Etiological classification of AKI
Understanding the causes of acute kidney injury is based on assessing the level of damage: the problem may be localized upstream of the organ, within the renal tissue itself, or along the urine outflow pathways. Based on this, three main forms are distinguished.
1. Prerenal AKI Develops due to a sharp disruption of blood inflow to the organ (reduced perfusion), while the renal parenchyma itself initially remains intact. This is the classical picture of the so-called "shock kidney", which occurs in:
- Massive blood or fluid loss (marked dehydration).
- Severe polytrauma and massive tissue breakdown.
- Conditions accompanied by hemolysis.
- Development of cardiogenic or bacteremic shock.
2. Renal AKI Associated with direct damage to the structures of the renal parenchyma. It occurs due to the following factors:
- Toxic kidney: the result of a direct detrimental effect of various nephrotoxins, a significant proportion of which are medications.
- Infectious-toxic kidney: develops due to the direct or indirect damaging effect of infectious agents.
- Vascular obstruction: occurs against the background of severe systemic vasculitis, malignant arterial hypertension, the development of true scleroderma kidney, or necrotizing papillitis.
3. Postrenal AKI Caused by impaired adequate urine outflow due to mechanical obstruction of the urinary tract. The main etiological factors are urolithiasis, as well as specific blockade of renal tubules by urate crystals.
Stages of the clinical course
The clinical presentation of acute kidney injury is not monotonous. Four sequential phases are clearly traceable in its development:
- Initial stage — the period during which the aggressive etiological factor directly exerts its effect (toxin, shock state, obstruction).
- Oliguric (azotemic) stage — the peak period of the disease. Diuresis drops sharply, nitrogen metabolism products rapidly accumulate in the blood, water-electrolyte balance disorders and acid-base balance disturbances develop.
- Diuresis recovery stage (polyuric) — the kidneys begin to produce urine, its volumes increase, but full concentration function has not yet been restored.
- Stage of complete functional recovery (convalescence) — gradual return of all organ function parameters to normal values.
Laboratory and instrumental diagnostics
Competent diagnostics allows not only to confirm AKI, but also to identify its cause.
- Laboratory markers: If a specific change in the form of eosinophiluria is detected in the urine, it in 95% of cases indicates the development of acute allergic interstitial nephritis. In the differential diagnosis of the causes of unusual urine coloration, attention is paid to the sediment: if pigmented casts and a positive occult blood test are detected, but erythrocytes are absent in the sediment, this is a sure sign of hemoglobinuria or myoglobinuria. In addition, microbiological control (urine culture) is mandatory for all patients without exception, since the susceptibility to infections increases sharply against the background of AKI.
- Instrumental methods:
- Ultrasound: the basic imaging method. Helps to promptly detect hydronephrosis, hydroureter, the presence of calculi, or an abdominal aortic aneurysm. Doppler ultrasound is used to assess blood flow quality.
- Arteriography: prescribed when there is a well-founded suspicion of vascular pathologies, such as renal artery stenosis or dissecting aortic aneurysm.
- CT and isotope methods: computed tomography is indispensable for confirming obstructive nephropathy. Isotope dynamic scanning is used for a detailed assessment of perfusion and identification of obstruction levels.
- Chromocystoscopy: used for visual assessment of the patency of the ureteral orifices and reliable exclusion of obstruction at this level.
Briefly: answers to topical questions
What etiological factors lead to the development of postrenal AKI?
Etiological factors of postrenal AKI include any conditions that disrupt urine outflow through the urinary tract. The main causes include:
- Urinary tract obstruction — kidney stones, tubular blockade by urate crystals, tumors, blood clots, foreign bodies (catheters), inflammatory edema, and urethral stenosis (including prostate hypertrophy).
- External compression of the urinary tract — abdominal tumors, enlarged uterus, prostate adenoma, pressure from ascites fluid.
- Kinking of the ureters — occurs with a migrating kidney (nephroptosis) or excessive ureteral length.
What water-electrolyte balance disorders develop in the oliguric stage of AKI?
In the oliguric stage of acute kidney injury, against the background of a sharp drop in diuresis, water-electrolyte disturbances rapidly increase:
- Hyperhydration — associated with impaired fluid excretion from the body.
- Hyperkalemia — the main threat during this period; it can lead to rhythm disturbances, blocks, ventricular fibrillation, and the risk of sudden death. The ECG sign is a tall peaked T wave.
What are the absolute indications for hemodialysis in acute kidney injury?
Absolute indications are not separately distinguished in the sources. The following indications and guidelines are specified for extracorporeal detoxification or hemodialysis in AKI:
- Inefficacy of drug therapy for AKI.
- Progression of uremia.
- Terminal uremia.
- Severe homeostasis disorders.
- Hyperkalemia — the potassium level serves as a guide for deciding on the initiation of hemodialysis.
- Anuria in AKI — hemodialysis is used when anuria develops.
Which groups of medications most frequently cause toxic renal AKI?
Nephrotoxic drugs and iatrogenic factors are indicated in the sources for toxic renal AKI, especially with inadequate dosage or unjustified use. These include:
- Certain antibiotics, with aminoglycosides specifically mentioned.
- Sulfonamides.
- Nonsteroidal anti-inflammatory drugs (NSAIDs).
- Cytostatics.
- Radiocontrast agents — associated with the risk of contrast-induced nephropathy.
To memorize the levels of damage (types of AKI), imagine a plumbing system: a problem at the pumping station before the filter is prerenal, a breakdown of the purification cartridge itself is renal, and a clog in the drain pipe after the filter is postrenal.
Class and exam questions
Is acute kidney injury reversible?
Yes, the prognosis is favorable in most cases. With the timely elimination of the damaging factor, the pathological process is reversible, and renal function is restored.
Why is a urine bacterial culture performed in all patients with AKI?
Patients with acute kidney injury exhibit an extremely high susceptibility to developing secondary urinary tract infections, which requires strict microbiological control.
What does the presence of blood on a test strip indicate in the absence of erythrocytes in the urine?
Such a picture (especially in combination with pigmented casts) indicates not bleeding, but hemoglobinuria or myoglobinuria — massive destruction of erythrocytes or muscle tissue.
What else to study on this topic
- Pathophysiology of azotemia and acid-base balance disorders in AKI
- Detailed clinical presentation of the oliguric stage (peak period)
- Features of the course of acute allergic interstitial nephritis
- Principles of differential diagnosis of obstructive nephropathy based on CT data
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