A cat that was normal an hour ago is now dragging both back legs, crying, and the pads feel cold. Treat that as a saddle thrombus until a vet says otherwise, and drive. The clock did not start when you found the cat: muscle and nerve damage becomes irreversible at roughly 4 hours of complete ischemia, so a cat seen at 90 minutes has a very different limb from one seen at 6 hours. Do not rub the legs, do not apply heat or ice, and do not give aspirin or any human painkiller. Call the clinic while the car is moving so oxygen and analgesia are waiting at the door.
What is actually blocked
A clot formed in the left atrium, usually in the blind pouch called the auricle, breaks loose and travels down the aorta until it wedges at the trifurcation, where the vessel splits toward the 2 back legs and the tail. Blood stops reaching the hind limbs, and often 1 forelimb too, because the same embolus can sit further forward. Cats recently treated for a bladder blockage belong to the same group, which is why catheter aftercare includes watching for sudden hind limb weakness. The muscle below the block runs out of oxygen within minutes; the nerve tissue follows.
Most of these cats have hypertrophic cardiomyopathy, and many have never had a murmur heard, so a first episode is often the first sign of heart disease.
5 signs that separate this from a sprain
Vets call these the 5 P's, and 4 can be checked at home in minutes.
Pain. Sudden, severe, loud: a yowl, then panting, then hiding. The cat may bite if the leg is touched.
Pallor. The pads and nail beds go grey-white or bluish. Compare with a front foot in the same light.
Pulselessness. Run 2 fingers along the inside of the thigh, high near the belly. The femoral pulse on the bad side is weak or absent; the good side is easy.
Poikilothermia. The limb is cold to the back of your hand. A pad reading more than 2 C below the other leg is a red flag; 3 to 5 C below is typical.
Paralysis or paresis. The cat cannot stand on the leg, drags the knuckles, or holds both legs straight out. Licking and biting at the foot is the tingling phase.
The 4-hour line, hour by hour
Ischemia is a stopwatch. In the first 1 to 2 hours the muscle is starved but recoverable. Between 2 and 4 hours, cells begin to die and the nerve injury starts to become permanent. Past 4 to 6 hours of complete blockage, much of the damage cannot be reversed even if flow is restored, and the risk shifts to what happens when it returns.
Reperfusion is the second hit. When the clot dissolves or is bypassed, oxygen returns to dead muscle and floods the circulation with potassium, myoglobin and acid. That surge peaks between 6 and 72 hours, which is why a cat that looks better at hour 8 can die at hour 30. Hyperkalemia above 7 mmol/L is the specific danger, and it is why potassium is checked every 2 to 4 hours on the first day.
What happens in the first minutes at the clinic
| Stage | Time from onset | Pad temperature | Femoral pulse | Action |
|---|---|---|---|---|
| Suspected at home | 0 to 30 min | 2 to 4 C below other leg | not checked | Call ahead, drive, no massage |
| Triage on arrival | 30 to 90 min | below 34 C | absent | Oxygen, opioid, echo, potassium |
| First hours of care | 1 to 4 h | 30 to 34 C | absent | Analgesia, heparin, warm the body only |
| Reperfusion watch | 6 to 12 h | may rise 1 to 2 C | faint or returning | Potassium every 2 to 4 h, urine output |
| Renal and cardiac phase | 24 to 72 h | variable | variable | Creatinine, fluid balance, heart failure check |
The first 3 rows are the ones a driver controls: a cat reaching triage at 45 minutes instead of 3 hours keeps more muscle.
3 things never to do on the way
Never massage or rub the leg. It does not move a clot wedged in the aorta, and it can break loose smaller fragments. Not every weak back leg is a clot, either: zinc poisoning from a swallowed coin causes weakness and pale gums with a warm limb.
Never apply heat or ice. Heat on a limb with no blood flow burns skin the cat cannot feel and worsens the reperfusion load; ice constricts what little flow remains. Keep the whole cat warm with a blanket and leave the leg alone.
Never give aspirin, ibuprofen, naproxen or acetaminophen. Cats lack the liver pathway to clear these drugs, and 1 human 325 mg aspirin tablet is 65 mg per kg in a 5 kg cat, far past any safe exposure. Aspirin does not dissolve an existing clot either.
What the vet will do, and what it costs in time
Analgesia comes first at a full opioid dose, because these cats are in pain that ranks with a fracture. Oxygen follows, then a fast echocardiogram, then heparin or a low molecular weight heparin to stop the clot growing. Thrombolytics exist and are used in some hospitals, but the bleeding risk is serious and the window narrow.
The decision point usually arrives between 12 and 48 hours, once it is clear how much limb function is returning and whether the kidneys are coping.
Prognosis, honestly
This is 1 of the few feline emergencies where euthanasia is a common and reasonable outcome. Many cats are euthanized at first presentation because of the pain, the heart disease and the recurrence risk; of those treated, a minority walk again. Recurrence is common within 6 to 12 months, because the atrium that made the first clot is still there.
What improves the odds: arrival inside 2 hours, a limb still warm at the hock, potassium under 7 mmol/L, and urine production restarting within 24 hours. What worsens them: a leg cold to the hip, no pulse at all, and a cat already struggling to breathe from heart failure.
5 steps to take right now
- Call the nearest 24-hour hospital and say suspected saddle thrombus, so a table and oxygen are ready.
- Wrap the cat in a towel, support the whole body, and lay it flat in a carrier.
- Drive; do not wait 1 hour to see whether the cat improves, because it will not.
- Give no medication, food or water on the way.
- Note the time you first saw the cat drag the leg, and say that number at the door.
Frequently asked
Can a cat recover from a saddle thrombus without treatment?
Rarely, and not in a way anyone can rely on. A small clot in a cat with good collateral flow can sometimes allow partial return of function over 2 to 6 weeks, but the limb is often shortened, stiff, or held oddly, and the underlying heart disease still needs treatment. Waiting also means the 4-hour window passes, which is when nerve damage becomes permanent.
How do I tell a saddle thrombus from a broken leg or a sprain?
Temperature and pulse do it. A fracture is painful and swollen but the foot stays warm and a femoral pulse is easy to find. In a saddle thrombus the pads read 2 to 5 degrees C colder than the other leg, the femoral pulse is weak or absent, and both hind legs are usually affected at once. Both need a vet within the hour.
Is aspirin safe to give while I drive to the clinic?
No. Cats clear aspirin far more slowly than dogs or people, and 1 human 325 mg tablet delivers about 65 mg per kg to a 5 kg cat, which is a toxic exposure rather than a treatment dose. Aspirin also does not dissolve a clot that already exists. Give nothing by mouth and spend the drive making the phone call instead.