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Pyelonephritis

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Overview

Acute pyelonephritis (AP) describes the inflammation and infection of one or both kidneys.

An upper urinary tract infection (UTI) describes infection of the upper urinary tract including pyelitis (infection of the proximal ureters) and pyelonephritis (infection of both the kidneys and proximal ureters).

Most cases of pyelonephritis occur following a lower urinary tract infection ascending to involve the upper urinary tract, although some patients may not have a recent history of it.

Epidemiology

  • The incidence of acute pyelonephritis is highest in young women aged 15-30 years old, infants, and older people
  • Acute pyelonephritis is less common in men, but its incidence in men increases after 65 years of age
  • Uncomplicated pyelonephritis is more common in younger women than men

Causes

Escherichia coli is the most common causative organism and makes up 80% of cases. Less common causes include:

  • Staphylococcus saprophyticus – 4%
  • Klebsiella pneumoniae – 4%
  • Proteus mirabilis – 4%
  • Candida albicans – rare, but associated with people who are immunosuppressed or have indwelling catheters

Risk Factors

  • Urinary tract infection:
    • Many people with AP have a preceding UTI
  • Female sex, infants, older age:
    • Due to differences in urinary tract structure and function
    • Although AP is less common in men, they are more likely to have complicated AP compared to women
  • Pregnancy:
    • Pregnant people are at an increased risk of complicated AP which is thought to be due to the uterus compressing the ureters and relatively lax pelvic ligaments
  • Anatomical/physiological abnormalities:
  • Diabetes mellitus:
    • Including other causes of glycosuria (e.g. drug-induced, such as SGLT-2 inhibitors)
  • Sexual intercourse:
    • Thought to spread bacteria around the perineum and facilitate entry into the urinary tract
  • Renal stones
  • Catheterisation
  • Immunosuppression

Presentation

Overview

Although acute pyelonephritis and a lower UTI can present similarly, the presence of systemic upset makes pyelonephritis more likely.

Features develop acutely over hours to days and are recognised as a classic triad of:

  • Flank pain – nearly always present
    • This may present as back pain and palpating the costovertebral (renal) angle may elicit tenderness
  • Fever – usually high fever (>38°C):
    • This can be absent or mild in the early stages or in older people
  • Nausea and vomiting – are common

Other features include:

  • Features of a lower UTI:
    • Such as dysuria, frequency, urgency, nocturia
  • Flu-like symptoms:
    • Such as malaise and myalgia

Similarly to lower UTIs, immunocompromised and older, frail patients may not have classic features including fever. In some, it may present as delirium or an acute worsening of dementia.

Examination findings

Additional examination findings may include:

  • Costovertebral (renal) angle tenderness
  • Suprapubic tenderness may be seen
  • Features of urosepsis may be present:
    • Hypotension with or without tachycardia
    • Temperature >38°C or <36°C
    • Altered consciousness
    • Rigours, profuse sweating
    • Pallor

Investigations

Overview

If patients have signs and symptoms of urosepsis they should be admitted to hospital.

A urine culture should be arranged in all people:

  • Ideally, this should be done before starting antibiotics unless the patient is severely unwell

Management

Overview

If features of urosepsis or severe AP are present, arrange hospital admission.

If admission is not indicated:

  • Non-pregnant people and people with indwelling catheters:
    • 1st-line: cefalexin, co-amoxiclav, or ciprofloxacin for 7-10 days
  • Pregnant people:
    • 1st-line: cefalexin for 7-10 days

Monitoring and Patient Advice

Overview

Patients are reassessed if their symptoms worsen or do not improve within 48 hours of taking antibiotics. This may suggest worsening of AP and urosepsis, antibiotic resistance, or an alternate diagnosis. 

Complications

  • Urosepsis
  • Renal parenchymal scarring – can lead to irreversible renal damage
  • Recurrent UTI
  • Renal abscess formation
  • Preterm delivery and low birth weight in pregnancy
  • Emphysematous pyelonephritis – necrosis of the kidneys leading to gas accumulating in the kidneys 

Prognosis

  • In most cases, prompt diagnosis and treatment lead to a complete and uncomplicated recovery within days to weeks
  • The prognosis is worse in those who are older, immunosuppressed, complicating factors, or those who have underlying renal disease

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