The role of magnetic resonance imaging in diagnosing optic nerve hypoplasia

Abstract

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.

Figure 1

Method of Intraorbital Optic Nerve Measurement for Cases with Optic Nerve Hypoplasia and Normal Controls using Magnetic Resonance Imaging of the Orbits. Axial T2 SPACE source image (Left) of the intraorbital segment of the optic nerve. Digital crosshair tool is placed on a magnified image of the intraorbital right optic nerve 5 mm posterior to the optic disc (Top right) exactly orthogonal to the plane of the nerve. A coronal transverse oblique image (Bottom right) is digitally generated which then allows for craniocaudal and transverse measurements of the optic nerve.

Figure 2

Method of Prechiasmatic Optic Nerve Transverse Measurement using Magnetic Resonance Imaging of the Orbits. Axial T2 SPACE source image (Left) of the prechiasmatic segment of the optic nerve immediately posterior to the optic canal. Digital crosshair tool is placed on a zoomed image of the prechiasmatic segment (Top right) exactly orthogonal to the transverse oblique plane of the nerve. A coronal transverse oblique image (Bottom right) is digitally generated which allows for transverse measurement of the optic nerve.

Figure 3

Method of Prechiasmatic Optic Nerve Craniocaudal Measurement using Magnetic Resonance Imaging of the Orbits. Sagittal T2 SPACE source image (Left) of the prechiasmatic segment of the optic nerve immediately posterior to the optic canal. Digital crosshair tool is placed on a magnified image of the prechiasmatic segment (Top right) exactly orthogonal to the craniocaudal oblique plane of the nerve. A coronal cranio-caudal oblique image (Bottom right) is digitally generated that then allows for craniocaudal measurement of the optic nerve.

Figure 4

Flow Diagram Outlining Method of Optic Nerve Hypoplasia Case Identification for Optic Nerve Measurements by Magnetic Resonance Imaging of the Orbits.

Figure 5

Association Between Optic Nerve Size Determined by Magnetic Resonance Imaging of the Orbits for Controls Compared to Cases of Optic Nerve Hypoplasia. Black boxes represent individual control optic nerve measurements. The solid black line shows the mean optic nerve size of controls by linear regression. The dashed black lines indicate the 95% prediction intervals of the mean optic nerve size of controls. Down-pointing triangles are measurements of optic nerves with clinical ONH. Up-pointing triangles represent the clinically unaffected eye of patients with clinically unilateral ONH. The contralateral optic nerve of ONH patients was generally smaller than control optic nerves. The bold gray line is a linear lower bound of the 95% prediction band [2.24 mm + 0.052 × (age in years)] mm that discriminated all but one of the cases of optic nerve hypoplasia.