Key Points
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Anal endosonography (AES) is simple to perform and visualizes the anal sphincter complex—notably the external and internal anal sphincters.
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AES is able to image sphincter tears and defects.
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AES can also characterize sphincter morphology and determine muscular quality.
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AES is the single most important investigation in patients with anal incontinence.
Introduction
First described in 1989, anal endosonography (AES) was the first technique to visualize the anal sphincter complex with enough spatial resolution to resolve the individual components of the sphincter mechanism. Despite the advent of magnetic resonance imaging (MRI), including endoanal receiver coils, AES generally remains the technique with the highest spatial resolution, and is also quick and easy to perform. The introduction of AES precipitated a significant reappraisal of the causes of anal incontinence (and its treatment), which had hitherto been thought to be mainly the result of pelvic neuropathy. When incontinent patients were studied with AES, it rapidly became clear that occult anal sphincter disruption was present in many cases. Patients with disrupted sphincters can be scheduled for surgical procedures that aim to restore integrity to the sphincter ring, whereas patients whose sphincters are intact, or whose muscles are thought to be of poor quality, can be directed toward conservative measures or alternative surgical approaches.
At present, AES has replaced physiologic testing as the pivotal examination in the clinical decision-making process for these patients, although both are usually performed. Although AES is probably most useful to characterize obstetric injury, it has also facilitated the characterization of other causes of fecal incontinence. For example, with AES, the examiner can identify neurogenic incontinence by way of specific patterns of sphincter atrophy and can identify occult and unintended sphincter damage following anal surgical procedures.
Equipment and Examination Technique
Although it is possible to perform AES using an echoendoscope, the best results by far are obtained using a dedicated anal ultrasound probe. The anus is a very superficial structure, and an echoendoscope is cumbersome and expensive when compared with a probe designed specifically for the purpose. AES first employed a 7.5-MHz transducer that had been designed initially for rectal cancer staging and prostatic imaging. The transducer was covered by a rubber balloon, then inserted through the anus into the rectum, the balloon inflated with degassed water, and the transducer rotated mechanically to produce 360-degree images of the rectal wall. Professor Clive Bartram of St. Mark’s Hospital, London, realized that by simply replacing the soft rubber balloon with a rigid plastic cone, the rotating transducer could be withdrawn into the anus safely. This maneuver was previously impossible because the balloon would be torn when compressed by the anus against the rotating metal transducer.
Modern probes encapsulate a fixed transducer within a permanent hard cover and are of higher frequency ( Fig. 19.1 ). Some also possess three-dimensional (3D) capacity, achieved either by withdrawing the probe during image acquisition (e.g., EUP-R54AW-19/33, Hitachi Medical Systems, Wellingborough, UK) or by incorporating a transducer that moves along the Z -axis of the probe, inside the exterior capsule, while the head is held stationary within the anal canal (e.g., 2052 transducer, BK Medical, Herlev, Denmark).
As noted earlier, anal imaging is best achieved using a dedicated rigid probe. However, an echoendoscope can be used if a dedicated probe is unavailable. When such systems were first introduced in the late 1980s, the only instrument available was the mechanical radial system (GF-UM3 and EU-M3, Olympus, Tokyo). Initially, attempts were made to utilize this instrument to evaluate the anal sphincter but this was problematic because the optimal focal zone for the UM3 was 1 to 2 cm from the transducer (i.e., beyond the sphincter muscles). The only way to bring the sphincters into this focal zone was to inflate the balloon surrounding the transducer but doing so distorts and compresses the sphincters and is very uncomfortable, and image interpretation is unreliable. In the early 1990s, units that wished to perform anal sphincter examinations had to purchase dedicated systems (as mentioned previously). In response, Olympus began to market rigid mechanical radial probes in 2000 (RU-75M-R1 and RU-12M-R1) intended to examine the sphincters and rectum and which were compatible with their existing radial endoscopic ultrasound processors. These rigid probes are still marketed today and are compatible with the EU-ME2 and EU-ME2 Premier Plus Ultrasound processors. With time, mechanical radial technology yielded to electronic array systems. The advantage of electronic radial technology is its variable focal length combined with very good resolution. Though not tested in comparative studies, many practitioners feel that the current electronic radial systems are adequate for anal sphincter evaluation.
