Aortic Thromboembolism

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Also Known As: ATE

Introduction

Cats that present with aortic thromboembolism almost invariably have significant underlying cardiac disease and often show overt signs of congestive heart failure. Treatment of the condition can be very challenging both in the prevention of reperfusion injury and management of existing heart problems.

Pathogenesis

Thromboemboli commonly lodge in the distal aorta and iliac arteries. They are formed within the heart - dislodging and entering the aorta. There are three factors that cause ischaemic damage in the limbs:

1) The thromboembolus itself

2) A series of subsequent vasoconstrictive events that decrease collateral circulation.

3) Reperfusion injury following the breakdown of the clot and return of collateral circulation.

The consequence of this is ischaemic neuromyopathy. The muscle is quite sensitive to ischaemic damage - with the clot causing a rigor mortis effect in the tissue. The peripheral nerves are fairly resistant to structural damage but they undergo rapid functional change. This functional failure can be reversed if blood flow is restored before structural nerve damage occurs (several hours later). The amount of structural nerve damage determines the level of permanent nerve damage.

Signalment

As mentioned, cats normally have underlying cardiac disease - most commonly hypertrophic cardiomyopathy (HCM).

Clinical Signs

The most common presenting signs are acute rear limb paralysis, depression, pain and dyspnoea. The paralysis may be unilateral or bilateral. On clinical exam there is an absence of femoral pulses in the affected limbs and the cranial tibial and gastrocnemius muscles feel firm on palpation. The footpads are cold and discoloured and there is no response to noxious stimuli or movement in the lower limb. Additional findings include a lack of tone in the anal sphincter, bladder distention and abdominal pain. Pain and underlying cardiac disease can cause tachycardia and tachypnoea. Additional signs of cardiac disease may be detected on cardiac auscultation such as a murmur, arrhythmia or gallop rhythm.

Reperfusion injury, which occurs several hours after clot formation may cause depression, arrhythmias and cardiac conduction abnormalities. This can be fatal. Swelling and oedema of the limbs is seen once reperfusion occurs.

Diagnosis

A provisional diagnosis may be made based on history and clinical signs however the following tests should be performed to confirm this diagnosis:

Blood Pressure Reading

Blood pressure readings should be taken on all four limbs. This allows comparison between the normal and affected legs and provides information on the cardiovascular status of the animal.

Echocardiography

Echocardiography is used to identify the underlying cardiac disease. It is possible to assess the level of hypertrophy, systolic and diastolic dysfunction. Intra-atrial blood stasis and spontaneous contrast (clots) may also be evident on echo.

Radiography

Thoracic radiography is useful to identify any pulmonary oedema or pleural effusions caused by the underlying cardiac disease.

Biochemistry

Many abnormalities may be seen including azotaemia, an increase in muscle enzymes and lactate, hyperglycaemia, hyperkalaemia, acidosis, hypocalcaemia and hyperphosphataemia.

Electrocardiography

An ECG should taken on arrival and following reperfusion. This is because hyperkalaemia is a common consequence of reperfusion and high potassium levels can cause significant cardiac rhythm disturbances.

Treatment

Immediate treatment should focus on both the management of both cardiac disease and the thromboembolism:

Congestive heart failure should be treated with the diuretic frusemide - which should clear pulmonary oedema and relieve respiratory signs. Fluid therapy is required for the treatment of cardiovascular shock and hyperkalaemia - however it is important to closely monitor and adjust the fluid rate to prevent the worsening of pulmonary oedema. Analgesia should be provided although acepromazine has been used historically, butorphanol is recommended in recent texts. The anticoagulant heparin should also be administered to prevent the clot expanding.

Surgery to remove the clot can be performed, however it must be done within hours of the thromboembolus forming and requires specialist facilities not usually available in general practice. Therefore it is not normally performed. Clot dissolution can be attempted using streptokinase and urokinase but again this is normally restricted to referral centres.

Hyperkalaemia and the associated cardiac rhythm and conduction disturbances that occur secondary to reperfusion injury should be treated with fluid therapy and appropriate drugs (calcium borogluconate, insulin, glucose, sodium bicarbonate or sodium chloride).

Anticoagulant therapy with warfarin, aspirin and clopidogrel may be given to the cat long term to prevent recurrence and cardiac medications such as frusemide, ACE inhibitors should be considered for the management of cardiac disease. Analgesia is not required at this point but physical therapy should be performed for a minimum of 4-6 weeks whilst the limb recovers. It is important to check the limb for any wounds or secondary infection and treat these appropriately.

Prognosis

Prognosis is entirely dependent on the level of ischaemic injury. Several features can give an indication of prognosis but ultimately only time will tell if the cat has the ability to regain function in the affected limb. Obviously less severe clinical signs on presentation suggests a ‘better’ prognosis. The type of cardiac disease present also affects prognosis, with a better prognosis in cases of HCM with minimal atrial enlargement and no congestive heart failure. It is reported that approximately one third of cats survive the initial thromboembolus and reperfusion injury. The condition is thought to recur in up to a quarter of cases.

Prevention

Cats with cardiac disease may have routine echocardiograms to monitor the progresion of disease. The presence of spontaneous contrast within an enlarged atria is seen in at-risk cats. These animals can be treated prophylactically with anticoagulants such as aspirin.

References

Gilson, SD (1998) Self-Assessment Colour Review Small Animal Soft Tissue Surgery Manson

Moise, NS (2007) Presentation and management of thromboembolism in cats In Practice 2007 29: 2-8




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