Although the day when we have cage side MRI isn't far away, for the moment most of us do not have easy access to an MRI. How do we diagnose brain herniation, with confidence, without advanced imaging? Since 1901 the Cushing's reflex has been used to describe a relationship between blood pressure, heart rate, and intracranial pressure and touted as a method for identification of eminent brain herniation. How well does it actually correlate in veterinary patients?
First, what is the Cushing's reflex?
The Cushing's reflex is described mathematically as: CPP = MABP - ICP (*CPP = cerebral perfusion pressure; MABP = mean arterial blood pressure; ICP = intracranial pressure). What this means in practical terms is that as the intracranial pressures rises over a specific threshold, mean arterial blood pressure rises to maintain brain perfusion. Baroreceptors then trigger bradycardia to account for the rise in systemic blood pressure. This elegant system is in place to keep all of the brain perfused and in place. When the system fails, the Monro-Kellie doctrine kicks in.
Okay, seriously, what is the Monro-Kellie Doctrine?
Remember when we learned that pressure = volume in a fixed space? That concept is named after Drs. Monro and Kellie (I haven't Googled it but I'm guessing they were male physicists from a few decades ago.) This concept applies to the cranial vault as well. As volume in the skull increases (hemorrhage, edema, tumor, etc), pressure increases. When pressure gets too high, herniation occurs. There are several types of brain herniation but the key point is that they all result in a shift of brain tissue from an area of high pressure to an area of lower pressure. By doing so, one or both areas often becomes compressed. If you do this too much, brain tissue will die. Now, I don't want to be narrow minded but since the brain is the best part of the body (sorry cardiology), if brain death occurs, organism death occurs. :)
Study Results
A recent study looked at the relationship between heart rate (HR), blood pressure (BP) and Modified Glasgow Coma Scales (MGCS). Dogs with brain herniation had significantly higher BP and lower MGCS compared to control dogs. Interestingly, HR was not significantly different between groups. The researches subtracted HR from BP and found a significant difference in this value between groups as well. Through ROC analysis a cut off BP of > 140, BP-HR >/= 80 and MGCS </= 14 identified herniation with a specificity of 100%, and a sensitivity of 24% I interpret this that these tests can identify who does NOT have brain herniation but are not highly sensitivity to detect those that DO have brain herniation. (DOI:10.1111/vec.13147)
What is the Take-Home Message?
When presented with a dog with decreased mentation, with or without a trauma history, perform a BP and HR measurement and use the reference cut-off values above to determine if intervention for elevated intracranial pressure and therefore brain herniation should be started. (ex: Mannitol or hypertonic saline.) Remember that these cut offs are great at determining if you shouldn't worry but did not have a high sensitivity. Err on the side of worrying and intervene earlier, rather than later!
Thanks for reading! I hope you have a great week and stay warm!