Management of a progressive post–Gamma Knife thalamic cyst using an Ommaya reservoir: illustrative case

BACKGROUND: Delayed cyst formation is a known complication of Gamma Knife (GK) stereotactic radiosurgery (SRS). Estimates of this phenomenon occurring after GK-SRS for arteriovenous malformation are 1.6%-3.6%. Management strategies include aspiration, craniotomy, and shunt placement. This case report describes the placement of an Ommaya reservoir for recurrent symptomatic thalamic cyst formation after GK thalamotomy for essential tremor. OBSERVATIONS: A 74-year-old female underwent left ventral intermediate nucleus GK thalamotomy for a medically refractory essential tremor in 2013 using a single 135-Gy shot. Eight years later, she developed right-sided motor symptoms and slurred speech. MRI demonstrated a left thalamic cyst with mass effect on adjacent thalamic structures and the posterior limb of the internal capsule. Initial burr hole biopsy and aspiration led to complete symptom resolution, but the cyst recurred. Her symptoms progressed and she underwent placement of a left Ommaya reservoir. She did well for approximately 1 year and then presented to the clinic with similar symptoms and worsened head tremor. CT showed a 2.8 × 2.1 × 1.5-cm cyst. Cyst aspiration via the Ommaya reservoir provided improvement in speech, gait, and head tremor. LESSONS: Ommaya reservoir placement may provide a minimally invasive and repeatable treatment option for recurrent symptomatic thalamic cysts after GK-SRS, particularly when intermittent decompression is preferred over permanent shunt placement or open surgical intervention. https://thejns.org/doi/10.3171/CASE26595.