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Imaging of the ischiorectal fossa
CT: THE JOURNAL
OF COMPUTED
TOMOGRAPHY
1985;
9:329-336
IMAGING OF THE ISCHIORECTAL FOSSA MADAN V. KULKARNI, MD, SNEHAL D. MEHTA, MD, DELAND D, BURKS, MD, AND MAX I. SHAFF, MD
A retrospective evaluation of 500 routine abdominal computed tomography scans revealed 21 cases in 16 patients. Pathoof ischiorectal fossa disease logic processes included involvement by primary and secondary neoplasms, usually of gynecologic origin; rectal prolapse; and abscess. These abnormalities can be missed if the ischiorectal fossae are not included in routine abdominal scans. The anatomy of the ischiorectal fossae and their relationship to other pelvic organs ore well demonstrated with high-resolution computed tomography scans as well as by magnetic resonance imaging. KEY WORDS:
Computed
tomography;
Pelvis, anatomy;
Fat
Although numerous articles have defined the role of computed tomography (CT) in detecting abnormalities in the pelvis (l--8), few case reports of abnormality in the ischiorectal fossa have been presented. The role of coronal and sagittal reconstruction of multiple axial CT images has also been reported (9, 10) and the importance of scanning the perineum in the evaluation of perineal pathology has been frequently emphasized. The CT evaluation of perineal and pelvic anatomy has also been mentioned in the literature (5). This paper provides additional anatomic details pertinent to CT scanning and mag-
From the Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, Tennessee. Address reprint rkquests to: Max I. Shaff, MD, Department of Radioloav and Radiological Sciences, Vanderbilt Universitv Medical-Center, Nashville, Tennessee 37232. Received December 1984. This paper was presented at the 69th Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, Illinois, November 13-18, 1983. 0 1985 by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Ave., New York, NY 10017 0149-936x/85/$3.00
netic resonance imaging of the ischiorectal fossa. A retrospective analysis of the different pathologic processes seen in the ischiorectal fossa is made. The fat content of the ischiorectal fossa povides inherent contrast in CT images. Similarly, magnetic resonance imaging also shows excellent signal acquisition in proton density images due to the predominantly fat content of the ischiorectal fossa. By contrast, and for the same reason, ultrasound examination of this area has limited value. Direct sagittal or coronal images can be obtained by magnetic resonance imaging scanning. High-resolution CT scans of the ischiorectal fossa and subsequent sagittal and coronal reconstruction can also be obtained. These can be used to evaluate pathology in the ischiorectal fossa and its relationship to normal anatomic landmarks such as the levator ani muscle and pelvic organs. This paper presents ischiorectal The normal anatomy fossa pathology in 16 patients. of the ischiorectal fossa as demonstrated by CT and magnetic resonance imaging is also described.
MATERIALS AND METHODS A retrospective analysis of 500 abdominal CT scans was made and 16 patients with ischiorectal fossa abnormalities were identified. As some of the patients had follow-up scans after therapy, the total number of abnormal scans was 21. The CT scans were performed either on a GE 8800 scanner (Milwauke, Wise.) or a Searle Photrox 4000 scanner (Des Plaines, Ill.) Routine pelvic scans were obtained using lo-mm thick slices at l-cm slice intervals. All scans were performed after administration of oral meglumine diatrizoate (Gastrografin, Squibb & Sons, Princeton, N.J.), which was given at least 2 to 4 hours before the CT scan. In some female patients, a vaginal tampon was inserted. In addition, some patients received between 140 and 300 mL of 2% meglumine diatrizoate (Gastrografin) as an enema immediately before scanning. Selected pa-
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tients were also given intravenous contrast medium by bolus infusion. Normal anatomy was demonstrated by high-resolution CT scan obtained on a GE 8800 scanner, using 5-mm slices at 3-mm slice intervals. Coronal as well as sagittal reconstructions were performed using algorithms provided by the General Electric Company. Magnetic resonance imaging was performed on a Technicare 0.5-T superconducting magnet (Teslacon, Technicare, Solon, Ohio). Saturation recovery images were obtained using pulse intervals of 0.5 seconds between successive 90” pulses. Spin echo images were obtained using pulse sequences of 30 or 60 msec. In some patients, inversion recovery images were obtained. Interpulse delay time was 450 msec between 180” and 90” pulses. The spin echo images with 30 msec pulse sequences gave predominantly proton density images. These images provide the best spatial resolution. Volume data collection was obtained for multiple axial sections. Direct coronal and sagittal scanning