Curative mechanisms of two tension‐free vaginal tape positions. Theoretical comments based on the urethral hanging theory of female stress urinary incontinence
Year: 2017
Type: article
Abstract: Dear Editor, It has been repeatedly postulated that the tension-free vaginal tape operation (TVT) is based on the integral theory (IT).1 According to the IT, urinary continence depends on adequate outlet resistance at the midurethra. Forces from three muscles stretch the anterior vaginal wall with the pubourethral ligaments (PUL) as a fulcrum. A well-supported intrinsic rhabdosphincter compresses the midurethra. During stress, the proximal urethra rotates dorsocaudally against the PUL and is kinked, elongated, and narrowed. Stress urinary incontinence (SUI) emerges when a defective PUL cannot counteract these rotating forces. A lax anterior vaginal wall descends, and the proximal urethra is funneled by external shearing forces when the posterior urethral wall is pulled down in relation to a better-supported anterior urethral wall. Due to the lax vagina, the rhabdosphincter is shortened and weak. At the abdominal leak point pressure (aLPP), the outlet resistance is overwhelmed, and the urethra opens and leaks. The TVT operation, introduced in 1996,2 claims to reconstruct a defective PUL and restore continence. The IT led to the conclusion of setting the tape 0.5 cm from the meatus externus (m.e.), which was later changed to 1 cm.3 Although the operation has high rates of success, it also fails 10-20% of the time.4 The pathophysiology of SUI is considered to be of multifactorial origins, all of which a single procedure cannot correct. Is this true? According to the urethral hanging theory (UHT),5, 6 the origin is “one-factorial,” such that a single specific procedure can correct the issue by enforcing a normal spatial relationship, at stress, between the proximal urethra and the bladder neck. These two theories—IT and UHT—both emphasize the PUL as important structures, but are else principally different. In the IT, the PUL designate a midurethral support, and in the UHT, they provide a proximal support—so the UHT, and not the IT, highlights the importance of the strong posterior components of the PUL which are attached to the proximal urethra, “the key site of control of continence” and the only parts of the the PUL with “fusion extensions to the levator ani muscle fascia.”7 In the IT, the PUL promote the proximal urethra to kink downwards and in the UHT they prevent the proximal urethra to descend downwards. In the IT, the high-pressure midurethral zone is flow-controlling, and in the UHT, a bladder neck sealing mechanism is flow-controlling. These theories also stipulate different tape positions to be curative. The IT stipulates a tension-free suburethral tape that starts 1 cm from the m.e. The UHT stipulates a tension-free suburethral tape that starts 1 cm from the bladder neck, but only in cases with hypermobile SUI. In cases with hypomobile SUI, the proximal urethra should be lifted above its resting position. Because suburethral lifting with a prolene net involves a high risk of obstructing complications, the “TVT technique” can be used to insert one tuned prolene net in the paraurethral tissues on each side of the vaginal point (v.p.) (Fig. 1) to create a lift without the risk of obstruction. The v.p. corresponds to the junction of the upper one-third and distal two-thirds of the urethra where the strong posterior PUL broadly attach to the paraurethral tissues and is also the midpoint of the intra-abdominal urethra. How these different tape positions lead to different curative mechanisms can be theoretically analyzed using a virtual biomechanical approach (Figs. 1-4). The pathophysiology of SUI according to the UHT is illustrated graphically in Fig. 1. At stress, the urethra is forced downward in relation to the bladder neck until it is stopped and funneled by hanging on the bladder neck. The hanging situation exerts one external shearing force (Fs) and enhances one internal outflow distending force (Fd). The miniscule meatus internus (m.i.) that is normally a perfect seal6 is forced open and the functional urethral length is shortened in proportion to the depth of the funnel. The urethral high-pressure zone can be engaged. Pascal's principle of fluid mechanics postulates that when the radius (r) of the m.i. widens, the Fd increases abruptly, and at aLPP, the urethra leaks. Fd = (aLPP + Pdet)*pi*r2 (Pdet = detrusor pressure). When funneling prevails at pressures <aLPP, the maximum urethral closure pressure (MUCP) resists the Fd and accordingly no leakage of urine but the enforced distension of the proximal urethra may provoke urge and frequency symptoms. With increasing age, this “secondary” continence mechanism weakens and the SUI worsens/debuts. When the proximal urethra is collapsed, the m.i. is closed and consequently the Fd is zero, even a low MUCP withstands very high Valsalva pressures; in fact, in the cases of a closed m.i., the midurethral high-pressure zone is not engaged at al. Analysis/comments: Hypermobile SUI (Fig. 1): The dorsocaudal rotation (dcr) is large; therefore, so too is the therapeutic window (t.w.)6. 