Meningoencephalitis of Unknown Origin

By Malcolm Weir, DVM, MSc, MPH; Catherine Barnette, DVM

What is meningoencephalitis of unknown origin?

Meningoencephalitis is a term referring to inflammation of the brain and the surrounding fluid and tissues. In canine patients, meningoencephalitis can often be further characterized into one of four types: granulomatous meningoencephalomyelitis (GME), necrotizing meningoencephalitis (NME), greyhound meningoencephalitis, and necrotizing leukoencephalitis (NLE). Each type of meningoencephalitis differs in the specific brain and spinal cord changes observed at the cellular level and there are slight variations in the management and prognosis for each disease. However, all four conditions are caused by an abnormal immune response directed against the patient’s own tissues.

Meningoencephalitis of unknown origin (MUO) is a term used to describe those cases of meningoencephalitis in which MRI and cerebrospinal fluid (CSF) analysis indicate inflammatory, non-infectious central nervous system (CNS) disease but diagnosis through microscopic analysis (histopathology) is not available. If the microscopic evaluation of nervous system tissue is impossible or impractical, meningoencephalitis of unknown origin may be diagnosed.

Are certain dogs at a higher risk for meningoencephalitis of unknown origin?

MUO is most commonly seen in small-breed dogs, suggesting a possible genetic basis for the condition; a genetic risk for the condition has been identified in the Pug and Maltese breeds. Large-breed dogs, however, can also occasionally develop the disease. Females are affected more frequently than males and affected dogs are typically over six months of age.

"...a genetic risk for the condition has been identified in the Pug and Maltese breeds."

What are the clinical signs of meningoencephalitis of unknown origin?

Clinical signs of MUO involve neurologic signs, which vary depending on which areas of the brain are affected by inflammation. In most cases, signs of MUO include seizures, muscle tremors, blindness, head tilt, vestibular signs (dizziness or falling over), or a head tilt. Affected dogs may also exhibit abnormal behaviors such as compulsively walking in circles. Some dogs may appear painful or become paralyzed.

These signs may develop suddenly, or they may progress slowly over a period of weeks to months.

How is meningoencephalitis of unknown origin diagnosed?

Your veterinarian will begin with a physical exam as well as blood work to assess your dog’s cell counts and internal organ function. If this blood work does not show any abnormalities that could explain the neurologic signs, your veterinarian may recommend infectious disease testing to rule out infections that could be causing them. If these initial tests are normal and no obvious causes for the neurologic abnormalities are found, your veterinarian will likely refer you to a veterinary neurologist.

The neurologist may perform two additional tests to aid in the diagnosis of meningoencephalitis: MRI and CSF analysis. Magnetic resonance imaging (MRI) is a type of imaging that uses a strong magnetic field to generate three-dimensional views of internal body structures. With the images provided through MRI, the neurologist can assess the internal structure of your dog’s brain and look for characteristic abnormalities that may suggest meningoencephalitis. This procedure is performed under general anesthesia because your dog must remain still for a prolonged period to obtain high-quality images.

"...the neurologist can assess the internal structure of your dog’s brain and look for characteristic abnormalities that may suggest meningoencephalitis."

While your dog is anesthetized, the neurologist will also perform a procedure known as a CSF tap. The brain and spinal cord are surrounded by cerebral spinal fluid, which acts as a shock absorber and protects the brain and spinal cord. In a CSF tap, a small amount of this fluid is removed using a long needle, and the fluid can be analyzed. The composition of the fluid will be tested, and the fluid will be examined under a microscope to assess what types of cells are present. The results of CSF analysis can be used to assess for the presence of meningoencephalitis.

How is meningoencephalitis of unknown origin treated?

MUO is typically caused by an underlying autoimmune condition; therefore, affected dogs are typically treated with immunosuppressant drugs. Commonly used drugs include prednisone (Deltasone®, Meticorten®), dexamethasone (Azium®, Decadron®, Dexasone®, Dexavet®), cyclosporine (Atopica®), and azathioprine (Imuran®).

Your veterinarian may also prescribe antibiotics to address the possibility of underlying infection and to prevent infections that could worsen the autoimmune disease. These antibiotics may include doxycycline (Vibramycin®, Monodox®), enrofloxacin (Baytril®), or clindamycin (Clindadrops®, Antirobe®, Cleocin®).

What is the prognosis for meningoencephalitis of unknown origin?

Multiple factors are associated with survival rates in patients with MUO, including the type of meningoencephalitis and clinical signs. Dogs who have confined focal lesions have a better prognosis than those with multifocal (widespread) disease. Dogs who present with seizures at the time of diagnosis often have a worse prognosis than those without seizures.

One of the best predictors of prognosis, however, is the response to therapy. Dogs showing an improvement in both MRI appearance and CSF analysis after three months of treatment typically have a good long-term prognosis. If dogs do not respond within the first three months of treatment, their prognosis is guarded.

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