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The importance of the urodynamic study is to identify if incontinence is of urethral cause, vesical or mixed and classify the type of urethral urinary incontinence .
Cystometry – That exam gives us information of the function of the bladder reservoir, allowing to quantify the capacity and the vesical compliance. It gives us information as to the existence of involuntary contractions of the detrusor (Fig. 1).
Abdominal leak point pressure (ALPP). The ALPP is defined as the vesical or abdominal pressure which leads to urinary loss in the absence of contractions of the detrusor. This exam confirms the diagnosis of urinary incontinence and in women it classifies the type of incontinence. The way to do this exam has still not been standardized and the used parameters. It is recommended that the patient do this exam standing.Several parameters were described to classify Urinary Incontinence in women. McGuire2 uses the following values:

ALPP < 60cm H2O intrinsic sphincter deficiency.
ALPP > 61< 120 cm H2O urethral hypermobility.

Other authors recommend3:
ALPP < 90 cm H2O Intrinsic sphincter deficiency.
ALPP: 90 – 100 cm H2O ndefinite.
ALPP > 100 < 150cm H2O urethral hypermobility.

This classification takes into consideration the physical characteristics of the patient, if she is obese or thin. The separation between urethral hypermobility and intrinsic sphincter deficiency is not clear.
Videourodynamics – Is the gold stand of urodynamic evaluation, allowing the measurement of vesical and urethral pressure and visualization of the inferior urinary tract. This exam must be used when you do not get a diagnosis through other ways.
When to make a urodynamic evaluation? In the cases of symptoms and a compatible physical exam with stress Urinary Incontinence, test of positive effort and a normal urine test is recommended to have a urodynamic study done. The urodynamic evaluation is a must before any surgical treatment, in the cases of symptoms and atypical physical exams for stress urinary incontinence, patients operated without success, recurrent urinary infection and associated illnesses like Parkinson disease, cerebral vascular accidents, diabetes.
The treatment of SUI for IIS consists in the application of some techniques, like an artificial sphincter, para-urethral injections and sling surgery, being the most used. Using one of these techniques is it possible to dispense the urodynamic evaluation? Theoretically, yes, because the sling technique may be used in SUI by hypermobility or ISD. The main motive to continue making urodynamic studies is to evaluate the vesical function, that is, if it presents instability of the detrusor. The instability of the detrusor must not be treated through surgery, because only 33% of the patients with this problem are cured the rest present an important worsening of the symptoms. The patients with mixed Urinary Incontinence IUE + detrusor instability must be treated and many times surgery is necessary. These patients must be warned of the possibility of persistence or the symptoms worsening and signs of detrusor instability, which can be interpreted as non therapeutic success by the patient.