Meningoencephalitis of Unknown Origin (MUO)

What is meningoencephalitis of unknown origin?

Meningoencephalitis is inflammation of the brain and the surrounding tissues and fluid. Meningoencephalitis of unknown origin (MUO) is a group of inflammatory brain diseases in which no infectious cause can be identified. In dogs, MUO can be divided into several types, including:

• Granulomatous meningoencephalomyelitis (GME)
• Necrotizing meningoencephalitis (NME)
• Greyhound meningoencephalitis
• Idiopathic eosinophilic meningoencephalitis (IEM)
• Necrotizing leukoencephalitis (NLE)s

Each type of meningoencephalitis causes different changes within the brain and spinal cord, and there are slight differences in treatment and prognosis among the various forms of the disease. However, all these conditions are caused by an abnormal immune response directed against the patient’s own tissues.

In many cases, a diagnosis of MUO is made when MRI and cerebrospinal fluid (CSF) analysis suggest inflammatory, non-infectious disease of the central nervous system (CNS), but a definitive diagnosis cannot be confirmed through microscopic examination of nervous tissue (histopathology). If microscopic examination of nervous system tissue is impossible or impractical, meningoencephalitis of unknown origin may be diagnosed.

Are certain dogs at a higher risk for MUO?

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

What are the signs of MUO?

Signs of MUO vary depending on which areas of the brain are affected by inflammation. Signs may appear suddenly or develop gradually over weeks to months. In most cases, signs of MUO include:

• Seizures
• Muscle tremors
• Blindness
• Vestibular signs (dizziness or falling over)
• A head tilt
• Abnormal behaviors, such as compulsively walking in circles
• Pain or sensitivity when touched
• Paralysis

How is MUO diagnosed?

Your veterinarian will begin with a physical exam and bloodwork to assess your dog’s cell counts and internal organ function. If this bloodwork 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. MRI uses a strong magnetic field to create three-dimensional images 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.

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 cerebrospinal fluid (CSF), 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 fluid is analyzed and examined under a microscope to determine which cells are present. The results of CSF analysis can be used to assess for the presence of meningoencephalitis.

How is MUO treated?

Affected dogs are generally treated with immunosuppressant drugs, because MUO is typically caused by an underlying autoimmune condition. Commonly used drugs include prednisone, dexamethasone, cyclosporine, and azathioprine. Your veterinarian may also prescribe antibiotics such as doxycycline or clindamycin if an infectious cause of MUO hasn’t been ruled out. Other medications may be needed to treat the signs of MUO, such as anti-seizure medications (i.e. phenobarbital, midazolam or levetiracetam).

What is the prognosis for MUO?

Survival rates in dogs with MUO depend on multiple factors, including the type of meningoencephalitis and clinical signs. Dogs with confined focal lesions have a better prognosis than those with multifocal (widespread) disease. Dogs that have seizures at the time of diagnosis often have a worse prognosis than dogs without seizures.

One of the best predictors of prognosis, however, is the response to therapy. Dogs that show improvement on MRI 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 (uncertain).