Owl eye sign | Radiology Signs

Answer:

The owl eye or snake eyes sign is a pair of symmetric, rounded T2 hyperintensities in the anterior horns of the spinal cord grey matter on axial imaging, resembling two eyes. It reflects selective injury to anterior horn cell regions and is most characteristic of anterior spinal artery territory infarction, in which the abrupt onset of flaccid paraparesis with dissociated sensory loss, sparing of vibration and proprioception, is the clinical counterpart. Supporting features of cord infarction are restricted diffusion within hours, an adjacent vertebral body infarct, and a cause such as aortic surgery or dissection, atherosclerosis, fibrocartilaginous embolism, vasculitis or profound hypotension. The same appearance is not unique to infarction: it is described in cervical spondylotic myelopathy, where it indicates chronic anterior horn damage from compression and carries a poorer surgical prognosis, and in poliomyelitis, enterovirus and West Nile flaccid myelitis, in amyotrophic lateral sclerosis, and in hyperextension injury. The distinctions are made by the clinical tempo, the diffusion findings, the presence of cord compression on the sagittal images and the cerebrospinal fluid results, so the sign should always be reported with its context and with the sagittal pattern, which in infarction is a longitudinally extensive pencil-like T2 hyperintensity.

Why is it called so?

The two round hyperintense anterior horns resemble the eyes of an owl, or a snake’s eyes, staring out of the axial image.

Pathophysiology

The anterior spinal artery supplies the anterior two-thirds of the cord, including the anterior horns, through sulcal branches, while the paired posterior spinal arteries supply the dorsal columns. Anterior horn motor neurons are large, metabolically demanding cells lying at the distal end of this supply, and they are the first population to die when perfusion falls, in the same way that hippocampal CA1 neurons are selectively vulnerable in the brain. Neuronal necrosis and the accompanying cytotoxic oedema increase tissue water content and produce the paired T2 hyperintense foci. In chronic compressive myelopathy the same neurons are damaged by repeated mechanical insult and ischaemia of the central grey matter, producing an identical cross-sectional appearance by a slower route.

Alternative names: Snake eyes sign; snake eye appearance

Other associated named signs: Pencil-like myelomalacia on sagittal imaging and the H-sign of central cord grey matter involvement

References

1. Alektoroff K, Kettner M, Papanagiotou P. [Spinal cord ischemia]. Radiologe. 2021;61(3):263-266. PMID: 33580807.