PHYSIOLOGY

The fundus is made up of three layers of crisscrossing smooth muscle, called the detrusor. These three smooth muscle layers extend down the proximal urethra and stop at the external sphincter, which comprises skeletal muscle. T10 to LI supplies the sympathetic innervation for the bladder; this causes the detrusor to relax and the bladder neck and posterior urethra to contract, effectively allowing storage of urine. S2 to S4 provide the parasympathetic innervation to the bladder primarily at the fundus and the neurotransmitter is acetylcholine causing contraction to empty the bladder. The sympathetic innervation is active during bladder filling, and the parasympathetic innervation is active during urination. Somatic nerves via the pudendal nerve (from the sacral cord S2-S4) innervate the skeletal muscle component of the external urethral sphincter: contraction occurs in the resting state for continence, and relaxation should occur during urination (112).

BLADDER CAPACITY

The prediction of normal bladder capacity aids the diagnosis of abnormal voiding patterns. It is typically accepted that the bladder capacity of a baby during the first year equals the weight of the baby in kilograms times 7 to 10 mL. A study with 200 children (132 with normal voiding, 68 frequent and infrequent voiders) demonstrated that from approximately 1 to 12 years of age, the sum of age in years plus 2-equals the bladder capacity in ounces (30 mL = one ounce). Teenagers (if normal size) assume adult-size bladders, typically around 400 to 500 mL. Clinically infrequent voiding may cause an increase in bladder capacity over normal values. Clinically frequent voiding may cause a decrease in functional bladder capacity (113). Postvoid residual (PVR) is generally accepted as less than 20% of bladder capacity, taking into account the appropriate bladder capacity for age.

DIAGNOSTICS

Checklist for Diagnosing Neurogenic Bladder

• Is the current management preserving the kidneys and renal function?

• If present, does bacteriuria represent infection or colonization?

• Are the bladder and kidney studies up to date?

• Is urination true voiding or overflow incontinence?

• Are bladder capacity and bladder compliance appropriate for age?

• Is PVR appropriate?

• Is the sphincter mechanism competent?

DIAGNOSTIC TESTS

Urinalysis

The nitrate test indirectly detects urine bacteria with enzymes that reduce nitrate to nitrite in urine (eg, Klebsiella, Enterobacteriaceae, Escherichia coli, and Proteus).

THE LEUKOCYTE ESTERASE TEST. While leukocytes in the urine can disintegrate and disappear rapidly, leukocyte esterase persists.

Urine culture (UC): Culture of urine for suspected clinical infection.

Serum creatinine: Indicator of renal function, but typically lags renal function by several days.

FUNCTIONAL STUDIES

• US—Ultrasound of the kidneys and bladder to determine any structural abnormalities, including hydronephrosis, trabeculation, stones, and PVR. If not on catheterization program, consider US to measure PVR.

• VCUG—Voiding cystourethrogram to detect vesicoureteral reflux (VUR), evaluate the bladder contour, and evaluate the urethra. For those without neurogenic bladders, the first study is a contrast VCUG for boys and girls. Subsequent VCUG studies, for boys and especially girls, can be radionuclear cystograms, as the radiation is reduced. However, for those with neurogenic bladders the fluoroscopic VCUG with contrast demonstrates greater detail of the bladder neck on all subsequent studies. Many centers perform cystograms as part of the urodynamic study, called videourodynamics. If videourodynamics are used, then a separate VCUG is not necessary.

• UDY—Urodynamics to determine detrusor leak point pressure, uninhibited bladder contractions now called detrusor over activity (114), detrusor sphincter dyssynergia, bladder capacity, PVR, and bladder compliance and sensation. The basic urodynamic formulas are:

Pressure detrusor = pressure vesical (bladder) -Pressure abdominal (rectum). On urodynamic testing, detrusor leak point pressure (p vesical - p abdominal) greater than 40 cm HzO, with a bladder capacity less than 33% of expected, was associated with renal damage (115).

Bladder compliance = change in bladder volume/ change in pressure

It is recommended that these tests (US, UDY, and VCUG) be performed in the neonatal period, as baseline studies. The VCUG can be excluded if videourodynamics are utilized. As growth of the infant is rapid in the first 12 months, US should be repeated every 3 to 6 months during infancy. Greater frequency is encouraged if initial studies are abnormal. A common protocol incorporates US twice yearly the second year and then yearly. Abnormalities on US will likely lag those found on UDY. UDY should be repeated during the first year if the initial study shows a high detrusor leak point pressure of greater than 40 cm HzO or detrusor sphincter dyssynergia. Many centers perform UDY yearly in all patients to aggressively detect potential bladder deterioration that could harm the kidneys, whereas others repeat only if renal changes are noted on US or continence is not obtained by school age (116). VCUG (or video UDY) are needed if VUR was noted or if there is a new onset of recurrent UTIs or hydronephrosis. UDY studies should be repeated with significant clinical changes in bowel or bladder incontinence, infections, or gait, all of which are potential signs of secondary tethered cord (117).

Other Studies

RENAL STONES. The excretory urogram (EXU) also known as intravenous pyelogram (IVP), once used to detect urinary tract (UT) stones, anatomic abnormalities, and obstruction, is rarely done anymore. Following diagnostic US for renal stones, the EXU has been replaced by CT without contrast for more precise imaging.

ANATOMIC VARIANTS. A magnetic resonance urogram with gadolinium is now used to demonstrate rare anatomic variants, particularly to delineate the ureters if ectopia is suspected. Diethylene triamine acetic acid (DTPA) and mercaptoacetyltriglycine (MAG3) are nuclear medicine studies used to evaluate differential function and excretion of each kidney; however, the MAG3 offers better imaging than DTPA to evaluate UT drainage/ obstruction. The DTPA and MAG3 radioactive tracers are filtered (DTPA) or excreted (MAG3) into the renal tubules. Dimercaptosuccinic acid (DMSA), a radiotracer that binds to the tubule, is not rapidly excreted, and it is the best test for imaging the functioning renal cortex to detect congenital hypoplasia or acquired scarring. This test should be done when there is abnormality on a renal US, a history of multiple urinary tract infections (UTIs), or pyelonephritis. Cystoscopy is recommended for persistent hematuria or bladder masses. There is controversy regarding the role of routine screening cystoscopy in patients with increased risk of malignancy due to either bladder augmentation surgery or long-term indwelling catheter use (118,119).

Figure 15.5 depicts a normal urodynamic study and Figure 15.6 is a tracing of a urodynamic study reflecting spastic bladder detrusor and sphincter dyssynergia.