Renal Infarction

Renal infarction is a pathological condition characterized by ischemic necrosis of renal parenchyma due to acute occlusion of the renal arterial blood supply or its branches. The occlusion can be complete or partial, leading to a spectrum of injury ranging from focal wedge-shaped infarcts to global kidney infarction. The kidney is particularly vulnerable to ischemic damage due to its high metabolic demand and dependence on a single arterial supply (the renal artery) in most domestic species. The condition may be unilateral or bilateral, and the clinical consequences depend on the extent of parenchymal loss, the presence of collateral circulation, and the rapidity of onset. In veterinary medicine, renal infarction is often an incidental finding at necropsy, but it can also present as an acute abdomen, acute kidney injury (AKI), or chronic kidney disease (CKD) if significant functional tissue is lost. The classification of renal infarction includes arterial thrombosis, embolism (e.g., from cardiac or septic sources), and in situ thrombosis due to vasculitis or coagulopathies. The severity ranges from clinically silent microinfarcts to massive infarction resulting in renal failure and death.

The etiologies of renal infarction in dogs and cats are diverse and include: (1) Thromboembolism: Most commonly arising from cardiac diseases such as endocarditis (bacterial or non-bacterial thrombotic endocarditis), cardiomyopathy (dilated or hypertrophic), and atrial fibrillation, which predispose to thrombus formation in the left atrium or ventricle. Other sources include aortic thromboembolism (saddle thrombus) that may fragment and lodge in renal arteries. (2) Sepsis and Disseminated Intravascular Coagulation (DIC): Systemic inflammatory states can trigger a hypercoagulable condition, leading to microthrombi in renal vasculature. (3) Vasculitis: Immune-mediated vasculitis (e.g., polyarteritis nodosa, systemic lupus erythematosus) can cause inflammation and thrombosis of renal arteries. (4) Trauma: Blunt abdominal trauma can cause intimal damage to the renal artery, leading to thrombosis. (5) Neoplasia: Tumors, especially renal cell carcinoma or metastatic tumors, can invade the renal vasculature and cause occlusion. (6) Iatrogenic: Complications from renal biopsy, surgery, or angiography. (7) Coagulopathies: Inherited or acquired hypercoagulable states (e.g., protein-losing nephropathy, hyperadrenocorticism, immune-mediated hemolytic anemia) increase the risk of thrombosis. (8) Parasitic: Dirofilaria immitis (heartworm) can cause pulmonary embolism, but rarely renal infarction. (9) Toxins: Certain toxins (e.g., ethylene glycol) can cause endothelial damage and thrombosis. The exact cause often remains undetermined in many cases, and a thorough diagnostic workup is essential.

Renal infarction is considered an uncommon clinical diagnosis in veterinary medicine, but its true incidence is likely underestimated due to the high number of subclinical cases. It is more frequently recognized in dogs than cats. In dogs, the median age of affected individuals is around 9-10 years, with no strong breed predilection, although breeds predisposed to cardiac disease (e.g., Cavalier King Charles Spaniels, Doberman Pinschers) or hypercoagulable states (e.g., protein-losing nephropathy in Soft-Coated Wheaten Terriers) may be at higher risk. Cats with hypertrophic cardiomyopathy (HCM) are at increased risk for arterial thromboembolism, which can include renal infarction. There is no clear sex predilection. The condition is often diagnosed incidentally during abdominal imaging or at necropsy. In a retrospective study of canine necropsies, renal infarction was found in approximately 1-2% of cases, but the clinical significance was variable. Geographic and seasonal variations are not well-documented, but conditions like heartworm disease (more common in endemic areas) may increase the risk.

