María Miguel-Garcés ^1,2* Rita Goncalves ³ Rodrigo Gutierrez-Quintana ⁴ Patricia Álvarez ^2,5 Katrin M. Beckmann ⁶ Emili Alcoverro ⁷ Melania Moioli ⁸ Edward Ives ^10,9 Megan Madden ¹¹ Sergio A. Gomes ¹² Evelyn M. Galban ¹³ R. Timothy Bentley ³ Koen Santifort ^14,15 An Vanhaesebrouck ¹⁶ Chiara Briola ^17,18 Patricia Montoliu ^19,20 Unai Ibaseta ²¹ Ines Carrera ²²

• ¹ Southern Counties Veterinary Specialists, Ringwood, United Kingdom
• ² IVC Evidensia (United Kingdom), Bristol, England, United Kingdom
• ³ Small Animal Teaching Hospital, University of Liverpool, Liverpool, North West England, United Kingdom
• ⁴ Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Glasgow, Scotland, United Kingdom
• ⁵ Pride Veterinary Centre, Derby, United Kingdom
• ⁶ Vetsuisse Faculty, University of Zurich, Zurich, Zurich, Switzerland
• ⁷ ChesterGates Veterinary Specialists, Norfolk, United Kingdom
• ⁸ Vetsuisse Faculty, University of Bern, Bern, Bern, Switzerland
• ⁹ Anderson Moores Veterinary Specialists, Winchester, England, United Kingdom
• ¹⁰ Linnaeus Veterinary Limited, West Midlands, England, United Kingdom
• ¹¹ Royal (Dick) School of Veterinary Studies, University of Edinburgh, Edinburgh, Scotland, United Kingdom
• ¹² Dovecote Veterinary Hospital, Castle Donington, England, United Kingdom
• ¹³ School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States
• ¹⁴ Evidensia Animal Hospital Arnhem, Arnhem, Netherlands
• ¹⁵ Evidensia Dierenziekenhuis Hart van Brabant, Waalwijk, Netherlands
• ¹⁶ Queen's Veterinary School Hospital, University of Cambridge, Cambridge, United Kingdom
• ¹⁷ The Ralph Veterinary Referral Centre, Marlow, United Kingdom
• ¹⁸ VetCT specialist (United Kingdom), Cambridge, Cambridgeshire, United Kingdom
• ¹⁹ Ars Veterinaria, Barcelona, Catalonia, Spain
• ²⁰ Hospital Veterinari Costa Brava, Girona, Spain
• ²¹ Hospital Veterinario menes, Asturias, Spain
• ²² VetOracle, Norfolk, United Kingdom

Metabolic/neurodegenerative encephalopathies encompass a wide list of conditions that share similar clinical and magnetic resonance imaging (MRI) characteristics, challenging the diagnostic process and resulting in numerous tests performed in order to reach a definitive diagnosis. The aims of this multicentric, retrospective and descriptive study are: (I) to describe the MRI features of dogs and cats with metabolic/neurodegenerative encephalopathies; (II) to attempt an MRI recognition pattern classifying these conditions according to the involvement of grey matter, white matter or both; and (III) to correlate the MRI findings with previous literature. A total of 100 cases were recruited, comprising 81 dogs and 19 cats. These included hepatic encephalopathy, myelinolysis, intoxications, thiamine deficiency, hypertensive encephalopathy, neuronal ceroid lipofuscinosis, gangliosidosis, fucosidosis, L-2-hydroxyglutaric aciduria, Lafora disease, spongiform leukoencephalomyelopathy and cerebellar cortical degeneration. None of the hepatic encephalopathies showed the previously described T1-weighted hyperintensity of the lentiform nuclei. Instead, there was involvement of the cerebellar nuclei (8/23), which is a feature not previously described. Dogs with myelinolysis showed novel involvement of a specific white matter structure, the superior longitudinal fasciculus (5/5). Thiamine deficiency affected numerous deep grey nuclei with novel involvement of the oculomotor nuclei (3/9), thalamic nuclei, subthalamus and cerebellar nuclei (1/9). Cats with hypertensive encephalopathy had a more extensive distribution of the white matter changes when compared to dogs, extending from the parietal and occipital lobes into the frontal lobes with associated mass effect and increased brain volume. Lysosomal storage disease showed white matter involvement only, with neuronal ceroid lipofuscinosis characterised by severe brain atrophy when compared to gangliosidosis and fucosidosis. All patients with L-2-hydroxyglutaric aciduria had a characteristic T2-weighted hyperintense swelling of the cerebral and cerebellar cortical grey matter, resulting in increased brain volume. Lafora disease cases showed either normal brain morphology (5/11) or mild brain atrophy (6/11). Dogs with cerebellar cortical degeneration had more marked cerebellar atrophy when compared to cats. This study shows the important role of MRI in distinguishing different metabolic/neurodegenerative encephalopathies according to specific imaging characteristics.