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: CT studies show the vertebral body is actually invaded first, but the pedicle is a thin cortical ring viewed end-on, so even a small extension into it is disproportionately conspicuous on the radiograph, making pedicle loss appear to be the earliest sign. 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. The usual primaries are breast, lung, prostate, renal and thyroid carcinoma. Classical teaching contrasts metastasis with multiple myeloma, in which the pedicle is said to be relatively spared early because myeloma fills the red-marrow-rich vertebral body first, though this is a tendency rather than a rule since advanced myeloma does destroy the posterior elements. A slow-growing intraspinal lesion such as a nerve sheath tumour erodes the pedicle from within, widening the interpediculate distance and scalloping the posterior vertebral body, and previous surgery or radiotherapy will do the same. Pedicle destruction also means loss of posterior column integrity, which matters in spinal instability scoring, and MRI, or CT or bone-specific PET, should follow because marrow replacement is visible long before cortical destruction.

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

Vertebral metastases spread haematogenously, often via the valveless Batson venous plexus, and CT and pathological studies show they truly begin in the posterior vertebral body rather than the pedicle; destruction of a pedicle is essentially never found without body involvement. The radiograph tells a different story only because of projection geometry: the pedicle is a thin, compact cortical ring seen end-on as a small dense oval, so a deposit that extends into it needs to destroy only a small volume of cortex to make the whole ring disappear. The cancellous vertebral body, by contrast, must lose roughly 30 to 50 per cent of its trabecular bone before any lucency is visible on a plain film. The pedicle therefore appears to fail first on the radiograph even though it is the second structure truly involved, which is why the winking owl sign is best understood as a radiographic illusion of conspicuity rather than a marker of the earliest tumour deposit. 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; pedicle sign; one-eyed owl sign; one-eyed vertebra

Other associated named signs: Blind vertebra when both pedicles are destroyed, draped curtain sign, vertebral collapse with a convex posterior border, ivory vertebra, and the pedicle sclerosis of contralateral stress hypertrophy

References

1. JACOBSON HG, POPPEL MH, SHAPIRO JH, GROSSBERGER S. The vertebral pedicle sign: a roentgen finding to differentiate metastatic carcinoma from multiple myeloma. Am J Roentgenol Radium Ther Nucl Med. 1958;80(5):817-21. PMID: 13583317.