Low-Field Magnetic Resonance Imaging Findings in 18 Dogs With Presumed Optic Neuritis

Abstract

Canine optic neuritis has been attributed to a focal or disseminated form of granulomatous meningoencephalitis (GME) amongst other etiologies. Magnetic resonance imaging (MRI) has been proven to help differentiate the structures within the optic nerve sheath and therefore could aid the diagnosis of optic neuritis in dogs. The objectives of this study were to describe and compare the MRI abnormalities affecting the optic nerve sheath complex and optic chiasm in dogs with clinically suspected optic neuritis as a component of meningoencephalitis of unknown etiology (MUE) or as an isolated form (I-ON). Retrospective evaluation of patient details, clinical signs, cerebrospinal fluid (CSF) analysis, and MRI findings of dogs with clinically suspected optic neuritis between January 2011 and May 2018 was performed. Eighteen dogs met the inclusion criteria. MRI findings included contrast enhancement of both optic nerves (11/18) and optic chiasm (6/18), changes within the CSF volume surrounding the optic nerve (10/18), changes to the optic disc (10/18), changes of size or signal affecting the optic chiasm (10/18), changes in the Short TI inversion recovery (STIR) signal of the optic nerve (7/15), retrobulbar changes (3/18), and concurrent brain lesions (13/18). A variety of subtle MRI features may indicate optic nerve involvement and low-field MRI is a sensitive method to detect changes within the optic nerve sheath complex in dogs with optic neuritis as an isolated form (I-ON) or as an extension of MUE.

Keywords: canine optic nerve; granulomatous meningoencephalitis; magnetic resonance imaging; meningoencephalomyelitis of unknown etiology; optic neuritis.

Figure 1

(A) T2W sagittal image of the brain showing a swollen optic chiasm (arrow). (B) T2W sagittal of the head, showing a bulging optic disc (arrow). (C) T1W post contrast transverse of the brain showing a strongly and slightly heterogeneous contrast enhancing of the optic chiasm (arrow). (D) Subtraction technique (digitally post-processed subtraction of the pre contrast T1W dorsal oblique sequence from the identical sequence obtained after gadolinium contrast administration), with arrows showing contrast enhancement of both optic nerves.

Figure 2

(A) T2W transverse of the brain of a dog showing a normal optic nerve sheath complex (arrows). (B,C) T2W transverse in two affected dogs, where there is absent CSF surrounding the optic nerve and generalized lack of differentiation of the structures within the optic nerve sheath complex (indicated by the arrows) suggesting swelling of the optic nerves.

Figure 3

(A,B) Dorsal T1W FS post contrast and STIR with arrows showing diffuse contrast enhancement and STIR hyperintensity of the retrobulbar structures. (C) STIR dorsal showing a hyperintense optic nerve (arrow). (D) T2W transverse images showing extensive T2W hyperintensity tracking along the white matter in a dog with MUE.