Granulomatous meningoencephalitis (GME) is an idiopathic, non-infectious inflammatory disease of the central nervous system (CNS) that primarily affects dogs but can rarely occur in cats.
Overview and Clinical Importance
Granulomatous meningoencephalitis (GME) is an idiopathic, non-infectious inflammatory disease of the central nervous system (CNS) that primarily affects dogs but can rarely occur in cats. In cats, this condition is more commonly referred to as Feline Meningoencephalomyelitis of Unknown Origin (FMUO) or simply Meningoencephalitis of Unknown Origin (MUO). The disease is characterized by granulomatous inflammation with perivascular cuffing of mononuclear cells within the CNS white matter.
While infectious causes of meningoencephalitis (such as FIP, toxoplasmosis, and cryptococcosis) are far more common in cats, understanding FMUO is crucial for the NAVLE because it represents an important diagnosis of exclusion that requires systematic rule-out of infectious etiologies and responds to immunosuppressive therapy.
High-YieldOn the NAVLE, when presented with a cat showing progressive neurological signs, always consider infectious causes FIRST (FIP, toxoplasmosis, cryptococcosis). FMUO/GME is a diagnosis of exclusion made only after infectious diseases have been ruled out.
| Histological Feature |
Description |
| Perivascular cuffing |
Dense aggregates of mononuclear cells (lymphocytes, macrophages, plasma cells) arranged in whorling patterns around blood vessels |
| Cell composition |
Predominantly lymphocytes and macrophages (epithelioid histiocytes), with variable numbers of plasma cells and occasional multinucleate giant cells |
| Location |
Predominantly white matter of cerebrum, cerebellum, brainstem, and spinal cord |
| Granuloma formation |
Perivascular cuffs may merge to form nodular granulomas that can mimic space-occupying masses |
Etiology and Pathogenesis
Unknown Etiology
The exact cause of GME/FMUO remains unknown. The condition is classified as idiopathic and non-infectious. Several hypotheses have been proposed:
- Autoimmune mechanism: Aberrant immune response directed against CNS antigens
- Delayed hypersensitivity: T-cell mediated inflammatory response
- Unidentified infectious trigger: Possible viral or other agent triggering immune dysregulation
- Genetic predisposition: Certain breeds (in dogs) show higher susceptibility
Histopathological Features
The hallmark histopathological finding in GME is perivascular cuffing with accumulation of inflammatory cells around blood vessels in the CNS. Key features include:
| Parameter |
Canine GME |
Feline MUO |
| Prevalence |
Common (up to 25% of CNS disorders) |
Rare (approximately 11% of feline encephalitis cases have no infectious agent) |
| Age |
Young to middle-aged (2-6 years) |
Variable (median 9.4 years in one study; range 1.7-17.8 years) |
| Sex predilection |
Female dogs more commonly affected |
No clear sex predilection in cats |
| Breed predisposition |
Small/toy breeds (Pugs, Maltese, Yorkshire Terriers, Chihuahuas) |
No breed predisposition; Domestic Shorthairs most commonly affected (reflects general population) |
Epidemiology and Signalment
Species Comparison
NAVLE TipUnlike dogs where GME is relatively common and shows clear breed predispositions, feline MUO is rare and must be differentiated from far more common infectious causes. Always rule out FIP, toxoplasmosis, and cryptococcosis before considering an immune-mediated cause in cats.
| Neuroanatomical Location |
Associated Clinical Signs |
| Forebrain (Cerebrum) |
Seizures, altered mentation, circling, head pressing, behavioral changes, contralateral visual deficits with normal PLR |
| Brainstem |
Cranial nerve deficits, vestibular signs (head tilt, nystagmus), altered consciousness, tetraparesis |
| Cerebellum |
Hypermetria, intention tremor, broad-based stance, vestibular ataxia, absent menace with normal PLR |
| Spinal cord |
Paresis/paralysis, proprioceptive deficits, spinal hyperesthesia, urinary/fecal incontinence |
| Multifocal |
Most common presentation; combination of above signs; asymmetric deficits |
Clinical Signs
Presentation Patterns
Clinical signs in cats with FMUO reflect the location and extent of CNS inflammation. The onset is typically acute to subacute with progressive deterioration over days to weeks.