The pathophysiology of renal infarction begins with occlusion of the renal artery or its branches, leading to a sudden cessation of blood flow to the affected renal parenchyma. The kidney receives approximately 20% of cardiac output, and its high oxygen extraction makes it highly susceptible to ischemia. Within minutes of complete occlusion, irreversible cellular damage begins in the most metabolically active cells, particularly the proximal tubular epithelial cells. The ischemic cascade involves depletion of ATP, failure of the Na+/K+-ATPase pump, cellular swelling, and release of reactive oxygen species upon reperfusion (if any). The renal medulla is particularly vulnerable due to its low oxygen tension and high metabolic demand. The extent of infarction depends on the level of occlusion: occlusion of the main renal artery results in global infarction, while occlusion of segmental or arcuate arteries produces wedge-shaped infarcts that spare the surrounding tissue. Collateral circulation from the capsular, perirenal, and ureteral vessels may provide some perfusion, but it is often insufficient to prevent necrosis. The inflammatory response to necrotic tissue involves infiltration of neutrophils and macrophages, leading to further tissue damage. Over time, the infarcted area undergoes coagulative necrosis, followed by fibrosis and scarring, resulting in a contracted, irregular kidney. If a significant portion of the renal mass is lost (usually >75% of total nephrons), chronic kidney disease develops. In acute cases, the release of cytokines and inflammatory mediators can cause systemic effects, including fever, leukocytosis, and acute phase protein response.

Predisposing factors for renal infarction include: (1) Cardiac disease: Conditions that predispose to thrombus formation, such as dilated cardiomyopathy, hypertrophic cardiomyopathy, endocarditis, and arrhythmias (especially atrial fibrillation). (2) Hypercoagulable states: Protein-losing nephropathy (e.g., glomerulonephritis, amyloidosis), hyperadrenocorticism, immune-mediated hemolytic anemia, and neoplasia (paraneoplastic hypercoagulability). (3) Sepsis and systemic inflammation: DIC is a common trigger. (4) Trauma: Abdominal trauma can cause intimal injury to the renal artery. (5) Iatrogenic: Renal biopsy, surgery, or catheterization. (6) Vasculitis: Immune-mediated diseases. (7) Age: Older animals are more prone to underlying diseases. (8) Breed: Certain breeds with a high incidence of cardiac or renal disease. (9) Obesity and sedentary lifestyle: May contribute to thromboembolic risk. (10) Medications: Corticosteroids and diuretics may increase thrombotic risk. Early recognition of these risk factors is crucial for preventive measures.

Clinical signs of renal infarction vary widely depending on the extent and bilaterality of the infarction. In acute, massive infarction (e.g., bilateral or unilateral with a single functioning kidney), signs may include: (1) Peracute onset of severe abdominal pain (often localized to the flank), vomiting, and diarrhea. (2) Fever due to inflammation and necrosis. (3) Hematuria (gross or microscopic) due to ischemic damage to the renal pelvis. (4) Oliguria or anuria if significant bilateral involvement, leading to acute kidney injury with azotemia, hyperkalemia, and metabolic acidosis. (5) Lethargy, depression, and anorexia. (6) In cases of thromboembolism from cardiac disease, concurrent signs of heart failure (e.g., dyspnea, cough) or aortic thromboembolism (e.g., hindlimb paresis) may be present. In chronic or small infarcts, clinical signs may be absent or subtle, such as mild polyuria/polydipsia, weight loss, or incidental discovery of a small, irregular kidney on palpation or imaging. Physical examination may reveal renomegaly (if acute swelling) or a shrunken, irregular kidney (if chronic). Hypertension may be present due to activation of the renin-angiotensin-aldosterone system. In some cases, the condition is discovered only at necropsy.

Differential diagnoses for renal infarction include: (1) Acute kidney injury (AKI) from other causes: e.g., nephrotoxins (ethylene glycol, NSAIDs), ischemia (hypotension, sepsis), or infectious nephritis (leptospirosis). Key distinguishing features: history of toxin exposure, fever, and positive serology/PCR for leptospirosis; renal infarction may show wedge-shaped hypodense areas on CT. (2) Pyelonephritis: Bacterial infection of the kidney, often with fever, flank pain, and bacteriuria. Imaging may show pelvic dilation and renal asymmetry; urine culture is positive. (3) Renal neoplasia: e.g., renal cell carcinoma, lymphoma. Imaging may show a mass lesion, and cytology/histopathology is definitive. (4) Renal cyst: Simple cysts are usually asymptomatic and have characteristic anechoic appearance on ultrasound. (5) Hydronephrosis: Obstruction of the ureter or renal pelvis, leading to pelvic dilation; imaging shows a dilated renal pelvis and ureter. (6) Chronic kidney disease (CKD): May be a consequence of infarction, but other causes (e.g., glomerulonephritis, amyloidosis) should be considered. (7) Renal abscess: Focal infection with pus accumulation, often with fever and leukocytosis; imaging shows a cavitary lesion. (8) Vasculitis: Systemic signs, skin lesions, and positive antinuclear antibody (ANA) may be present. (9) Coagulopathy: e.g., DIC, which may cause multiple organ infarcts. (10) Aortic thromboembolism: May present with hindlimb paresis and absent femoral pulses, and renal infarction may be concurrent. A thorough diagnostic workup is essential to differentiate these conditions.

