Objectives: To demonstrate the importance of thorough investigation of patients with Horner syndrome, and to explain the relevant anatomy.
Case report: A 46-year-old man presented with right-sided Horner syndrome. No other abnormality was found. Magnetic resonance imaging showed calcification of the stylohyoid ligament, with a pseudoarthrosis in the mid-portion of the ligament. This pseudoarthrosis was displacing and compressing the internal carotid artery and the adjacent sympathetic chain, causing Horner syndrome.
Conclusion: In this case, magnetic resonance imaging was invaluable in elucidating the cause of the Horner syndrome. This is the first described case of pseudoarthrosis of a calcified stylohyoid ligament causing Horner syndrome.