was performed to demonstrate the anatomic relationship of the ischiorectal fossa to surrounding structures. Histologic and surgical diagnoses were obtained in 12 of 16 patients. These 12 patients included those with primary or secondary malignancy and infection. The single diagnosis of fistula was made on CT after meglumine diatrizoate enema. The patient with herniation of the bowel into the ischiorectal fossa was diagnosed by consensus among three CT radiologists. ANATOMY The ischiorectal fossae are bilateral, symmetric, paired structures that lie between the anal canal and the lateral walls of the pelvis. Their apices are situated anteriorly and their bases are directed backward and downward. The two ischiorectal fossae communicate with each other posterior to the anal canal around the external anal sphincter. Consequently, an abscess in one ischiorectal fossa may communicate with the other and involve a semicircular area around the posterior aspect of the anus. The apex of the roof of each wedge-shaped fossa lies superiorly under the pelvic diaphragm where the levator ani muscles arise from the obturator fascia. The apex is situated about 6 cm superior to the ischial tuberosity. The base or floor of the wedgeshaped ischiorectal fossa is formed by the skin and the deep fascia of the perineum. Anteriorly, the ischiorectal fossa continues superior to the urogenital diaphragm as the anterior recess of the ischiorectal
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fossa. This space is filled with loose areolar tissue. There is also a posterior recess lateroposteriorly where the gluteus maximus muscle overhangs the ischiorectal fossa (Figure 1). The ischiorectal fossae of the two sides communicate with each other above the anococcygeal ligament. Anteriorly, each fossa is continuous with the lesser sciatic foramina superior to the sacrotuberous ligament. The ischiorectal fossa is bounded laterally by the ischium and the inferior part of the obturator internus, and medially by the rectum and the anal canal, to which the levator ani and sphincter ani are attached. The overlying gluteus maximus muscle forms the posterior boundary; the anterior extent is defined by the base of the urogential diaphragm. The contents of the ischiorectal fossa are mainly fat, which is traversed by many tough fibrous bands and septi. At body temperature, this fat is liquid, thereby allowing the anus to expand. This pad of fat supports the anal canal but is readily displaced to allow feces to pass through the terminal part of the digestive tract. The internal pudendal vessels and the pudendal nerve run forward in the lateral wall of the fossa in the fibrous pudendal or Alcock’s canal (Figure 2).
COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING ANATOMY On axial CT sections, the ischiorectal fossae are seen as fat-filled, triangular structures situated on either side of the midline. The posterior communication of both ischiorectal fossae is well-defined on CT. The levator ani muscle, which is seen forming a sling around the prostate, vagina, and rectum, separates these structures from the ischiorectal fossae. The gluteus maximus muscle forms an incomplete wall posterior to the ischiorectal fossae. The fat in the ischiorectal fossae communicates with the perineal fat at the medial margin of the ischiorectal fossa (Figure 3). A transverse section on magnetic resonance imaging shows similar anatomy. The fat in the ischiorectal fossae produces intense signal on spin echo images (Figure 6). Coronal plane reconstructions from multiple axial CT sections also show wedge-shaped ischiorectal fossae (Figure 4), with apices pointing upward. The levator ani muscle is seen forming the medial border and the obturator muscle forms the lateral border. The levator ani muscle separates the fat in the ischiorectal fossae from that in the supralevator space. Coronal reconstructions can be useful to determine if pathology seen in the ischiorectal fossae extends into the supralevator space. Sagittal recon-
OCTOBER
1985
1. Schematic diagram of a transverse section through the ischioretal fossa. The levator ani muscle (L) is shown with its relationship to the bladder, vagina, rectum, and ischiorectal fossa. The posterior communication of the ischiorectal fossa behind the levator sling is also shown (arrow). The lateral margin of the ischiorectal fossa is formed by the obturator internus muscle (0).
FIGURE
FIGURE 2. Schematic diagram of a coronal section through the ischiorectal fossa shows the relationship of the ischiorectal fossa to the supralevator space (SLS). The levator ani muscle (L) separates the ischiorectal fossa from the supralevator space. The prostate gland is also separated from ischiorectal fossa by the levator ani muscle. The neurovascular bundle traverses Alcock’s (pudendal) canal in the lateral margin in the ischiorectal fossa near the obturator internus (0) muscle and the ischium.