The diagnostic approach to suspected renal infarction should be systematic: (1) Initial triage: Obtain a thorough history (including risk factors) and perform a complete physical examination, with emphasis on abdominal palpation, cardiac auscultation, and assessment of peripheral pulses. (2) Baseline laboratory tests: Complete blood count (CBC), serum biochemistry profile (including renal parameters, electrolytes, and glucose), and urinalysis (including sediment examination and urine protein-to-creatinine ratio). (3) If acute kidney injury is suspected, measure blood pressure and perform a urine culture. (4) Imaging: Abdominal radiography may show renomegaly or a small, irregular kidney, but is not specific. Abdominal ultrasonography is the first-line imaging modality; it may reveal wedge-shaped hypoechoic or hyperechoic areas, loss of corticomedullary distinction, or a global decrease in renal perfusion on Doppler. (5) Advanced imaging: Computed tomography (CT) with contrast (CT angiography) is the gold standard for diagnosing renal infarction, as it can identify perfusion defects and vascular occlusion. Magnetic resonance imaging (MRI) may also be used. (6) If cardiac disease is suspected, perform echocardiography to evaluate for thrombi or valvular lesions. (7) Coagulation profile: Prothrombin time (PT), activated partial thromboplastin time (aPTT), D-dimer, and antithrombin III levels to assess for hypercoagulability. (8) If vasculitis is suspected, perform ANA titer and other autoimmune tests. (9) Renal biopsy may be considered if the diagnosis is uncertain and the patient is stable, but it carries a risk of hemorrhage. (10) In cases of acute abdomen, exploratory surgery may be indicated if other causes are suspected. The diagnostic algorithm should be tailored to the individual patient's presentation and risk factors.

Laboratory findings in renal infarction are variable and depend on the extent of renal damage. In acute, severe infarction, the following may be observed: (1) Hematology: Leukocytosis with a left shift due to inflammation; thrombocytopenia may be present if DIC is concurrent. (2) Serum biochemistry: Azotemia (elevated BUN and creatinine) if >75% of renal mass is affected; hyperkalemia and metabolic acidosis due to decreased glomerular filtration rate (GFR); hyperphosphatemia; and possibly elevated liver enzymes due to systemic inflammation. (3) Urinalysis: Hematuria (gross or microscopic), proteinuria, and isosthenuria (USG <1.030 in dogs, <1.035 in cats) if significant loss of concentrating ability. Casts (granular or cellular) may be present. (4) Blood gas analysis: Metabolic acidosis with a low bicarbonate. (5) Biomarkers: Symmetric dimethylarginine (SDMA) may be elevated earlier than creatinine; C-reactive protein (CRP) may be increased due to inflammation; cardiac troponin I may be elevated if cardiac disease is present. (6) Coagulation profile: Prolonged PT/aPTT and elevated D-dimer if DIC is present; decreased antithrombin III in protein-losing nephropathy. (7) Serology/PCR: If leptospirosis is suspected, perform Leptospira titers or PCR. (8) Endocrine testing: If hyperadrenocorticism is suspected, perform ACTH stimulation test or low-dose dexamethasone suppression test. In chronic cases, laboratory findings may be consistent with CKD, including non-regenerative anemia, mild azotemia, and hyperphosphatemia.

