Internal carotid artery (ICA) dissection is a rare and potentially devastating cause of cerebral ischaemia, initiated by an intimal tear or rupture of the vasa vasorum, that can lead to an intraluminal thrombus, vascular stenosis, occlusion, or dissecting aneurysm formation. Management is challenging due to its complex pathophysiology and non‐specific nature of symptoms. In this case–control study, we were able to document the clinical presentation and management of an ICA dissection in a hypertensive, 50‐year‐old male triathlete following an acute bout of intermittent apnoeic (pyramid breathing) swimming. He developed blurred vision in his left eye, ipsilateral headache, pulsatile tinnitus and later noticed left‐sided ptosis and pupil miosis consistent with Horner's syndrome, prompting specialist referral. Neuroimaging confirmed a dissection of the left ICA and incidental pseudoaneurysm of the distal right ICA. The patient recovered well due to a combination of pharmacological/dietary management of hypertension and graduated, structured return to physical activity and competition, culminating in significant re‐expansion of the ICA true lumen calibre. We also conducted a laboratory‐based, dry‐land, static swimming simulation in an age‐ and physical activity‐matched healthy male control. This demonstrated that exercise‐induced ICA shear stress was more exaggerated during dynamic apnoeic breathing compared to normal breathing, which, in the setting of the patient's hypertension, may have been a precipitating factor underlying ICA dissection. Collectively, these findings provide unique insights into the pathophysiology and management of this rare condition while highlighting the inherent risks associated with this mode of exercise training in susceptible individuals with hypertension
