Herniation is not one event. It is four routes with four sets of consequences, and the imaging finding that matters most to you is the one that makes a lumbar puncture lethal.
Four routes: subfalcine under the falx, transtentorial down through the incisura, transtentorial up through it from below, and tonsillar out through the foramen magnum. Each compresses something different, so each produces a different deficit and a different consequence — the descending route compresses the posterior cerebral artery, the ascending route compresses the aqueduct, and the tonsillar route compresses the medulla. If tonsillar herniation is present on imaging, lumbar puncture must not be performed.
An enlarging mass, a haemorrhage or oedema pushes brain out of the compartment it belongs in. Where it goes determines what gets compressed, and what gets compressed determines the deficit, the imaging appearance, and what will kill the patient. Herniation is worth learning as four separate events rather than as one word.
Before the routes, the setting. The imaging features of intracranial mass effect are sulcal effacement, midline shift, basal cistern effacement, obstructive hydrocephalus and herniation — and because haemorrhages frequently progress and large contusions often develop delayed haemorrhage or oedema, serial imaging is usually indicated, particularly if the neurological status changes.
Subfalcine. A hemispheric mass pushes the cingulate or supracingulate gyri beneath the falx. It is the easiest to recognize — deviation of the falx and extension of hemispheric structures across the midline — and it is what most people mean when they say “midline shift.”
Descending transtentorial. A supratentorial mass displaces the medial temporal lobe down through the tentorial incisura. It may involve the uncus anteriorly, the parahippocampal or lingual gyri posteriorly, or be complete. On imaging, the herniated tissue widens the ipsilateral subarachnoid cistern and obliterates the contralateral one as the brainstem is rotated and displaced to the opposite side.
The consequence to anticipate is vascular: the posterior cerebral artery may be compressed between the temporal lobe and the crus cerebri, producing an occipital infarct that can be ipsilateral or contralateral.
Ascending transtentorial. An infratentorial mass pushes the pons, vermis and adjacent cerebellum upward through the incisura. The cisterns are effaced symmetrically, the upper pons is pushed forward against the clivus, and — this is the one to carry — the Sylvian aqueduct is often compressed, causing acute hydrocephalus. The mass effect generates a second mass effect.
Tonsillar. The cerebellar tonsils are displaced inferiorly through the foramen magnum into the cervical spinal canal, compressing the medulla and producing dysfunction of the respiratory and cardiac rhythm centres. MRI, with sagittal imaging, is the primary modality for demonstrating it; on CT the tonsils can be seen below the level of the foramen magnum.
If this finding is present, lumbar puncture must not be performed. The source states that it can result in the death of the patient. That is today’s pitfall and it is the only categorical instruction on the page.
There has been a lot of effort to turn herniation into a number, and the results are genuinely mixed — which is worth knowing before you quote millimetres at someone.
In subfalcine herniation, the degree of displacement of the septum pellucidum from the midline is predictive of prognosis, and a correlation between the degree of midline shift and level of consciousness has been shown. But in descending transtentorial herniation, the degree of vertical descent did not always correlate well with neurological signs.
The useful summary: the imaging is reliable about what is happening and less reliable about how bad it is. For an anaesthetist that is the right way round, because the route tells you what to anticipate and the patient tells you how fast.
Communicating hydrocephalus results from excessive CSF production by a choroid plexus tumour, or — far more commonly — from obstruction to absorption at the arachnoid villi after subarachnoid haemorrhage, meningitis or leptomeningeal carcinomatosis. CT and MR show symmetric enlargement of the lateral, third and fourth ventricles with effacement of the sulci.
Obstructive hydrocephalus follows a blockage somewhere between the lateral ventricles and the outlet of the fourth. It looks similar with one diagnostic difference: the ventricles dilate proximal but not distal to the obstruction. Aqueductal obstruction enlarges the third and lateral ventricles while the fourth stays normal, which localizes the lesion for you.
MRI is preferred for evaluating obstruction — multiplanar images show the foramen of Monro, the aqueduct and the fourth ventricle; tumours are readily seen and webs or aqueductal atresia can occasionally be identified; and CSF flow techniques help classify the obstruction.
One physiological detail links this back to Neuro 1. Under elevated pressure, CSF leaks from the ventricles into the brain, producing interstitial oedema. That is the third column of day 09, and it is why the treatment is drainage rather than a drug.
Acute haemorrhage is hyperdense on CT, and blood products lose density over time — becoming isodense to brain and then hypodense. A pseudonegative CT does not exclude haemorrhage: severely anaemic patients can present with isodense extra-axial or intraparenchymal collections, where only the presence of mass effect gives the collection away.
So in a patient who is both anaemic and neurologically abnormal, the absence of bright blood on CT is weaker reassurance than it looks, and the four features of mass effect are what you should be looking at instead.
No randomized trial addresses this.
The anatomy and the imaging appearances are settled. Each route of herniation has a described appearance on CT or MRI, and the structures each one compresses follow from where the displaced tissue goes. None of that is controversial.
The quantitative correlations are weaker than they look. Efforts have been made to correlate measured herniation — degrees of shift — with clinical outcome, and the results are mixed. In subfalcine herniation, displacement of the septum pellucidum from the midline is predictive of prognosis, and a correlation between degree of midline shift and level of consciousness has been shown. But in descending transtentorial herniation, the degree of vertical descent did not always correlate well with neurological signs.
So the imaging tells you reliably what is happening and less reliably how bad it is. That is the right way round for an anaesthetist: the route determines what you should worry about next, and the patient in front of you determines how urgently.
One recommendation on this page is categorical rather than graded. Where tonsillar herniation is demonstrated, lumbar puncture should not be performed, because it can result in the death of the patient.
Most of this day is orientation. One part of it is a hard stop.
Where tonsillar herniation has been demonstrated — inferior displacement of the cerebellar tonsils through the foramen magnum — lumbar puncture should not be performed, and the source states plainly that doing so can result in the death of the patient. MRI with its sagittal imaging is the primary modality for showing it, and on CT the tonsils can be seen below the level of the foramen magnum.
The reason this belongs in an anaesthesia curriculum rather than a neurology one is that you are frequently the person asked to do the procedure, or asked to sedate someone so that another team can. "Can you help us get an LP on this patient" arrives without the imaging attached, and the person asking may not have looked either.
The same reasoning extends to the lumbar drain. Neuro 1 day 12 noted that patency of the CSF pathways should be established before using a lumbar drain to estimate pressure, and Neuro 2 day 03 will note that lumbar CSF drainage is used cautiously and only with the dura open, because of the risk of causing acute herniation. All three are the same fact in three settings: taking fluid from below a pressure gradient moves brain downward.
The habit worth building is small. When someone asks you to help with a lumbar procedure in a patient who has had brain imaging, look at the imaging first — and if it has not been done, say so out loud rather than assuming somebody upstream considered it.
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A patient has an infratentorial mass. Imaging shows the cerebellar vermis bulging upward through the tentorial incisura with symmetrical effacement of the subarachnoid cisterns. What additional problem should you expect?
You are asked to provide sedation so that a medical team can perform a lumbar puncture on a drowsy patient with headache and vomiting. What is the most important thing to establish first?
What is the correct reading of the relationship between quantitative measures of herniation on imaging and clinical status?
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