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FIGURE 3. Supine axial CT scan of the ischiorectal fossa in a normal patient shows the levator ani (arrow), obturator internus (0), and gluteus maximus (G) muscles. The posterior communication of the ischiorectal fossa across the midline is also demonstrated (white arrow).
structions
(Figure
5) show
the
communication
of
inferior to the gluteus maximus muscle. The levator ani muscle is shown separating the bladder from the ischiorectal fossae. Magnetic resonance imaging displays direct coronal and sagittal images to better advantage and acquisition of images by this means has improved spatial resolution over similar sagittal and coronal reconstructions obtained from multiple the ischiorectal
fossae
with
perineal
fat situated
FIGURE 4. Coronal reconstruction from multiple axial CT images in a normal patient shows the rectum (R). The pyramid-shaped ischiorectal fossae are seen with their apices at the junction of the levator ani (white arrow) and the obturator internus (0) muscle. Separation of the ischiorectal fossa and the supralevator spaces (black arrow] by the levator ani muscle is seen.
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FIGURE 5. Sagittal reconstruction in the same normal volunteer as in Figure 5 shows the fat in the ischiorectal fossa communicating with the perineal fat inferior and medial to the gluteus maximus muscle (solid arrow). The fat in the supralevator retroperitoneal space is also shown
(open arrow).
axial CT sections (Figure 7). Although the pudendal nerves and vessels are not seen on the regular CT scans, their location in the lateral wall of the ischiorectal fossae should be borne in mind when performing needle biopsies.
RESULTS In all patients with ischiorectal fossa abnormalities, the detection of pathology was unquestionable on CT scan. Of the three patients with ischiorectal
FIGURE 7. A direct coronal section using spin echo sequence shows the triangular shape of the ischiorectal fossa. The separation of fat in the ischiorectal fossa and supralevator space by the levator ani muscles [arrow) is shown. 0 = obturator internus.
fossa abscesses, two had fever of unknown origin and one had a history of sudden onset of pelvic and perineal pain. The diagnosis of the abscesses was made on CT by the presence of abnormal soft tissue density as well as the presence of air in the ischiocommunicarectal fossa (Figure 8). The posterior tion of the two ischiorectal fossa abscesses and the spread
FIGURE 6. A magnetic resonance image of the ischiorectal fossa in the transverse image using a spin echo sequence (TR = 0.5 seconds, TE = 30 msec). The fat in the ischiorectal fossa shows intense signal due to high proton density. Lower intensity signal is obtained from the muscles obturator internus (0) and gluteus maximus [G).
of
infection
across
the
midline
was
also
FIGURE 6. A transverse CT scan shows abnormal soft tissue density in the ischiorectal fossa with areas of scattered air density within it. A diagnosis of ischiorectal fossa abscess was made. Also note the extension of the abscess across the midline.
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. demonstrated in one case. All three cases were treated and confirmed by surgery. The patients with carcinoma of the cervix and endometrium were worked up for history of abnormal vaginal bleeding. The involvement of the ischiorectal fossa was diagnosed by abnormal soft tissue extension into the normal fat content of the ischiorectal fossa (Figure 9). These patients subsequently underwent surgery and histologic confirmation was obtained in all cases. The carcinoma of the vagina and rectum extending into the ischiorectal fossa were diagnosed during workup for the recurrence of these patients’ known tumors. The patient with carcinoma of the vagina also had a large pelvic mass and underwent radiation treatment. The carcinoma of the rectum was diagnosed by CT-directed thin needle biopsy. The patient with metastasis in the ischiorectal fossa presented with a history of a palpable painless mass in the perineum. There was also a history of significant recent weight loss. The CT scan showed a 2cm soft tissue mass in the ischiorectal fossa fat (Figure 10A). This turned out to be a large, retroperitoneal leiomyosarcoma with metastasis to the ischiorectal fossa at the time of surgery (Figure 10B). The patient with a fistula had a history of carcinoma of the rectum treated previously with radiation and chemotherapy. The CT scan showed an abnormal, well-defined fluid density which, after meglumine diatrizoate enema, filled with contrast medium. Chondrosarcoma (Figure ll), osteomyelitis, and chloroma of the ischium were diagnosed and proved histologically at the time of those pa-
