Fig. 7.1
Solid sphere expulsion test device
7.7 Anorectal Manometry and Sensory Tests
Anorectal manometry consists of the measurement of pressure in the anal canal and the rectal ampulla, and it is useful as a diagnostic tool in patients with FI and outlet obstruction due to pelvic floor dyssynergy, to help understand the pathophysiology of the patients’ symptoms. The measurement of the pressures in the anorectal tract can be performed using various techniques that differ in the type of probe used (water-perfused probe, solid-state probe, microtransducer probe) and the mode of detection.
The pressure detectors can be placed longitudinally, helically, or radially on the probe. At the end of the probe there is a latex balloon placed in the rectum, which is inflated with air or water, mimicking the presence of feces in the rectum. This allows the patient to evoke the rectoanal inhibitor reflex, to determine the compliance and sensitivity of the rectum.
Water-perfused probes are connected to a low-compliance water infusion system, and to a water-filled pressure transducer, linked to a digital multichannel recorder. The pressure is converted to mmHg.
Solid-state probes are directly connected to a pressure transducer, linked to multichannel recorder. This method is more practical, fast, and reliable than water-perfused systems, but more expensive; the pressure is expressed in mmHg.
The rectum must be prepared with a hypertonic phosphate enema (130 cm3) 2 h before the manometry examination, and the patient is placed in a left lateral decubitus position, with legs bent at 90° and superimposed on the trunk (Sims’ position). The probe is positioned with all solid state sensors or opentips in the rectal ampulla and retracted with a rapid pull-through technique using an automatic retractor or with a stationary pull-through technique, in which the probe is stopped for 20 s every 5 mm, allowing to the operator to obtain the pressure profile of the anal canal. In the assessment of voluntary functions, the sensors should be placed all along the anal canal. In the study of reflexes, anorectal sensitivity and compliance, the probe is positioned with the end of the balloon 8–10 cm from the anal margin.
The parameters that are usually considered to have clinical utility are described in the following sections [11].
7.7.1 Resting Anal Pressure
This is mainly influenced by the tone of the internal anal sphincter (IAS). High anal pressure has been observed in patients with anal fissures or anal pain. Pressure reduction, occurring as lone symptom, is usually present in patients with incontinence, but measurements have low specificity and sensitivity.
7.7.2 Squeezing Anal Pressure
This is expressed in peak size and duration, and is produced by the contraction of external anal sphincter (EAS), with possible contributions by the accessory muscles of the perineum (puborectalis and gluteus). A decrease in peak pressure is caused by a weak EAS and may be myogenic (usually iatrogenic or obstetric) or neurogenic. A decrease in duration of squeeze (< 45 s) may indicate pudendal nerve damage.
7.7.3 Rectoanal Inhibitory Reflex
The distention of the rectum elicits an intrinsic reflex (i.e., via the myoenteric plexus) that produces a relaxation of the IAS. This reflex can be elicited by inflating a rectal balloon with 20–40 mL of air. Absence of the reflex is pathognomonic of Hirschsprung’s disease, and is also found in patients after low rectal resection and coloanal anastomosis, when it can disappear.
7.7.4 Cough Reflex
An intra-abdominal pressure increment induces a reflex contraction of the EAS. This parameter is particularly useful in cases of damage of the EAS, resulting in muscular weakness. It can be used to evaluate possible damage to the sacral reflex arc. In patients with lesion of the sacral reflex arc, the contraction of EAS has a lower peak size and duration; in subjects with spinal cord injury above the sacral level, the cough reflex is preserved.
7.7.5 Canal Pressure in Response to Defecatory Attempts
This maneuver determines an inhibition of the EAS. The failure to inhibit the tone of EAS or even its paradoxical contraction is typical of pelvic floor dyssynergia.
7.7.6 Compliance of the Rectum and Sensory Thresholds in Response to Balloon Distention
Rectal compliance is expressed by the pressure/volume ratio during continuous rectal distention obtained by using an inflatable low-compliance balloon. It is influenced by the size of the rectum, the tone of the rectal wall muscles, the elastic properties of the rectum, the integrity of parasympathetic innervations, and the mobility of the pelvic organs, which may limit rectal distention. A normal range of value has never been determined; in our laboratory a normal range is said to be between 2 and 15.
During progressive rectal distention, the sensitivity of the rectal ampulla can be evaluated by asking the patient about his first sensation of rectal distention (threshold of rectal sensitivity), his desire to defecate (threshold of the stimulus to defecate), and pain or impending desire to defecate (maximum tolerable volume). Some authors consider that the sensitive threshold measurement is a suitable test for the identification of patients with rectal hyposensitivity in cases of constipation. In contrast, hypersensitivity can be present in cases of fecal urge incontinence.
7.7.7 Vector Anal Manometry
A more sophisticated application of anal manometry is vector anal manometry, which shows the pressure profile in the anal canal in three dimensions. The methodology requires dedicated software, with automatic retraction of a microtip probe. Vector anal manometry can best assess the asymmetry of the anal pressure in cases of anal sphincter damages, in both resting and squeezing states [12].
7.8 Electromyography
Electromyography (EMG) of the anal sphincter is a neurophysiological examination used to identify the presence and the characteristics of myoelectric activity in the anal sphincters and levator ani. This investigation has lost its importance in the functional assessment of anorectal function because mapping the anal sphincters (IAS and EAS) by EMG, which is aimed at identifying sphincter injury (scar), is now performed by endoanal ultrasound, and because the diagnosis of acute or chronic denervation or re-innervation potentials indicating a pudendal neuropathy have been found to have no prognostic implications. EMG can be performed by three different types of electrode: concentric needle electrode, single fiber electrode, or surface electrode. In routine diagnosis, needle electrodes are used (these are more accurate than surface electrodes).
The standard technique applied with a concentric needle electrode consists of introducing the electrode into each quadrant of the EAS. Introducing the electrode at a greater depth enables study of the puborectalis muscle. Use of an intramuscular needle can identify the electrical activity of the membrane potential in the muscle fibers.
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