Non‐rim‐enhancing intracranial abscess in a cat confirmed by MRI and histopathology

A 6-year-old male neutered domestic shorthair cat presented with progressive lethargy and abnormal mentation. Neuroanatomical localisation was multifocal, including the right forebrain and brainstem. Haematology and serum biochemistry were consistent with a systemic inflammatory response. Thoracic radiographs revealed a cranial mediastinal mass with pleural effusion and a mixed broncho-interstitial lung pattern with pulmonary nodules. Brain MRI (1.5T, Philips Intera) revealed a solitary right occipital intra-axial mass with marked vasogenic oedema and mass effect. The lesion was hyperintense on T2w, FLAIR and DWI, with corresponding low signal on ADC map, and was T1w hypointense without contrast enhancement (Fig 1C,D). A thin peripheral hypointense rim was identified in all sequences. Differentials included intracranial abscess or, less likely, intra-axial neoplasia (e.g. low-grade glioma). The cat was humanely euthanised. Post-mortem and histopathological examination of the right occipital mass revealed necro-suppurative encephalitis with neutrophilic perivascular cuffing and moderate gliosis, with absence of a fibrovascular capsule (Fig 1G,H), consistent with an intracranial abscess. Concurrently, there were pulmonary and mediastinal pyogranulomas and pleuritis with non-acid fast intralesional bacteria; the intracranial abscess was speculated to be secondary following haematogenous spread. The lack of rim enhancement may have reflected the histological absence of a fibrovascular capsule. To the authors' knowledge, this is the first reported non-enhancing intracranial abscess in a cat confirmed by MRI and histopathology, highlighting that intracranial abscessation may present without classical ring-enhancing features. A. Vila Cabaleiro: Conceptualization; investigation; writing – original draft; methodology; validation; visualization; writing – review and editing; resources. C. Llanos: Conceptualization; investigation; methodology; validation; visualization; writing – review and editing; supervision. H. G. Miller: Conceptualization; investigation; methodology; validation; visualization; writing – review and editing; supervision. S. Wyatt: Supervision; methodology; validation; visualization; writing – review and editing; investigation; conceptualization. G. O. Bertoldi: Conceptualization; investigation; methodology; validation; visualization; writing – review and editing. E. Fitzgerald: Supervision; methodology; validation; visualization; writing – review and editing; conceptualization; investigation. The authors declare no conflicts of interest.