tients’ surgery. However, extension into the ischiorectal fossa was demonstrated only on CT. The patient with granulomatous colitis extending into the ischiorectal fossa was diagnosed during follow-up CT scan examination. The soft tissue abnormality in the ischiorectal was not present on the previous CT scan. Histologic proof was not obtained. Similarly, the patient with bowel herniation were diagnosed by the presence of well-localized air in the ischiorectal fossa by consensus of three CT radiologists. Table 1 lists the diagnoses in patients with ischiorectal fossa abnormalities. DISCUSSION Several series have been published demonstrating the role of CT in the diagnosis of pelvic neoplasms and abscesses (1-8). The information obtained by CT of the pelvis and ischiorectal fossa is, as yet, unmatched by any other modality in diagnostic radiology. Magnetic resonance imaging of the pelvis and ischiorectal fossa demonstrates great potential for diagnosing ischiorectal fossa pathology. Sagittal and coronal reconstruction by CT scanning improve the diagnosis of ischiorectal fossa abnormalities and their relationship to normal anatomic structures surrounding the ischiorectal fossa. Similar anatomic information can also be obtained by direct twodimensional sagittal and coronal images by magnetic resonance imaging. The presence of normal fat in the ischiorectal fossa aids imaging by CT and proton density magnetic resonance imaging. There has been a previous report of a metastatic bronchogenic carcinoma in the ischiorectal fossa
TABLE 1. Ischiorectal
Fossa
Abnormalities
Pathology Neoplastic Cervical carcinoma Rectal carcinoma Vaginal carcinoma Endometrial carcinoma Leiomyosarcoma metastasis Chloroma Chondrosarcoma
FIGURE 9. Transverse CT image of a patient with a history of vaginal bleeding. Vaginal examination revealed a large necrotic mass arising from the cervix. The CT scan shows the abnormal soft tissue density with a necrotic center. The abnormal density extends in the left ischiorectal fossa. At the time of surgery, a large squamous cell carcinoma of the cervix was seen extending through the left levator ani sling into the ischiorectal fossa.
Inflammatory Abscess Granulomatous Osteomyelitis Miscellaneous Hernia Pararectal fistula Total
colitis
No. of cases
3
1 1 1
1 1
1 3 1 1
1 1
16
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10. (A) A rnjiddleaged woman with a pal pable mass in the nerineum and weight loss. i‘he axial CT slice through the ischiorectal fossa shows a soft tissue mass in the right ischiorectal fossa. Biopsy revealed a metastatic leiomyosarcoma. (B) This demonstrates a large, primary leiomyosarcoma in this patient [arrow]. FIGURE
A
(11). Although the exact mechanism of the spread is uncertain, it is probably related to the tendency of malignant cells in metastasize to an area of trauma or subclinical infection (12). A diagnosis of unsuspected metastasis in the ischiorectal fossa may alter the therapeutic management of the primary malignancy. The direct extension of the primary pelvic malignancies into the ischiorectal fossa also has major therapeutic implications. Coronal and sagittal reconstructions help the surgeon to understand extension into, the ischiorectal fossa by
primary pelvic disease and its anatomic relationship to the muscles and bony boundaries of the ischiorectal fossa. It is also important that radiation therapy portals cover the extension of the pelvic malignancies into the ischiorectal fossa. This CT information is extremely valuable and is unobtainable by other conventional radiographic techniques. A case of extension of infection into the ischiorectal fossa from a prosthetic hip joint has been described in the literature (13). Abscesses may also
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FIGURE
11. An axial CT slice shows a large mass with matrix mineralization extending into the obturator internus and externus muscles. The mass extends through the obturator internus muscle into the right ischiorectal fossa (arrow).
reach the ischiorectal fossa following inflammation of anal sinuses, from downward extension of the peripelvirectal abscess, following a tear in the mucous membrane, or from a penetrating wound in the anal region, The natural history of ischiorectal fossa abscesses is that they open spontaneously into the anal canal, the rectum, the skin of the perineum near the anus or anterior abdominal wall, or into all fossae of these areas (15). Because both ischiorectal communicate with each other posterior to the anal canal and around the external sphincter, an abscess in one ischiorectal fossa may spread across the midline and involve a semicircular area around the posterior aspect of the anus. According to the prospective survey done by Read and Abcarian (la), 31% of ischiorectal fossa abscesses rupture spontaneously, 9% are bilateral, and 7% have supralevator extension. Coronal and sagittal reconstructions will detect the supralevator extension preoperatively. When pus is formed in the ischiorectal fossa, it may extend forward, producing edema and spasm of the urethral sphincter. In these cases, urinary difficulties and retention may be the presenting symptoms (16). Rarely, an ischiorectal fossa abscess may actually point above the pubis (15). Because of the marked distensibility of the ischiorectal fossa as well as the paucity of nerves in the fat-filled space, ischiorectal fossa abscesses are not especially painful lesions. The correct CT diagnosis of extension of known granulomatous disease or of herniation of bowel into the ischiorectal fossa may prevent unnecessary surgery in these patients, who can be treated conservatively and their progress monitored on repeat CT scans.
