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.
Highlights
Optic neuritis (ON) is an inflammation of the optic nerve [1] and in dogs presents as a clinical syndrome characterized by unilateral or bilateral acute vision loss and altered pupillary light reflexes [2]
For the three cases included without cerebrospinal fluid (CSF), these cases did not have a CSF sample taken due to the opinion of the attending neurologist: in two cases it was considered contraindicated due to a suspicion of raised intracranial pressure and the other case had previously been diagnosed with meningoencephalitis of unknown etiology (MUE) 2 years before, at which point the dog had signs of central nervous system (CNS) neurolocalisation without any visual deficits, Magnetic resonance imaging (MRI) Findings
Contrast enhancement of the optic nerves was the most common finding in our study, present in 61% (11/18) of the cases, and contrast enhancement of the optic chiasm was seen in 31%, which is described in previous reports of optic neuritis [9,10,11]
Summary
Optic neuritis (ON) is an inflammation of the optic nerve [1] and in dogs presents as a clinical syndrome characterized by unilateral or bilateral acute vision loss and altered pupillary light reflexes [2]. Optic neuritis has recently been classified in multifocal meningoencephalitis of unknown etiology (MUE-associated ON), isolated optic neuritis (I-ON) and “others” [2]. Described etiologies of canine optic neuritis include infectious diseases, inflammatory central nervous system (CNS) diseases [such as granulomatous meningoencephalitis (GME)], trauma, orbital diseases, neoplastic processes, toxins, vitamin A deficiency, and idiopathic [1]. In a previous review of GME cases [3], 15 out of 151 dogs with GME showed concurrent signs of optic neuritis. GME is a histopathological diagnosis of a subtype of MUE [4, 5] and can develop in a focal, disseminated or ocular form; both the ocular and disseminated forms can have concurrent CNS and optic nerve lesions [6].
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