Brain herniation is diagnosed when one part of the brain shifts to another part of the cranial vault. The four most common types of herniation include: 1) Midline falx herniation (from left to right side of the forebrain), 2) caudal transtentorial herniation (cerebrum herniations under the tentori cerebelli onto the cerebellum and brainstem, 3) rostral transtentorial herniation (cerebellum herniates rostrally into forebain and 4) foramen magnum herniation (cerebellum exits out the foramen magnum). Brain herniation may be acute, secondary to a sudden shift in intracranial pressure or gradual secondary to a chronic or slow rising intracranial pressure.
The most common cause of acute herniation is trauma, resulting in cytotoxic and vasogenic edema or hemorrhage with a subsequent sudden increase in intracranial pressure. Brain tumors account for the majority of cases with a gradual increase in intracranial pressure.
The Cushing's Reflex, identified by Dr. Harvey Cushing, describes a reflexive bradycardia and hypertension with a reduced level of consciousness, induced by increased intracranial pressure. Increased pressure can result in poor cerebral perfusion, and ultimately brain herniation.
A recent article evaluated cats with brain herniation noted on MRI to cats with intracranial disease WITHOUT brain herniation for signs of the Cushing's Reflex, and other clinical factors that might predict the presence of herniation. They evaluated: age, weight, heart rate, respiratory rate, temperature, blood pressure, level of mentation, Glasgow Coma Scale gait assessment and brainstem reflexes. Can you guess what they found?
There was no significant difference in any of these factors between the two groups except as related to level of consciousness and age. Cats with brain herniation were significantly older, and had a reduced level of consciousness compared to cats with intracranial disease without evidence of herniation. Wow!
Why didn't we see a Cushing's Reflex? Well, one option is that the majority of cats were diagnosed with intracranial neoplasia which we know is a slow growing process. This may have provided ample time for compensatory mechanisms so that a significant difference couldn't be determined in this study. Another option is that clinical evidence of brain herniation is simply less obvious or prevalent in cats. One prior study of cats with normal brains found incidental herniation in up to 40% of the cats!
What's the take away? Cats with brain herniation may present very similarly to cats without brain herniation . Therefore, any cat with an intracranial neuroanatomic lesion localization undergoing general anesthesia should be managed as if they have increased intracranial pressures. Do not administer anesthetic drugs that are known to markedly increased or decrease blood pressure as this may negatively affect the cerebral perfusion pressure. When in doubt, assume they have increased intracranial pressure.
Thanks for reading! This week is the 4H Dane County Fair so if you need me I'll be watching horse shows and eating cotton candy! Feel free to text or email anytime, however my live consult availability is very limited due to my kid's show schedules. Have a great week!