This topic has come up in Question 18 from the second paper of 2016. Previous papers have never featured it, which is probably surprising. Brain herniation can be subfalcine herniation, lateral ("midline shift"), uncal,  tonsillar, upward or downward central transtentorial, or transcalvarial (i.e. out of a defect in the skull). Coma seems to be a common feature, and in most (unilateral) cases there is a ipsilateral third nerve palsy with the affected eye not doing very much in response to a doll's eye manoeuvre. There is, of course, more detail. The following point-form summary takes the salient features of Plum and Posner, adding various bits of wisdom from Radiopedia.org and whatever other web pundits had to say about this topic. Wherever possible, references were attributed- but unfortunately a lot of this stuff is found in various neurosurgical presentation slides and "learning packages" from universities, rather than peer reviewed material.

As far as external FOAM resources go, one cannot go past the excellent article in LITFL.  It has that magical combination of brevity and completeness. Again, the summary offered below cannot claim either. Most of this material was derived from Plum and Posner's Diagnosis of stupour and coma, the definitive resource for all information related to head injury. There, on page 95 of my tattered 4th edition one may find Herniation syndromes: intracranial shifts in the pathogenesis of coma. The link is to a Googlebook which may not be complete.

Anyway: there are eight major patterns of herniation:

  • Falcine herniation
  • Lateral displacement of the diencephalon ("midline shift")
  • Uncal herniation
  • Central transtentorial herniation
  • Rostrocaudal brainstem deterioration
  • Tonsillar heniation
  • Upward brainstem herniation
  • Transcalvarial herniation (i.e. out of a defect in the skull)

In brief summary, with only the most important features picked out:

Herniation Radiological features Clinical features
Falcine
  • Displacement of the cingulate gyrus under the falx cerebri
  • Leg weakness
Midline shift
  • Midline shift of the septum pellucidum
  • A decreased level of consciousness, proportional to the degree of shift.
Uncal
  • Uncus and medial temporal lobe displaced medially
  • Effacement of the suprasellar cistern
  • The hippocampus obliterates the quadrigeminal cistern
  • midbrain effaced and displaced laterally
  • Ipsilateral fixed dilated pupil (3rd nerve palsy)
  • Decreased level of consciousness
  • Hemiparesis
  • Cortical blindness
Central tentorial
  • Obliteration of basal subarachnoid cisterns
  • Increased brainstem sagittal diameter 
  • Inferior displacement of the basilar artery
  • Coma​
  • Parinaud's syndrome:
  • Diabetes insipidus
Tonsillar
  • Cerebellar tonsil below the foramen magnum
  • Coma
  • Apnea
  • Hypertension
Upward
  • Flattened quadrigeminal cistern
  • "Spinning top" midbrain
  • Hydrocephalus
  • Coma
  • Miosis (reactive)
  • Absent or assymmetric doll's eye
  • Decerebrate posuring
Transcalvarial
  • Depends where the defect is
  • Depends where the defect is
     
     

In more detail:

Falcine herniation

  • Caused by lateral supratentorial masses
  • This pushes the cingulate gyrus under the falx cerebri
  • Callosomarginal and pericallosal arteries are compressed (squished under the herniating gyrus)
  • The medial wall of the ipsilateral cerebral hemisphere becomes ischaemic as a result
  • As it infarcts, it swells- increasing the ipsilateral intracranial pressure, and thus pushing the cingulate gyrus even further uder the falx.
  • In the worst case scenario, the anterior cerebral artery (ACA) is also compressed, infarcting more of the ipsilateral hemisphere
  • Contralateral hydrocephalus thenoccurs  due to obstruction of the foramen of Monro, as CSF is no longer able to leave the healthy hemisphere

Radiological features of falcine nerniation

  • Displacement of the cingulate gyrus under the falx cerebri
  • Midline shift of the septum pellucidum
  • Effacement of the anterior horn of the lateral ventricle (affected side)
  • Enlargement of the lateral ventricle on the unaffected side
  • Compression of the anterior cerebral artery against the falx
  • Evidence of medial cerebral hemisphere infarction on the afected side

