What causes the hummingbird sign on midsagittal MRI of the brain?
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Answer:
The hummingbird or penguin sign is the profile of the brainstem on midsagittal T1-weighted MRI when the midbrain tegmentum atrophies while the pons is preserved: the flattened, concave upper midbrain forms the beak and the bulky pons forms the body of the bird. It is the classic appearance of progressive supranuclear palsy. On axial images the same atrophy produces the morning glory sign, a concave lateral margin of the midbrain tegmentum, and the Mickey Mouse appearance of the shrunken midbrain with preserved cerebral peduncles. Because visual assessment is subjective, quantitative measures are preferred: a midsagittal midbrain area below about 100 to 110 mm² , a midbrain-to-pons area ratio below roughly 0.15 to 0.20, and the magnetic resonance parkinsonism index, which combines the pons-to-midbrain ratio with the middle and superior cerebellar peduncle widths. The differential is the rest of the atypical parkinsonian syndromes: multiple system atrophy shows the hot cross bun sign and pontine and middle cerebellar peduncle atrophy, corticobasal degeneration shows asymmetric perirolandic atrophy, and Parkinson disease shows none of these. Recent work confirms that these measurements overlap with other tauopathies and with amyotrophic lateral sclerosis, so the sign supports but does not establish the diagnosis, which remains clinical.
Why is it called so?
The atrophic midbrain forms the slender beak and the intact pons the plump body of a hummingbird in profile.
Pathophysiology
Progressive supranuclear palsy is a four-repeat tauopathy in which neurofibrillary tangles, tufted astrocytes and neuronal loss affect the midbrain tegmentum, subthalamic nucleus, globus pallidus, substantia nigra and superior cerebellar peduncle. The rostral midbrain, containing the vertical gaze centres of the rostral interstitial nucleus of the medial longitudinal fasciculus, is affected early and severely, which is why vertical supranuclear gaze palsy and midbrain atrophy appear together. The pontine base, dominated by transverse pontocerebellar fibres, is relatively spared, so the disproportion between a shrunken midbrain and a normal pons develops, and it is this disproportion rather than absolute atrophy that the ratios capture.
Alternative names: Penguin silhouette sign
Other associated named signs: Morning glory sign and Mickey Mouse sign on axial images, and the hot cross bun sign of multiple system atrophy
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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