What causes the eye-of-the-tiger sign on brain MRI?
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Answer:
The eye-of-the-tiger sign is a central focus of T2 hyperintensity within an otherwise markedly T2-hypointense globus pallidus, seen bilaterally and symmetrically, best on T2-weighted and susceptibility-weighted sequences. The hypointense surround is iron accumulation; the hyperintense centre corresponds to gliosis, spongiosis and vacuolation, that is tissue destruction and oedema in the anteromedial pallidum. It is the classic finding of pantothenate kinase-associated neurodegeneration (PKAN), caused by PANK2 mutation, the commonest form of neurodegeneration with brain iron accumulation. It is not, however, pathognomonic: it has been reported in fucosidosis, neuroferritinopathy, multiple system atrophy, corticobasal degeneration and other conditions, particularly in adult-onset cases, and it may be absent very early in the disease and can disappear late as the hyperintense centre is replaced by iron. Genetic confirmation is therefore required, and the imaging finding should be interpreted with the clinical phenotype of dystonia, rigidity, dysarthria and pigmentary retinopathy.
Why is it called so?
The bright central spot within the dark globus pallidus resembles the pupil of a tiger’s eye.
Pathophysiology
PANK2 encodes the mitochondrial enzyme pantothenate kinase, which catalyses the first step of coenzyme A synthesis. Its deficiency causes accumulation of cysteine-containing substrates that chelate iron and generate free radicals, and iron therefore deposits preferentially in the globus pallidus, producing marked T2 shortening and susceptibility effect. At the same time the oxidative injury destroys tissue in the anteromedial pallidum, where gliosis, spongiform change and increased free water produce T2 prolongation. The juxtaposition of the destroyed, water-rich centre with the iron-laden surround creates the two-tone appearance.
Alternative names: Tiger eye sign
Other associated named signs: Face of the giant panda sign of Wilson disease, the hot cross bun sign and putaminal rim sign of multiple system atrophy, and the hummingbird sign
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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