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Bullmastiff mitochondrial fission encephalopathy, MRI imaging shows olivary nuclei changes as a distinct feature.

VRU 2024

Emma Suiter, Kerstin Baiker, Adriana Kaczmarska, Matthias Christen, Tosso Leeb, Alejandro Ororbia, Carlo Anselmi, Juan Minguez, Rodrigo Gutierrez-Quintana

Background
This case report describes a 9-month-old male Bullmastiff cross dog diagnosed with mitochondrial fission encephalopathy (MFE), a rare neurodegenerative condition. The condition was associated with a homozygous frameshift mutation in the mitochondrial fission factor (MFF) gene. This is the first documented case to reveal novel MRI and histopathological findings distinct from previously reported Bullmastiffs with MFE.

Methods
Clinical and neurological examinations identified progressive proprioceptive ataxia, forebrain and vestibulo-cerebellum involvement, and behavior changes. Serial MRI scans were conducted using a 1.5-T unit. Histopathological analysis of brain and cardiac tissues was performed postmortem, accompanied by genetic analysis using formalin-fixed brain samples to confirm the MFF mutation.

Results
Initial MRI revealed mild diffuse widening of cerebral and cerebellar sulci, ventricular dilation, and pinpoint T2-weighted (T2W) hyperintense foci in the olivary nuclei. A follow-up MRI six months later showed progression, with rounded, bilaterally symmetrical lesions in the olivary nuclei. Histopathology revealed extensive neuronal disintegration, vacuolation, and gliosis in the olivary nuclei, alongside mild Purkinje cell degeneration. Cardiac pathology indicated adipocyte replacement and fibrosis, consistent with a primary cardiomyopathy. Genetic testing confirmed the MFF homozygous frameshift mutation.

Limitations
The absence of diffusion tensor imaging (DTI) limited further exploration of early microstructural changes in affected areas. Additionally, the natural progression of MFE lesions across different brain regions remains unclear, as this case diverges from previously documented regional vulnerabilities.

Conclusions
This study expands the phenotypic spectrum of MFE in dogs, identifying olivary nuclei changes as a distinct feature. The findings underscore the need for advanced imaging techniques and multimodal diagnostics in suspected MFE cases, as well as the potential for systemic involvement, such as cardiomyopathy.

Figure 2. Transverse T2W images at the level of the olivary nuclei at first MRI (A, B) and second MRI performed 6 months later (C, D). More evident in the latter MR images there are small, rounded, and well-defined bilaterally symmetrical T2W hyperintense lesions in area of the olivary nuclei (green arrow). In panel D two subtle pinpoint areas of hyperintensity can be seen dorsolateral to the olivary nuclei, which are considered artefactual due to lack of corresponding histopathological changes.

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