Last updated on:October 19th, 2021
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Acute kidney injury (AKI) occurs when a sudden decrease in kidney function causes the accumulation of the products of metabolism; it can be classified into prerenal AKI, intrinsic AKI, and postrenal AKI. Prerenal AKI is a consequence of renal hypoperfusion; and postrenal AKI is due to obstruction of the urinary tract. Both are a consequence of extrarenal disease. Intrinsic AKI represents true kidney disease, and can be categorized by the component of the kidney that is primarily affected: tubular, glomerular, interstitial, or vascular. Importantly, prerenal AKI can precipitate intrinsic AKI, if prolonged severe hypoperfusion causes ischemic injury to the renal tubules; this is termed acute tubular necrosis (ATN).
Most patients with mild to moderate AKI are asymptomatic. Such individuals are diagnosed with AKI only after laboratory testing.
Prerenal AKI: reduced fluid intake
Reduced fluid intake causes intravascular volume depletion, and may also impact renal arterial pressures. Renal perfusion is correspondingly lowered, resulting in a reduced glomerular filtration rate (GFR) and prerenal AKI.
Prerenal AKI: volume depletion
Severe volume loss can occur due to vomiting, diarrhea, diuretic overuse, burns, or hemorrhage. The intravascular volume depletion and resulting reduction in renal arterial blood pressures then gives rise to prerenal AKI.
Prerenal AKI: medications
Medications such as angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARB), and nonsteroidal anti-inflammatory drugs (NSAIDs) can cause prerenal AKI due to volume depletion or impairment of renal perfusion.
Prerenal AKI: heart failure
Prerenal AKI can occur as a consequence of heart failure; this is termed “cardiorenal syndrome.”
Prerenal AKI: liver failure
Liver failure can give rise to prerenal AKI; this is termed “hepatorenal syndrome.”
Prerenal AKI: vascular causes
Vascular conditions causing renal hypoperfusion (e.g., renovascular disease, hemolytic uremic syndrome, etc.) can give rise to prerenal AKI.
Intrinsic AKI: glomerular disease
All glomerular diseases can give rise to intrinsic AKI—especially (but not exclusively) those giving rise to nephritic syndrome. Rheumatological conditions (e.g., systemic lupus erythematosus) and autoimmune disorders (e.g. Goodpasture's syndrome) can also give rise to intrinsic AKI. Their symptoms should be looked for.
Intrinsic AKI: medications
Antibiotics such as aminoglycosides, acyclovir, phenytoin, interferon, proton pump inhibitors (PPI), and NSAIDs can cause ATN and acute interstitial nephritis (AIN).
Intrinsic AKI: contrast agent use
Use of an intravascular contrast agent (as an adjunct for imaging studies) can result in contrast-induced AKI, a form of intrinsic AKI. Risk factors include co-existing chronic kidney disease (CKD), diabetes mellitus, NSAID therapy, a low circulatory volume, and the use of a large amount of contrast.
Postrenal AKI: voiding symptoms
Symptoms such as urgency, hesitancy, intermittent flow, anuria, or polyuria may indicate postrenal AKI. In this, an obstruction to the urinary flow causes increased intra-tubular pressure and a correspondingly decreased GFR. The obstruction can be at any level of the urinary system, from the renal tubule to the urethra. Benign prostatic hypertrophy (BPH) is the most common such etiology.