Unusual Blood Clots: When Vein Thrombosis Strikes in Unexpected Places
"Exploring rare venous thrombosis locations and what they reveal about underlying health conditions."
Deep vein thrombosis (DVT) is a serious condition where blood clots form in the veins, most commonly in the legs. However, DVT can also occur in less typical locations, such as the jugular, cerebral, or mesenteric veins. These unusual occurrences, known as venous thrombosis of an unusual site, prompt a more in-depth investigation to identify underlying causes.
Several studies presented at the 78th Congress of Internal Medicine in Grenoble shed light on the diagnostic and etiological aspects of these atypical thromboses. Understanding these rare cases is crucial because they often indicate underlying health issues that might otherwise go unnoticed. This is especially important for younger individuals, those without typical risk factors, or when thrombosis occurs in multiple sites.
This article synthesizes findings from these studies to provide a comprehensive overview of unusual venous thromboses, focusing on their clinical presentation, potential causes, and the role of diagnostic tools in uncovering these rare conditions.
VTE’s Reach and Burden
The CDC says the precise number of people affected by deep vein thrombosis (DVT) or pulmonary embolism (PE) is unknown, but as many as 900,000 people could be affected each year in the United States. An older CDC data page estimates 60,000–100,000 Americans die of DVT/PE annually. A 2024 AHA/ASA journal article reports that thrombosis is responsible for 1 in 4 deaths worldwide and identifies age as a major risk factor.
Uneven Evidence for Unusual Clots
Lower-limb DVT and pulmonary embolism are the most commonly encountered forms of venous thrombosis and have a strong evidence base for management. By contrast, thrombosis at unusual sites is documented, but the guideline source describes a paucity of robust studies and says management has often lacked that level of evidence. The American Venous Forum guideline on upper-extremity DVT likewise notes that limitations stem from a lack of evidence.
Why Location Matters: Uncovering Clues from Atypical Thrombosis Sites
When DVT occurs in an unusual location, it raises critical questions about its origin. Unlike typical leg DVT, which is often linked to prolonged immobility or surgery, unusual sites suggest other factors are at play. Here's what different locations might indicate:
- Cerebral Veins: Thrombosis here can indicate a range of conditions, from infections to autoimmune disorders.
- Mesenteric Veins: Clots in these veins may point to intra-abdominal infections, inflammatory conditions, or even certain cancers.
- Upper Extremity Veins: Thrombosis in the arms can be associated with central venous catheters, thoracic outlet syndrome, or hypercoagulable states.
A Rare and Heterogeneous Group
A review of unusual-location venous thromboembolism describes these events as rare, accounting for less than 10% of all venous thromboses. It notes that the affected population is heterogeneous depending on the clot’s location, which limits the data available to guide care. A review of splanchnic vein thrombosis describes portal cavernoma—multiple small collateral vessels around portal vein thrombosis—as a main sign of chronic thrombosis, detectable by Doppler ultrasound or CT.
Why Unusual Clots Resist Simple Rules
A review of unusual-site venous thrombosis says these clots can occur in cerebral, splanchnic, upper-extremity, renal, ovarian, and retinal veins. Their clinical features, symptoms, and risk factors are heterogeneous and often differ from those of lower-extremity thrombosis; treatment approaches also vary widely. Separately, a review of DVT and PE notes that the exact pathway by which clots form around venous valves is not entirely clear.
Evidence on Splanchnic Clot Treatment
A systematic review and meta-analysis identified 4,312 records and analyzed 97 studies involving 7,969 patients with splanchnic vein thrombosis. An individual-patient-data meta-analysis reports incidence rates of 5.3 recurrent venous thromboembolic events, 4.4 major bleeds, and 13.0 deaths per 100 patient-years. These figures describe different outcomes and do not, by themselves, establish how treatment compares with another approach.
The Broader Significance: What Uncommon Clots Tell Us
The occurrence of DVT in unusual locations is relatively rare but serves as an important indicator of underlying health problems. Identifying the root cause requires a comprehensive approach, including detailed medical history, physical examination, and targeted diagnostic testing. This may involve imaging studies, blood tests for thrombophilia, and investigations for cancer or autoimmune disorders.
Site-Specific Care and Limited Evidence
A practical review explains that unusual-site venous thromboses have distinctive presentations, pathophysiology, workups, and treatments compared with more common lower-extremity thrombosis and cardiovascular disorders. Another review says treatment for clots at unusual sites is much less common and is typically guided by expert opinion, observational studies, or small randomized trials. Together, these sources emphasize that care must account for the affected site while recognizing the limited evidence base.
Broader Measures of VTE Outcomes
A 2023 article argues that evaluating venous thromboembolism care should include more than conventional clinical outcomes. It identifies death, recurrent VTE, and bleeding as conventional binary outcomes, alongside patient-centered and society-level outcomes. The article presents combining these perspectives as a route toward outcome-driven, patient-centered health care.
Different Risk Factors by Clot Location
A review of unusual-site venous thrombosis identifies hematologic disorders, autoimmune diseases, and hormonal therapy as common systemic risk factors associated with Budd–Chiari syndrome. For portal and mesenteric vein thrombosis, it instead identifies local factors including solid abdominal cancer, liver cirrhosis, intra-abdominal inflammatory conditions, and surgery. The listed risk factors therefore differ by the affected venous territory.
An Unusual DVT Presentation
A 2023 case report describes a middle-aged man with DVT who presented with foot drop and no clearly identifiable DVT risk factors. Its authors state that they found no previous reports in the literature of DVT patients presenting with foot drop. The report highlights that symptom variation can make DVT diagnosis challenging, but a single case does not establish how commonly this presentation occurs.
For instance, a study correlated D-dimer levels with the extent of thrombosis in lower limb DVT, suggesting that elevated D-dimer levels might indicate more extensive clots and a higher risk of pulmonary embolism. This highlights the potential for using biomarkers to assess the severity and guide management of DVT.
Ultimately, recognizing and understanding unusual DVT locations is crucial for early diagnosis and treatment of underlying conditions, improving patient outcomes and preventing potentially life-threatening complications.