The treatment of ICVT is still controversial as conflicting international guideline strategies show. Evidence suggests that propagation rates of ICVT without risk factors are low. Unless ICVT propagates to proximal veins anticoagulation can safely been withheld, thereby avoiding a bleeding risk for the patient. However, the impact of different risk factors on propagation rates is not well understood. This gap in knowledge can only be filled by a natural history study of ICVT without anticoagulation. Since patients are safe as long as no propagation occurs close ultrasound surveillance is an adequate safety umbrella for the natural history study period. In addition, it provides an objective outcome measure to define the impact of risk factors.

For the purpose of this study, ICVT is defined as any thrombus being present in the calf muscle veins and/or in the deep paired calf veins as long as they do not affect the confluence (trifurcation) area or the popliteal vein.

Patients presenting with symptomatic ICVT will be examined by CCUS for both legs (V1). Inclusion/exclusion criteria will be checked, and patients will be informed about the nature of the study. In case patients provide informed consent, no anticoagulation therapy is given. Compression stockings at the calf length will be prescribed for 30 days.

CCUS will be repeated on day 5±2 (V2), day 10±2 (V3), and day 30±4 (V4). If the ICVT has propagated into the trifurcation area, the popliteal vein, or into more proximal veins therapy will be implemented according to current guidelines for proximal DVT. If no propagation to proximal veins has occurred anticoagulation will be withheld further on

Inclusion Criteria:

• Presence of symptomatic ICVT proven by CCUS
• Absence of proximal DVT or PE
• Age of 18 or older
• Outpatient and inpatient status
• Written informed consent

Exclusion Criteria:

• Bedridden patients
• Prior unprovoked VTE
• Active malignancy (diagnosis within the last 6 months, and/or incomplete resection, and/or ongoing radiotherapy or chemotherapy, and/or metastatic disease).
• Pregnancy
• Therapeutic anticoagulation therapy for reasons other than VTE

Patients presenting with symptomatic ICVT will be examined by CCUS for both legs. Inclusion/exclusion criteria will be checked. No anticoagulation therapy is given.