An Adolescent Football Player With Persistent Headaches Following a Concussion: A Case of Primary Arachnoid Cyst.

Post-concussion syndrome (PCS) is a common sequela of mild traumatic brain injury in adolescent athletes, typically resolving within weeks. However, persistent or atypical symptoms warrant further investigation to exclude structural pathology. Arachnoid cysts, though often incidental, can become symptomatic following trauma and may mimic or exacerbate PCS. A previously healthy adolescent American football player presented with persistent headaches and cognitive symptoms 14 weeks after a sports-related concussion. Despite completing a return-to-play protocol, his symptoms worsened. A brain MRI revealed a large left-sided arachnoid cyst with a 5 mm midline shift, consistent with a primary arachnoid cyst. The patient was admitted to the medical unit of the pediatric hospital for monitoring on the medical floor with 48 hours of observation and repeat imaging to verify that the MRI findings were not worsening. Serial imaging over three months demonstrated regression of the cyst and resolution of mass effect. He remained asymptomatic at follow-up but was permanently restricted from contact sports. This case underscores the importance of considering structural brain lesions in athletes with prolonged or atypical post-concussive symptoms. MRI played a critical role in identifying a potentially life-threatening condition that mimicked PCS. Conservative management may be appropriate in select cases of arachnoid cysts with favorable clinical and radiographic progression. Surgical management may be necessary in cases of increased intracranial pressure, neurologic deficit, or radiographic progression.