Meningeal null cell lymphoma causing diffuse pachymeningeal contrast enhancement in a dog.

Diffuse pachymeningeal contrast enhancement is an uncommon imaging finding in dogs and current understanding of its aetiologies in veterinary medicine is limited. A 2-year-old female neutered Pug presented with chronic progressive vestibular signs, facial nerve paralysis, obtundation and episodic decerebellate rigidity. A magnetic resonance imaging study of the head revealed diffuse pachymeningeal thickening and contrast enhancement involving the caudal fossa and falx cerebri. Cerebrospinal fluid analysis revealed marked neutrophilic pleocytosis. Infectious disease testing was negative and a tentative diagnosis of intracranial idiopathic hypertrophic pachymeningitis was made. Immunosuppressive treatment resulted in a short period of clinical improvement. The patient subsequently suffered multiple relapses which failed to respond to alternative immunosuppressive strategies, necessitating euthanasia. Histopathological findings supported a meningeal null cell lymphoma. While rare, neoplastic causes of diffuse pachymeningeal thickening and contrast enhancement should be considered.