On MRI, Variable Intensities Among Optic Nerve Segments Can Be Normal

Changes in optic nerve signals on MRI are used to diagnose many diseases, including ischemic optic neuropathy and demyelinating disease. However, variability of optic nerve MRI signal intensity may occur in the normal population. An analysis of MRI scans from a cohort of patients with an ocular motor nerve palsy but no optic neuropathy found that pre-chiasmatic regions of the optic nerve showed greater intensity than mid-orbital segments, suggesting that signal differences on MRI may not necessarily be an indication of disease.

Study Design

This was a retrospective study involving 75 patients who underwent MRI of the brain for oculomotor palsy without clinical evidence of optic neuropathy. Quantitative intensity measurements were taken by a neuroradiologist pre- and post-contrast of both the mid-orbital segment and the pre-chiasmatic segment of the optic nerve. The objective was to identify any differences in intensity between the two segments on MRI in patients without optic neuropathy.

Outcomes

Despite normal clinical appearance of the optic nerve, the signal intensity of the pre-chiasmatic segment was significantly higher than that of the mid-orbital segment in pre- and post-contrast images in most of the 75 cases, with larger differences seen in pre-contrast vs post-contrast images. Patients with third nerve palsy showed the most significant differences in signal intensity between mid-orbital and pre-chiasmatic segments.

Limitations

Some limitations of this study include its retrospective nature, the use of different MRI machines, and irregular fat saturation.

Clinical Significance

This study highlights the importance of understanding variations of normal in order to detect what is abnormal. Knowing that greater intensity of the pre-chiasmatic optic nerve segment on MRI is a “normal” finding could reduce the risk of overcalling pre-chiasmatic enhancement. That said, it is also important not to risk undercalling what might be true enhancement. It is well known that conditions such as multiple sclerosis, myelin oligodendrocyte glycoprotein disease, and neuromyelitis optica show differences in enhancement of the optic nerve due to optic neuritis. Overall, this study suggests that comparing signal intensity between mid-orbital and pre-chiasmatic segments of the optic nerve could have diagnostic benefit in determining patients with true optic nerve enhancement. Further studies adding more points of measurement and comparison would assist in distinguishing normal vs abnormal MRI findings.

Financial Disclosures: Dr. Dana Albreiki discloses no financial relationships.