Most Common Clinical Signs in Feline MUO
- Ataxia (most common neurological sign - approximately 84% of cases)
- Proprioceptive deficits
- Seizures (often the presenting complaint)
- Spinal hyperesthesia
- Vestibular signs (head tilt, nystagmus)
- Depression/altered mentation
Extraneural Signs
Some cats with FMUO may present with systemic signs including fever/hyperthermia, weight loss, hyporexia, and peripheral leukocytosis. These findings may initially suggest an infectious etiology and emphasize the importance of thorough diagnostic workup.
High-YieldSeizures are a major presenting complaint in feline MUO. In young to middle-aged cats with new-onset seizures and no prior history, inflammatory CNS disease (including MUO) should be high on the differential list.
| Category |
Differential |
Key Distinguishing Features |
| Viral |
Feline Infectious Peritonitis (FIP) |
Most common infectious cause; young cats; hyperglobulinemia; CSF protein greater than 2 g/L; periventricular enhancement on MRI; FCoV antibodies in CSF |
| Protozoal |
Toxoplasmosis |
Concurrent systemic signs (pneumonia, hepatitis); uveitis; outdoor/hunting history; positive serology; responds to clindamycin |
| Fungal |
Cryptococcosis |
Often begins with upper respiratory signs; nasal discharge; positive latex agglutination test; organisms visible in CSF |
| Bacterial |
Bacterial meningitis |
Often extension from otitis media/interna; neutrophilic pleocytosis; positive culture; bacteria visible on cytology |
| Neoplastic |
Lymphoma |
Second most common intracranial neoplasm; FeLV association; atypical cells on CSF cytology; discrete mass on MRI |
| Parasitic |
Cuterebra larva migration |
Seasonal (summer/fall); acute onset; asymmetric signs; eosinophilic pleocytosis |
Differential Diagnosis
The differential diagnosis for feline meningoencephalitis is extensive. Infectious causes are far more common in cats than immune-mediated disease and must be systematically ruled out.
| CSF Parameter |
Normal Feline Values |
Findings in FMUO |
| Nucleated cell count |
Less than 5 cells/μL |
Mildly to moderately increased (median 62-70 cells/μL); may be normal in some cases |
| Protein |
Less than 25-45 mg/dL |
Mildly to moderately increased; albuminocytologic dissociation may occur |
| Pleocytosis type |
N/A |
Mixed (most common), lymphocytic, or mononuclear pleocytosis |
| PCR testing |
Negative |
Negative for FCoV, Toxoplasma, and other infectious agents (required for MUO diagnosis) |
Diagnostic Approach
A systematic approach is essential to rule out infectious causes before diagnosing FMUO. FMUO is a diagnosis of exclusion that requires negative infectious disease testing, compatible clinical findings, and characteristic imaging/CSF results.
Initial Workup
- Complete blood count (CBC): May show leukocytosis; rule out lymphoma
- Serum biochemistry: Hyperglobulinemia suggests FIP
- FeLV/FIV testing: Rule out retroviral infection and lymphoma risk
- Toxoplasma gondii serology: IgM and IgG titers
- Cryptococcus antigen test: Latex agglutination in serum
Cerebrospinal Fluid (CSF) Analysis
CSF analysis is essential for diagnosis of inflammatory CNS disease. CSF should be collected from the cerebellomedullary cistern (atlantooccipital) after ruling out elevated intracranial pressure.
High-YieldIn FIP, CSF protein is typically greater than 2 g/L (200 mg/dL) with greater than 100 cells/μL and predominantly neutrophils. In contrast, FMUO shows milder changes with mixed or lymphocytic pleocytosis. Some FMUO cases may have normal CSF (albuminocytologic dissociation only).
Magnetic Resonance Imaging (MRI)
MRI is the imaging modality of choice for evaluating CNS inflammatory disease. While MRI cannot definitively diagnose FMUO, it helps characterize lesions and differentiate from neoplasia.