Table 1 demonstrates that, on CT, more patients are seen with extension of primary or secondary malignancies into the ischiorectal fossa than infections. This is because patients with neoplasms are usually worked up and followed up by CT scans, whereas patients with ischiorectai abscesses are frequently diagnosed clinically and treated surgically without further radiologic workup. Use of CT scanning of the pelvis and ischiorectal fossa is an invaluable tool in the diagnosis and further management of ischiorectal fossa pathology. It is unmatched by conventional radiologic procedures. Magnetic resonance imaging also demonstrates excellent ischiorectal fossa evaluation and aids in the diagnosis of ischiorectal fossa diseases. The authors express their appreciation to Bonnie Norman for the artwork and to Mary Henry for preparing and editing the manuscript.
REFERENCES 1. Wolverson MK, Jagannadharao B, Sundaram a primary diagnostic method in evaluating abscess. Am J Roentgen01 1979;133:1089-95,
M, et al.: CT as intraabdominal
2. Jaques PF, Staab E, Richey W, et al.: CT assisted abdominal aspiration biopsies in gynecological cies. Radiology 1978;128:651-5.
pelvic and malignan-
3. Hagga JR, Alfidi RJ, Havilla TR, et al.: CT detection and aspiration of abdominal abscesses. Am J Roentgen01 1977;15:441-8. 4. Redman HC: Computed tomography Clin North Am 1977;15:441-8.
of the pelvis.
Radio1
5. Tisnado J, Amendola MA, Walsh JW, et al.: Computed tomography of the perineum. Am J Roentgen01 1981;136:47581.
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6. Brizel HE, Livingston PA, Grayson EV: Radiotherapeutic application of pelvic computed tomography. J Comput Assist Tomogr 1979;3:453-66. 7. Callen PW: Computed tomographic evaluation and pelvic abscess. Radiology 1979;131:171-5.
of abdominal
8. Levitt RG, Sage1 SS, Stanley RJ, Evans RG: Semin Roentgen01 1978;8:193-200. MP, Moss AA, Boyd DD, Royal SA: Coronal and sagittal reconstruction using a 4.8 set CT body scanner. Am J
9. Federle
Roentgen01
1979;135:625-32.
10. Pevsner PH, Kreel L, King DL, Wilson P: Multiple axis image reconstructions from axial transverse data. J Comput Assist Tomogr 1979;3:279-81. 11. Levack B, McCollum CN: Broncheal carcinoma to the ischiorectal fossa. Br Med J 1977;1137-8.
metastasizing
11. Levack B, McCollum CN: Broncheal carcinoma metastasizing to the ischiorectal fossa. Br Med J 1977;1137-8. 12. Fisher B, Fisher ER, Feduska N: Trauma tumor cells. Cancer 1967;20:23-9.
and localization
of
13. Briggs RD, McLauchlan J, Davidson AI: Late infection of a total hip prosthesis presenting as an ischiorectal abscess. Br J Surg 1979;66:291-2. 14. Read DR, Abcarian H: A prospective survey of 474 patients with anorectal abscess. J Dis Colon Rectum 1979;22:566-9. 15. Sherlock D, Ward A: Complications Br J Clin Prac 1981;203-4.
of ischiorectal
abscess.
16. Cassell P: Urinary retention associated with infection in the ischiorectal and supralevator spaces. Br J Surg 1969;56:91820.
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CONTINUING MEDICAL EDUCATION QUESTIONS
Which of the following is not a true statement? a. The ischiorectal fossae communicate with each other posteriorly. b. The ischiorectal fossae are transversed by pudendal vessels and nerves. c. Ischiorectal fossa abscesses are especially painful due to rich nerve supply. d. Ischiorectal fossa abscesses commonly point at the anterior abdominal wall. On CT, the most common chiorectal fossa is: a. Infection. b. Neoplastic. c. Hernia. d. Fistula.
pathology
involving
Which of the following primary malignancies commonly showed extension to the ischiorectal on CT examination? a. Rectum. b. Prostate. c. Leiomyosarcoma. d. Uterine cervix.
the is-
most fossa
Ischiorectal fossa abscesses most commonly: a. Extend to the contralateral ischiorectal fossa. into the rectum and anal b. Drain spontaneously canal. C. Form fistulous communications with the bladder. and present with septid. Spread hematogenously cemia.
