Dr Hector Thomson
Emergency Registrar

Peer review: Dr Luke Phillips
Editor: Dr David McCreary

The Case

A 37-year-old man has been ejected from his car after crashing at high speed. He was initially mildly confused but then rapidly dropped his GCS to 8. The ambulance crew on scene have intubated him and whisk him into your emergency department. During your primary survey you noticed his left pupil is dilated and non-reactive.

What is happening inside this patient’s head?

In this post, we will discuss some concepts of intracerebral pressure regulation and outline some brain herniation syndromes.

The Basics of the Monro-Kellie Doctrine

You’ll have seen this a few times at medical school, in primary exams and on the ATLS/EMST courses, but let’s refresh ourselves on it. A couple of 18th century Scottish surgeons Alexander Monro and George Kellie noted that the brain is enclosed in non-expandable bone and is nearly incompressible.

 “it does not appear very conceivable how any portion of the circulating fluid can ever be withdrawn from the cranium, without its place being simultaneously occupied by some equivalent; or how any thing new or exuberant can be intruded, without an equivalent displacement” – Kellie 1924.

The sum of the volumes of the brain (1400ml), cerebrospinal fluid (150ml) and intracerebral blood (150ml) is constant. An increase in one should cause a reciprocal decrease in either one or both of the remaining two. Aside from being primary examination fodder this principle explains why small changes in volume can have marked changes in intracranial pressure.

The Monro-Kellie model for the contents of the intracranial compartment. ‘Brain tissue’ includes neurons, glia, extracellular fluid and cerebral microvasculature. ‘Venous’ and ‘Arterial blood’ represents the intracranial blood volume in macro-vasculature and cerebral venous sinuses. ‘CSF’ includes ventricular and cisternal CSF. If into this fixed box we have a haemorrhage such as an extradural haematoma, swelling from an infarct, a tumour or an abscess then CSF and venous blood will be pushed out with an initial small increase in pressure. Extra CSF production or blockage of CSF drainage can also raise the intracranial pressure (ICP). Beyond a certain point, the pressure will rapidly increase. Squeezing brain tissue into another compartment like the last bit of toothpaste in the tube. As the pressure rises the ability of the cerebral vascular to regulate cerebral blood flow reaches a critical point, resulting in decreased compliance as cerebral ischemia.

Image: Pressure Volume Curve for ICP(1)

What are the common examination findings of raised ICP?

Initial signs of raised ICP are nonspecific including headache, nausea, vomiting, agitation or conversely increased drowsiness then obtundation. Examination findings of brain herniation can include:

  • Dilated and nonreactive pupils
  • Asymmetric pupils
  • Extensor posturing or not response to painful stimuli
  • Progressive decline in neurologic condition (decrease in GCS > 2 points) that are not associated with non-TBI causes
  • Cushing reflex (hypertension, bradycardia, irregular respirations)

What causes these examination findings and what are the important herniation syndromes I need to know?

It depends on which bit of the brain is being squished. While you can also herniate upwards (reverse coning – yes, that’s a thing) or out a hole in your skull (trans-calvarial), these are both extremely rare, so let’s focus on the three most common and important herniation syndromes:

  1. Subfalcine
  2. Transtentorial
  3. Tonsillar

Image: Types of herniation syndromes – courtesy of Dr Matt Skalski, Radiopaedia.org, rID: 45683

1. Subfalcine Herniation

The two cerebral hemispheres are separated by a sheet of dura called the falx cerebri. If there is a unilateral mass on one side the cingulate gyrus can be pushed down and under.

Image: Anatomy of the falx cerebri and tentorium cerebelli(2)

What do I look for on CT?

The CT brain will have midline shift. Draw a line between the anterior and posterior attachments of the falx to the inner table of the skull and measure at the level of the foramen of Monroe as seen on the CT below. The degree of shift has prognostic implications (<5 mm has a good prognosis while >15mm generally do very poorly.)

Image: Case courtesy of Assoc Prof Frank Gaillard, Radiopaedia.org. From a case rID: 15823 These axial slices show a huge acute subdural overlying the right cerebral hemisphere as well as filling the floor of the middle cranial fossa and tracking along the posterior aspect of the falx (blue arrow), resulting in marked mass effect and midline shift (red). There is marked uncal herniation (yellow) with significant distortion of the midbrain (blue dotted line). Following the craniectomy (orange arrow) and evacuation of the haematoma, an anterior cerebral artery infarct has developed on the right (green *).

What examination findings manifest from this syndrome?

This shift can cause compression of the anterior cerebral artery resulting in a stroke syndrome with the most common manifestation being contralateral leg weakness. Hydrocephalus can also develop as the foramen of Monro is compressed (yep same bloke as the doctrine).

2. Transtentorial/Uncal herniation

The tentorium cerebelli covers over the posterior cranium. Descending transtentorial herniation can occur in two patterns: lateral and central. We will discuss the more common lateral pattern. With increasing pressure, the uncinate process of the temporal lobe can herniate into the anterior part of the opening, placing pressure on the midbrain. The brainstem will squeeze down through the opening in the tentorium cerebelli as shown below.

