We have talked about meningitis a lot over the years! Meningitis of unknown origin (MUO) is an antemortem diagnosis that is made when a dog has clinical signs, MRI findings and CSF results, along with negative infectious disease testing suggestive of immune mediated inflammation of the CNS. Once we perform a necropsy on these patients we can call it GME, NME or any of the other alphabet-soup based terms used to describe specific inflammatory diseases in the CNS. The typical presentation for MUO is young to middle aged small breed dogs with multifocal clinical signs. A recent study investigated MUO in dogs over 8 years of age and compared it to dogs diagnosed at less than 8 years old. I think it highlights a need for us to remember this important disease in our older patients. Read on to learn more...

Study design

Multicenter retrospective case-control study
Two UK referral hospitals (Liverpool and Royal Veterinary College)
Cases from 2008–2023
71 dogs ≥8 years old
142 dogs <8 years old
Presumptive MUO diagnosis based on:

neurological examination
MRI
CSF analysis (when possible)
exclusion of infectious disease

Histopathology available in only a minority of cases in this study(14 necropsies). Remember, histopathology confirms a non-infectious, non-cancer cause.

What are the take away messages?

1. Older dogs present differently

Compared with younger dogs, older dogs were significantly more likely to have:

behavioral abnormalities
cranial nerve deficits
concurrent systemic disease (comorbidities)

Independent predictors of late-onset MUO included:

behavioral changes
cranial nerve deficits

This suggests that we should avoid dismissing MUO in geriatric dogs simply because of age. Behavior change in an older dog is commonly attributed to canine cognitive dysfunction, intracranial neoplasia and metabolic disease but this paper demonstrates that immune-mediated encephalitis should remain an important differential diagnosis.

2. Survival is not worse in older dogs

Median survival:

Younger dogs: 24 months
Older dogs: 16 months

Difference:

Not statistically significant (P = 0.48)

3. Relapse rates were similar

Overall relapse:

Younger dogs:

Older dogs:

Despite the numerical difference, multivariable analysis showed:

Age was not an independent predictor of relapse. Importantly, it was comorbidity—not age—that adversely affected survival!

So, what do we take away from this study? We should remember that older dogs with cranial nerve deficits or signs of cognitive dysfunction might benefit from a steroid trial. If they markedly improve, consider MUO. A neurology consult is another great idea. 😊

Thanks for reading! I hope you have a good week and look forward to working with you soon!