OF COMPUTED
TOMOGRAPHY
1985;
9:329-336
IMAGING OF THE ISCHIORECTAL FOSSA MADAN V. KULKARNI, MD, SNEHAL D. MEHTA, MD, DELAND D, BURKS, MD, AND MAX I. SHAFF, MD
A retrospective evaluation of 500 routine abdominal computed tomography scans revealed 21 cases in 16 patients. Pathoof ischiorectal fossa disease logic processes included involvement by primary and secondary neoplasms, usually of gynecologic origin; rectal prolapse; and abscess. These abnormalities can be missed if the ischiorectal fossae are not included in routine abdominal scans. The anatomy of the ischiorectal fossae and their relationship to other pelvic organs ore well demonstrated with high-resolution computed tomography scans as well as by magnetic resonance imaging. KEY WORDS:
Computed
tomography;
Pelvis, anatomy;
Fat
Although numerous articles have defined the role of computed tomography (CT) in detecting abnormalities in the pelvis (l--8), few case reports of abnormality in the ischiorectal fossa have been presented. The role of coronal and sagittal reconstruction of multiple axial CT images has also been reported (9, 10) and the importance of scanning the perineum in the evaluation of perineal pathology has been frequently emphasized. The CT evaluation of perineal and pelvic anatomy has also been mentioned in the literature (5). This paper provides additional anatomic details pertinent to CT scanning and mag-
From the Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, Tennessee. Address reprint rkquests to: Max I. Shaff, MD, Department of Radioloav and Radiological Sciences, Vanderbilt Universitv Medical-Center, Nashville, Tennessee 37232. Received December 1984. This paper was presented at the 69th Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, Illinois, November 13-18, 1983. 0 1985 by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Ave., New York, NY 10017 0149-936x/85/$3.00
netic resonance imaging of the ischiorectal fossa. A retrospective analysis of the different pathologic processes seen in the ischiorectal fossa is made. The fat content of the ischiorectal fossa povides inherent contrast in CT images. Similarly, magnetic resonance imaging also shows excellent signal acquisition in proton density images due to the predominantly fat content of the ischiorectal fossa. By contrast, and for the same reason, ultrasound examination of this area has limited value. Direct sagittal or coronal images can be obtained by magnetic resonance imaging scanning. High-resolution CT scans of the ischiorectal fossa and subsequent sagittal and coronal reconstruction can also be obtained. These can be used to evaluate pathology in the ischiorectal fossa and its relationship to normal anatomic landmarks such as the levator ani muscle and pelvic organs. This paper presents ischiorectal The normal anatomy fossa pathology in 16 patients. of the ischiorectal fossa as demonstrated by CT and magnetic resonance imaging is also described.
MATERIALS AND METHODS A retrospective analysis of 500 abdominal CT scans was made and 16 patients with ischiorectal fossa abnormalities were identified. As some of the patients had follow-up scans after therapy, the total number of abnormal scans was 21. The CT scans were performed either on a GE 8800 scanner (Milwauke, Wise.) or a Searle Photrox 4000 scanner (Des Plaines, Ill.) Routine pelvic scans were obtained using lo-mm thick slices at l-cm slice intervals. All scans were performed after administration of oral meglumine diatrizoate (Gastrografin, Squibb & Sons, Princeton, N.J.), which was given at least 2 to 4 hours before the CT scan. In some female patients, a vaginal tampon was inserted. In addition, some patients received between 140 and 300 mL of 2% meglumine diatrizoate (Gastrografin) as an enema immediately before scanning. Selected pa-
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tients were also given intravenous contrast medium by bolus infusion. Normal anatomy was demonstrated by high-resolution CT scan obtained on a GE 8800 scanner, using 5-mm slices at 3-mm slice intervals. Coronal as well as sagittal reconstructions were performed using algorithms provided by the General Electric Company. Magnetic resonance imaging was performed on a Technicare 0.5-T superconducting magnet (Teslacon, Technicare, Solon, Ohio). Saturation recovery images were obtained using pulse intervals of 0.5 seconds between successive 90” pulses. Spin echo images were obtained using pulse sequences of 30 or 60 msec. In some patients, inversion recovery images were obtained. Interpulse delay time was 450 msec between 180” and 90” pulses. The spin echo images with 30 msec pulse sequences gave predominantly proton density images. These images provide the best spatial resolution. Volume data collection was obtained for multiple axial sections. Direct coronal and sagittal scanning was performed to demonstrate the anatomic relationship