Tau sign | Radiology Signs

Answer:

The tau sign is the beaked configuration of the tectal plate seen on axial ultrasound, CT or MRI, in which the fused, elongated colliculi point posteriorly and the normal quadrigeminal plate outline is replaced by a shape resembling the Greek letter tau. It is a feature of the Chiari II malformation and, on the midsagittal image, the same abnormality is called tectal beaking. It should be reported as one element of a constellation, since Chiari II is a whole-hindbrain malformation: a small posterior fossa with a low tentorial insertion and torcular, downward displacement of the vermis, brainstem and fourth ventricle through the foramen magnum, cervicomedullary kinking, an open neural tube defect, hydrocephalus, callosal dysgenesis, colpocephaly, stenogyria, an enlarged massa intermedia and a lacunar skull in the neonate. Tectal beaking correlates with clinical findings including nystagmus, and the degree of hindbrain descent correlates with brainstem symptoms. The distinction from Chiari I, which is isolated tonsillar descent without a neural tube defect, and from Chiari III, in which the hindbrain herniates into an occipital or high cervical encephalocele, is straightforward once the posterior fossa and the spine have been assessed together, and prenatal repair of the myelomeningocele modifies the hindbrain findings on follow-up imaging.

Why is it called so?

The beaked, posteriorly pointing tectum outlines the shape of the Greek letter tau.

Pathophysiology

In the Chiari II malformation, leakage of cerebrospinal fluid through the open neural tube defect during fetal life prevents normal distension of the rhombencephalic vesicle, so the posterior fossa fails to enlarge. The developing hindbrain then grows within a chamber that is too small, and the cerebellum and brainstem are displaced downward through the foramen magnum and upward through the tentorial incisura. Compression of the midbrain within the small incisura fuses and elongates the colliculi posteriorly, producing the beak. The same mechanical crowding accounts for the kinked cervicomedullary junction, the towering cerebellum and the low-lying torcular, and it explains why the malformation is a consequence of the spinal defect rather than an independent anomaly.

Alternative names: Tectal beaking; beaked tectum

Other associated named signs: Banana sign and lemon sign on prenatal ultrasound, and the lacunar skull of the neonate

References

1. Callen AL, Stengel JW, Filly RA. Supratentorial abnormalities in the Chiari II malformation, II: tectal morphologic changes. J Ultrasound Med. 2009;28(1):29-35. PMID: 19106353.