Small breed dogs can suffer from idiopathic inflammatory disorders of the brain. MR imaging has become very useful in diagnosing these conditions and differentiating between them. The location, involvement of gray and white matter, and severity are all helpful in making this diagnosis and offering a prognosis for the animal. Here is an excerpt from the Atlas of Small Animal CT & MRI describing the imaging features of GME, NME, and NLE.
Introduction
Although a number of noninfectious inflammatory disorders of the brain of the dog and cat have been reported, two entities, granulomatous meningoencephalitis and necrotizing encephalitis, are the most common and best described. Necrotizing encephalitis can be further subdivided into necrotizing meningoencephalitis and necrotizing leukoencephalitis, and it is possible that these represent different manifestations of the same disease. All three of these conditions are thought to be autoimmune disorders.
Granulomatous meningoencephalitis
Granulomatous meningoencephalitis (GME) is an idiopathic inflammatory disorder of the central nervous system, characterized by perivascular mononuclear cell infiltrates. Young to middle‐aged (4–5 years), female, small‐ and toy‐breed dogs are predisposed, while the disorder is less common in large‐breed dogs and rare in cats. Lesion distribution may be focal, disseminated (multifocal), or ocular, with the focal and disseminated forms predominating. Lesions primarily involve white matter, but gray matter and meninges may also be affected. This disorder most often affects the forebrain, brainstem, or spinal cord, with the cerebellum and optic nerves less frequently involved.
Depending on the extent of associated edema, lesions may have ill‐defined hypoattenuation on unenhanced CT images and will variably contrast enhance. Enhancement can be absent, heterogeneous and ill-defined, or may sometimes reveal a well-delineated mass. Lesions are typically T1 iso‐ to hypointense and T2 hyperintense and have similar contrast enhancement characteristics as described for CT.
Meningeal involvement is documented in many patients with GME, and abnormal imaging findings are sometimes limited to prominent meningeal enhancement. In a minority of patients, imaging may be normal or lesions may not contrast enhance.
Necrotizing encephalitis
Necrotizing meningoencephalitis Necrotizing meningoencephalitis, sometimes referred to as Pug dog encephalitis, is a nonsuppurative, necrotizing, inflammatory brain disorder. Small‐ and toy‐breed dogs are predisposed, and Pug, Maltese, and Chihuahua breeds are highly overrepresented. Median age of onset is 1.5–3 years, and females are affected more commonly than males. Lesions may be focal or asymmetrically multifocal and involve both gray and white matter of the cerebral hemispheres and overlying meninges. Cerebellar and brainstem involvement, though uncommon, has been reported. Grossly, lesions are frequently cavitary and associated with significant brain swelling from inflammation and edema.
Lesions may be hypoattenuating on unenhanced CT images when cavitary or when associated with significant brain edema. Edema may also induce midline shift, brain herniation, and other features of mass effect. Contrast enhancement is variable on CT, ranging from absent to moderate, but when present, the enhancement pattern is heterogeneous and margins may be poorly delineated. On MR images, lesions are T1 iso‐ or hypointense and T2 hyperintense, involve the cerebral gray and white matter, and typically have indistinct margins. About half to two thirds of lesions contrast enhance on MR, but enhancement is minimal to moderate and nonuniform, when present. Meningeal enhancement is evident in about 50% of patients.
Necrotizing leukoencephalitis
Necrotizing leukoencephalitis is also a nonsuppurative, necrotizing, inflammatory brain disorder affecting both gray and white matter. Grossly, there are subcortical regions of liquefaction and cavitation. Microscopically, lesions are characterized by mononuclear infiltrates, gitter cells, and frank necrosis. Descriptions of the anatomic distribution of this disorder are sparse, but lesions can be focal, asymmetrically multifocal, or regionally diffuse with a predilection for the cerebral hemispheres, although brainstem lesions have also been reported.
Lesions are iso‐ to hypoattenuating on unenhanced CT images and may appear contiguous with the ventricles. Contrast enhancement is absent to moderate and nonuniform and ill defined, if present. On MR images, brain lesions are T1 hypointense and T2 hyperintense and minimally to moderately contrast enhance. When enhancement is present, it is typically nonuniform and sometimes peripheral.
Homework
Compare some of the images in your collection with the examples in this chapter to hone your interpretation skills. The Atlas of CT & MRI is available through Amazon, and has over 700 figures showing case examples to further your learning. Check it out if this article was helpful!
See you next time,
Allison Zwingenberger
Co-Author, Atlas of Small Animal CT and MRI
P.S.
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