of the ischiorectal fossa to surrounding structures. Histologic and surgical diagnoses were obtained in 12 of 16 patients. These 12 patients included those with primary or secondary malignancy and infection. The single diagnosis of fistula was made on CT after meglumine diatrizoate enema. The patient with herniation of the bowel into the ischiorectal fossa was diagnosed by consensus among three CT radiologists. ANATOMY The ischiorectal fossae are bilateral, symmetric, paired structures that lie between the anal canal and the lateral walls of the pelvis. Their apices are situated anteriorly and their bases are directed backward and downward. The two ischiorectal fossae communicate with each other posterior to the anal canal around the external anal sphincter. Consequently, an abscess in one ischiorectal fossa may communicate with the other and involve a semicircular area around the posterior aspect of the anus. The apex of the roof of each wedge-shaped fossa lies superiorly under the pelvic diaphragm where the levator ani muscles arise from the obturator fascia. The apex is situated about 6 cm superior to the ischial tuberosity. The base or floor of the wedgeshaped ischiorectal fossa is formed by the skin and the deep fascia of the perineum. Anteriorly, the ischiorectal fossa continues superior to the urogenital diaphragm as the anterior recess of the ischiorectal
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fossa. This space is filled with loose areolar tissue. There is also a posterior recess lateroposteriorly where the gluteus maximus muscle overhangs the ischiorectal fossa (Figure 1). The ischiorectal fossae of the two sides communicate with each other above the anococcygeal ligament. Anteriorly, each fossa is continuous with the lesser sciatic foramina superior to the sacrotuberous ligament. The ischiorectal fossa is bounded laterally by the ischium and the inferior part of the obturator internus, and medially by the rectum and the anal canal, to which the levator ani and sphincter ani are attached. The overlying gluteus maximus muscle forms the posterior boundary; the anterior extent is defined by the base of the urogential diaphragm. The contents of the ischiorectal fossa are mainly fat, which is traversed by many tough fibrous bands and septi. At body temperature, this fat is liquid, thereby allowing the anus to expand. This pad of fat supports the anal canal but is readily displaced to allow feces to pass through the terminal part of the digestive tract. The internal pudendal vessels and the pudendal nerve run forward in the lateral wall of the fossa in the fibrous pudendal or Alcock’s canal (Figure 2).
COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING ANATOMY On axial CT sections, the ischiorectal fossae are seen as fat-filled, triangular structures situated on either side of the midline. The posterior communication of both ischiorectal fossae is well-defined on CT. The levator ani muscle, which is seen forming a sling around the prostate, vagina, and rectum, separates these structures from the ischiorectal fossae. The gluteus maximus muscle forms an incomplete wall posterior to the ischiorectal fossae. The fat in the ischiorectal fossae communicates with the perineal fat at the medial margin of the ischiorectal fossa (Figure 3). A transverse section on magnetic resonance imaging shows similar anatomy. The fat in the ischiorectal fossae produces intense signal on spin echo images (Figure 6). Coronal plane reconstructions from multiple axial CT sections also show wedge-shaped ischiorectal fossae (Figure 4), with apices pointing upward. The levator ani muscle is seen forming the medial border and the obturator muscle forms the lateral border. The levator ani muscle separates the fat in the ischiorectal fossae from that in the supralevator space. Coronal reconstructions can be useful to determine if pathology seen in the ischiorectal fossae extends into the supralevator space. Sagittal recon-
OCTOBER
1985
1. Schematic diagram of a transverse section through the ischioretal fossa. The levator ani muscle (L) is shown with its relationship to the bladder, vagina, rectum, and ischiorectal fossa. The posterior communication of the ischiorectal fossa behind the levator sling is also shown (arrow). The lateral margin of the ischiorectal fossa is formed by the obturator internus muscle (0).
FIGURE
FIGURE 2. Schematic diagram of a coronal section through the ischiorectal fossa shows the relationship of the ischiorectal fossa to the supralevator space (SLS). The levator ani muscle (L) separates the ischiorectal fossa from the supralevator space. The prostate gland is also separated from ischiorectal fossa by the levator ani muscle. The neurovascular bundle traverses Alcock’s (pudendal) canal in the lateral margin in the ischiorectal fossa near the obturator internus (0) muscle and the ischium.
