Postrenal acute kidney injury: Clinical sciences
Introduction 0:00–0:44
Acute kidney injury, or AKI, refers to a sudden decline in kidney function that results in electrolyte imbalances, extracellular dysregulation, and the accumulation of nitrogenous waste, such as ammonia and uric acid.
Based on the underlying cause, acute kidney injury can be subdivided into prerenal-, renal, and postrenal acute kidney injury.
Postrenal acute kidney injury is further classified based on the location of the obstruction in the urinary tract as postrenal acute kidney injury due to upper and lower obstruction of the urinary tract.
Now, if your patient presents with chief concerns suggesting postrenal AKI, perform an ABCDE assessment to determine if they are unstable or stable.
Unstable patient 0:44–1:23
If unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access, which might include dialysis access, and put your patient on continuous vital sign monitoring and cardiac telemetry!
Finally, if you identify hyperkalemia, metabolic acidosis, volume overload, or symptomatic uremia, start emergent hemodialysis!
Now, let’s go back to the ABCDE assessment and take a look at stable patients. In stable individuals, obtain a focused history and physical exam, which is going to help you differentiate different types of AKI.
Stable Patient 1:23–3:13
Patients with post-renal AKI typically report pelvic discomfort, often in combination with slow stream or dribbling. In acute cases, the patient could report sudden pelvic pain and urinary retention.
Additionally, history might reveal sudden flank pain and bloody urine or conditions like prostate disease and nephrolithiasis.
With these findings, you should suspect postrenal causes of AKI. Next, order a basic metabolic panel and urinalysis with microscopy, measure the patient’s urine output over time, and check renal ultrasound!
In all types of AKI, labs will reveal a rise in serum creatinine of 0.3 milligrams per deciliter or more over 48 hours; a rise of serum creatinine 1.5 times the baseline or more in the last 7 days, or urine output less than 0.5 milliliters per kilogram per hour for six hours.
However, in postrenal AKI, the urinalysis with microscopy will show bland urine sediment and the renal ultrasound will reveal obstruction and hydronephrosis.
At this point, diagnose postrenal AKI, so your next step is to order a post-void residual, or PVR, bladder scan. Post-void residual volume below 100 milliliters suggests upper urinary tract obstruction, so order abdominal and pelvic CT to assess the underlying cause.
Upper urinary tract obstruction/Nephrolithiasis 3:13–4:02
If the CT confirms hydronephrosis and shows bilateral ureteral stones, diagnose nephrolithiasis. Bilateral ureteral obstruction in healthy individuals is rare.
However, conditions like hyperuricemia and hyperparathyroidism increase the risk of kidney stones, so these individuals have a higher chance of developing bilateral obstruction.
Next, if the CT scan reveals a solid mass surrounding or infiltrating the ureters, the most likely cause of obstruction is malignancy!
Malignancy 4:02–4:35
In order to confirm the diagnosis and determine the specific type of malignancy you should always perform a biopsy. Treatment generally depends on the type of malignancy, but in most cases, includes surgical consultations for ureteral stenting and tumor resection.
Often, your patient will require additional therapy, such as chemo- or radiation therapy. Finally, if the CT scan reveals a mass external to the colonic lumen with fluid loculations, think of an intraabdominal abscess.
Intra-abdominal abscess 4:35–4:57
In this case, treatment includes intravenous antibiotics and surgical consultations for ureteral stenting, abscess drainage, and possible surgical resection.
Now, let’s go back and focus on individuals with a post-void residual volume greater than 100 milliliters, which indicates lower urinary tract obstruction and urinary retention.
Lower urinary tract obstruction 4:57–5:37
In this case, your first step is to immediately insert a urinary catheter to drain the accumulated urine and relieve the pressure.
Now, here’s a clinical pearl to keep in mind! If a patient with AKI has a urinary catheter in place, make sure to check for proper placement and patency of the catheter!
Flush or replace an obstructed urinary catheter and repeat labs to ensure AKI resolution. Next, assess the digital rectal exam findings because prostate enlargement and palpable mass suggest prostate disease.
Prostate disease 5:37–6:00
Common causes include benign prostate hyperplasia and prostate cancer. In both cases, you should consult your surgery team for possible treatment options, such as resection or complete prostatectomy.
No prostate disease 6:00–6:34
Moreover, order an abdominal X-ray to check for the presence of calcium phosphate and calcium oxalate stones, which are radiopaque!
However, uric acid stones are radiolucent, so you can’t identify them using X-ray. Instead, consider ultrasound, cystoscopy, or CT, which can also help you identify possible tumors or anatomical abnormalities, like strictures.
Bladder outlet obstruction 6:34–6:53
Urethral obstruction 6:53–7:12
Alternatively, if the underlying cause is urethral stricture, consult your surgery team for urethral dilation. Finally, if imaging findings are normal, but your patient is taking anticholinergic medications, like atropine, or medications with known anticholinergic side effects, such as antihistamines, diagnose medication-induced urinary retention.
Medication-induced urinary retention 7:12–7:33
In this case, you should discontinue the offending medication. On the flip side, if imaging findings are normal and history reveals central nervous system conditions, like multiple sclerosis, autonomic neuropathy, which is common in diabetes, or a spinal cord injury, think of neurogenic bladder!
Neurogenic bladder 7:33–8:15
Management primarily relies on treating the underlying condition. Keep in mind that these conditions are often chronic and irreversible, so you will also need to educate your patient on placing urinary catheters.
Alternatively, you could place an indwelling urinary catheter indefinitely or consult your surgery team for the placement of a suprapubic catheter.
This way, you are going to facilitate urinary drainage and avoid future AKI. Alright, as a quick recap… Once you diagnose post-renal AKI, obtain a PVR bladder scan to assess for urinary retention.
Review 8:15–9:02
If the post-void residual volume is less than 100 milliliters, there’s no evidence of urinary retention in the bladder, so diagnose upper urinary tract obstruction.
In this case, consider conditions like nephrolithiasis, malignancy, abscess, and retroperitoneal fibrosis. On the flip side, if the post-void residual volume is greater than 100 milliliters, diagnose lower urinary tract obstruction and immediately insert a urinary catheter.
Conditions associated with lower urinary tract obstruction include prostate disease, bladder outlet- and urethral obstruction, medication-induced urinary retention,
- "Acute Kidney Injury: Diagnosis and Management. " Am Fam Physician. (2019;100(11):687-694. )
- "Urinary Retention in Adults: Evaluation and Initial Management. " Am Fam Physician. (2018 Oct 15;98(8):496-503. PMID: 30277739. )
- "Harrison's Principles of Internal Medicine, 21e." McGraw Hill (2022. )
- "Acute Kidney Injury: Medical Causes and Pathogenesis. " J Clin Med. (2023 Jan 3;12(1):375. )
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