Bridging vein thrombosis

Last revised by Travis Fahrenhorst-Jones on 3 May 2023

Bridging vein thrombosis describes thrombosis of the fragile bridging veins that cross the subarachnoid and subdural spaces. The presence of bridging vein thrombosis on imaging has a strong association with non-accidental injury and can hint towards a traumatic etiology behind subdural hematoma and subdural hygroma in the context of suspected abusive head trauma 1,2.

A study of 45 infants who suffered abusive head trauma found that 44% had imaging evidence of bridging vein thrombosis 3.

Given the anatomical position of the bridging veins, the vessels are prone to rupture with acceleration, deceleration, and rotation of an infant's head 4. The high relative weight of the infant's head in combination with weak musculature of the neck allows for an increase in the amount of torque leading to a tendency to shear and rupture small vessels such as the bridging veins within the subdural and subarachnoid space. Within hours, thrombosis within or surrounding the damaged bridging vein begins to form 4.

The presence of bridging vein thrombosis is linked secondary to a shaking mechanism, in opposition to axial loading (or slamming) 2,4.

Bridging vein thrombosis appears on cross-sectional imaging as a tubular shape in the parasagittal plane representing the thrombosed vessel 4. Termed the tadpole sign, the round shape of the tadpole represents thrombotic material within the subarachnoid or subdural space 4. The bent tail of the “tadpole” represents the torn bridging vein itself expanded by thrombosis.

While visible on CT imaging as a hyperdense linear structure, findings are better demonstrated with gradient-eco or susceptibility weighted MRI sequences 4,5. In unclear cases, contrast enhanced MRI demonstrates a lack of enhancement within the damaged bridging vein 4.

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