Feline aortic thromboembolism

What it is

Arterial thromboembolism is the occlusion of an artery by a thrombus that formed elsewhere and travelled in the blood. In the cat the great majority are cardiogenic: in the dilated left atrium of a cat with cardiomyopathy (above all hypertrophic cardiomyopathy) blood stagnates, the endothelium is altered and platelets are highly reactive, so a thrombus forms. When it breaks free it usually lodges at the aortic trifurcation (the "saddle" thrombus), where the aorta gives off the external iliac arteries and continues as the internal iliac and median sacral arteries. Besides the mechanical obstruction, the thrombus releases vasoactive substances (serotonin, thromboxane) that constrict the collateral circulation, so the ischaemia is more severe than the blockage alone would explain. Less often the embolus obstructs a brachial artery (a single thoracic limb, usually the right), or a renal, mesenteric or cerebral artery. A minority of cases are associated with neoplasia, especially of the lung.

Who it affects

Adult cats, more often males, with a cardiomyopathy that has often not been diagnosed: in many cats thromboembolism is the first sign of the heart disease. The risk rises with left atrial size, atrial dysfunction, spontaneous echo contrast or a thrombus on echocardiography, and previous episodes.

Anatomy involved

• Left atrium and auricle: where the thrombus forms.
• Terminal abdominal aorta: the embolus lodges at the trifurcation and blocks flow to the external iliac arteries and, from them, to the femoral arteries, which lose their pulse.
• Pelvic limb muscles: ischaemia mainly affects the distal muscles; the gastrocnemius and the cranial crural muscles become hard and painful from ischaemic contracture. The sciatic nerve and its branches are also ischaemic, which explains the paralysis.

Clinical signs

Sudden paresis or paralysis of one or both pelvic limbs, with severe pain and vocalisation. The "five Ps": paralysis, pain, pallor (pale or cyanotic pads and nail beds), pulselessness (no femoral pulse) and poikilothermia (cold limbs). The crural muscles are firm, and clipped nails do not bleed or bleed dark blood. Tachypnoea is common from pain, stress or concurrent heart failure. Cats are often hypothermic, which worsens the prognosis.

Diagnosis

• Diagnosis is mainly clinical: absent femoral pulses, cold limbs and pain in a cat with acute paralysis.
• Glucose and lactate in blood from the affected limb compared with a normal limb or the central circulation: glucose is lower and lactate higher in the ischaemic limb.
• Doppler or ultrasound of the terminal aorta to confirm absent flow.
• Echocardiography for the underlying cardiomyopathy, atrial size and residual thrombi; thoracic radiographs and NT-proBNP for heart failure.
• Blood work: hyperkalaemia, increased creatine kinase and azotaemia; potassium must be monitored closely as the limbs reperfuse.

Treatment

Analgesia with opioids from the start, oxygen and treatment of heart failure if present. Antithrombotic therapy is started: clopidogrel together with an anticoagulant (unfractionated or low-molecular-weight heparin, or a factor Xa inhibitor such as rivaroxaban). Thrombolysis and surgical removal of the thrombus are little used because of reperfusion injury, in particular life-threatening hyperkalaemia. Recovery needs nursing care and physiotherapy; motor function, when it returns, does so over days to weeks, and distal necrosis or contracture may remain.

The prognosis is guarded. Many cats are euthanised at initial presentation. Survival is better when a single limb is affected, when some motor function is preserved and when the cat is not hypothermic; concurrent heart failure worsens it. Recurrence is common, so lifelong clopidogrel prophylaxis is maintained; in the FAT CAT trial clopidogrel was superior to aspirin in preventing further episodes.

Key studies

Presentation and prognosis

• Smith et al. (2003), retrospective study of 127 cats at a referral hospital. Hypothermia (66%) and absent motor function (66%) were common, and 93% of the cats examined had an enlarged left atrium. Of 87 cats treated, 45% survived to discharge; rectal temperature, heart rate, phosphorus, motor function and the number of limbs affected differed between survivors and non-survivors, and a model based on rectal temperature predicted a 50% probability of survival at 37.2 °C. Median survival after discharge was 117 days, shorter in cats with heart failure (77 versus 223 days).
• Borgeat et al. (2014), retrospective study of 250 cats in 3 general practices in the United Kingdom. 61.2% were euthanised at presentation. Hypothermia predicted death or euthanasia within 24 hours and, in cats that survived longer, so did not receiving aspirin, clopidogrel or both. Cats that survived 7 days had a median survival of 94 days, and 6 were alive a year later.

Diagnosis

• Klainbart et al. (2014), prospective controlled study of 22 cats with thromboembolism, 10 cats paralysed for other reasons and 10 ambulatory cats (and dogs in parallel). The difference between systemic glucose and glucose in the affected limb identified thromboembolism well: a difference of 30 mg/dL had 100% sensitivity and 90% specificity in cats.

Treatment and prevention

• Guillaumin et al. (2022, BLASTT), double-blind, randomised, placebo-controlled trial of tissue plasminogen activator in 40 cats with thromboembolism of 2 or more limbs, treated within 6 hours. Only 20 cats survived to 48 hours. Limb scores, survival to discharge (45% with thrombolysis, 30% with placebo) and complications such as acute kidney injury or reperfusion injury did not differ significantly; the high mortality left the trial with little statistical power.
• Hogan et al. (2015, FAT CAT), double-blind randomised trial in 75 cats that had survived a cardiogenic thromboembolism. Clopidogrel reduced recurrence compared with aspirin (median time to recurrence 443 versus 192 days) and also the combined outcome of recurrence or cardiac death. Both drugs were well tolerated.
• Brainard et al. (2025, SUPERCAT), double-masked randomised trial in 45 cats that had recovered from cardiogenic thromboembolism. Recurrence was similar with clopidogrel (7 of 19 cats, 37%) and rivaroxaban (10 of 26, 39%), as was the time to death. The number of cats is small for a comparison of this kind.