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 three types:

  • Granulomatous Meningoencephalomyelitis (GME)
  • Necrotizing Meningoencephalitis (NME)
  • Necrotizing Leukoencephalitis (NLE)

Different types of meningoencephalitis

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 three 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 histopathological analysis is not available as this can only be done post mortem

Breeds affected

Meningoencephalitis of unknown origin is most commonly seen in small-breed dogs but can occur in all sizes of patients. Females are affected more frequently than males and affected dogs are typically over six months of age.

Clinical signs of MUO

Clinical signs of meningoencephalitis involve neurologic signs, which vary depending on which areas of the brain or spine are affected with inflammation. Common signs of meningoencephalitis include seizures, muscle tremors, blindness, or vestibular signs (head tilt, dizziness or falling over). Affected dogs may also exhibit abnormal behaviours, such as compulsive walking in circles, head pressing. 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?

Diagnosis is made by a thorough clinical examination, full blood profile MRI and cerebrospinal fluid (CSF) analysis.

Magnetic resonance imaging (MRI) is a type of imaging that uses a strong magnetic field to generate three dimensional views of the brain and spine. MRI 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 Anaesthesia, because your dog must remain still for a prolonged period of time in order to obtain high-quality images.

mri scan

While your dog is anesthetized we will perform a procedure known as a CSF tap. The brain and spinal cord are surrounded by cerebrospinal, 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. This fluid can then be sent to a laboratory for analysis. 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. Infectious causes can also be tested for in the CSF.

Treatment of MUO

Meningoencephalitis of unknown origin is typically caused by an underlying autoimmune condition; therefore, affected dogs are typically treated with immunosuppressant drugs. Commonly used drugs include prednisolone, dexamethasone, cytosine arabinoside, cyclosporine, azathioprine, and other medications. These drugs have possible side effects and precautions.

Often antibiotics are also used to address the possibility of underlying infection and to prevent infections that could trigger worsening of the autoimmune disease. These antibiotics may include doxycycline, enrofloxacin, or clindamycin.

Prognosis

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

Perhaps one of the best predictors of prognosis, however, is the response to therapy. Dogs showing an improvement in both MRI appearance and CSF fluid 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.