Winking owl sign | Radiology Signs

Answer:

The winking owl sign is loss of one pedicle on a frontal spine radiograph. The two pedicles normally project as the owl’s eyes and the spinous process as its beak; when one pedicle is destroyed or absent, the owl appears to wink. The commonest cause is an osteolytic metastasis, because the pedicle is rich in red marrow and is often involved early, and the differential also includes tuberculosis and other infection, lymphoma, myeloma, primary and intraspinal tumours, previous radiotherapy, neurofibromatosis and congenital absence. Congenital absence is rare and asymptomatic and can be recognised by its associated features: a pseudo-enlarged neural foramen, dysplastic ipsilateral facets, deviation of the spinous process to the opposite side and compensatory stress hypertrophy and sclerosis of the contralateral pedicle. The radiograph is insensitive, requiring roughly 50% loss of bone mineral before a lesion is visible and missing up to 40% of metastases, so a negative film never excludes disease and MRI is the appropriate next test.

Why is it called so?

The two pedicles and the spinous process form the eyes and beak of an owl; a missing pedicle makes it appear to wink.

Pathophysiology

The vertebral pedicle contains haematopoietic marrow with a rich sinusoidal blood supply and is a favoured site for haematogenous tumour deposits, particularly through the valveless Batson venous plexus. A deposit growing within the pedicle resorbs its thin cortical shell, and because the pedicle is imaged end-on as a small dense oval, complete loss of that cortex removes the ring entirely rather than merely altering its density. In congenital absence there was never a pedicle to destroy, so instead of a purely absent structure the adjacent bone shows the adaptive changes of chronic altered load, which is what allows the two to be distinguished.

Alternative names: Absent pedicle sign, one-eyed owl sign, blind vertebra

Other associated named signs: Draped curtain sign, vertebral collapse with a convex posterior border, ivory vertebra, and the pedicle sclerosis of contralateral stress hypertrophy

References

1. Ridley LJ, Han J, Ridley WE, Xiang H. Winking owl sign: Unilateral absent pedicle. J Med Imaging Radiat Oncol. 2018;62 Suppl 1:168. PMID: 30309088.