Partial Horner's Syndrome Secondary to Carotid Dissection With an Associated Folate Deficiency.

Horner's syndrome is an uncommon clinical finding characterized by the triad of ptosis, miosis, and anhidrosis. It typically results from disruption along the oculosympathetic pathway. When anhidrosis is absent, the partial Horner's syndrome suggests a lesion distal to the superior cervical ganglion and is an important clinical clue. One of the most important causes in young patients is carotid artery dissection, a potentially serious but often under-recognized condition. We report the case of a 34-year-old woman who presented with a left-sided headache and new-onset ptosis. Examination revealed a partial Horner's syndrome. Initial blood tests, CT of the head, and chest X-ray were unremarkable, but CT angiography demonstrated a left internal carotid artery dissection with near-occlusion. She was treated with dual antiplatelet therapy. Further investigations into the cause revealed a folate deficiency and mild hyperhomocysteinemia. This case emphasizes the diagnostic value of recognizing partial Horner's syndrome in localizing the lesion and the importance of urgent vascular imaging to rule out a carotid pathology, especially in a young patient. Additionally, while carotid dissections are often spontaneous or trauma-related, nutritional deficiencies such as folate deficiency with resulting hyperhomocysteinemia represent potentially modifiable risk factors. Early recognition and a comprehensive stroke workup are essential to guide management and reduce the risk of a recurrence.