Trochlear Nerve Palsy: A Systematic Review of Etiologies and Diagnostic Insights

Background/Objectives: Trochlear nerve palsy (TNP) is a clinically significant neuro-ophthalmic disorder with a broad and heterogeneous etiologic spectrum. Due to the trochlear nerve (TN)'s long intracranial course and its proximity to key neurosurgical corridors, it is particularly susceptible to injury. This systematic review aimed to synthesize contemporary evidence on TNP etiologies and highlight diagnostic considerations. Methods: Following PRISMA 2020 guidelines (PROSPERO registration: CRD420251150614), we systematically searched PubMed through July 2025 for studies reporting TNP etiologies. Given substantial heterogeneity in study populations and methodologies, a qualitative synthesis was performed examining study characteristics, patient demographics, etiological distribution, and clinical outcomes. Results: Thirty-three studies (n = 5785) met the inclusion criteria. Reported etiologies clustered into seven categories: congenital, vascular/ischemic, infectious/inflammatory, structural, traumatic, iatrogenic, and idiopathic. Congenital cases frequently demonstrated absence of the TN or superior oblique hypoplasia. Microvascular ischemia predominated in older adults with vascular risk factors and typically exhibited spontaneous recovery. Structural lesions (e.g., tumors, aneurysms) and trauma were major acquired causes, often associated with more persistent deficits. Iatrogenic palsy predominantly followed skull base and petroclival procedures; most cases resolved, although lasting dysfunction occurred after complex or radiosurgical interventions. A proportion of cases remained idiopathic, and many resolved spontaneously. Conclusions: TNP displays a broad etiologic spectrum with distinct clinical profiles and prognostic trajectories. Accurate etiologic classifications supported by targeted neuroimaging and focused clinical evaluation are essential for optimizing management and informing neurosurgical decision-making.