Trochlear Nerve Schwannoma: An Easily Missed Diagnosis

Identification of the trochlear nerve on magnetic resonance (MR) imaging is challenging because it is the thinnest cranial nerve and courses horizontally parallel to the transverse imaging plane. These anatomical features lead to difficulties in detecting small trochlear nerve schwannomas. We identified four cases of presumed small trochlear schwannomas which had been underdiagnosed at outside hospitals due to suboptimal imaging protocols - specifically the absence of gadolinium administration or the lack of high-resolution imaging tailored to the trochlear nerve. In our repeat MR imaging, we employed a high-resolution imaging (voxel size: 0.3 × 0.3 × 0.25 mm), enabling visualization of the entire nerve path within the perimesencephalic cistern. Additional sequences included thin-section postcontrast T1-weighted imaging and orbital coronal T2-weighted imaging. All tumors exhibited clear contrast enhancement and were accurately localized to the trochlear nerve on high-resolution imaging. Orbital coronal images showed ipsilateral atrophy of the superior oblique in all patients. Imaging and clinical findings were reviewed and compared with previously reported cases in the literature. Trochlear nerve schwannomas are rare and often overlooked due to their small size and inconspicuous imaging findings. High-resolution MR imaging tailored for the trochlear nerve is a robust technique to diagnose small trochlear nerve schwannomas.