Normative measurements of the frontal nerve by magnetic resonance imaging in an Australia cohort.

Purpose: To report the normative dimensions of the frontal nerve (FN) on fat-suppressed suppressed gadolinium (fs-gad) enhanced magnetic resonance imaging (MRI).

Method: A retrospective cohort study of patients who underwent coronal fs-gad T1-weighted MRI. Orbits were excluded if there was unilateral or bilateral pathology of the FN or optic nerve sheath (ONS), incomplete MRI sequences, poor image quality or indiscernible FN on radiological assessment. The maximum diameter of the FN and ONS was measured.

Results: The mean age of participants was 58 ± 16 years and 50% were females (n = 42). The mean normative measurements (mean ± standard deviation) on coronal T1-weighted imaging: optic nerve sheath, 5.08 ± 0.67mm. On coronal fs-gad T1-weighted imaging: frontal nerve, 0.74 ± 0.18mm. No significant differences were found between male or female participants in both the frontal nerve (p = 0.913) or optic nerve sheath (p = 0.646). There was no significant correlation between age and mean diameter of the frontal nerve (r = 0.14, p = 0.067) or optic nerve sheath (r = 0.075, p = 0.336). Additionally, no significant difference was identified between the mean diameter of the frontal nerve (p = 0.075) and optic nerve sheath (p = 0.120) across age groups. The mean frontal nerve to optic nerve sheath ratio was 0.15 ± 0.04.

Conclusion: Normative dimensions of the FN may provide quantitative cut-offs that can aid the diagnosis of FN enlargement seen in instances such as IgG4-related ophthalmic disease (IgG4-ROD) and neoplastic perineural spread.