The images below are from a 2-year-old Cavalier King Charles spaniel with severe epileptic seizures since 4 months of age. Despite ending up on three antiseizure medications at recommended dosages, the seizures continued to get worse with 3 to 4 episodes of severe cluster seizures a month lasting 24 to 48 hrs each. Relevant MR images are shown opposite and reveal a well-defined nodular lesion protruding within the right lateral ventricle at the level of the parietal lobe. This lesion was homogeneous with similar signal intensity as gray matter on all sequences and it was well-demarcated from the adjacent white matter. There was no associated contrast enhancement or perilesional edema. The MR images were considered strongly suggestive of subependymal neuronal heterotopia.

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Gray matter heterotopia is classified as a subgroup of malformation of cortical development (MCD) and involves clusters of normal neurons in abnormal locations, mainly due to abnormal neuronal migration. Neurons start migrating from the ventricular and subependymal zones after completing their final division and having established their polarity. Migration occurs either radially or, less commonly, tangentially. Too early or too late migration leads to disruption of the normal neuronal migration and associated disorders. Although histopathology would be required to confirm the diagnosis, subependymal neuronal heterotopia was suspected in this case. This type of malformation is very rare in dogs and people affected with MCDs show a high incidence of drug-resistant epilepsy. It remains to be established in veterinary medicine if surgical excision, as is sometimes considered in people, can help with seizure control.​

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Courtesy of Dr Kaloyan Voychev at MVC Hospital in Bulgaria