Clinical features of falcine herniation

  • Initially, possibly no symptoms
  • As hernation progresses, headache develops
  • With compression of ipsilateral ACA and various callosal arteries, leg weakness develops

Lateral displacement of the diencephalon ("midline shift")

  • "Lateral displacement of the diencephalon" is what Plum and Posner call it; these days pretty much everybody else uses the colloquilaism "midline shift".
  • Degree and speed of displacement determine whether there will be clinical features.
  • Acutely, 0 to 3mm is associated with no signs, 3 to5mm with drowsiness, and 6 to 8mm with what used to be called "stupour", a state where "only vigorous and repeated stimuli will arouse the individual".
  • So that one is not embarrassed by having to look up what the diencephalon is, the definition given by Gray's anatomy is "structures lateral to the third ventricle", which includes the thalamus hypothalamus and pineal body, along with a bunch of of other more exotic structures. The lateral boundaries are formed by the internal capsule.

Radiological features of "midline shift"

  • Midline shift of the septum pellucidum
  • Effacement of the sulci on the affected side
  • Effacement of the ventricle on the affected side
  • Asymmetry of the basal CSF cisterns
  • Subfalcine, transtentorial or tonsillar herneation may be present
  • Hydrocephalus on the unaffected side

Clinical features of "midline shift"

  • Initially, possibly no symptoms
  • As midline shift progresses, the level of consciousness decreases

Uncal herniation

  • This happens when the uncus of the temporal lobe herneates medially and downward into the tentorial notch.
  • Thus, the initial features are those caused by the compression of the third nerve in that notch.
  • The pressure also decreased midbrain bloodflow, affecting the the ascending arousal systems as they pass through the midbrain. This gives rise to coma.  Plum and Posner generally view uncal herniation with an intact level of consciousness as something of a unicorn,  "rare enough to be the subject of case reports"
  • The uncus, as it descends upon the brain stem, puts pressure on the cerebral peduncles, thereby causing one-sided weakness. It could be ipsilateral (classically) or contralateral (if the midbrain gets pushed by the uncus enough to compress the opposite peduncle).
  • The posterior cerebral artery can get compressed by the uncus, giving rise to cortical blindness

Radiological features of  uncal herniation

  • Uncus and medial temporal lobe displaced medially
  • Effacement of the suprasellar cistern
  • The hippocampus obliterates the quadrigeminal cistern
  • midbrain effaced and displaced laterally

Clinical features of uncal herniation

  • Ipsilateral fixed dilated pupil (3rd nerve palsy)
  • Decreased level of consciousness
  • Hemiparesis
  • Cortical blindness

Rostrocaudal deterioration in uncal herniation

  • This is  a clinical syndrome which describes the progression of signs as more and more structures are compromised in the course of uncal herniation.
  • The signs progress from rostral structures to caudal structures.
  • As the herniation progresses, four distinct stages are seen.
  • These are best described in a table. 
Stage Pupils Respiration Oculocephalic Motor examination
Early third nerve stage Dilated
ipsilateral pupil;
sluggishly reactive
Normal Present
Dysconjugate (ipsilateral eye does not move)
Increased tone
Contralateral upgoing plantar
Late third nerve stage Dilated ipsilateral pupil; unreactive Hyperventilation or Cheyne-Stokes Dysconjugate (ipsilateral eye does not move)
 
Decorticate or decerebrate
Midbrain and upper pons Both pupils fixed and unreactive Hyperventilation or Cheyne-Stokes Absent (eyes fixed) Decerebrate posturing, or nothing
Lower pons and medilla Both pupils fixed and unreactive Apnoea Absent (eyes fixed) Unresponsive, flaccid

Central (downward) transtentorial herniation

  • This happens when there is downward pressure on the diencephalon. The alternative form is where the brainstem structures herniate upward (strictly speaking this would still be called central and transtentorial, as central structures traverse the tentorium).
  • The pressure stretches delicate branches of the Circle of Willis which supply diencephalic structures
  • The resulting ischaemia affects the ascending arousal system, producinf coma
  • If the pressure is sufficient, the midbrain is also affected, causing characteristic features of Parinaud's syndrome (a dorsal midbrain compression syndrome, chiefly characterised by being unable to move the eyes up due to pressure on the vertical gaze centre).
  • As the pituitary stalk is stretched, pituitary blood supply is interrupted and diabetes insipidus may develop
  • Bilateral uncal herniation often accompanies this syndrome(as the usual causes are diffuse bilateral cerebral swelling).
  • Unlike most of this material, there is a specific reference for the radiological findings (Hahn and Gurney, 1985)

