Megaureter

Megaureter is abnormal, significant dilation of the ureter, generally defined by a diameter exceeding 7 millimetres. It is a descriptive radiological finding with a range of causes rather than a single disease entity.

Primary megaureter reflects an intrinsic abnormality of the distal ureteric segment. Most commonly this is an adynamic, functionally obstructive segment near the ureterovesical junction that impairs peristaltic transport. Less commonly it is primary vesicoureteral reflux producing retrograde distension.

Secondary megaureter is ureteric dilation resulting from a distinct underlying cause, such as bladder outlet obstruction, a neurogenic bladder, or high-grade vesicoureteral reflux from a specific cause.

Accurate classification of the mechanism is essential, because management differs substantially between causes.

Classification and pathophysiology

• Primary obstructive megaureter is the commonest primary form. A short, adynamic segment of distal ureter near the ureterovesical junction fails to transmit peristaltic contractions effectively. This is a functional, not anatomical, obstruction to urine flow, with proximal ureteric dilation.
• Primary refluxing megaureter reflects significant vesicoureteral reflux. Retrograde flow of urine from the bladder into the ureter during voiding produces the dilation. The mechanism is reflux rather than obstruction (see vesicoureteral reflux).
• Secondary megaureter is dilation resulting from an identifiable underlying process. Bladder outlet obstruction is particularly relevant in male infants with posterior urethral valves. A neurogenic bladder is associated with an underlying spinal cord abnormality, including spina bifida. Severe vesicoureteral reflux may follow a specific predisposing cause.
• In secondary megaureter, management is directed at the underlying cause rather than the ureteric dilation in isolation.

• The great majority of cases are now identified antenatally on routine ultrasound, as incidental ureteric dilation, often with some hydronephrosis.
• The severity and pattern of dilation, and any associated finding such as bladder wall thickening suggesting bladder outlet obstruction, give important initial diagnostic guidance.
• Where not identified antenatally, postnatal presentation may include urinary tract infection, a common presenting feature that may reflect an unidentified structural abnormality predisposing to infection. Less commonly there is an abdominal mass or, in older children, flank pain. The principles are broadly those under congenital anomalies of the kidneys and urinary tract.
• Assess carefully for findings suggesting a secondary cause. A palpable bladder or poor urinary stream suggests bladder outlet obstruction, and neurological findings suggest a neurogenic bladder. This distinguishes primary from secondary megaureter, which are managed very differently.

• Postnatal renal and bladder ultrasound is performed at the standard interval described under congenital anomalies of the kidneys and urinary tract. It confirms and characterises the pattern and severity of ureteric dilation and assesses the kidneys and bladder for associated findings.
• Micturating cystourethrography is essential to assess and characterise any vesicoureteral reflux, distinguishing primary refluxing from primary obstructive megaureter.
• It also identifies evidence of bladder outlet obstruction, including posterior urethral valves in a male infant. This suggests a secondary cause that needs its own targeted management.
• Nuclear medicine renal scanning (MAG3) assesses differential renal function and drainage from the affected renal unit. It distinguishes a genuinely obstructive process needing intervention from a dilated but adequately draining system that may be managed conservatively. It directly informs management decisions.

• Mild to moderate primary obstructive megaureter, particularly in infants, is managed in the great majority of cases by observation and periodic surveillance (serial ultrasound and, where indicated, repeat nuclear medicine assessment). Many cases improve spontaneously over the first years of life as the ureter matures and the functional obstruction resolves.
• Surgery is reserved for deteriorating renal function on serial assessment, recurrent significant urinary tract infection despite appropriate management, or significant, progressive dilation. The indications are broadly those under pelviureteric junction obstruction.
• Surgery is ureteric reimplantation: excision of the adynamic distal segment and reimplantation of the ureter into the bladder using an anti-reflux technique.
• Primary refluxing megaureter is managed according to the principles under vesicoureteral reflux, directed at the reflux rather than the ureteric dilation in isolation.
• Secondary megaureter is managed by addressing the cause: surgical relief of any bladder outlet obstruction, or structured management of an underlying neurogenic bladder. The dilation itself generally improves once the cause is addressed.
• Antibiotic prophylaxis may be considered in selected infants pending assessment and definitive management planning, particularly with a history of previous urinary tract infection or significant risk factors. The considerations are broadly those under congenital anomalies of the kidneys and urinary tract.
• Long-term structured follow up incorporates serial imaging and functional assessment, with prompt investigation and treatment of any urinary tract infection.
• As with congenital urinary tract anomalies generally, the goal is preservation of long-term renal function through timely, targeted intervention where genuinely required. Unnecessary intervention is avoided in the substantial proportion of cases with a favourable, self-resolving course.