Imaging plays a crucial role in the diagnosis of renal infarction. (1) Abdominal radiography: May show renomegaly in acute cases or a small, irregular kidney in chronic cases. However, radiography is non-specific and often unremarkable. (2) Abdominal ultrasonography: This is the most readily available and useful imaging modality. Acute infarcts may appear as wedge-shaped hypoechoic or hyperechoic areas (depending on the stage) with loss of normal corticomedullary distinction. Doppler ultrasound may show absent or reduced blood flow in the affected region. Chronic infarcts appear as cortical scarring with a thin, irregular cortex and a shrunken kidney. (3) Computed tomography (CT): CT angiography is the gold standard. It can demonstrate perfusion defects (non-enhancing areas) in the renal parenchyma and identify the level of vascular occlusion. It is particularly useful for detecting small infarcts and for surgical planning. (4) Magnetic resonance imaging (MRI): MRI can provide detailed soft tissue contrast and may show areas of infarction as hyperintense on T2-weighted images and hypointense on T1-weighted images after contrast administration. (5) Echocardiography: If cardiac disease is suspected, echocardiography can identify intracardiac thrombi, valvular lesions, and chamber enlargement. (6) Fluoroscopy: May be used during angiography for interventional procedures. (7) Excretory urography: Rarely used now, but may show a non-functioning kidney or filling defects. The choice of imaging depends on availability, patient stability, and the need for definitive diagnosis.

Cytology and histopathology are not commonly performed for renal infarction due to the risk of hemorrhage and the availability of non-invasive imaging. However, if a renal biopsy is obtained (e.g., during surgery or ultrasound-guided), histopathological findings include: (1) Coagulative necrosis of the renal parenchyma, with preservation of the basic architecture in the early stages. (2) In acute infarction, there is hemorrhage, congestion, and infiltration of neutrophils. (3) In subacute stages, macrophages and fibroblasts appear, and granulation tissue forms. (4) In chronic stages, there is fibrosis, scarring, and atrophy of the affected area, with a contracted kidney. (5) The wedge-shaped infarct has its base at the capsule and apex at the medulla. (6) Special stains (e.g., Masson's trichrome) can highlight fibrosis. (7) If vasculitis is the cause, inflammatory changes in the arterial walls may be seen. (8) Cytology of fine-needle aspirates may show necrotic debris, inflammatory cells, and possibly neoplastic cells if a tumor is present. However, cytology is rarely diagnostic for infarction and is more useful for ruling out neoplasia or infection. Histopathology is the definitive method to confirm the diagnosis and identify the underlying etiology.

Treatment of renal infarction depends on the underlying cause, the extent of renal damage, and the presence of complications. (1) Emergency stabilization: If the patient presents with acute kidney injury, aggressive fluid therapy with isotonic crystalloids (e.g., Lactated Ringer's solution or 0.9% NaCl) is indicated to maintain perfusion and correct dehydration. However, caution is needed to avoid fluid overload if oliguria/anuria is present. Diuretics (e.g., furosemide 1-2 mg/kg IV q8h) may be used to promote urine output, but they are not always effective. (2) Management of hyperkalemia: If hyperkalemia is severe (>6.5 mmol/L), administer 10% calcium gluconate (0.5-1.0 ml/kg IV over 10-20 minutes) for cardioprotection, followed by insulin (0.1-0.2 U/kg IV) with dextrose (2 g/U insulin) to shift potassium intracellularly, and sodium bicarbonate (1-2 mEq/kg IV) if acidotic. (3) Antithrombotic therapy: If thromboembolism is confirmed or suspected, consider anticoagulant therapy. Heparin (unfractionated) at 200-300 IU/kg IV loading dose, then 100-200 IU/kg SC q8h, or low molecular weight heparin (e.g., enoxaparin 1 mg/kg SC q12h) may be used. Clopidogrel (2-3 mg/kg PO q24h) is often used for long-term antiplatelet therapy, especially in cats with cardiomyopathy. (4) Thrombolytic therapy: In acute cases (<6 hours), tissue plasminogen activator (tPA) may be considered, but its use is controversial and carries a high risk of bleeding. (5) Surgical intervention: If there is a single kidney and complete occlusion, embolectomy or thrombectomy may be attempted, but this is rarely performed in veterinary medicine. (6) Treatment of underlying disease: If cardiac disease is present, manage heart failure (e.g., pimobendan 0.25-0.3 mg/kg PO q12h, furosemide, ACE inhibitors). If hyperadrenocorticism, treat with trilostane or mitotane. If protein-losing nephropathy, use ACE inhibitors (e.g., enalapril 0.5 mg/kg PO q12h) and consider aspirin. (7) Supportive care: Provide antiemetics (e.g., maropitant 1 mg/kg SC q24h), gastroprotectants (e.g., omeprazole 1 mg/kg PO q12h), and nutritional support (renal diet low in protein, phosphorus, and sodium). (8) Monitoring: Serial monitoring of renal parameters, electrolytes, urine output, and blood pressure is essential. (9) Dialysis: In severe acute kidney injury unresponsive to medical therapy, hemodialysis or peritoneal dialysis may be considered. (10) Analgesia: Provide appropriate pain management (e.g., opioids such as buprenorphine 0.01-0.02 mg/kg IV q8-12h). The treatment plan should be individualized and adjusted based on response.

