Aetiology

The majority of bacterial pathogens implicated in UTIs in children are gram-negative.

Escherichia coli is the most common cause, accounting for 85% to 90% of paediatric UTIs.[7]

Other potential bacterial pathogens include:[13]​​​[14][15][16][17]

  • Proteus mirabilis in uncircumcised males

  • Staphylococcus saprophyticus in female adolescents

  • Pseudomonas species in congenital anomalies of the kidneys and urinary tract

  • Serratia marcescens, Citrobacter species, and Staphylococcus epidermidis, which may cause infections in patients with malformation or dysfunction of the urinary tract

  • Klebsiella aerogenes and Enterococcus species.

Candida species may cause UTI in immunocompromised children, those with complex congenital anomalies of the kidneys and the urinary tract, post-operatively, and those with stents and urinary catheters for prolonged periods of time.

Schistosoma haematobium infection may affect children in endemic regions.

UTIs can be classified as uncomplicated or complicated. The aetiology of complicated UTI may be due to a structural abnormality or a functional abnormality.​[1]

  • Structural abnormality is mostly due to the presence of posterior urethral valves, strictures, or stones.

  • Functional abnormality most commonly results from lower urinary tract dysfunction of neurogenic (e.g., spina bifida) or non-neurogenic (e.g., voiding dysfunction) origin, as well as dilating vesicoureteral reflux (VUR).

Pathophysiology

Colonisation of periurethral mucosa with genitourinary bacteria is hypothesised to precede UTI. Ascending infection into the bladder is the mechanism for most episodes of cystitis. Shorter urethral length in girls predisposes to ascending infection.

Vesicoureteral reflux facilitates infection of the ureters and kidneys. In the absence of vesicoureteral reflux, uropathogenic Escherichia coli are able to inhibit ureteric peristalsis, facilitating infection of the upper urinary tract.[18] Bacterial infection is more likely if abnormalities in bladder emptying exist (e.g., bladder and bowel dysfunction, chronic constipation, vesicoureteral reflux), because elimination of bacteria from the bladder after micturition is incomplete.[19]​​

Constipation also facilitates development of UTI by increasing the number of uropathogenic organisms in the gastrointestinal tract.[19]

E coli isolates from UTI more commonly express virulence factors. Adhesins such as the type 1 pilus and P fimbriae may mediate attachment to uroepithelial receptors, and aerobactin may enhance bacterial growth through iron acquisition.[20][21][22]

Classification

Classification according to site of infection[1][4][5]

Lower UTI (cystitis)

  • Lower tract symptoms only, including frequency, urgency, dysuria, haematuria, malodorous urine, enuresis, and suprapubic pain.

Upper UTI (pyelonephritis)

  • Abrupt onset with systemic signs and symptoms, including fever (≥38°C [≥100.4°F]), chills, flank pain, and costovertebral angle tenderness.

Classification according to severity of infection[1]

Non-severe UTI

  • Mostly lower UTIs.

  • Child has mild pyrexia at most, is able to take fluids and oral medication, and is only slightly or not dehydrated.

Severe UTI

  • Mostly upper UTIs.

  • Child has a high fever (>39°C [>102.2°F] ), feels unwell, is persistently vomiting, and is moderately to severely dehydrated.

Classification according to complicating factors[1]​

Uncomplicated UTI

  • UTI in a child who has a structurally and functionally normal urinary tract, normal renal function, and a competent immune system.

Complicated UTI

  • UTI in a child who has a structural or functional abnormality of the urinary tract.

Classification according to episode of infection[1][4]

First UTI

  • May be indicative of anatomical anomalies and so anatomical evaluation is warranted.

Recurrent UTI

  • May be due to unresolved infection or persistent infection.

    • Unresolved infection: initial treatment is inadequate for elimination of bacteria in the urinary tract.

    • Persistent infection: caused by re-emergence of bacteria in the urinary tract due to a site of persistent infection that cannot be eradicated (e.g., infected stones or fistulas).

  • Same pathogen is implicated in each recurrent infection.

  • In the UK, the National Institute for Health and Care Excellence (NICE) classifies recurrent UTIs as follows:

    • ≥2 episodes of acute upper UTI, or

    • 1 episode of acute upper UTI plus ≥1 episode of lower UTI, or

    • ≥3 episodes of lower UTI.

Breakthrough UTI

  • May be seen in patients undergoing antibiotic prophylaxis.

  • Usually the result of resistant strains of the infecting pathogen.

  • May also be due to a parent or guardian's non-compliance with their child's treatment, and/or severe urogenital anomalies.

Re-infection

  • Unlike recurrent UTI, re-infection involves different types of pathogens or different serotypes of the same pathogen.

Classification according to clinical presentation[1][4]

Atypical UTI

  • Child is seriously ill, with signs and symptoms including:

    • Poor urine flow

    • Abdominal or bladder mass

    • Raised creatinine

    • Septicaemia

    • Failure to respond to treatment with 48 hours

    • Infection with non-Escherichia coli organisms.

Asymptomatic UTI

  • Child has leukocyturia without any other symptoms.

Symptomatic UTI

  • Child may have irritative voiding symptoms and suprapubic pain (cystitis) or fever and malaise (pyelonephritis).

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