Intrinsic sphincter deficiency: what is it and does it matter anymore?
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There was a time when intrinsic sphincter deficiency (ISD) had a definition that was clinically relevant and guided the therapeutic decision-making process. However, over time the definition has become muddied and has somewhat morphed into a vague term defined in various ways with various tests. Currently, I doubt that any two clinicians use the same definition, and ISD has become more of a euphemism than a defined disease. So how did we get here? It may be the result of some technology introduced in the mid 1990s that has been described by my colleague T. Fleming Mattox as “The single most important practice altering technology in urogynecology in our lifetime,” the tension-free vaginal tape (TVT).
SUI may also be caused by an intrinsic urethral sphincter deficiency (ISD) … In women, ISD is commonly associated with multiple incontinence surgical procedures, as well as with hypoestrogenism, aging, or both. In this condition, the urethral sphincter is unable to generate enough resistance to retain urine in the bladder, especially during stress maneuvers… Patients with ISD often leak continuously or with minimal exertion. In some patients, stress incontinence results from coexisting ISD and hypermobility of the urethra and bladder neck .
Then in the mid 1990s came literature on the tension-free vaginal tape demonstrating that ISD, as well as “typical” stress urinary incontinence with a hypermobile urethra, could both be effectively treated with a TVT. At this point, the definition of ISD began to become muddled. For a time ISD was defined as stress incontinence with a well-supported bladder neck (lack of hypermobility) regardless of the VLPP or MUCP. As noted above, in the 1996 AHCPR document, a mention of lack of hypermobility being a component of ISD was noted. In the International Classification of Disease, 9th edition (ICD-9), ISD was defined as a disorder of the urethra “…due to intrinsic sphincter damage in which the urethra is usually well supported but there is posterior rotation and opening of the bladder neck and posterior urethra during straining” . It was then felt that in subjects with ISD (without hypermobility) periurethral bulking agents were the treatment of choice, as these subjects did not do well with a Burch retropubic urethropexy. Here, again, the TVT dispelled this myth, as subjects with or without urethral hypermobility seem to do equally well with a TVT . The final use of the definition of ISD came following the introduction of the transobturator tape tension-free sling (TOT). In subjects with ISD defined by a low-pressure urethra (MUCP < 20 cm H2O), the TVT seemed more efficacious than the TOT, but recently that concept has been questioned [6, 7].
So now we are left with almost any form of stress urinary incontinence being treatable with either a TOT or TVT. Therefore, the reason for having the definition of ISD seems to be of historical significance only. Even the most recent International Urogynecologic Association (IUGA)/International Continence Society (ICS) joint report on the terminology for female pelvic floor dysfunction ignores the diagnosis of ISD and the most recent International Consultation on Incontinence has omitted the definition of ISD for no longer being useful .
Is this to say, then, that our surgical management of stress urinary incontinence has evolved to the point where all comers are effectively managed with either a TOT or TVT or perhaps a single incision sling? The answer to that question is inevitably “no.” So what is the clinician left with in making surgical decisions about how patients with stress urinary incontinence are best managed? I don’t think the answer to that question is currently available. I think that we need to have another look at how we view the urethral sphincter mechanism. I don’t think we can dichotomize it into either a “normal” sphincter or one with an “intrinsic deficiency.” Instead, we need to look at the urethral sphincter mechanism as a having a continuum of functional capabilities or deficiencies. In the recent TOMUS trial the investigators could not identify at what point subjects undergoing a TOT or TVT were at greatest risk of failure, but instead noted that as the ability of the urethral sphincter to generate a closing pressure, or force, declined, the risk of failure (with either a TVT or TOT) increased. In other work, the distensibility of the urethra to stretch and not its ability to generate a closure pressure is being evaluated as a measure of its functionality [9, 10]. This work—while somewhat helpful in counseling patients about the risk of success or failure—will undoubtedly stimulate further research into alternative therapies. Currently, it cannot help us decide how best to treat our patients.
So, is the definition of ISD still helpful, or is it a relic of the past that should be discarded? That question may not yet be answerable, but we should not let old definitions continue to influence our research. Instead, we should view afresh the problem of surgical failure in our patients with stress urinary incontinence and how to best prevent it.
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