Strange blood clots are clots that form in unusual places, such as the veins of the brain, liver, or intestines, or that appear in people without the usual risk factors. In haematology we take them seriously because they often point to an underlying cause, such as an inherited clotting tendency, antiphospholipid syndrome, a bone marrow disorder, or a hidden cancer. They are treated with anticoagulants like more common clots, but the search for the cause is what makes them different.
In my practice, a patient referred with an “unusual clot” usually needs two things: prompt treatment of the clot itself and a careful workup to find out why it happened. This guide explains both.
What Makes a Blood Clot “Strange”?
Most clots are deep vein thrombosis (DVT) in the leg or pulmonary embolism (PE) in the lungs. Doctors call a clot unusual or atypical thrombosis when it has one or more of these features:
- An unusual site, such as the brain’s venous sinuses or the abdominal veins
- A young patient, often under 50, without an obvious trigger
- Recurrence despite treatment, or clots in both veins and arteries
- Unusual lab findings, such as a clot with a low platelet count
For a broad introduction to how clotting normally works, see our blood clotting guide.
Common Unusual Sites and Their Symptoms
Symptoms depend on where the clot forms. The table below summarizes the sites I see most often.
| Site | Medical name | Typical symptoms |
|---|---|---|
| Veins draining the brain | Cerebral venous sinus thrombosis | Severe or worsening headache, blurred vision, seizures, weakness |
| Portal and mesenteric veins | Splanchnic vein thrombosis | Abdominal pain, bloating, nausea, sometimes bloody stools |
| Hepatic veins | Budd-Chiari syndrome | Abdominal pain, enlarged liver, fluid in the abdomen |
| Arm and shoulder veins | Upper-extremity DVT | Arm swelling, heaviness, bluish discoloration |
| Eye veins | Retinal vein occlusion | Sudden painless blurring or loss of vision in one eye |
| Arteries in a young person | Arterial thrombosis | Stroke symptoms, a cold painful limb, or chest pain |
Why Strange Blood Clots Happen
Clots form when one or more of three factors, known as Virchow’s triad, tip the balance: slow blood flow, damage to the vessel wall, or blood that clots too easily. Unusual clots often have a specific cause behind them.
Inherited thrombophilias
Genetic conditions such as Factor V Leiden, the prothrombin gene mutation, and deficiencies of natural anticoagulants (protein C, protein S, and antithrombin) make blood more likely to clot.
Acquired conditions
- Antiphospholipid syndrome, an autoimmune condition causing venous and arterial clots and pregnancy complications
- Myeloproliferative neoplasms, such as polycythemia vera and essential thrombocythemia, often linked to a JAK2 mutation and a classic cause of abdominal vein clots
- Paroxysmal nocturnal hemoglobinuria (PNH), a rare blood disorder causing hemolysis and clots in unusual sites
- Cancer, which can make blood more prone to clot, sometimes before the cancer is diagnosed
- Heparin-induced thrombocytopenia, where an immune reaction to heparin lowers platelets while causing clots
Local and hormonal triggers
Estrogen-containing contraceptives, pregnancy, liver cirrhosis, abdominal infection or surgery, central venous catheters, and repetitive arm strain can all trigger clots in unusual places. Smoking and inactivity add further risk.
How Strange Blood Clots Are Diagnosed
First, imaging confirms the clot. Ultrasound works well for limbs; CT or MRI with venography is used for the brain and abdomen.
Second comes the search for a cause. Depending on the situation, tests may include:
- A complete blood count and blood smear, looking at red cells, white cells, and thrombocytes
- Antiphospholipid antibodies, tested twice at least 12 weeks apart
- JAK2 mutation testing for abdominal vein clots
- Flow cytometry for PNH
- Inherited thrombophilia testing, when results would change management
- Age-appropriate cancer screening
Timing matters. Some clotting tests are unreliable during an acute clot or while taking anticoagulants, so your specialist may schedule them later.
Treatment Options
Anticoagulation is the mainstay. It stops the clot growing and lets the body gradually break it down. Options include heparin injections, warfarin, and direct oral anticoagulants (DOACs) such as apixaban and rivaroxaban.
The choice depends on the cause. Warfarin is generally preferred in high-risk antiphospholipid syndrome, and heparin-induced thrombocytopenia requires stopping all heparin and using a non-heparin anticoagulant. Clot-busting drugs or catheter procedures are reserved for severe or life-threatening cases.
How long treatment lasts depends on whether the trigger was temporary or persistent. A clot linked to a short-term trigger may need a few months of treatment; a persistent cause may mean long-term anticoagulation. Treating any underlying condition, such as a myeloproliferative neoplasm with measures to control platelet counts, is also part of care.
When to See a Doctor
Call emergency services for a sudden severe headache, confusion, seizures, weakness or numbness on one side, sudden vision loss, chest pain, or sudden breathlessness. Seek prompt care for unexplained arm or leg swelling, persistent abdominal pain, or unusual bruising together with swelling. If you have had a clot before, tell every doctor who treats you.
Frequently Asked Questions
Are strange blood clots hereditary?
Sometimes. Inherited thrombophilias raise clot risk, but many unusual clots have acquired causes such as antiphospholipid syndrome, hormones, or a blood disorder. A family history of clots is worth sharing with your doctor.
Can a strange clot be a sign of cancer?
It can be, which is why doctors often recommend age-appropriate cancer screening after an unexplained clot. Most people with an unusual clot do not have cancer, but it should be considered.
Will I need blood thinners for life?
Not necessarily. The duration depends on the cause, the site, bleeding risk, and whether the trigger is still present. Your haematologist will review this at regular intervals.
Are large clots in menstrual blood the same thing?
No. Menstrual clots are pieces of shed uterine lining and blood, not clots inside blood vessels. Frequent clots larger than about a quarter (2.5 cm) can signal heavy menstrual bleeding and deserve assessment, including a check for anemia.
Key Takeaways
Strange blood clots form in unusual places or unusual people, and they often signal an underlying cause. Prompt imaging and anticoagulation treat the clot, while careful testing uncovers conditions such as antiphospholipid syndrome, myeloproliferative neoplasms, PNH, or cancer. Seek urgent care for warning symptoms, and work with a haematologist to plan treatment length and prevention.
