To demonstrate the importance of thorough investigation of patients with Horner syndrome, and to explain the relevant anatomy.
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.
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.
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