The examination is simple, well tolerated by the patient, and very rapid when performed by an experienced operator. No special patient preparation is required. The patient is told that discomfort, if any, will be similar to having a small finger in the anus, and the procedure will likely be much less uncomfortable than digital rectal examination by a doctor. To the patient, the probe is potentially quite a frightening piece of equipment, so it is worth mentioning that only the distal few centimeters will enter the anus (as opposed to rectal endosonography, in which insertion is obviously deeper).
Men are examined in the left lateral patient position, but the prone patient position is preferable for examining women. Placing women in the left lateral position can occasionally distort anterior perineal anatomy and can induce an asymmetrical image, which makes it difficult to distinguish perineal scarring from normal anatomic features. In the past, it was necessary to fill the transducer head with degassed water to achieve acoustic coupling, accomplished by injection using a syringe through a side port and then maneuvering the probe so that all air was expelled through a pinhole located at the tip of the cone. However, modern probes merely require the tip to be lubricated with ultrasound jelly and then covered with a condom, which is itself lubricated to facilitate insertion. The probe tip is then inserted into the anus, and image acquisition commenced. The aim is to insert the probe so that the transducer lies just into the distal rectum. The probe is then withdrawn gently and slowly to examine the anal sphincters.
As for all ultrasound examinations, clinical findings are generally based on the image displayed on the monitor screen in real time (with the exception of 3D acquisition, in which case the examination in its entirety can be replayed later). However, still images are usually required for archival purposes, and the author finds it is convenient to obtain these still images at three levels: the proximal, middle, and distal anal canal (see later). These three anatomic levels are imaged at standard magnification, and the examination is then repeated at a higher magnification, for a total of six images, three at each magnification. The probe is oriented so that anterior (i.e., the 12 o’clock position) is uppermost, which mimics the standard used for axial medical imaging. The examination is normally very quick, perhaps only a minute or so for the experienced operator who is familiar with normal and abnormal anatomy, and especially if the sphincters are normal.
Anal Sphincter Anatomy
Clearly a sound understanding of basic anal anatomy is a prerequisite for accurate interpretation of endosonographic findings. There are two anal sphincters: the external anal sphincter (EAS) is composed of striated muscle, whereas the internal anal sphincter (IAS) is smooth muscle. These form two cylindrical layers, with the IAS innermost ( Fig. 19.2 ).
The EAS arises from the striated muscles of the pelvic floor and is composed of three cylindrical bundles lying on top of one another (deep, superficial, and subcutaneous) that are difficult to distinguish in practice. The deep portion is fused with the puborectalis (or pubococcygeus) muscle, which itself merges with the levator plate of the pelvic floor. The EAS extends approximately 1 cm distal to the IAS, where it forms the subcutaneous part of the EAS muscle. Anteriorly the EAS is closely related to several surrounding structures, such as the superficial transverse muscle of the perineum and the perineal body. Posteriorly it is continuous with the anococcygeal ligament, a structure that is often more prominent in men and should not be mistaken for a posterior sphincter defect. The EAS is much shorter anteriorly (in the craniocaudal direction; i.e., longitudinal) in women than in men, and this feature should not be confused with a sphincter defect.
