Urinary Tract Infections of the Kidney

Urinary tract infections can affect the lower urinary tract (bladder, and prostate in men) or the upper urinary tract (kidneys and ureters).  Infections of the lower urinary tract can be problematically symptomatic - symptoms include dysuria (pain with urination), foul-smelling urine, hematuria (blood in the urine), frequency and urgency.  However, lower tract infections are not often dangerous.  Infections of the upper tract can be extremely dangerous as bacteria in the extremely vascular kidneys can quickly spread around the body, leading to massive infection.

Infections of the upper tract manifest as a number of distinct entities, which we will review in this blog:

Acute Pyelonephritis

Acute Pyelonephritis (AP) refers to an infection and inflammation of the kidney and renal pelvis (where the urine collects).  AP can range from a mild, self-limited infection to life-threatening urosepsis (septic shock caused by urinary bacteria in the blood).  Most cases of AP are caused by E.coli (80%), most of which are equipped with P. pili - a special bacterial growth that helps the infection "ascend" the urinary tract in the kidney.  Occasionally, AP can be associated with obstructing kidney stones.  This circumstance is known as obstructive pyelonephritis and is a surgical emergency.

Symptoms can vary, but usually include fever, chills and flank pain, although can include nausea, vomiting and nonspecific abdominal pain.  AP is a clinical diagnosis defined by the triad of:

• fever
• flank pain
• leukocytosis (elevated white blood cell count in the blood)
• bacteriuria if often, but not reliably present

Therefore, the work-up involves routine blood work (complete blood count, basic metabolic panel) and urine tests (urinalysis and urine culture).  Imaging can either be CT scan or ultrasound and is indicated if suspicion of a kidney stone, anatomic abnormalities, recent urologic surgery or recurrent pyelonephritis.  The role of imaging is mainly to rule out obstructive pyelonephritis by looking for hydronephrosis.  Other non-specific findings on imaging can be renal enlargement, hyper- or hypoenhancing parenchyma and perinephric stranding (a sign of inflammation around the kidney).

AP can be stratified into uncomplicated and complicated forms of the disease.  In addition to distinguishing the different presentations in these groups, these classifications describe different forms of treatment.

Uncomplicated Acute Pyelonephritis

A patient with uncomplicated AP has a normal urinary tract by history and is clinically stable (no sepsis).  Therefore imaging is not warranted and treatment is outpatient therapy with an oral antibiotic (fluoroquinolone, amoxicillin or augmentin; NOT bactrim) for seven days.  Urine cultures can be repeated five days after treatment and/or two weeks after completing therapy to ensure eradication of the bacteriuria.

Complicated Acute Pyelonephritis

A patient with complicated AP is either acutely ill or has:

• an anatomic abnormality (either congenital or after reconstructive surgery) 
• recent urologic surgery
• an obstructing ureteral stone
• recurrent pyelonephritis 
• uncomplicated AP with no response to therapy after 72 hours

These patients should be admitted to the hospital, undergo axial imaging (CT) and treated with 14 days of antibiotics.  Broad-spectrum, intravenous antibiotics should be initiated and may be transitioned to oral antibiotics when clinically stable to complete the 14-day course.  If any consideration of obstruction (i.e. hydronephrosis), the blockage should be relieved either with ureteral stent(s) or nephrostomy tube(s).  Patients with complicated AP should have repeat urine cultures at 5 days and 2 weeks after completing therapy as up to 30% will relapse after the initial 14-day course. 

Infected Hydronephrosis / Pyonephrosis

This entity is defined by bacterial infection in a hydronephrotic (or blocked, swollen collecting system) kidney.  There may be air in the collecting system and the treatment is conservative, including antibiotics (3-14 days depending on presentation) and relief of obstruction when applicable.  This should be distinguished from emphysematous pyelonephritis (see below) - which is a life-threatening bacterial infection of the kidney where air may appear in the renal parenchyma.

Acute Focal Bacterial Nephritis / Renal Abscess

Acute Focal Bacterial Nephritis is a localized infection of kidney characterized by heavy leukocyte infilatration and may represent early abscess formation.  It is diagnosed only by imaging in a patient with symptoms similar to AP.  Treatment is 7-14 days of antibiotics with consideration of repeat cultures and repeat imaging to make sure the process is resolving and not progressing to a renal abscess.  A renal abscess is a localized, purulent collection in the kidney.  A perirenal abscess is a purulent collection adjacent to the kidney while a pararenal abscess refers to a collection near the kidney but outside of Gerota's fascia (often unrelated to a kidney infection).  Most often, abscesses resolve with antibiotics and conservative treatment, however large abscess may be aspirated or percutaneously drained to facilitate resolution of illness.  

Xanthogranulomatous Pyelonephritis 

Xanthogranulomatous Pyelonephritis (XGP) can be considered chronic, end-stage pyelonephritis.  XGP is a process caused by severe, chronic obstruction and infection with resultant destruction of the kidney.  As most (83%) cases of XGP are associated with an obstructing kidney stone, the offending bacteria is E.coli or proteus species in most cases.  Patients with XGP often present with a smoldering case of flank pain, persistent UTI and can be associated with intermittent fevers, weight loss, malaise or a palpable mass if the kidney is large.  Radiographically, an XGP kidney typically appears with the triad of renal enlargement, poor function, and large pelvic calculus; although occasionally XGP can resemble renal cell carcinoma.  However, the diagnosis is pathological: the kidney parenchyma will have lipid-laden macrophages in a background of active infection.  Treatment is nephrectomy.

Emphysematous Pyelonephritis

Emphysematous Pyelonephritis (EP) is an acute, gas-forming infection of the kidney parenchyma.  It is most often caused by an ascending E.coli infection.  EP presents like AP in an acutely ill patient with air in the parenchyma of the kidney.  Patients often have comorbidities, chronic illnesses or are immunosuppressed; the most common comorbidity is diabetes.  Historically, the mortality of this entity approached 50%.  However, with better diagnostic capabilities and intensive care, the mortality from this disease has decreased dramatically.  Treatment involves broad-spectrum, intravenous antibiotics; supportive therapies and percutaneous drainage of the kidney with urgent nephrectomy if failure to respond.

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