other names: Meningoencephalitis of Unknown Origin (MUO), Immune-Mediated Meningoencephalitis, Non-Infectious Meningoencephalitis, Inflammatory Brain Disease
Overview
Meningoencephalomyelitis of Unknown Etiology (MUE) is a group of diseases where the body’s immune system mistakenly causes inflammation in the brain, spinal cord, and the protective layers around them. It usually affects dogs but can also happen in cats. Because it involves multiple parts of the nervous system, it can cause a wide variety of neurological problems.
Causes
Meningoencephalomyelitis of Unknown Etiology is an immune-mediated disease, meaning it arises from an inappropriate or misdirected immune response in which the body attacks its own central nervous system (CNS) tissues. Unlike infections caused by bacteria, viruses, fungi, or parasites, this disorder is not linked to any known infectious agents.
There is no single known cause of MUE. It’s likely the result of multiple factors that can vary between dogs. Possible contributors include genetic predispositions, reactions related to vaccines, or inappropriate responses to things they eat or are exposed to in their environment.
Clinical Signs
Symptoms of Meningoencephalomyelitis of Unknown Etiology can appear suddenly and often get worse quickly. Signs vary depending on which part of the brain or spinal cord is affected, but common ones include seizures, changes in behavior, confusion or disorientation, trouble walking, a head tilt, unusual eye movements, or neck pain.
MUE is most often seen in young, small-breed dogs—such as Maltese, Shih Tzus, Pugs, Yorkshire Terriers, Toy Poodles, and Chihuahuas—but it can affect dogs of any age, size, or breed, and even cats.
Diagnosis
Diagnosing MUE usually requires advanced testing, including an MRI and analysis of the cerebrospinal fluid (CSF). An MRI of the brain and spinal cord can show patterns of inflammation that are typical for MUE. CSF analysis, obtained through a spinal tap, is an important part of the process and often reveals inflammation along with abnormal cell and protein levels, which help support the diagnosis. In some cases, the brain inflammation is so severe that performing a spinal tap isn’t safe, and a presumptive diagnosis is made based on MRI findings alone.
Because some infections can look very similar to MUE on both MRI and CSF analysis, a panel of specialized blood tests is often run to rule out these infectious diseases.
Treatment
Treatment is primarily focused on suppressing the overactive immune response that drives the condition. Steroids are the mainstay of therapy. Initial treatment usually involves giving high doses of steroids to quickly reduce inflammation. The dose is then slowly decreased over several months, and the patient may be kept on a low dose of prednisone long term to help prevent relapse.
To improve long-term disease control and minimize steroid-related side effects, additional immunosuppressive agents are often introduced that more precisely regulates immune function. These include Cytosar (cytarabine arabinoside), Atopica (cyclosporine), lefulonmide, or other immunomodulants.
Cytosar is an injectable immunosuppressant typically given every three weeks for the first few months while the steroid is slowly tapered. This medication has the strongest evidence supporting its effectiveness in treating MUE. It can be administered either as an IV infusion or as a subcutaneous injection (under the skin). If the patient stabilizes neurologically and does well during steroid wean, the interval between Cytosar doses can be gradually extended. The dosing schedule is guided by the patient’s response, with the goal of using the lowest effective steroid dose and the least frequent Cytosar schedule that still controls symptoms. In some—though not all—cases, prednisone or Cytosar may eventually be discontinued, but this usually isn’t considered until at least a year after starting treatment.
Oral immunosuppressants like Atopica or leflunomide are daily pills that can also be used to treat MUE. Unlike Cytosar, these medications don’t require frequent hospital visits and are often more affordable. However, they may have more side effects, and there is less research available to confirm how well they work compared to Cytosar.
Additional medications may be required to manage specific clinical signs. These can include anti-seizure medications if seizures are present, pain medications for any observed discomfort, or other supportive therapies as needed.
Prognosis
The overall response to treatment for MUE is variable, though encouraging in many cases with prompt and aggressive medical intervention. Approximately 80-90% of affected animals demonstrate clinical improvement when appropriate treatment is initiated early.
Most of these patients experience lasting neurological improvement and go into sustained remission with medical treatment. Many of these pets can eventually have their medications slowly reduced to a very low dose—or even stopped completely. However, some pets initially improve on high-dose immunosuppressive therapy but relapse when the medication is reduced. These patients often need higher-dose, long-term treatment to keep the disease under control and maintain a good quality of life.
Unfortunately, the remaining 10-20% of casesdo not respond to therapy and may experience continued neurologic worsening despite aggressive treatment, ultimately resulting in euthanasia or death due to progressive disease.