Image: Anatomy of the tentorium cerebelli – Emergency Medicine Ireland

How do I look for this on CT?
  • Shift of brainstem and distortion of adjacent cisterns
  • Dilatation of the contralateral temporal horn
  • PCA territory infarct due to compression of the posterior cerebral artery as it crosses the tentorium

Image: Transtenorial Herniation(2) 

The axial CT slice below shows an acute left subdural causing transtentorial hernation. There is widening of the left basal cistern (arrow) with effacement of the right basal cistener (dashed line). There is also dilatation of the temporal horn of the right lateral ventricle (*).
What examination findings manifest from this syndrome?

The classical presentation is: Blown pupil + Coma + Contralateral hemiparesis As the brain squeezes down this places pressure on the oculomotor nerve as shown below. This results in an ipsilateral non-reactive, dilated pupil. As the parasympathetic fibres are on the outside there will be dilatation before the motor effects causing a down and out pupil. This is the earliest and most reliable sign.

Image: Anterior view of transtentorial uncal herniation caused by a large haematoma under a skull fracture.

Coma develops due to compression of the reticular activating system in the brainstem. The descending corticospinal tracts will also be compressed as shown in the MRI below. This usually results in contralateral hemiparesis. Confusingly this can also be ipsilateral (false lateralising sign) in 25% if lateral displacement compresses the opposite cerebral peduncle (Kernohan’s phenomena). If untreated, this may eventually compress the midbrain (causing bilateral fixed & dilated pupils, with decorticate or decerebrate posturing).

Image: Descending corticospinal tracts when compressed will cause contralateral hemiparesis – Emergency Medicine Ireland

3. Tonsillar Herniation

Tonsillar herniation involves the inferior descent of the cerebellar tonsils below the foramen magnum. Any mass effect on the brain can displace the posterior cranial fossa structures inferiorly. This then leads to the brain stem compression against the clivus (the surface of the skull base anterior to the foramen magnum – thanks Radiopaedia). Squeezing the brainstem impairs the function of the pons and the medulla (labelled below), which are pretty important for, you know, keeping you alive. Obstructive supratentorial hydrocephalus may result from fourth ventricle compression.

How do I look for this on CT?

Draw a line on the sagittal slice across the foremen magnum (yellow dash) then measure the inferior descent of the cerebellar tonsils (red arrow). The radiographic definition is >3mm. Normal can vary with age and definitions vary.

What are the examination findings?

Tonsillar herniation can manifest initially as headache and irritability. The posterior fossa is a tight place and this places pressure on cardiac and respiratory centres. As the pressure increases this results in the Cushing reflex which is comprised of:

  1. Irregular breathing
  2. Bradycardia
  3. Hypertension

Hypertension is the brainstem’s response to decreased cerebral blood flow, and its attempt to restore cerebral blood flow. Unfortunately, hypertension can cause a vicious cycle of increased blood volume and worsening cerebral oedema, which in turn can worsen cerebral blood flow. Finally, the patient will fall into a deep coma with low blood pressure and tachycardia before apnea and death.

With the pathophysiology now covered, in Part 2 we will cover the practical aspects of how to manage this at the bedside.

References / Resources

FOAM Resources

Internet Book of Critical Care: Raised ICP – detailed notes on all things raised ICP by Josh Forkas

Anatomy for Emergency Medicine: Brain Herniation – if you do nothing else, watch Andy Neill’s video on the anatomy of brain herniation (and all of his other videos while you’re there)

Life in the Fast Lane: Brain Herniation Radiopedia: Brain Herniation

  1. Harary M, Dolmans RGF, Gormley WB. Intracranial Pressure Monitoring—Review and Avenues for Development. Sensors Basel Switz. 2018;18(2):465. doi: 3390/s18020465
  2. Berta Riveros Gilardi, José Ignacio Muñoz López, Antonio Carlos Hernández Villegas, Juan Alberto Garay Mora, Oralia Cristina Rico Rodríguez, Roberto Chávez Appendini, Marianne De la Mora Malváez, Jesús Antonio Higuera Calleja. Types of Cerebral Herniation and Their Imaging Features. (2019) RadioGraphics. 39 (6): 1598-1610. doi:10.1148/rg.2019190018– Pubmed
  3. Demetriades, D., & Benjamin, E. (2021). Head Injury. In D. Demetriades, C. Chudnofsky, & E. Benjamin (Eds.), Color Atlas of Emergency Trauma (pp. 1-23). Cambridge: Cambridge University Press. doi:10.1017/9781108776622.003
Hector Thomson

Hector Thomson

Emergency Registrar

Hector (the one on the left) is an Emergency Medicine Advanced Trainee at The Alfred. He’s still clinging to the basic science knowledge he gained during primary exam prep and enjoys shoulder dislocations, trauma, rugby union, fresh pasta and good gin. He doesn’t like vague allergies or cats.