The same disease in places the Wells score does not cover. Each has its own trap: the arm that is blamed on the line, the headache that is called migraine, the abdominal pain scanned for something else, and the phlebitis that is a DVT in waiting.
1. The arm
Around one DVT in twenty is in the arm, in the deep veins under the collarbone and in the armpit. Most are line-related: a central venous catheter, a PICC line, a pacemaker lead or a chemotherapy port sits in the vein and clot forms around it. The rest are effort thrombosis (Paget–Schroetter syndrome): a young, often muscular person develops a swollen arm after heavy or repetitive overhead activity, because the vein is compressed between the collarbone and the first rib.
- Diagnosis: ultrasound of the arm veins; if negative and suspicion is high, CT or MR venography, because the central veins are hard to see on ultrasound.
- Treatment: anticoagulation for at least three months, with the same drugs as for a leg. A functioning line that is still needed can usually stay in while the clot is treated; it is removed if it is infected, no longer needed, or the arm does not improve.
- Effort thrombosis is often referred for catheter-directed thrombolysis and then surgery to remove the first rib, because the compression will otherwise recur; the evidence is observational, and the decision is made at a vascular centre.
- Pulmonary embolism from an arm clot is less common than from a leg but does happen; post-thrombotic symptoms in the arm affect a minority.
2. The abdomen: portal, mesenteric and hepatic veins
Clots in the veins that drain the gut and liver are rare and usually found on a CT done for abdominal pain. They matter because they signal something: cirrhosis, a cancer of the liver or pancreas, inflammation of the bowel or pancreas, recent abdominal surgery, or a blood disorder. The two blood disorders that most often present this way are a myeloproliferative neoplasm (found by testing for the JAK2 mutation) and paroxysmal nocturnal haemoglobinuria, and both are tested for in any abdominal vein clot without an obvious local cause. Treatment is anticoagulation for at least three months and often longer; a clot in the liver's outflow veins (Budd–Chiari syndrome) can need a shunt procedure and is managed at a liver centre.
3. The brain: cerebral venous thrombosis
A clot in the large veins and sinuses that drain the brain. It is uncommon — a few cases per 100,000 a year — and it affects younger adults, three women for every man, because of the combined pill, pregnancy and the weeks after birth. Other triggers: dehydration, infections of the ear or sinuses, head injury, cancer, thrombophilia, and, rarely, the immune reaction after certain vaccines seen in 2021.
How it presents. Headache in about nine in ten: new, often building over days, worse lying flat, on coughing or straining, sometimes thunderclap. Then, in a proportion, blurred or double vision, seizures, weakness on one side, confusion or drowsiness. The trap is that it is called migraine or tension headache for days.
How it is found. A plain CT is often normal. The test is a CT venogram or MR venogram, which any emergency department can arrange. A D-dimer that is normal makes it less likely but does not exclude it.
How it is treated. Anticoagulation, started at once with low-molecular-weight heparin, even when the scan shows bleeding into the brain: the bleeding is caused by the blocked vein, and the anticoagulant treats the cause. That is counter-intuitive and is the point at which treatment is sometimes wrongly withheld. Both the European Stroke Organisation and the American Heart Association guidance say so. After the first days, a DOAC is now an accepted alternative to warfarin (a trial of dabigatran and a large observational comparison found similar outcomes), for three to twelve months depending on the cause. Clot removal through a catheter is reserved for people deteriorating despite anticoagulation; surgery to relieve pressure is used for large bleeds with swelling. Most people recover well; the combined pill is stopped for good.
4. The skin: superficial vein thrombosis
A clot in a surface vein, usually a varicose vein of the leg: a firm, tender, red cord that you can feel. It used to be treated as a local nuisance with anti-inflammatories. Two things changed that. Around one in five people with it has a DVT at the same time, and a clot in the long saphenous vein of the thigh that reaches to within 3 cm of where it joins the deep vein behaves as a DVT.
- Every superficial thrombosis of the leg gets a deep-vein ultrasound, which also measures how long the clot is and how close it lies to the junction.
- Within 3 cm of the junction: treated as a DVT, with full-dose anticoagulation for three months (the British Committee for Standards in Haematology position).
- Further from the junction but 5 cm or longer: a preventive dose of an injected anticoagulant — fondaparinux 2.5 mg daily for 45 days, or a prophylactic dose of low-molecular-weight heparin for around 30 days. The CALISTO trial in 3,002 people found that fondaparinux cut the combined risk of extension, DVT, pulmonary embolism or death from 5.9% to 0.9%, with no more bleeding. A rivaroxaban 10 mg daily alternative was as effective in a smaller trial.
- Short and far from the junction: anti-inflammatories, a stocking if it helps, and a repeat scan if it spreads.
- A thrombophlebitis with no varicose vein and no drip site, especially if it recurs or moves around, is a reason to look for an underlying cancer or clotting disorder.
5. Where UK and US guidance differ
- Superficial thrombosis. The UK uses the BCSH rule (3 cm and 5 cm); the American CHEST guideline suggests fondaparinux or prophylactic LMWH for 45 days for clots 5 cm or longer. The same trial, the same drug, slightly different thresholds.
- Cerebral venous thrombosis. Both anticoagulate through haemorrhage; the US statement of 2024 is more explicit in endorsing DOACs.
- Arm and abdomen. The UK follows the British Society for Haematology's unusual-site guidance; the US positions in CHEST are the same in substance.