ABSTRACT

The video demonstrates that the current concept of total pelvic floor relaxation preceding micturition is only partly correct. It is only the anterior part of pelvic floor which relaxes. The posterior muscles stretch the bladder/urethral smooth muscle backwards to tension it, prior to active opening of the posterior urethral wall by the downward angulation of the anterior border of levator plate by the conjoint longitudinal muscle of the anus. This action expands the diameter of the urethra, vastly reducing the resistance to flow and therefore, the head of pressure required to be generated by the detrusor to empty the bladder.

INTRODUCTION

Text adapted from: Petros P, Lynch W, Bush M. Surgical repair of uterosacral/cardinal ligaments in the older female using the tissue Fixation system improves symptoms of obstructed micturition and residual urine. Pelviperineology 2015;34:112-116.

Inability to adequately evacuate the bladder is a major source of repeated urinary infection and pathology.¹ Catheter-associated urinary tract infection (UTI) is the most common nosocomial infection, accounting for >1 million cases in hospitals and nursing homes. The risk of UTI increases with increasing duration of catheterization. In non-institutionalized elderly populations, UTIs are the second most common form of infection, accounting for nearly 25% of all infections with a cost of 1 billion dollars p.a.¹ The traditional view of the mechanism of micturition was described by Messelink et al:¹ “The pelvic floor muscles must relax in order to remove the passive continence mechanisms, thereby favouring normal micturition”. A recent Review of voiding dysfunction^2,3 shed little light on the problem. It stated, “There remains a lack of consensus regarding a precise diagnosis and definition of voiding abnormalities in women”. The Review’s statement of causation² was limited to “detrusor underactivity and outflow obstruction which could be either physiological or iatrogenic”. Two studies reporting improvement of bladder emptying following cystocele and fascial repair were mentioned, but no anatomical explanations were forwarded as to why.² We believe that the answer to these conundra is to be found in urethral resistance to urine flow which is exponentially determined and is instantaneously modified by an external striated muscle mechanism first described in 1990.⁴

Micturition Video: https://www.youtube.com/watch?v=eiF4G1 mk6EA&feature=youtu.be

This mechanism, since validated with electromyography and video X-ray studies stretches open the posterior urethral walls during micturition,^5,6 Figure 1 and 2, and is in turn ultimately dependent on competent posterior suspensory ligaments in the position of the cervix (CX), Figure 1.^5,6 The external opening mechanism, Figure 1 and 2, was described as follows: immediately prior to commencement of voiding, the forward closure vector (m. pubococcygeus) relaxes; relaxation of m. pubococcygeus releases the closure pressure of the hammock on the posterior urethral wall, thereby freeing the posterior vectors (levator plate and the conjoint longitudinal muscle of the anus, arrows, Figure 1, to actively open the urethra prior to detrusor contraction; this causes the urethra to funnel, exponentially lowering the resistance to flow immediately prior to the expulsive action of the detrusor.⁶ According to,⁴ the keystone of this mechanism is the requirement for firm anchoring points for the downward opening vector, the uterosacral/cardinal ligaments (CL/USL) at CX, Figure 1: the downward opening vector (white arrow, Figure 1) contracts against the CL/USL: if the USL is loose, the vector weakens;⁷ the vector cannot open out the posterior urethral wall; the detrusor contracts against an unopened urethra and therefore, a high urethral resistance; a higher detrusor pressure is required for expulsion, Figure 3; the patient will have bladder emptying difficulties because of greatly increased resistance to flow.⁸ Resistance to flow is highly sensitive to this opening mechanism, as it is exponentially determined (Poiseuille’s Law). For non-laminar flow, it is approximately inversely related to the 4^th power of the radius (r).⁸ With reference Figure 1, there appears to be almost doubling of the urethral diameter during micturition. The pressure flow relationship as determined by direct laboratory measurement and computer modelling is shown in Figure 3.⁸ At a diameter of 3.5 mm, a pressure head of approximately 170 cm H₂O is required to achieve a flow rate of 50 ml/sec, Figure 3. If the urethra can be opened out from 3.5 mm to 6 mm by the proposed external mechanism, the head of pressure required for a 50 ml/sec flow falls to 20 cm H₂O.