The IAS is the distal termination and condensation of the circular smooth muscle of the gut tube. It extends from the anorectal junction to approximately 1 to 1.5 cm below the dentate line (see Fig. 19.2 ). The longitudinal muscle of the gut tube also terminates in the anal canal, but it is less apparent than the IAS. The longitudinal muscle interdigitates between the EAS and the IAS and terminates in the subcutaneous EAS and subcutaneous anus. Its exact sphincteric action, if any, is much less clear than that of the EAS and IAS, and it is thought that its main purpose is to brace the anus and thus prevent anal eversion during defecation. Lying between the EAS and the longitudinal muscle is a potential plane, the intersphincteric space, which may contain fat. The components of the anal sphincter are surrounded by the ischioanal space (often referred to by surgeons as the ischiorectal fossa), which contains fat predominantly.
Directly anterior to the anal sphincter is the central perineal tendon or perineal body. In men, this lies posterior to the bulbospongiosus and corpus cavernosum and their related muscles, whereas in women, it lies within the anovaginal septum. Many structures insert fibers into the perineal body, such as the EAS, the deep and superficial transverse muscles of the perineum, the bulbocavernous muscle, and the puborectalis muscle. These structures should not be confused with sphincter defects. For example, normal variants of anal sphincter anatomy have been identified, such as differing relationships between the superficial transverse perineal muscle and the EAS.
The distal anal canal is lined with stratified squamous epithelium, richly supplied by sensory receptors. These receptors are most concentrated at the dentate line, which demarcates the junction with proximal columnar epithelium. The anal subepithelial tissues are relatively thick, and this lining and its underlining vascular spaces—the anal cushions—also play a role in maintaining continence.
Normal Endosonographic Findings
Because the anus and surrounding sphincter muscles are cylindrical, a 360-degree field of view is optimal, and the axial plane is also the most relevant surgically when considering sphincter defects. As stated earlier, it is convenient to obtain baseline images at three levels at a minimum: the proximal, middle, and distal anal canal.
The proximal anal canal is primarily identified by the puborectalis and transverse perineal muscles ( Fig. 19.3A ). The puborectalis slings around the anorectal junction and can be distinguished from the EAS, with which it blends imperceptibly, because its anterior ends splay outward as they travel toward their fusion with the pubic arch (see Fig. 19.3A ). The IAS is visible as a continuous hypoechoic ring and is generally the easiest structure to differentiate from other adjacent anal canal components because it is normally very hyporeflective. The subepithelial tissues, EAS, and longitudinal muscle all normally show varying degrees of hyperreflectivity, and their margins can often be difficult to define precisely, although direct comparisons with endoanal MRI have helped tremendously. Increases in transducer frequency that improve spatial resolution have also helped clarify sonographic anatomy, as has 3D imaging.
In their seminal early studies, Sultan et al. carefully imaged cadaveric specimens following sequential histologic dissection of anal layers and thereby validated the sonographic appearances. These investigators found that the echogenicity of normal muscle changed as its orientation was altered with respect to the transducer. Thus normal variant striated muscle slips may appear hypoechoic, depending on their orientation to the transducer, and should not be confused with sphincter tears or scars.
If the probe is withdrawn just a centimeter or so from the proximal anal canal position, the anterior ends of the normal puborectalis muscle will converge anteriorly as they segue imperceptibly into the EAS. The mid-anal canal is thus defined where the EAS forms a complete ring anteriorly (see Fig. 19.3B ). The IAS is also normally thickest and best seen at this level. At this level, the intersphincteric plane and longitudinal muscle may also be resolved as two distinct layers, with the longitudinal muscle forming distinct bundles of smooth muscle fibers.
Withdrawing the probe slightly more will move the field of view into the subcutaneous EAS (see Fig. 19.3C ). This lies below the termination of the IAS, which is either not visualized or only partially visualized if its termination is irregular (a common normal variant). It is usually impossible to visualize the longitudinal muscle reliably at this level because it has thinned out as it interdigitates into the EAS, and it is mainly composed of fibroelastic tissue rather than the smooth muscle found more proximally.
