Improvements in the general care of children with spinal cord injury and myelodysplasia led to longer survival. This advance, in turn, led to the observation that some of these survivors would later suffer from significant urological complications, including hydronephrosis, chronic urinary tract infection, stone formation, renal damage and loss of renal function. Others would fare better, and in 1981 one of the earliest insights into the physiological basis of this clinical difference came with the description by McGuire et al of the detrusor leak point pressure (DLPP). ¹ He found that in 42 myelodysplastic patients followed for a mean of 7.1 years, none with intravesical storage pressures under 40 cm H ₂O developed vesicoureteral reflux (VUR), whereas 60% of those with pressures above 40 cm H ₂O did. This allowed urodynamics to emerge from an academic investigative exercise to an objective clinical tool to predict renal decline, and that this deterioration was linked to a loss of bladder compliance, thus guiding treatment and helping to establish the pressure based management of neurogenic bladders.

This concept was proactive, and now, rather than waiting for upper tract deterioration to occur before intervening, urologists could monitor and act early before there was lasting damage. ¹ Persistent high pressure begets loss of compliance. High bladder pressure causes inflammation and fibrosis of the bladder wall, leading to a loss of bladder compliance, which, in turn, only makes the bladder storage pressure worse: a vicious feed-forward situation. Ultimately, if unchecked, storage pressure is sufficient to work against ureteral peristalsis, leading to urinary stasis, infections, stone formation and, in its terminal stages, direct pressure effects on the kidney. This is the underlying logic for urodynamic testing to predict the loss of compliance.

Early critics of this concept expressed concerns for overtreatment when proactively treating all children with elevated DLPP. In 1990, Klose et al reported a greater than 90% resolution rate of radiological changes (hydronephrosis and reflux) with initiation of treatment. ² Why not then just treat once pathology was visually evident? This reactive approach was made obsolete with the recognition that high storage pressures could be monitored, successfully identified and treated. The timely use of anticholinergic drugs and clean intermittent catheterization had immediate effects with lowered rates of VUR, hydronephrosis, renal insufficiency and failure among these patients. There was also seen a lowering of the rate of augmentation cystoplasty, and for those who still progressed to reconstruction, the lower rates of renal failure were important in decreasing overall morbidity and mortality. ^3,4 Today, this opposition seems as odd as if one were arguing to wait on treating vascular hypertension until there was end organ damage.

Despite the general appreciation of the concept, predicting which neurogenic bladder patients will progress to a poorly compliant neuropathic (so called “hostile”) bladder remains imperfect. One of the first challenges was to standardize terminology and definitions. It is hard to communicate and share findings if it is unclear if everyone is actually discussing the same issues and concerns. In 2015, the International Children’s Continence Society created a standardization report that helped to create some degree of uniformity and transparency, and ease the process of pooling and sharing results. ⁵

Figure. Time line of EKG and urodynamics

The second challenge is the current nature of urodynamics for bladder compliance. The urodynamic study (typically a multichannel cystometrogram with or without electromyography, and video fluoroscopy) is one of the urologist’s best objective tools in the management of neurogenic bladders. However, one of its great limitations is the need for contextual interpretation by well trained and experienced personnel. ⁶ The term “urodynamacist” was coined to describe this role of simultaneously conducting and interpreting the study. Interpretation is nuanced, and it has been shown that when this contextual presence is removed, many urologists have difficulty consistently interpreting readings from other centers or even their own units. Pathological findings, normal physiological reactions and test artifacts can become confused. ^7,8 This stands in stark contrast to a test like the electrocardiogram (EKG), where a technician attaches the leads and presses a button, and voila, a fully computerized tracing is generated complete with interpretation. Despite these limitations, urodynamics remains a key decision making factor when we consider the many options now available for treating poor bladder compliance, including new anticholinergic drugs, botulinum toxin injection, neurostimulation and bladder augmentation.

The 40 cm H ₂O value used as a cutoff by McGuire et al was developed at a time when it was still common to use water manometer cystograms. ¹ This methodology could not provide the number of data points that later electronic systems could yield. It obscured the point that the 40 cm H ₂O was a useful rule of thumb but one still had to look at the urodynamic tracing and clinical history. Casual readers may have misunderstood that any value under 40 cm H ₂O was safe. Others have noted that while 40 cm H ₂O may be useful when one is trying to look at large groups of patients, individual patients may benefit from lower pressures, such as 20 to 25 cm H ₂O. Tarcan et al noted in their study of myelodysplastic children that a lower cutoff value of 20 cm H ₂O may have greater sensitivity in predicting upper tract changes. ⁹ Backhaus et al noted that hydrostatic pressures less than 40 cm H ₂O can cause alterations in some of the molecular determinants of the bladder matrix that affect bladder compliance. ¹⁰ In addition, other factors such as the presence of VUR and bladder trabeculations (physical manifestations of poor compliance) if present already portend a high risk of renal deterioration. ^11,12

Pressure based management with urodynamics, despite its 40-some years of expanded use, remains a young concept, and has continued to evolve as a predictor of which neurogenic bladders will clinically deteriorate. While we have noted its limitations, it is worth recalling that Einthoven first introduced the idea of an elektrokardiogramm at the Dutch Medical Association meeting in 1893, although it was not until 50 years later that the now familiar 12-lead EKG was standardized. ^13-16 When viewed in this historical context, the current efforts to standardize terminology, methodology and interpretation of urodynamics portend a good future (see figure).