One of the most common methods of measuring anorectal pressure utilizes a closed balloon system. The pressure result obtained with balloon recording is an average of all pressures acting into the balloon; radial asymmetry cannot be detected. However, larger balloons are unphysiologic as the probe itself may cause reflex contractions but the results obtained are more representative of a greater area of the sphincter. It is of tremendous importance to understand that a compliance hysteresis phenomenon exists in the anal canal, wherein both pressure-radial curves (distention/deflation) are not superimposable, and the level of pressure is lower on return, for similar levels of distention due to compliance adaptation processes. Thus, probes with a larger diameter generate greater pressures in the same patient than do smaller balloons.2 Furthermore, more rapid distention records higher pressures. Air is compressible; therefore the use of water for the balloon filling may be more reproducible.3
Air-Filled Balloon Technique (Schuster)
In the 1960s, Schuster4 invented a simple method for measuring anorectal pressure changes. The device consisted of a metal cylinder around which double-molded latex balloons are tied forming two compartments (Fig. 8.1). These balloons were connected with separate catheters through a hole in the cylinder to either inflate air or to record pressure changes, when appropriate. The inner balloon has a doughnut shape when inflated with 7 to 10 ml of air; the outer balloon has a pear-shaped structure. Through the metal cylinder, further balloons (rectal and/or colonic) can be inserted. Thus, rectal pressures can be measured or the rectoanal inhibitory reflex can be elicited.
This device is inserted into the anus and positioned where the inner balloon lies attached to the internal anal sphincter and the pear-shaped balloon to the outer bundle of the external anal sphincter. Thus, theoretically, the pressures of the internal and external sphincter can be independently measured; however, the overlap of these two portions is too wide to allow adequate differentiation. Normally, recording is performed with an aneroid manometer. The advantage is that recording is done from a larger surface reflecting overall pressures of a large area of the sphincters. Furthermore, the technique is simple and cost-effective, and the balloon stays fixed in place, thereby requiring only one operator. The disadvantage is that larger balloons create more artifacts during recording; therefore, this method is better for evaluating pressure changes than for evaluating actual pressures. Balloons and cylinders are available in three sizes: infant, pediatric, and adult. Although the device is not widely used for diagnostic evaluation, it may have a therapeutic role as a biofeedback catheter.4,5
Another form of measuring anorectal pressures is with small tubes. The advantage of this technique is that only minor changes and irritation of the sphincters by the device itself are obtained.6 Standardization is very important in order to obtain reliable and reproducible results;
a three-balloon system is generally used. While the water-filled microballoons (diameter 5-7mm) are placed in the upper and lower anal canal, the third balloon on top of the catheter is distended by air to elicit the rectoanal inhibitory reflex in the distal rectum (Fig. 8.2).
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