central_herniation [Operative Neurosurgery]

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====== Central herniation ======


In the first phase of [[central herniation]], the [[diencephalon]] and the medial parts of both [[temporal lobe]]s 
are forced through a notch in the [[tentorium cerebelli]].

Caused by diffuse [[brain edema]] as seen in patients with severe traumatic brain injury

CT Scan shows effacement of the perimesencephalic cisterns and loss of gray-white matter differentiation.

Early diencephalic stage (reversible)
----

Decreasing level of consciousness with difflculty concentating, agitation and drowsiness

Pupils are small (1-3 mm) but reactive

Pupils dilate briskly in response to a pinch of the skin on the neck (ciliospinal reflex)

Oculocephalic reflexes are intact (Doll’s eyes)

Plantar responses are flexor

Respirations contain deep sighs, yawns and occasional pauses then progress to Cheyne
-Stokes 

----

Late diencephalic stage

Patient becomes more difficult to arouse

Localizing motor responses to pain disappear and decorticate posturing appears with eventual progression to 
decerebrate posturing

Progressive 
diencephalic 
impairment is thought to 
be the result of 
stretching of the small 
penetrating vessels of 
the posterior cerebral 
and communicating 
arteries which supply 
the hypothalamus and 
thalamus
----
As herniation progresses to the 
midbrain stage signs of oculomotor 
failure appear

The pupils become irregular and then fixed 
at midposition

Oculocephalic movements become more 
difficult to elicit

Extensor posturing appears spontaneously

Motor tone is increased and plantar 
responses are extensor
----
The progression of symptoms indicates 
irreversible ischemia and therefore 
intervention must occur before the 
midbrain stage to prevent permanent 
deficits from central herniation

===== Clinical characteristics differentiating uncal from central herniation =====

● decreased consciousness occurs early in [[central herniation]], late in [[uncal herniation]]

● uncal herniation syndrome rarely gives rise to decorticate posturing