• Meningitis: inflammation of the meninges (the pia, arachnoid and dura mater; protective membranes covering the brain
and spinal cord).
• Encephalitis: inflammation of the brain.
• Myelitis: inflammation of the spinal cord.
• Meningoencephalitis: inflammation of the meninges and brain.
• Meningomyelitis: inflammation of the meninges and spinal cord.
Meningoencephalitis and meningomyelitis are common problems in small animal medicine. They may affect any breed of dog or
cat, and animals of multiple ages, although certain disease processes are more common in specific breeds and age ranges (see
below). Small and toy breed dogs in particular are frequently afflicted with inflammatory central nervous system (CNS) disorders.
A variety of clinical signs may be seen with inflammatory CNS disease, depending on what area of the nervous system is affected.
Involvement of the forebrain (cerebral cortex and thalamus) may result in seizures, altered mentation (stupor, coma, delirium,
head pressing), vision abnormalities, and compulsive pacing or circling. Involvement of the brainstem (pons and medulla oblongata)
may result in altered mentation, cranial nerve deficits (e.g., vestibular dysfunction [head tilt, nystagmus], facial nerve
paralysis, swallowing difficulties, tongue paralysis), ataxia, and proprioceptive deficits. Spinal cord involvement leads
to ataxia, paresis, and possibly segmental spinal reflex deficits. Most inflammatory CNS conditions result in pain due to
involvement of the meninges. A hallmark of these conditions is the presence of multifocal neurologic signs; that is, involvement
of multiple areas of the CNS (e.g., forebrain, brainstem and spinal cord).
The majority of small animals with meningoencephalitis or meningomyelitis have signs restricted to the nervous system. However,
involvement of other organ systems is possible with some infectious agents (e.g., systemic fungal or bacterial infections,
rickettsial disease). Contrary to what many believe, fever is quite uncommon in most meningoencephalitis patients.
The diagnosis of inflammatory CNS diseases in small animal patients involves consideration of the signalment, reported and
observed clinical signs, and results of diagnostic testing. The most useful tests are advanced diagnostic imaging (particularly
magnetic resonance imaging [MRI]), and cerebrospinal fluid (CSF) analysis. Although computed tomography (CT) is of some use
in demonstrating brain lesions, MRI is the gold standard for brain imaging, and is much more sensitive in showing inflammatory
lesions of the brain and spinal cord. Inflammatory lesions are typically hyperintense on T2-weighted, STIR, and FLAIR images,
hypointense or isointense on T1-weighted images, and enhance to varying degrees after intravenous contrast administration
(gadolinium compounds). Analysis of CSF in these patients typically shows elevated white blood cell numbers (pleocytosis)
and protein. Cytology should be performed to determine the types of inflammatory cells present. Electrodiagnostic testing
is less frequently used, but may provide useful functional information to complement the imaging and CSF results. Electroencephalography
(EEG) and brainstem auditory evoked response (BAER) provide such information about the cerebral cortex and brainstem pathways
respectively. Finally, testing for infectious disease agents may involve quantification of antibody titers or polymerase
chain reaction (PCR)-based testing on blood and CSF samples.
The therapy of meningoencephalitis and meningomyelitis should be tailored to the specific etiology or suspected syndrome,
while taking into account the severity of the clinical signs, comorbid disease processes, and financial constraints of the
owner. Patients with identified infectious etiologies should be treated with appropriate antimicrobial medications, which
may include antibiotics, antifungals, or antiprotozoals. Whenever possible, medications with good penetration of the blood-brain
barrier should be selected for therapy. Such choices often include doxycycline, sulfonamide drugs, and fluconazole.
Corticosteroids are a mainstay of therapy for inflammatory CNS conditions, regardless of etiology. Although immunosuppression
and long-term treatment with corticosteroids is usually contraindicated for infectious processes, short-term therapy can dramatically
reduce the inflammatory response, and can be life-saving. In addition, corticosteroids form the cornerstone of therapy for
the inflammatory conditions without an underlying identifiable etiology. Although many animals benefit from anti-inflammatory
doses of these medications, immunosuppressive doses are often required for disease remission. The doses of corticosteroids
are generally reduced over an extended period of time to the lowest dose that controls clinical signs.
Many clinicians add additional immunomodulatory or cytotoxic medications to the therapeutic regimen of patients with meningoencephalitis
of unknown etiology (MUE). Such drugs include cytosine arabinoside (cytarabine, Ara-C), cyclosporine A, leflunomide, mycophenolate
mofetil, lomustine (CCNU), azathioprine, and procarbazine. These medications are useful to provide additional immunosuppression
through alternate mechanisms of action, and also to reduce the side effects associated with glucocorticoid therapy, which
include polyuria, polydipsia, polyphagia, weight gain, panting, gastrointestinal ulceration, coagulopathies, pancreatitis,
and steroid hepatopathy.