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Patients with obstruction of a solitary kidney or bilateral obstruction present with acute oligoanuric renal failure. Incomplete obstruction can result in fluctuating urine output. Pain is related to the location, duration, and severity of obstruction. Acute obstruction can result in severe pain due to distention of the collecting system or renal capsule. Renal colic due to calculi is often sudden and severe, with pain beginning in the flank and radiating into the ipsilateral groin. This can be accompanied by nausea and vomiting. Patients with renal colic prefer to be in motion, compared to patients with peritonitis, whose pain is worsened with movement.
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A distended bladder on physical examination or the presence of a flank mass is suggestive of obstruction. Hypertension can be seen in obstructive uropathy due to volume expansion and activation of the renin–angiotensin–aldosterone system.
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Table 16–1 lists the common laboratory abnormalities in obstructive uropathy. The kidneys lose their ability to concentrate the urine early in obstruction. Later, they cannot concentrate or dilute urine well (isosthenuria). Defects in distal urinary acidification result in a hyperchloremic metabolic acidosis (distal renal tubular acidosis). This can be accompanied by hyperkalemia. Patients will have renal insufficiency if there is bilateral obstruction or obstruction to a solitary functioning kidney. Often the ratio of blood urea nitrogen to serum creatinine will be greater than 10:1 due to increased urea reabsorption throughout the collecting system. Patients with a partial obstruction can have nephrogenic diabetes insipidus (resistance to antidiuretic hormone) and develop hypernatremia. Patients may have polycythemia due to excess erythropoietin production, or may be anemic with more advanced renal impairment. Urinary stasis can result in urinary tract infection with urea splitting bacteria like Proteus and Staphylococcus. This results in an alkaline urine pH and is associated with struvite (magnesium, ammonium, phosphate) calculi.
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Abdominal x-ray is useful in identifying radiopaque calculi. However, it is often omitted as part of the evaluation for obstructive uropathy because spiral computed tomography (CT) without contrast is often performed as part of the initial evaluation.
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Ultrasound is 90% sensitive and specific for detecting hydronephrosis. It can be falsely negative in the presence of an early obstruction, volume depletion, or if the ureters are encased due to a retroperitoneal process. Ultrasound of the bladder reveals a thickened bladder wall with trabeculations in chronic bladder outlet obstruction. Duplex Doppler can also be performed. A high resistive index is seen from increased vascular resistance in obstruction.
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Spiral Computed Tomography
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Spiral CT (noncontrast) has become the radiographic study of choice to evaluate a patient for stone disease. It is a very short procedure that is highly sensitive for detecting renal and ureteric calculi.
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Magnetic Resonance Urography (MRU)
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MRU is available at some centers and can be performed without the use of contrast agents. Although T2-weighted MRU can determine the severity and location of obstruction, it has a sensitivity of only 70% for detecting calculi. This fact combined with its cost will likely limit the utilization of MRU.
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Radioisotope Renography
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Radioisotope renography with the administration of furosemide can be used to differentiate mechanical from functional obstruction.
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Intravenous Pyelography (IVP)
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IVP should be avoided in patients with renal insufficiency due to the risk of radiocontrast-induced acute renal injury. It may be used in patients in whom ultrasound or noncontrast spiral CT has been unable to determine the exact location of obstruction, and is helpful in diagnosing papillary necrosis. However, the use of IVP has been largely replaced by contrast-enhanced CT, which has been found to be more sensitive than IVP in determining the cause of obstruction.
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Documenting postvoid residual urine by bladder catheterization or ultrasound is an integral part of the initial evaluation of a patient with suspected obstructive uropathy. Patients may also need rectal and/or pelvic examinations to determine the presence of a mass (cervix, rectal, prostate) and the size of the prostate. Cystoscopy with retrograde ureterograms and percutaneous nephrostomy with antegrade ureteropyelograms are diagnostic and therapeutic procedures done to determine the site of and potentially relieve the obstruction.