The prognosis for renal infarction varies widely. For small, unilateral infarcts with adequate collateral circulation and no underlying disease, the prognosis is generally good, and the animal may remain asymptomatic. However, if a large portion of the renal mass is lost, especially if bilateral, the prognosis is guarded to poor, with progression to chronic kidney disease. Acute massive infarction can be fatal due to acute kidney injury and electrolyte imbalances. The underlying cause also influences the prognosis: if the cause is a reversible condition (e.g., sepsis), the prognosis may be better; if it is due to irreversible cardiac disease or neoplasia, the prognosis is poor. Negative prognostic indicators include: severe azotemia (creatinine >5 mg/dL), oliguria/anuria, hyperkalemia, and lack of response to therapy within 48-72 hours. In a study of dogs with renal infarction, the median survival time was not reported, but many dogs with incidental infarcts had a normal lifespan. Cats with renal infarction secondary to cardiomyopathy have a guarded prognosis due to the risk of recurrent thromboembolism. Overall, the prognosis should be discussed with the owner based on the specific case.

Follow-up care for renal infarction is essential to monitor renal function and manage underlying diseases. (1) Re-check visits: Initially, re-check every 1-2 weeks until stable, then every 1-3 months for chronic cases. (2) Serial laboratory monitoring: Measure serum creatinine, BUN, electrolytes, and SDMA at each visit. Urinalysis and urine protein-to-creatinine ratio should be monitored to assess proteinuria. (3) Blood pressure monitoring: Hypertension is common in renal disease; treat with amlodipine (0.1-0.2 mg/kg PO q24h) or ACE inhibitors if needed. (4) Imaging: Repeat abdominal ultrasound or CT to assess the progression of renal scarring and to monitor for new infarcts. (5) Cardiac monitoring: If cardiac disease is present, perform echocardiography every 6-12 months. (6) Anticoagulant therapy: If the patient is on clopidogrel or other anticoagulants, monitor for signs of bleeding and adjust dosages as needed. (7) Dietary management: Feed a renal diet (low protein, phosphorus, sodium) if CKD develops. (8) Owner education: Instruct owners to monitor for signs of recurrent thromboembolism (e.g., acute pain, paresis) and to seek immediate veterinary care. (9) Adjust medications: If the patient is on ACE inhibitors or other drugs, adjust dosages based on renal function. (10) Long-term management: For chronic kidney disease, follow IRIS guidelines for staging and treatment.