Radiological features of central transtentorial herniation

  • Complete obliteration of the basal subarachnoid cisterns
  • An increase in sagittal diameter of the brainstem
  • Inferior displacement of the basilar artery
  • Small lateral ventricles (generalised brain swelling)

Clinical features of central transtentorial herniation

  • Coma
  • Parinaud's syndrome:
    • Failure of upward gaze
    • Mid-dilated pupil (Pseudo-Argyll Robertson pupils)
    • Eyelid retraction
    • Conjugate down gaze ("setting sun sign")
  • Diabetes insipidus

Rostrocaudal deterioration in central herniation

Stage Consciousness Respiration Cranial nerves Motor examination
Early diencephalic Stupor Normal Normal reactive pupils
Roving eye movement
Absent doll's eye
Increased tone
Late diencephalic Coma Cheyne-Stokes Small eactive pupils
Present doll's eye
Restriction of upward gaze
(Parinauds' syndrome)
 
Decorticate posturing
Midbrain and upper pons Coma Hyperventilation Mid-size unreactive pupils
Present doll's eye
Restriction of upward gaze
(Parinauds' syndrome)
Decerebrate posturing
Lower pons and medilla Coma Ataxic breathing Mid-size unreactive pupils
Absent doll's eye
Weak decerebrate
Medulla Coma Apnoea Absent brainstem reflexes Unresponsive, flaccid

Tonsillar heniation

  • This is the consequence of a pressure gradient across the foramen magnum: either the pressure above has increased, or the pressure below has decreased
  • The cerebellar tonsil then compresses the fourth ventricle (blocking CSF outflow) and puts pressure on the medulla. 
  • Clearly, this is bad.

Radiological features of tonsillar herniation

  • Inferior descent of the cerebellar tonsil below the foramen magnum

Clinical features of tonsillar herniation

  • Coma
  • Apnea
  • Hypertension (or more generally, a labile cardiovascular state)
  • Neck stiffness

Upward brainstem herniation

  • This is alo sometimes known as "ascending teranstentorial herniation". A posterior fossa lession forces infratentorial contents against the tentorium, and then up through the tentorial notch.
  • With this,  the cerebellar vermis and the midbrain are pushed upward.
  • This puts prssure on the dorsal midbrain, again potentially giving rise to Parinaud's syndrome. However, one needs to remember that the infratentorial structures are experiencing the maximum pressure here, and so everything from the midbrain down will be severly affected. Cuneo et al (1979) described seven cases in which the majority only had third nerve function left.
  • The cerebral aqueduct is also compressed (hydrocephalus may ensue)
  • Small vessels which supply the midbrain are also compressed, producing coma.
  • If it weren't for the coma, sign associated with the cerebellar vermis are typically related to gait, eg. failure of tandem gait seen in alcoholic cerebellar degeneration.
  • The superior cerebellar artery may be trapped against the tentorial edge, infarcting the superior brainstem

Radiological features of upward brainstem herniation

  • "Frowny" flattened quadrigeminal cistern (it is usually smile-shaped)
  • Obliteration of the quadrigeminal and superior cerebellar cistern in severe cases
  • Compression of the posterior midbrain (apparently, that gives it a "spinning top" appearance)
  • Hydrocephalus

Clinical features of upward brainstem herniation

  • Coma
  • Miosis (reactive)
  • Absent or assymmetric doll's eye
  • Decerebrate posuring

References

Hahn, F. J., and J. Gurney. "CT signs of central descending transtentorial herniation." American Journal of Neuroradiology 6.5 (1985): 844-845.

Brazis, P., J. Masdeu, and Jose Biller. "Localization in Clinical Neurology." 2007 Lippincott Williams & Wilkins

Cuneo, Richard A., et al. "Upward transtentorial herniation: seven cases and a literature review." Archives of neurology 36.10 (1979): 618-623.