Pancake brain | Radiology Signs

Answer:

Pancake brain describes a thin, flattened rim of residual cerebral tissue compressed against the inner table by a massive fluid collection filling the cranial cavity. It is the end result of extreme long-standing hydrocephalus and of hydranencephaly, and the distinction between those two is the diagnostic question the sign poses. In severe hydrocephalus the compressed mantle is continuous cortex under pressure, a falx is present and, critically, some tissue remains that can re-expand after shunting, so the appearance is not by itself a reason to withhold treatment. In hydranencephaly the cerebral hemispheres have been destroyed in utero, usually by bilateral internal carotid territory infarction, and what remains is a membranous sac containing cerebrospinal fluid with preserved thalami, brainstem and cerebellum, which are supplied by the posterior circulation, and often a preserved falx; there is no cortical mantle to recover. Severe alobar holoprosencephaly is the third consideration and is separated by an absent falx and interhemispheric fissure, a monoventricle with a dorsal cyst, fused thalami and midline facial anomalies. Ultrasound in the neonate, MRI for tissue characterisation and, where needed, imaging of the arterial supply distinguish them, and the distinction determines whether shunting is done for head size control alone or with the expectation of neurological gain.

Why is it called so?

The residual brain is squashed into a thin flat layer against the skull, like a pancake.

Pathophysiology

When cerebrospinal fluid pressure exceeds the pressure that the cerebral mantle can resist, the ventricles expand and the parenchyma is progressively compressed, first with reversible periventricular interstitial oedema and later with irreversible axonal loss and gliosis. In hydranencephaly the mechanism is entirely different: bilateral supraclinoid internal carotid occlusion in the second trimester, from thrombosis, infection such as toxoplasmosis or cytomegalovirus, or twin-twin transfusion, causes liquefactive necrosis of the hemispheres, which are resorbed and replaced by fluid within a leptomeningeal sac. Because the posterior circulation is intact, the thalami, brainstem and cerebellum survive, and this preserved posterior fossa anatomy is the anatomical fingerprint that separates it from holoprosencephaly.

Alternative names: Pancaked brain; compressed cortical mantle

Other associated named signs: Monoventricle of alobar holoprosencephaly and the interhemispheric fissure sign used to separate it from hydranencephaly