What causes the trident sign on post-contrast MRI of the spinal cord?
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Answer:
The trident sign is a pattern of enhancement on axial post-contrast T1-weighted images of the spinal cord in which enhancement of the dorsal subpial region on both sides combines with enhancement of the central canal region, producing a three-pronged appearance. It was described in spinal cord sarcoidosis, where it is a useful discriminator against the principal alternatives in a patient with longitudinally extensive cord signal change. Multiple sclerosis produces short, peripherally located, wedge-shaped lesions occupying less than two vertebral segments and rarely enhances in this configuration; neuromyelitis optica spectrum disorder produces longitudinally extensive central cord lesions with a bright spotty appearance on T2 and patchy enhancement; and spinal cord tumours enhance as a focal expansile mass. Supporting features of neurosarcoidosis are dorsal subpial extension over several segments, leptomeningeal and nerve root enhancement, associated intracranial leptomeningeal disease and cranial neuropathy, and thoracic nodal or pulmonary involvement on chest CT or FDG PET. The same term is used loosely for a three-pronged pontine appearance in the perivascular enhancement of CLIPPERS, so the anatomical site should be stated when the sign is reported. Diagnosis rests on demonstrating non-caseating granulomas in an accessible site, since cord biopsy carries significant risk.
Why is it called so?
The two dorsal subpial bands of enhancement flanking the central canal enhancement form the three prongs of a trident.
Pathophysiology
Neurosarcoidosis involves the central nervous system by granulomatous infiltration of the leptomeninges, from which it extends into the parenchyma along the Virchow-Robin spaces. In the spinal cord this centripetal spread from the pial surface explains the subpial distribution of the enhancement, and the perivascular route along the central sulcal arteries and the central canal region explains the third prong. Because the granulomas disrupt the blood-cord barrier, they enhance intensely, while the surrounding oedema produces the longitudinally extensive T2 hyperintensity. Corticosteroids suppress the enhancement rapidly, which is why imaging performed after treatment has started may be unrevealing and why the pattern should be sought on the pre-treatment study.
Alternative names: Trident-shaped enhancement
Other associated named signs: Bright spotty lesions of neuromyelitis optica and the peppering of the pons in CLIPPERS
References
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