Correct interpretation of AES is possible only if the operator has a firm grasp of normal sonographic anatomy. Pathology is defined by either muscular discontinuity (i.e., from sphincter tears or lacerations, secondary to a variety of causes) or abnormal muscular quality (which is usually caused by neuromuscular atrophy or degeneration). To appreciate muscular quality correctly, it is important to realize that normal sonographic appearances are contingent on both age and sex. Frudinger et al. examined 150 nulliparous women with high-frequency AES to define normal age-related differences in sphincter morphology and found a highly significant positive correlation between IAS thickness and increasing age. In contrast, EAS thickness showed a highly significant negative correlation with increasing age. Some evidence also suggested that IAS reflectivity increased with age. No significant correlation was noted between age and thickness of subepithelial tissues, the longitudinal muscle, or the puborectalis muscle.
On average, the IAS measures 2 to 3 mm thick in normal adults (measured at either the 3 or 9 o’clock position at mid-anal canal level). A thin IAS has more significance in an older person with symptoms (see later sections). In addition, although the IAS can be measured easily because it contrasts with adjacent structures, other muscles may be more difficult to measure and are subject to greater interobserver variation. Gold et al. measured anal canal structures in 51 consecutive referrals: intraobserver agreement was superior to interobserver agreement and the 95% limits of agreement for EAS measurements spanned 5 mm versus 1.5 mm for IAS measurements. Interobserver agreement for diagnosis of sphincter disruption and IAS echogenicity was very good, suggesting that AES has generalizable diagnostic utility ( κ = 0.80 and 0.74, respectively).
Clear sonographic differences exist between men and women with respect to the dimensions of anal canal structures and their sonographic appearances. Most importantly, the anterior complete ring of the EAS is shorter in women in the craniocaudal (longitudinal) direction. This difference has been widely appreciated for some time: Williams et al. used 3D AES to show that craniocaudal EAS length was approximately 17 mm in women versus 30 mm in men. A short anterior EAS in a woman should not be misinterpreted as a sphincter defect. In addition, in men the various muscular components generally show a more striated appearance ( Fig. 19.4 ).
Anal Sphincter Function
Most clinical referrals for AES are in response to patients complaining of anal incontinence, either to gas alone or to both gas and feces. It is therefore important to have some basic understanding of normal anal sphincter function.
The anal sphincter is the most complex sphincter in the human body. Continence is maintained by a multifaceted interrelationship between anal and pelvic floor musculature, integrating somatic and autonomic nervous pathways, the effects of which must be temporarily overcome during the act of defecation. The IAS is innervated by sympathetic presacral nerve fibers and is not under conscious control. It is primarily responsible for closing the anal canal at rest, at which time it is in a state of continuous involuntary contraction. Despite being striated muscle, the puborectalis and EAS also display some resting tone and can contract rapidly without conscious control in response to any sudden increase in intraabdominal pressure, to prevent anal incontinence. The EAS is innervated by the pudendal nerves (S2, S3, and S4).
Defecation is initiated by colonic smooth muscle contractions—for example, those provoked by waking and eating. These contractions propel stool from the sigmoid colon into the normally empty rectum and stimulate rectal sensory nerves that produce an urge to defecate. These nerves are also able to determine the nature of rectal content (i.e., solid, liquid, or gas). The sensation of a full rectum and the ability to discriminate among gaseous, liquid, and solid content are important components of continence, in addition to sphincter integrity. Sensation is retained after rectal excision, a finding suggesting that some sensory receptors reside in the pelvic floor itself. Rectal filling causes reflex IAS relaxation (via the rectoanal inhibitory reflex), rectal contraction, and contraction of the puborectalis and the EAS, both of which are heavily modulated by conscious control. Stool within the anal canal contacts sensory receptors concentrated at the dentate line and greatly intensifies the urge to defecate, which is resisted by vigorous striated muscle contraction until circumstances for defecation are appropriate. When this is so, pelvic floor relaxation and increased intraabdominal pressure create a positive pressure gradient from rectum to anus and evacuation ensues.
