The causes of internal carotid artery (ICA) and vertebral artery (VA) dissection, as well as its provoking factors, remain understudied. The aim of this paper was to explore morphological changes in the ICA/VA walls, factors provoking dissection, clinical signs and biomarkers of connective tissue (CT) damage. A total of 271 patients were examined, of whom 54% were women. The mean age of the participants was 37.0 ± 10 years. Clinical signs and biomarkers of CT damage (matrix metalloproteinase 9, tissue inhibitor of metalloproteinase 1, hydroxyproline, sulphated glycosaminoglycans) were analyzed in 82 patients and 40 healthy volunteers. Histologic examination of dissected and seemingly intact arteries conducted in 5 cases revealed signs of arterial wall dysplasia similar to those characteristics of fibromuscular dysplasia: thinning and splitting of the internal elastic membrane, areas of fibrosis, irregular orientation of myocytes, and their necrosis in the tunica media. Clinical signs and biomarkers of CT dysplasia (CTD) were more pronounced in patients with arterial dissection than in the controls. The major provoking factors were head turns and physical activity (42%), minor head injury (10%), and acute respiratory infection in the month preceding arterial dissection (14%). We conclude that arterial wall dysplasia is a predisposing factor for ICA/VA dissection, both spontaneous and provoked. The analysis of CTD biomarkers and clinical signs suggests connective tissue pathology in patients with ICA/VA dissection.