1. Long urethra of 4.5 cm. Based on the IT (Fig. 2a): The tape is set tension-free at 1 cm from the m.e. and is “flat” at rest and “curved” at stress.8 The operation is successful because the dcr/t.w. is large even in the distal part of the urethra. The tape delivers acute kinking and compression. This closure mechanism is effective but not physiologic. The proximal urethra is not supported. During stress, funneling persists proximal to the tape, which implies an increased risk for postoperative urge and frequency symptoms9, 10 and the recurrence of SUI.10, 11 Moreover, the proximal urethra is chronically burdened by withstanding the intra-abdominal pressure and is eventually traumatized. Based on the UHT (Fig. 2b): The tape is set tension-free at 1 cm from the bladder neck and is “flat” at rest and “curved” at stress. The operation is successful because the proximal urethra is prevented from reaching a hanging position. Some urethral kinking presents, which is unnecessary to achieve continence. There is no hanging and no funneling. The miniscule m.i. is a perfect seal. 2. Short urethra of 3 cm. The operations based on the IT and the UHT (Fig. 3): Both operations are successful because the tape, for obvious reasons, ends up at about the same midurethral position where the dcr/t.w. are large. There is no hanging and no funneling. The m.i. is closed. Hypomobile SUI (Fig. 4): The dcr/t.w. is small. 1. Long urethra of 4.5 cm. Based on the IT: The failure rate is very high because the dcr/t.w. is very small. The correct setting of the tape is highly restricted. During stress, minor kinking and minor compression occur. The proximal urethra is not supported, and funneling persists proximal to the tape, implying an increased risk for postoperative urge and frequency symptoms and the recurrence of SUI. The proximal urethra is chronically burdened by withstanding the intra-abdominal pressure and is eventually traumatized. Based on the UHT: The failure rate is high. The dcr/t.w. is small. The correct suburethral setting of the tape is restricted. The failure rate is high if the surgeon does not lift the v.p. above its resting position. 2. Short urethra of 3 cm. Based on the IT and the UHT: The dcr/t.w. is small. The tape, for obvious reasons, ends up at about the same midurethral position (Fig. 3). The surgeon should lift the v.p. above its resting position. Approximately 80-90% of SUI are hypermobile SUI, promoting high success rates irrespective of the TVT position, and 10-20% are hypomobile SUI (ie, “fixed urethra” and many of cases defined as “ISD/intrinsic sphincter deficiency”), indicating high failure rates if the tape is set without lifting the v.p. above its resting position. The bladder neck/urethra rotates dorsocaudally during stress. The tape uses this mobility to stop leakage. A distal tape kinks the urethra. A proximal tape enforces a correct spatial relation between the urethra and the bladder neck. Distally from the v.p., urethral mobility decreases and is nearly zero at the m.e. Therefore, the therapeutic window decreases and failure increases the more distally the tape is set. Nevertheless, a tape positioned at or above the level of the bladder neck has high failure rates,12 even when the dcr is large, because hanging is not prevented. Urethral mobility can be measured by holding a fingertip a short distance under the v.p. at rest and asking the patient to make a slow Valsalva maneuver. The maximal “curative” distance is the t.w. The t.w. concept assists the surgeon in selecting the proper surgical procedure. It is noninvasive, rapid, without cost and is not inconvenient for the patient. Customary preoperative urodynamic testing is invasive, time-consuming, expensive, and not without inconvenience for the patient. Moreover, “it is not apparent that either LPP measurement or UPP (urethral pressure profilometry) can accurately predict which patients will achieve the best outcomes from surgical treatment for SUI.” ICS Book 2013.13 Conjecturally, a short urethra has a foreshortened extra-abdominal part; consequently, the posterior PUL attachment (v.p.) to the vaginal wall may be found at approximately the same distance from the bladder neck and equally at the midpoint of the intra-abdominal urethra. In the cases of a short urethra the term MUS (mid-urethral sling) for TVT/TOT procedures is correct, else it is a misnomer. These theoretical comments are also valid for tension-free obturator tape operations (TOTs). However, TVT and TOT are dissimilar. TVT is similar to a tight loop, and TOT is more similar to a 5-8 cm long14 springy hammock. At stress, the TOT may sway outside a small t.w., making TOT less successful in cases of hypomobile SUI.15 Yours sincerely, Bo S. Bergström1,* 1Specialist in obstetrics and gynecology, Stockholm, Sweden *Correspondence Bo S. Bergström, Specialist in obstetrics and gynecology, Karlavägen 27A 11431, Stockholm, Sweden. [email protected]
Source: Neurourology and Urodynamics
DOI: 10.1002/nau.23229
Author: Bo S. Bergström
Language: English
Subfield: Urology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W2586459986