The normal function and contribution of the EAS and IAS to anal continence can be used to predict which muscles are abnormal in incontinent patients. For example, IAS abnormality generally results in passive incontinence (i.e., the patient is unaware that leakage is about to occur), whereas EAS abnormality more frequently manifests as urge incontinence (i.e., the patient is unable consciously to defer defecation).
Anorectal Physiologic Testing
Sphincter integrity and function can also be determined by anorectal physiologic testing, which evaluates nervous integrity, conduction, and muscular performance. Few physiologic tests are absolutely diagnostic, and most need to be considered together with symptoms, clinical findings, and imaging. For example, low pressures could be due to sphincter defects or neurologic deficit, and conversely, it is possible to have a sphincter defect and normal pressures. However, anorectal physiology provides valuable complementary information and continues to be requested in combination with AES, and endosonographers should be aware of their implications. Normal values vary among laboratories.
Manometry
Manometry determines rectal and anal pressures more precisely than simple digital examination. Complexity varies, from simple balloons connected to a pressure transducer to multichannel probes capable of measuring pressures at several sites simultaneously and displaying pressures in 3D, and even ambulatory systems recording over several hours. The pressure recorded rises when a rectal catheter is withdrawn into the anus, and falls again when it reaches the anal margin. This high-pressure zone defines the functional anal canal length (as opposed to anatomic length, which is usually shorter). The pressure zone is frequently diminished in incontinent patients. A static anal probe measures resting pressure, which predominantly reflects IAS function; reduced resting pressure broadly reflects IAS disease. In contrast, the squeeze pressure is the incremental rise over resting pressure elicited when the patient is asked to contract their anus voluntarily, and broadly reflects EAS function. Squeeze pressure is frequently reduced when incontinence results from EAS laceration, as occurs following obstetric injury. Dual-sphincter disease is implicated when both resting and squeeze pressures are abnormal, and neither finding is absolutely specific in an individual patient.
Pudendal Nerve Latency
Pudendal nerve terminal motor latency can be determined from the time taken for a digitally delivered pudendal nerve stimulus to elicit anal sphincter contraction. A common system employs a disposable glove with a stimulating electrode at its fingertip, coupled with a pressure sensor at its base. The nerve is stimulated near the ischial spine and has both sensory and motor components. Slow conduction is thought to predominantly result from stretch-induced injury (e.g., following childbirth or chronic straining ) and can even be demonstrated transiently in normal individuals when asked to strain excessively. The clinical relevance of pudendal neuropathy remains unclear, especially because the degree of neuropathy, pelvic floor descent, and anal sensation should be directly related, but studies cannot demonstrate this. Nevertheless, patients with abnormal latencies but intact sphincters usually have their incontinence attributed to neuropathic sphincter degeneration, and sphincter repair is less successful if underlying neuropathy is present.
Electromyography
A needle electrode inserted into the EAS can determine both its activity and muscular quality. Sphincter denervation is followed by reinnervation via neighboring healthy axons, which can be quantified electromyographically because the recorded action potentials become polyphasic. Until the advent of AES, electromyography was the only reliable way to diagnose sphincter tears directly (as opposed to indirectly via reduced pressures, for which there are multiple causes); the needle was inserted into the suspected defect, which was confirmed if no muscular potentials could be recorded subsequently (also possible if the needle tip missed normal muscle because of incorrect placement—easily done when insertion is blind!). Sphincter defects were mapped out by “blind” needle passes made circumferentially around the anus. Electromyography is painful because local anesthetic interferes with recording so isn’t used. Fortunately, AES is superior for detecting sphincter defects when the two modalities are compared directly.
