Microsurgical Resection of a Giant Posterior Fossa Aneurysmal Malformation in a 21-Month-Old.

Pediatric aneurysms commonly occur in the vertebrobasilar circulation with complex morphologies. "Aneurysmal malformations," or fistulous vessel dilations without a nidus, have also been described. Vessel friability and sensitivity to blood loss can complicate surgery. A 21-month-old male with motor and speech delay was found to have a giant posterior fossa aneurysmal malformation. He was lethargic, with minimal speech, and moved all extremities with mild hypotonia. Imaging demonstrated a 6.9 × 5.1 × 4.6 cm aneurysm arising from a fenestrated right V4 segment. This communicated via a single connection with the deep venous system, draining through the superior vermian cistern veins, posterior mesencephalic vein, basal vein of Galen, and inferior sagittal sinus, consistent with an arteriovenous fistula with secondary aneurysmal dilatation. Endovascular sacrifice was not feasible, in addition to concern for swelling after embolization. Three-dimensional modeling confirmed close proximity of the single inflow and outflow tracts. A suboccipital and left far lateral craniotomy for clip trapping and excision of the aneurysmal arteriovenous malformation was performed in a lateral position to completely decompress the brainstem (Video 1). Angiography before closure and postoperative vascular imaging demonstrated complete aneurysmal resection and fistula disconnection, with patency of normal vasculature. The postoperative course was notable for transient swallowing difficulties likely from lower cranial nerve irritation and refractory hydrocephalus requiring a shunt. The patient was meeting all developmental milestones at 2-year follow-up. This case highlights the complex vascular pathology often seen in pediatric patients, as well as the importance of presurgical planning and careful microsurgical technique in achieving a successful outcome.