Non-infectious Causes of Meningitis and Encephalitis

Granulomatous Meningoencephalomyelitis (GME)

GME is a well recognized idiopathic disease of the CNS (10 white matter). The incidence is lowest in very young and very aged dogs, although dogs of any age and sex can be affected. The age range is typically 6 months to 11 years of age (30% of dogs in one study were <2 years of age). There is no significant breed predisposition, with a wide range of purebred and mongrel dogs being affected.

The etiology is currently unknown. Immune-mediated, infectious, and neoplastic causes have been theorized, although autoimmunity is the most scientifically supported theory at this time. MHC class II and CD3 antigen + lymphocytes are prominent in this disease, suggesting a T cell mediated delayed type hypersensitivity reaction.

There are 3 different forms of GME: Focal GME, Disseminated/Multifocal GME, and Ocular GME. Clinical signs reflect the location and nature of the lesion. The most common neuroanatomic sites for GME are the optic nerves, the cerebrum, the vestibular-cerebellar system, and the cervical spinal cord.

Common clinical sign combinations
1. cervical pain +/- lateralizing hemiparesis
2. nystagmus, head tilt, ataxia, tight circling (facial and/or trigeminal paralysis)
3. blindness with absent or decreased PLRs
4. seizures, pacing, wide circling, behavior change, head pressing

Disseminated GME often has an acute onset with rapid progression of signs (1-8 weeks). Focal GME is more insidious with progression over 3-6 months. GME can clinically resemble many other CNS diseases.

Diagnosis: CSF Analysis
1. Cisternal:
• WBC: 800 +/- 300 cells/μl
• protein: 255+/- 98 mg/dl
2. Lumbar: 
• WBC: 533 +/- 256 cells/μl
• protein: 163 +/- 25 mg/dl
3. Protein  
• acute: gamma globulin
• chronic: albumin
4. WBC differential varies - 98% lymphocytes & 2% monos to 7% lymphocytes & 42 % monos
5. Occasional plasma cells
6. Neutrophils – 2/3 cases: 18%-20% (occ. >60%)
7. Anaplastic reticulum cells - occasionally
8. NO association between severity of meningeal involvement of degree of necrosis in lesions and WBC count, differential, or protein level [...]