Sonographic Findings in Anal Incontinence
As mentioned earlier, most clinical referrals for AES are to investigate anal incontinence. Anal incontinence has a variety of causes, many of which relate to the integrity and quality of the sphincter mechanism. AES has assumed a central role in the diagnostic workup for assessment of this problem because AES reliably identifies those patients who have a sphincter tear, selects individuals likely to benefit from surgery that aims to restore integrity to the sphincter ring, and prevents unnecessary surgery in others. Physical examination cannot detect anal sphincter defects reliably, and although anal canal pressures can help determine whether sphincter function is abnormal, they cannot indicate precisely whether the cause is loss of sphincter integrity or neuropathy.
Anal incontinence is common, especially in women, and its prevalence increases with age. Two percent of the general population older than 45 years have anal incontinence, with prevalence rising to 7% of persons more than 65 years old. In retirement homes or hospitals, approximately one-third of individuals have anal incontinence. Prevalence is likely to be even higher because of underreporting. Anal incontinence has considerable economic impact. A 1988 study estimated that more than $400 million annually was spent on incontinence appliances in the United States alone, and anal incontinence was the second most common cause of placement in a nursing home. Several clinical grading systems for anal incontinence have been developed.
Obstetric Injury
Childbirth is a common cause of anal incontinence, either directly, from anal sphincter laceration, or indirectly, from damage to sphincter innervation. Obstetric anal sphincter injury is often termed “OASIS.” Until the advent of AES, it was assumed that neuropathy resulting from damage to sphincter innervation was the primary cause of obstetric-related incontinence because impaired pudendal nerve conduction can be demonstrated after vaginal delivery, presumably from stretch-induced injury. Anal sphincter laceration was thought to be a relatively rare event because it was identified clinically in only 1 out of 200 vaginal deliveries. However, AES revealed that anal sphincter tears were far more common than assumed. An early sonographic study of 11 women with a diagnosis of neurogenic fecal incontinence revealed that four had also sustained unsuspected anal sphincter tears. A further study of 62 women whose incontinence was related to childbirth found EAS tears in 56 (90%).
In a landmark study, Sultan et al. used AES to study 202 consecutive unselected women before and after vaginal delivery, finding anal sphincter tears in 28 of 79 (35%) of primiparous subjects and in 21 of 48 (44%) of multiparous subjects. Furthermore, endosonographic evidence of sphincter laceration was associated with symptoms of anal incontinence 6 weeks following delivery and correlated positively with impaired physiology—namely reduced anal resting and squeeze pressures. No primiparous woman had a sphincter defect before childbirth, and no subject undergoing cesarean section developed a new defect. These findings confirmed that sphincter injury was caused by vaginal delivery, especially forceps extraction. Moreover, the study confirmed that clinical examination of the perineum immediately after vaginal delivery missed most sphincter tears.
Anal incontinence occurs immediately after delivery if trauma is substantial, but many women present years later, presumably because the cumulative effects of multiple deliveries, progressive neuropathy, aging, and menopause overcome their compensatory mechanisms. Many women are also too embarrassed to complain, or they or their doctors believe that the condition is incurable. The accuracy of endosonography has been validated both histologically and intraoperatively, and approaches 95%. For example, a study of 44 patients found that all 23 EAS defects and 21 of 22 IAS defects visualized on preoperative AES were confirmed subsequently by surgery.
The sphincters are cylindrical structures, so discontinuity is diagnostic of a sphincter tear. A break in the hypoechoic IAS ring indicates an IAS defect, whereas EAS defects are defined by discontinuity of the more heterogeneous EAS, located peripheral to the intersphincteric plane and the longitudinal muscle. In severe disruptions, the entire sphincter mechanism is completely absent anteriorly, with a cloacal defect between the vagina and anal canal ( Fig. 19.5 ). Obstetric injury is practically always anterior, because this is where the vagina lies. Because the EAS and IAS are in very close proximity, it is usual for obstetric injury to involve both sphincters. Isolated EAS injury is relatively uncommon, and isolated IAS injury is rarely the result of obstetric injury alone.
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