Primary oculomotor nerve malignant peripheral nerve sheath tumor in an infant: reflex-mediated biopsy failure and tubular port-assisted diagnostic escalation.

backgroundIntracranial malignant peripheral nerve sheath tumors (MPNSTs) are exceptionally rare, and primary origin from cranial nerves is particularly uncommon. Involvement of the oculomotor nerve represents an extraordinarily rare entity. Their deep mesencephalic location, nonspecific imaging characteristics, and proximity to critical neurovascular structures pose major diagnostic challenges. In addition, surgical manipulation near the third cranial nerve may trigger trigeminocardiac or oculocardiac reflex (TCR/OCR), resulting in severe hemodynamic instability and potentially limiting minimally invasive biopsy techniques. CASE DESCRIPTION: A 6-month-old infant presented with progressive ophthalmoplegia and hemiparesis. Magnetic resonance imaging demonstrated a deep mesencephalic lesion adjacent to the cisternal segment of the oculomotor nerve, associated with obstructive hydrocephalus. Two minimally invasive diagnostic procedures (stereotactic and endoscopic biopsy) were interrupted due to recurrent episodes of profound bradycardia consistent with TCR/OCR, and insufficient tissue was obtained. A third procedure using a planned open tubular port-assisted biopsy with endoscopic guidance allowed safe acquisition of representative samples without hemodynamic compromise. Definitive diagnosis of MPNST was established through DNA methylation profiling, supported by focal S-100/SOX10 positivity, elevated Ki-67 index, and loss of H3K27me3 expression.

conclusionsPrimary MPNST of the oculomotor nerve represents an exceptionally rare and surgically challenging entity. In selected deep cranial nerve-related lesions complicated by reflex-mediated instability, tubular port-assisted biopsy with endoscopic guidance may constitute a safe and effective diagnostic escalation strategy.