Meningoencephalomyelitis of unknown etiology (MUE) is a challenging antemortem diagnosis. MUE is the term used when we diagnosed inflammatory, non-infectious disease in the CNS. In the old days, we would have called this GME, NME or NLE, depending on the breed. However, those three inflammatory conditions cannot be distinguished on clinical picture, diagnostic testing, or treatment response, alone. Histopathology is needed to determine if a patient has GME, NME, and NLE. Without a brain biopsy, or post mortem sample, we cannot call it GME, NME or NLE. Several years ago, the term MUE started getting tossed around as an antemortem term for inflammatory brain disease, without a biopsy.
To diagnose MUE, a patient must have:
1) a neurologic examination with deficits identified (or a history of seizures) and
2) have a MRI with classical changes and
3) and CSF tap with a lymphocytic or monocytic pleocytosis and/or elevated protein level and
4) negative geographically specific infectious disease panel. If the pet meets these criteria, they can be diagnosed with MUE.
MUE does not show up in routine blood work. Inflammatory leukograms are rare, elevated body temperature is not typically reported and pets are not systemically, clinically ill. However, some immune mediated diseases (such as MS) have garnered attention for a neutrophil to lymphocyte ratio present on routine CBC.
A recent study out of Korea evaluated the ratio of neutrophils to lymphocytes in a standard CBC to see if we could predict a diagnosis of MUE. As it turns out, you can divide the neutrophil count by the lymphocyte count, and get a significantly higher number in dogs with MUE than healthy dogs. Furthermore, the ratio was significantly higher than dogs with idiopathic epilepsy but NOT different than dogs with brain tumors. The neutrophil-lymphocyte ratio was higher in dogs with MUE than all other forebrain diseases (hydrocephalus, idiopathic epilepsy, and brain tumors) but not always significantly so.
What does this mean for us?
Unfortunately, nothing...yet. This is an interesting concept and a step towards a much needed tool for diagnosing MUE without invasive diagnostic testing. Performing a CBC, and calculating the ratio, is both simple and readily available in most (all?) veterinary practices today. However, if the ratio is high, it does not mean the pet has MUE. It may support a diagnosis of forebrain disease (which you already know from your neuro exam) and may support structural brain disease (i.e. not idiopathic epilepsy). Very importantly - this study did not find a significant difference between dogs with brain tumors and those with MUE. Sadly, brain tumors and MUE are often our top differentials for pets with forebrain disease so this ratio does not (yet) give us another non-invasive tool to differentiate between these two diseases.
I enjoyed reading this study and learning about inflammatory markers for humans with MS (the human counterpart to MUE) and how this neutrophil-lymphocyte ratio could be used in the future for pets. If you'd like to read more, please see the link at the bottom of the page.
Thanks for reading! I hope you have a wonderful week and look forward to working with you soon!
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Reference: https://onlinelibrary.wiley.com/doi/epdf/10.1111/jvim.16512