Urodynamics in Children

Drzewiecki and Bauer from Boston Children’s Hospital provide a review of urodynamics (UDS) in children.1 First, a history, physical examination, and a 3-day voiding and bowel diary are obtained. A renal sonogram noting bladder volume, residual volume, and bladder wall thickness is then performed.2 Most children with nonneurogenic bladder dysfunction are potty trained but subsequently present with lower urinary tract symptoms. Most children with urgency, frequency, and incontinence can be managed with behavioral therapy and anticholinergic medications. UDS is useful when there is no improvement. Kaufman and colleagues have shown a high yield (63%) of pathologic findings following UDS in the refractory pediatric patient with incontinence.3 Uroflowmetry can be useful in children with dysfunctional voiding who contract their external sphincters or pelvic floor muscles during micturition. Baseline and periodic UDS are performed in neurogenic bladder dysfunction (NBD) including myelomeningocele (MM, 90%) occult spinal dysraphism, sacral agenesis, imperforate anus, cloacal malformation, traumatic spinal cord injury, and central nervous system disorders. Infants with MM have three voiding patterns: synergic (26%), dyssynergic with or without diminished bladder compliance (37%), and complete denervation (36%).4 Detrusor sphincter dyssynergia (DSD) with associated high-end filling pressures (≥ 40 cm of water) and highvoiding pressures of ≥ 80 to 100 cm of water leads to reflux and hydronephrosis unless UDS is performed along with early intervention with clean intermittent catheterization (CIC). UDS for NBD is repeated following a change in pharmacotherapy or surgery, new onset incontinence or hydroureteronephrosis, or recurrent symptomatic infections. Because deterioration in bladder function may occur silently, changes in the orthopedic or neurological examination warrant reassessment with UDS. Only one-third of infants with occult spinal dysraphism will have abnormal UDS irrespective of the neurological findings on examination. With increasing age, symptoms become more evident and include bowel and bladder dysfunction and alterations in lower extremity function. Recently, detrusor overactivity has been shown in all age groups with occult tethered cord syndrome.5–7 The earlier the surgical intervention, the greater the likelihood for functional improvement. Children with sacral agenesis involving partial or complete absence of vertebral bodies can remain silent until late childhood when incontinence, difficulty potty training, or urinary tract infection are evaluated. A flattened buttock with absence of the upper gluteal cleft may not be evident on early physical examination. UDS will often demonstrate detrusor acontractility and urethral sphincter denervation or overactive bladder (OAB) with DSD.8 Anorectal malformations may have genitourinary and spinal abnormalities, including tethered cord or iatrogenic injury, but may also have NBD without obvious etiology. These children may exhibit OAB with or without DSD (upper motor neuron lesion) or detrusor acontractility with sphincter denervation (lower motor neuron lesion).9 Posterior urethral valves (80%) often have bladder dysfunction with detrusor overactivity and diminished bladder compliance.10,11 Myogenic failure may be due to infrequent bladder emptying in conjunction with increased urinary output and is more often seen in the older age group. Uroflowmetry is noninvasive and can be used in patients who void spontaneously. The flow pattern is accurate as long as the volume is > 50% of maximum voided volume.12 The shape of the flow curve denotes the detrusor function, outlet resistance, or external sphincter dysfunction during micturition.13 Voiding patterns include a bell-shaped (normal), tower (OAB), plateau (outlet obstruction), staccato (sphincter activity during voiding), and interrupted curve (acontractile or underactive bladder).2 Perineal patch electromyography (EMG) can be used as an adjunct in determining the etiology of an abnormal flow pattern or postvoid residual urine.14 Postvoid residuals (PVRs) using bladder scanning should show residuals of ≤ 20 cc or abnormal emptying is suspected in children. PVR is useful in patients on anticholinergic therapy. Invasive UDS is performed in the sitting or supine positions. Rectal and urethral catheters provide intraabdominal and intravesical pressures, respectively. The difference in these pressures is the detrusor pressure. A PVR is obtained in a non-CIC patient and patch EMG electrodes are positioned perineally in boys or paraurethrally in girls.15 EMG provides information on individual motor units at rest in response to sacral reflexes and during bladder filling and emptying with suspected or previously diagnosed NBD.9 During bladder filling, saline infusion at a temperature of 21°; to 37°;C is performed at a rate of 5% to 10% of the expected bladder capacity/minute.16,17 Bladder capacity for children is determined from the Hjalmas equation: expected bladder capacity (mL) 5 × 1 (age in years × 30).16 For children with MM, the formula 24.5 × age (years) + 62 should be used.18 Children on CIC use the largest catheterized volume during the day over several days. At least two cycles of filling are required unless the child has no sensation and an NBD. The bladder has been sufficiently filled when the child has a strong urge to urinate, is uncomfortable, voiding starts, bladder pressures are > 40 cm of water, or the volume infused is > 150% of the expected capacity. The fluoroscopic video portion of the test permits correlation of detrusor pressures with urinary incontinence or reflux and also provides information on the bladder shape and the state of the sphincter during filling and emptying. Intrinsic or bladder neck dysfunction can also be assessed. Detrusor overactivity occurring during bladder filling is defined as an involuntary detrusor contraction > 15 cm of water from baseline.13 Bladder underactivity is also abnormal and recognized in patients who are filled to > 150% of their expected bladder capacity and have a poor or absent detrusor contraction. During filling, normal detrusor compliance is 10 cm of water at capacity, or 5% of the child’s normal capacity per cm of water, or about 20 cm of water at expected bladder capacity.12,13 Infants tend to have higher voiding pressures than children, and boys tend to have higher voiding pressures (by 5 to 15 cm of water) than girls.2,19 Urethral obstruction is suggested when there are high voiding pressures accompanied by poor flow rates. EMG pads may show a staccato voiding pattern. A low flow rate may be indicative of an anatomical obstruction and bladder emptying should be assessed. This review of pediatric urodynamics is comprehensive and provides an excellent source of classic references.