Sonographic reference values of cranial nerve size: A systematic review and meta-analysis

Ultrasonography of the cranial nerves has recently gained attention for assessment of inflammatory, compressive, or degenerative neuropathies. However, sonographic reference values of cranial nerves have received less attention than those of peripheral nerves. In this systematic review and meta-analysis we aimed to provide current evidence of sonographic reference values for cranial nerve size.

By searching Medline (via PubMed), Scopus, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), and Web of Science, we conducted a systematic review and meta-analysis of studies that reported ultrasound measurements of the facial, spinal accessory, and hypoglossal nerves in healthy adults. We included studies that reported either the sonographic cross-sectional area (CSA) or the nerve diameter; the included nerves were subgrouped according to the site of nerve measurement.

Fourteen studies with a total of 661 participants and 1437 ultrasound nerve measurements met the inclusion criteria. The anatomical sites for each nerve were combined to provide single-nerve mean measurements. We found an overall mean nerve diameter of 0.80 mm for the facial nerve, 0.63 mm for the spinal accessory nerve, and 1.82 mm² for hypoglossal nerve CSA.

This meta-analysis provides reference values for the diameter and cross-sectional area of the facial, spinal accessory, and hypoglossal nerves at different sites, which can be used as guidance in clinical practice to detect pathological changes in cranial nerve size in cranial neuropathies. We recommend further validation in large-scale studies as well as standardization of the scanning protocols.