The role of magnetic resonance imaging in diagnosing optic nerve hypoplasia.

PURPOSE

To establish objective lower limits of normal optic nerve (ON) size in children based on high-resolution orbital magnetic resonance imaging (MRI).

DESIGN

Case-control study of patients with ON hypoplasia vs normal controls.

METHODS

A neuroradiologist made 4 measurements of each ON at 2 locations (5 mm posterior to the optic disc and just posterior to the optic canal) in patients with ON hypoplasia and controls aged 0-17 years from an academic eye center and children's hospital. Primary analyses were performed using mixed linear models.

RESULTS

Measurements were made in 26 cases of clinically confirmed ON hypoplasia and 31 controls (median age: ON hypoplasia, 1 year; controls, 5.5 years). Nine of 26 cases (35%) and 19 of 31 controls (61%) underwent high-resolution T2-weighted imaging of the orbits. Mean ON diameter was 1.36 mm (95% CI: 1.19-1.54; P < .001) smaller for clinically hypoplastic ONs than for controls. Optic nerve diameter increased by 0.05 mm per year of age (95% CI: 0.03-0.07; P < .001). A lower bound to the 95% prediction interval for normal optic nerves was (2.24 mm + 0.052 × [age in years]) mm and excluded all but 1 case.

CONCLUSIONS

Age was independently associated with normal ON size by MRI and should be taken into consideration when evaluating ON hypoplasia, based on MRI criteria. We have provided a formula to assist clinicians in objectively determining if ON hypoplasia is present.