2013 Radiology Exam - CT/MRI Case 1

HISTORY
A 2 year old, spayed female spayed Yorkshire terrier presented with ataxia, circling to the right for 6 weeks, blind in left eye, decreased to absent conscious proprioception in the left front and hind limbs.

IMAGES
Sequences provided in the transverse plane included: FSE T2W, T2 FLAIR and pre- and post-contrast T1-W +Gd. Only 4 selected images are presented here due to space constraints.

FINDINGS
In the right cerebrum, specifically the frontal, parietal and occipital lobes, there is intra-axial, irregularly-shaped abnormal hyperintensity on the T2 images. The hyperintensity is greatest in the white matter, with extension into adjacent gray matter areas. There are multiple areas of this hyperintensity that are hypointense on the T2 FLAIR images. On the pre-contrast images, the abnormal parenchyma is predominantly hypointense. Following contrast administration, there is heterogeneous areas of marked enhancement, indicative of blood brain barrier breakdown. The largest area of enhancement is characterized by peripheral enhancement around the areas that were hypointense on the T2 FLAIR images. Additional multifocal small regions of enhancement are present in the right frontal lobe. The lateral ventricles are mildly enlarged, with the right lateral ventricle being mildly asymmetrically enlarged in comparison to the left.

SYNTHESIS
Right-sided, cavitated encephalitis, with mild asymmetric hydorcephalus interna (R > L).
Based on the signalment and the MRI features, a necrotizing encephalitis is most likely - specifically Yorkie necrotizing leukoencephalitis (NLE). Necrotizing meningoencephalitis (NME) or granulomatous meningoencephalitis (GME) are also considered, but are less likely. Atrophy of the right cerebral cortex secondary to inflammation is considered for the asymmetric enlargement of the lateral ventricles, but a variant of congenital/development hydrocephalus is possible or contributory in this breed.

Candidates that did well in the findings section correctly defined lesion localization being predominantly in the white matter, and they described other MR features, such as central areas of suppression of the T2 FLAIR images consistent with fluid-filled cavitations. Some erroneously identified the temporal lobe as the dorsally affected lobe (instead of parietal), representing an error in neuroanatomic knowledge. Those candidates correcting identifying the breed affected, predominant white matter distribution and cavitated areas appropriately generated an accurate prioritized differential diagnosis (1st NLE, with NME or GME also considered; focal infectious meningitis unlikely). Candidates that did not consider the breed, lesion distribution or the lack of mass effect would consider other disease processes, such as focal infectious encephalitis or neoplasia. Some candidates considered a chronic infarction, confusing the the central hypointensity on the pre-contrast T1 images and ignoring the lesion hyperintensity on T2 images. Points were earned by recommending collection of cerebrospinal fluid (CSF), with consideration given to assessment of the sagittal plane images for herniation or noting that no herniation was seen on the provided images and therefore was likely a safe procedure. This is a case in which all candidates are expected to describe the MRI features of a classic clinical presentation, generate a breed-specific list of differentials and appropriate and safe management options.