable. Ophthalmoscopy and visual-field testing during a neuro-ophthalmologic consulta-tion did not reveal any additional abnormalities. At this point, the main diagnostic considerations included a cervical mass lesion within the carotid sheath, such as dissec-tion of the carotid artery, which could cause a mass effect on the adjacent ascending sympathetic plexus or on cranial nerve XII at the base of the skull, beyond the point of emergence from the hypoglossal foramen. Such a mass would cause ipsi-lateral Horner syndrome (see Box 1) and deviation of the tongue. Alternatively, a left-sided lesion of the brainstem Cases Dissection of the internal carotid artery causing Horner syndrome and palsy of cranial nerve XI