NAVLE TipIn FIP, look for periventricular and ependymal enhancement, ventricular dilation, and meningeal enhancement. FMUO typically shows more parenchymal involvement with ill-defined margins. However, significant overlap exists, and definitive differentiation requires CSF analysis and infectious disease testing.
| MRI Sequence |
Findings in Inflammatory CNS Disease |
| T1-weighted |
Iso- to hypointense lesions; may appear normal |
| T2-weighted |
Hyperintense, ill-defined intraparenchymal lesions (seen in approximately 50% of cats) |
| FLAIR |
Hyperintense lesions; may be more conspicuous than T2 |
| T1 post-contrast |
Contrast enhancement (seen in approximately 71% of cats); may be focal, multifocal, or meningeal; homogeneous enhancement typical |
| Distribution |
Multifocal lesions most common; may involve forebrain, brainstem, cerebellum, or spinal cord |
Treatment
Treatment of FMUO is based on immunosuppression. Since the underlying etiology is suspected to be immune-mediated, the goal is to suppress the aberrant inflammatory response in the CNS. Treatment is typically lifelong.
Corticosteroid Therapy
Corticosteroids are the mainstay of treatment for FMUO. Initial immunosuppressive doses are gradually tapered to the lowest effective maintenance dose.
Secondary Immunosuppressive Agents
Addition of a second immunosuppressive agent is indicated for cats that fail to respond to corticosteroids alone, have frequent relapses, or develop intolerable steroid side effects. These drugs allow steroid-sparing effects.
Supportive Care
- Anticonvulsants: Phenobarbital (2-4 mg/kg PO q12h) or levetiracetam (20 mg/kg PO q8h) for seizure control
- Gastroprotection: Omeprazole (1 mg/kg PO q24h) for cats on high-dose corticosteroids
- Physical therapy: For cats with residual neurological deficits
- Environmental modifications: Shallow litter boxes, non-slip surfaces, padded bedding
| Drug |
Initial Dose |
Maintenance |
Notes |
| Prednisolone |
1-2 mg/kg PO q12h |
Taper over weeks to lowest effective dose (0.5-1 mg/kg q24-48h) |
First-line therapy; use prednisolone (not prednisone) in cats due to better bioavailability |
| Dexamethasone |
0.15-0.25 mg/kg IV/PO q24h |
Switch to prednisolone for long-term |
May be used initially for severe/acute cases; greater potency but more side effects |
Prognosis
The prognosis for cats with FMUO is guarded to fair with appropriate treatment. Recent studies suggest that with early diagnosis and aggressive immunosuppression, many cats can achieve clinical remission.
High-YieldGood prognosis in feline MUO is highlighted by recent studies showing that most cats survive to discharge with appropriate treatment, and more than half achieve clinical remission. This contrasts with the historically poor prognosis for all cats with inflammatory CSF (77% dying within 1 year) - but that statistic includes FIP and other infectious causes with worse outcomes.
| Drug |
Dose |
Mechanism |
Monitoring |
| Cytarabine (Cytosar-U) |
200 mg/m² SC divided over 2 days, q3-6 weeks |
Antimetabolite; good CNS penetration |
CBC before each treatment; bone marrow suppression |
| Cyclosporine (Atopica) |
5-10 mg/kg PO q12-24h |
Calcineurin inhibitor; T-cell suppression |
Trough levels; GI side effects; gingival hyperplasia |
| Lomustine (CCNU) |
50-70 mg/m² PO q6-8 weeks |
Alkylating agent; crosses blood-brain barrier |
CBC 7-10 days and before each dose; cumulative hepatotoxicity |
| Prognostic Factor |
Outcome Association |
| Response to initial therapy |
Cats showing improvement within 2-3 weeks generally have better long-term outcomes |
| Lesion distribution |
Focal lesions may have better prognosis than multifocal disease |
| Survival with treatment |
Median survival 5-12+ months; some cats survive years with maintenance therapy |
| Relapse rate |
Relapses are common; many cats require lifelong immunosuppression |