Arachnoid cysts (ACs) are relatively common developmental anomalies, which are frequently diagnosed incidentally by intracranial imaging in pediatric patients. ¹ The majority are asymptomatic with rare cases of rupture resulting in a subdural hematoma or subdural hygroma typically presenting with localized headache, behavioral changes, or ataxia. ²  

We present the case of an eleven-year-old boy with a known history of a large previously asymptomatic AC presenting with acute onset of right facial droop, hemiplegia, and expressive aphasia. Shortly after arrival to the emergency department he had complete resolution of right-sided hemiplegia but developed headache, nausea, and had persistent word-finding difficulties. Prior to symptom onset while in class at school, there is an absence of reported jerking movements, headache, photophobia, fever, or trauma. At the time of neurology consultation, physical exam was notable of mildly delayed cognitive processing but was otherwise unremarkable. The patient underwent MRI scan of the brain which revealed left convexity subdural hematohygroma and perirolandic cortex edema resulting from ruptured left frontoparietal AC. Head and neck MRA were normal.  He was evaluated by neurosurgery and managed expectantly. He recovered uneventfully and was discharged two days after presentation remaining asymptomatic on subsequent visits. 

There are case reports for traumatic cyst rupture, but spontaneous cyst rupture is rare. However, mostly they have presented with headaches and nonspecific neurological findings.  To our knowledge, this is the first case of spontaneous AC rupture presenting with acute onset of focal neurological signs. This could have either resulted from the mass effect of fluid from rupture or vasospasm caused by the release of proteinaceous fluid.