Pancake brain | Radiology Signs

Answer:

Pancake brain describes a thin, flattened rim of cerebral cortex draped over a single midline monoventricle, reaching only as far as the edge of a dorsal cyst; it is one of three configurations (ball, cup and pancake, in decreasing order of cortical coverage) used to describe the most severe form of the holoprosencephaly spectrum, alobar holoprosencephaly. It results from failure of the prosencephalon to cleave into two hemispheres, so there is a single fused monoventricle, no falx cerebri or interhemispheric fissure, fused thalami, and absent septum pellucidum and corpus callosum, usually with a posterior dorsal cyst and midline facial anomalies ranging from cyclopia to a single central incisor. Two mimics must be excluded, and this differential is the diagnostic question the sign poses. In hydranencephaly the cerebral hemispheres have been destroyed in utero, usually by bilateral internal carotid territory infarction, and what remains is a membranous sac of cerebrospinal fluid; the falx is present, the thalami are not fused, and the thalami, brainstem and cerebellum are preserved because they are supplied by the posterior circulation. In extreme long-standing hydrocephalus the compressed mantle is continuous cortex under pressure with a normal falx and, critically, some tissue remains that can re-expand after shunting, so the appearance is not by itself a reason to withhold treatment. Ultrasound in the fetus or neonate raises the possibility, MRI is used for tissue characterisation and to confirm the fused thalami and absent midline structures, and, where needed, imaging of the arterial supply distinguishes hydranencephaly from the other two. 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 cortex is squashed into a single thin, flattened rim over the fused monoventricle, like a pancake.

Pathophysiology

Holoprosencephaly results from failure of the prosencephalon to cleave into two cerebral hemispheres during the fifth gestational week, a process driven by sonic hedgehog signalling from the underlying prechordal plate; the alobar form is the most severe end of the spectrum and is associated with sonic hedgehog pathway mutations, chromosomal abnormalities such as trisomy 13, and maternal diabetes. Because the hemispheres never separate, the thalami remain fused in the midline, no falx or interhemispheric fissure develops, and the residual cortex, unable to form normal gyral architecture, is draped as a thin rim around the single monoventricle, becoming the ‘pancake’ when coverage is at its most minimal. This is mechanistically unrelated to hydranencephaly, where bilateral supraclinoid internal carotid artery occlusion in the second trimester, from thrombosis, infection such as toxoplasmosis or cytomegalovirus, or twin-twin transfusion, causes liquefactive necrosis of hemispheres that had cleaved normally; they are resorbed and replaced by fluid within a leptomeningeal sac while the posterior-circulation-supplied thalami, brainstem and cerebellum survive intact and unfused. It is equally unrelated to severe hydrocephalus, where cerebrospinal fluid pressure exceeding what an otherwise normally formed cerebral mantle can resist causes progressive ventricular expansion and parenchymal compression, first with reversible periventricular oedema and later with irreversible axonal loss and gliosis.

Alternative names: Pancake configuration of alobar holoprosencephaly; compressed cortical mantle

Other associated named signs: Monoventricle, absent interhemispheric fissure, and the horseshoe (boomerang) brain configuration, all seen in alobar holoprosencephaly

References

1. Gomez GD, Corrêa DG, Trapp B, Figueiredo IR, Zuppani HB, Kingston S, et al. Holoprosencephaly spectrum: an up-to-date overview of classification, genetics and neuroimaging. Jpn J Radiol. 2025;43(1):13-31. PMID: 39259418.