The "hot cross bun sign" refers to a cruciform hyperintensity seen on MRI in patients with multisystem atrophy (MSA), particularly MSA-C. It is caused by selective loss of myelinated pontocerebellar fibers and neurons in the pons. The sign resembles a hot cross bun due to its cross-shaped appearance. It indicates pontine atrophy associated with MSA.

1. HOT CROSS BUN SIGN

4. Hot cross bun sign refers to the cruciform-
shaped hyperintensity on T2W axial magnetic
resonance images (MRI) in MULTISYSTEM
ATROPHY (MSA), more commonly in MSA-C.

5. It is called so because of its resemblance to Hot Cross
Bun.(Sweet spiced bun baked by the Christian church on
last Thursday before Easter and marked with a cross on
top, with the four quarters representing the four quarters of
the year. See the Images

6. The exact cause is not known but it is mostly due to selective loss of
myelinated transverse pontocerebellar fibers and neurons in the pontine
and sparing of the pontine tegmentum and corticospinal tracts.
Multisystem atrophy (MSA) is a sporadic progressive
neurodegenerative disorder of adult onset, involving the basal ganglia and
the olivopontocerebellar complex to varying degrees.
It commonly presents with Parkinsonian symptoms and cerebellar ataxia
and depending on which of these symptoms predominates, is classified as
MSA-Parkinsonian predominant(MSA-P) and MSA-Cerebellar predominant
(MSA-C) respectively.

7. Hot cross bun sign is a manifestation of pontine
atrophy in MSA and is more commonly found in
MSA-C.

8. Hot Cross Bun Appearance of Skull on the other hand is
seen in cases where there is prominence of suture lines of
skull and prominence of frontal and parietal eminences
leading to 'bossing' of head ('hot-cross-bun appearance')
newborns. Its usually seen in
1. Congenital Syphilis
2. Rickets

9. https://blog.acrosspg.com/2013/03/hot-cross-bun-sign-hot-cross-bun.html