In venous intravasation, contrast transits from the uterine cavity, through the myometrium, and directly into draining pelvic veins. Contrast may also drain into the lymphatic system (lymphatic intravasation).

Although relatively rare, it is a potential pitfall in the interpretation of HSGs. As in the above case, it remains imperative to distinguish venous intravasation from free intraperitoneal contrast spillage which would occur from either patent fallopian tubes, or in the setting of uterine perforation.

On HSG, intravasation is characterized by the opacification of myometrial vessels which appear as a fine lace-like network surrounding the uterine cavity. The contrast then enters larger pelvic veins and is washed out. This is unlike free intraperitoneal contrast spillage, which does not demonstrate these characteristics and, in particular, does not washout.

Also, since intravasated contrast remains within the circulation, renal excretion of contrast may be observed on delayed images whereas in the case of intraperitoneal contrast which persists within the peritoneal cavity, negligible renal clearance is demonstrated.

Lastly, ultrasound may potentially be used to confirm free intraperitoneal spillage of contrast as it is a sensitive modality for detecting fluid. However, this particular application of ultrasound has not been studied, and it is uncertain how much free intraperitoneal spillage of contrast must occur before it is clearly distinguishable from physiologic fluid which may often be present at baseline.

Venous intravasation can cause pulmonary embolism along with associated systemic side effects (this is the most significant risk). In cases where oil based contrast media are used, fat embolism may occur.