Spinal metastases are common in malignant tumours. Up to 80% of cancer patients have spinal metastasis at autopsy and the spine is the principle site for bony metastasis (50%).
Plain XR of the spine are a valuable and readily available initial imaging study. Plain x-ray film findings include pedicle erosion, paraspinal soft-tissue shadows, wedge compression, and pathological fracture–dislocation. One of the first signs of vertebral metastases is disappearance of the pedicle on the AP X-ray. This is known as the ‘absent pedicle‘ or ‘winking owl sign‘.
The winking owl sign is a reliable sign of osteolytic spinal metastases and is classically seen on AP radiographs of the thoracolumbar spine. The loss of the normal pedicle contour and unilateral pedicle absence has been likened to that of a winking owl with the missing pedicle being the closed eye, the contralateral pedicle being the open eye and the spinous process being the beak.
Anteroposterior and lateral radiographs demonstrate abnormal findings in up to 90% of patients with symptomatic spinal metastasis. Lytic lesions and vertebral collapse are common; however, both osteoblastic and sclerotic alterations also occur, especially with breast and prostatic metastasis. Intervertebral disc margins are invariably spared in metastatic tumor invasion, contrasting with the disc erosion commonly observed with infectious entities.Jacobs, 2001
In 1992, Algra et al, reviewed a series of patients with proven metastatic disease. They found the most common X-ray finding in vertebral metastatic disease was destruction of the pedicle (30% of cases). Cross sectional imaging revealed the most common site as the posterior vertebral body, with the pedicular destruction seen on X-ray representing extension of tumour into the pedicle
Differentials include congenital aplasia/hypoplasia, neurofibromatosis, radiation therapy, tuberculosis and neoplasms (spinal metastases, intraspinal malignancies, and lymphoma)
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- Ridley LJ, Han J, Ridley WE, Xiang H. Winking owl sign: Unilateral absent pedicle. J Med Imaging Radiat Oncol. 2018 Oct;62 Suppl 1:168
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