Spontaneous late-onset myoclonic epilepsy in cats: 15 cases (2015–2023)

Objectives The aim of this study was to describe spontaneous late-onset myoclonic epilepsy (SLOME) in older cats. Methods A retrospective, two-centre study was conducted on cats diagnosed with spontaneous (non-audiogenic) head myoclonus between 2015 and 2023. Results A total of 15 cats were included. Breeds included domestic shorthair (11/15, 73%) and one each (7%) of domestic longhair, Somali, Burmese and Russian Blue. Of the 15 cats, eight (53%) were castrated males and seven (47%) were spayed females. All cats were older, with a median age at presentation of 13.2 years (range 8.9–17). The median duration between the first episode and presentation was 5.5 months (range 1–24). All cats (15/15, 100%) presented with spontaneous, non-audiogenic myoclonus of the head, some of which had additional thoracic limb (3/15, 20%) or full body (1/15, 7%) involvement. Myoclonus frequency was progressive, with multiple daily or weekly episodes in half of the population (7/15, 47%). Two cats (2/15, 13%) had concurrent generalised tonic–clonic seizures. Eight cats (53%) had advanced imaging that was normal in all but three (37%) cats with concurrent brain atrophy, otitis media/externa and a pituitary mass. A diagnosis of SLOME was made in all cats. Levetiracetam (7/15, 47%) and phenobarbital (1/15, 7%) were administered, and all cats had a positive response, displaying either resolution or significant reduction in frequency of the myoclonus at a median follow-up time of 128 days (range 30–300). Conclusions and relevance SLOME is a novel epileptic seizure phenotype prevalent in older cats, unrelated to audiogenic triggers, and occasionally accompanied by generalised tonic–clonic seizures. Levetiracetam could result in dramatic improvement.