Anorectal manometry measures the pressures in the anal canal (internal anal sphincter [IAS] and the external anal sphincter [EAS]) and the distal rectum and screens for functional outlet obstruction (nonrelaxing pelvic floor), Hirschsprung’s disease (absence of rectoanal inhibitory reflex [AIR]), and damage to sacral reflex arc (absence of cough reflex and predicting responses to biofeedback training and to objectively evaluate pressures before and after surgical intervention).

There are four essential components in anorectal manometry equipment: (1) a probe for measuring intraluminal pressure, (2) a pressure-recording device (amplifier/recorder, pneumohydraulic pump, and pressure transducers), (3) a balloon for inflation inside the rectum, and (4) a monitor/printer/storage system.

The probes can be of different types, including solid state, water perfused, air charged, or microballoon. The diameter of the probe should not exceed 5–6 mm and the probe usually includes sensors radially distributed to measure several pressures at each level. Calibration of the probe and the recorder is critical for accurately measuring and obtaining reproducible results.

The stationary pull-through technique is common and measures the resting and squeeze pressures in increments from 6 to 1 cm from the anal verge by pulling the probe out 1 cm at a time. Allowing a waiting period between each measurement minimizes artifacts.

The dynamic pull-through technique measures pressures as the catheter is withdrawn in a continuous motion. It may create a reflex sphincter contraction, due to the stimulation generated by the probe, potentially resulting in an artificial increase of anal pressures.

The anal resting pressure reflects the tonic activity of the IAS (55 %), the EAS (30 %), and the anal cushions (15 %).

The IAS has an oscillating tonic activity with both slow waves of low amplitude and ultraslow waves of high amplitude.

The anal resting tone is usually measured with the stationary pull-through technique and the maximum resting pressure (MRP) is usually defined as the highest recorded resting pressure.

Due to radial asymmetry in different parts of the anal canal, the pressures in the four quadrants are averaged. The pressure profiles also vary according to gender, age, and measuring technique (Fig. 4.1).

The length of the functional anal canal or high pressure zone is defined as the length of the anal canal with resting pressures exceeding 30 % of the rectal pressure.

Patients with fecal incontinence tend to have lower anal resting tone than continent patients or normal controls. The clinical value of measuring basal anal canal pressures alone is limited, since patients with low pressures may have normal continence and patients with incontinence may have normal pressures. There is also a lack of defined values of what is the normal range for the anal resting tone.

Squeeze increase of the anal canal pressure is generated by contraction of the EAS and can be calculated as the increase in pressure from the anal canal resting tone during maximal anal squeeze (Fig. 4.2).

The squeeze increase is usually measured with the stationary pull-through technique, by asking the patient to maximally squeeze the sphincter at each level and hold this squeeze for 3 s. Instructing the patient to avoid contraction of accessory muscles, particularly the gluteal muscles, or to avoid increasing the intra-abdominal pressure reduces the risk of measuring false high squeeze increase.

The maximum voluntary squeeze pressure (MSP) is usually defined as the highest pressure recorded above the baseline (zero) at any level of the anal canal during maximum squeeze effort. An alternative measurement is the highest pressure recorded above the resting pressure during maximum squeeze effort (the increment of pressure above the resting tone).

Decreased squeeze pressures are frequently correlated to injuries in the EAS, neurologic damage, or just poor patient compliance/voluntary control. If the latter problem is suspected, the results of decreased squeeze pressure can be interpreted in context with the EAS response to coughing (see the section “Cough Reflex”).

The susceptibility for fatigue of the EAS can be estimated by measuring the patient’s ability to sustain the squeeze effort over time. The squeeze duration is often reduced in patients with incontinence. The squeeze durability can be measured as a fatigue index (the coefficient of maximum squeeze pressure and the gradient of decay).

Rectal distension or attempted defecation results in an inhibition of the tonic activity of the IAS and reflex relaxation of this muscle. The RAIR is mediated via the myenteric plexus and it is modulated by the spinal cord.