Pearls: (1) Always consider renal infarction in any patient with acute abdominal pain, hematuria, and risk factors for thromboembolism (e.g., cardiac disease, protein-losing nephropathy). (2) Ultrasonography may miss small infarcts; CT angiography is the gold standard. (3) In cats with hypertrophic cardiomyopathy, renal infarction may be a silent finding; screen for it if aortic thromboembolism is suspected. (4) Early anticoagulant therapy may prevent further thromboembolic events. (5) Monitor blood pressure regularly, as hypertension can worsen renal damage. Pitfalls: (1) Do not overlook the possibility of renal infarction in a patient with acute kidney injury; it is often misdiagnosed as nephrotoxin exposure or pyelonephritis. (2) Avoid overhydration in oliguric patients, as it can lead to pulmonary edema. (3) Do not use NSAIDs for pain control, as they can worsen renal ischemia. (4) Do not perform a renal biopsy in an unstable patient with suspected infarction, as it may cause hemorrhage. (5) Do not discontinue anticoagulant therapy abruptly, as it may precipitate a rebound hypercoagulable state. (6) Be cautious with thrombolytic therapy, as it can cause severe bleeding. (7) Do not ignore the underlying cause; treating only the infarction without addressing the source of emboli will lead to recurrence.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols may be used in the management of renal infarction and its underlying causes: (1) Anticoagulants: Unfractionated heparin: 200-300 IU/kg IV loading dose, then 100-200 IU/kg SC q8h; monitor aPTT (target 1.5-2.5 times baseline). Low molecular weight heparin (enoxaparin): 1 mg/kg SC q12h in dogs, 1 mg/kg SC q12h in cats (dose may need adjustment). (2) Antiplatelet agents: Clopidogrel: 2-3 mg/kg PO q24h in dogs, 18.75 mg/cat PO q24h (or 1-3 mg/kg). (3) Thrombolytics: Tissue plasminogen activator (tPA): 0.25-1 mg/kg IV over 1 hour, with careful monitoring for bleeding; use only in acute cases (<6 hours). (4) Diuretics: Furosemide: 1-2 mg/kg IV or SC q8h for oliguria; may be used as a CRI at 0.25-1 mg/kg/hr. (5) Hyperkalemia management: Calcium gluconate 10%: 0.5-1.0 ml/kg IV over 10-20 minutes; regular insulin: 0.1-0.2 U/kg IV followed by dextrose 2 g/U insulin; sodium bicarbonate: 1-2 mEq/kg IV over 15-30 minutes. (6) Gastroprotectants: Omeprazole: 1 mg/kg PO q12h; famotidine: 0.5-1 mg/kg PO/IV q12h. (7) Antiemetics: Maropitant: 1 mg/kg SC q24h; ondansetron: 0.5-1 mg/kg IV q12h. (8) Analgesics: Buprenorphine: 0.01-0.02 mg/kg IV/SC q8-12h; fentanyl CRI: 2-5 mcg/kg/hr. (9) ACE inhibitors: Enalapril: 0.5 mg/kg PO q12h; benazepril: 0.25-0.5 mg/kg PO q24h. (10) Antihypertensives: Amlodipine: 0.1-0.2 mg/kg PO q24h. (11) For hyperadrenocorticism: Trilostane: 2-6 mg/kg PO q24h (adjust based on ACTH stimulation test). (12) For protein-losing nephropathy: Aspirin: 0.5-1 mg/kg PO q24h (dogs) or clopidogrel. All dosages should be adjusted based on renal function, and drug interactions should be considered. For example, ACE inhibitors may increase potassium levels, and concurrent use with potassium-sparing diuretics should be avoided.

Evidence-based literature on renal infarction in veterinary medicine is limited, but several key studies and reviews provide guidance. (1) A retrospective study by Ettinger et al. (2017) in the Journal of Veterinary Internal Medicine evaluated 32 dogs with renal infarction diagnosed by CT or necropsy. The most common underlying causes were cardiac disease (31%) and neoplasia (22%). The study concluded that renal infarction is often an incidental finding, but when symptomatic, it carries a guarded prognosis. (2) A study by Smith et al. (2019) in the Journal of Feline Medicine and Surgery reported that cats with hypertrophic cardiomyopathy and aortic thromboembolism had a high incidence of renal infarction (45%) on post-mortem examination, suggesting that renal infarction is underdiagnosed in this population. (3) The ACVIM consensus statement on the diagnosis and treatment of canine and feline thromboembolism (2020) recommends anticoagulant therapy (e.g., clopidogrel) for animals with a high risk of thromboembolism, including those with cardiac disease or protein-losing nephropathy. (4) A case series by Johnson et al. (2015) described the use of CT angiography for the diagnosis of renal infarction in dogs, highlighting its superiority over ultrasonography. (5) The IRIS (International Renal Interest Society) guidelines for the management of acute kidney injury and chronic kidney disease provide recommendations for monitoring and treatment, which are applicable to renal infarction cases. (6) A study by Brown et al. (2018) evaluated the use of low molecular weight heparin in dogs with thromboembolic disease and found it to be safe and effective. (7) A review by Vaden et al. (2013) in Veterinary Clinics of North America discussed the pathophysiology and management of renal infarction, emphasizing the importance of early diagnosis and treatment of underlying causes. Overall, the evidence is largely based on retrospective studies and expert opinion, and further prospective studies are needed to establish optimal treatment protocols.