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FIGURE 3. Supine axial CT scan of the ischiorectal fossa in a normal patient shows the levator ani (arrow), obturator internus (0), and gluteus maximus (G) muscles. The posterior communication of the ischiorectal fossa across the midline is also demonstrated (white arrow).
structions
(Figure
5) show
the
communication
of
inferior to the gluteus maximus muscle. The levator ani muscle is shown separating the bladder from the ischiorectal fossae. Magnetic resonance imaging displays direct coronal and sagittal images to better advantage and acquisition of images by this means has improved spatial resolution over similar sagittal and coronal reconstructions obtained from multiple the ischiorectal
fossae
with
perineal
fat situated
FIGURE 4. Coronal reconstruction from multiple axial CT images in a normal patient shows the rectum (R). The pyramid-shaped ischiorectal fossae are seen with their apices at the junction of the levator ani (white arrow) and the obturator internus (0) muscle. Separation of the ischiorectal fossa and the supralevator spaces (black arrow] by the levator ani muscle is seen.
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FIGURE 5. Sagittal reconstruction in the same normal volunteer as in Figure 5 shows the fat in the ischiorectal fossa communicating with the perineal fat inferior and medial to the gluteus maximus muscle (solid arrow). The fat in the supralevator retroperitoneal space is also shown
(open arrow).
axial CT sections (Figure 7). Although the pudendal nerves and vessels are not seen on the regular CT scans, their location in the lateral wall of the ischiorectal fossae should be borne in mind when performing needle biopsies.
RESULTS In all patients with ischiorectal fossa abnormalities, the detection of pathology was unquestionable on CT scan. Of the three patients with ischiorectal
FIGURE 7. A direct coronal section using spin echo sequence shows the triangular shape of the ischiorectal fossa. The separation of fat in the ischiorectal fossa and supralevator space by the levator ani muscles [arrow) is shown. 0 = obturator internus.
fossa abscesses, two had fever of unknown origin and one had a history of sudden onset of pelvic and perineal pain. The diagnosis of the abscesses was made on CT by the presence of abnormal soft tissue density as well as the presence of air in the ischiocommunicarectal fossa (Figure 8). The posterior tion of the two ischiorectal fossa abscesses and the spread
FIGURE 6. A magnetic resonance image of the ischiorectal fossa in the transverse image using a spin echo sequence (TR = 0.5 seconds, TE = 30 msec). The fat in the ischiorectal fossa shows intense signal due to high proton density. Lower intensity signal is obtained from the muscles obturator internus (0) and gluteus maximus [G).
of
infection
across
the
midline
was
also
FIGURE 6. A transverse CT scan shows abnormal soft tissue density in the ischiorectal fossa with areas of scattered air density within it. A diagnosis of ischiorectal fossa abscess was made. Also note the extension of the abscess across the midline.
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. demonstrated in one case. All three cases were treated and confirmed by surgery. The patients with carcinoma of the cervix and endometrium were worked up for history of abnormal vaginal bleeding. The involvement of the ischiorectal fossa was diagnosed by abnormal soft tissue extension into the normal fat content of the ischiorectal fossa (Figure 9). These patients subsequently underwent surgery and histologic confirmation was obtained in all cases. The carcinoma of the vagina and rectum extending into the ischiorectal fossa were diagnosed during workup for the recurrence of these patients’ known tumors. The patient with carcinoma of the vagina also had a large pelvic mass and underwent radiation treatment. The carcinoma of the rectum was diagnosed by CT-directed thin needle biopsy. The patient with metastasis in the ischiorectal fossa presented with a history of a palpable painless mass in the perineum. There was also a history of significant recent weight loss. The CT scan showed a 2cm soft tissue mass in the ischiorectal fossa fat (Figure 10A). This turned out to be a large, retroperitoneal leiomyosarcoma with metastasis to the ischiorectal fossa at the time of surgery (Figure 10B). The patient with a fistula had a history of carcinoma of the rectum treated previously with radiation and chemotherapy. The CT scan showed an abnormal, well-defined fluid density which, after meglumine diatrizoate enema, filled with contrast medium. Chondrosarcoma (Figure ll), osteomyelitis, and chloroma of the ischium were diagnosed and proved histologically at the time of those pa-
tients’ surgery. However, extension into the ischiorectal fossa was demonstrated only on CT. The patient with granulomatous colitis extending into the ischiorectal fossa was diagnosed during follow-up CT scan examination. The soft tissue abnormality in the ischiorectal was not present on the previous CT scan. Histologic proof was not obtained. Similarly, the patient with bowel herniation were diagnosed by the presence of well-localized air in the ischiorectal fossa by consensus of three CT radiologists. Table 1 lists the diagnoses in patients with ischiorectal fossa abnormalities. DISCUSSION Several series have been published demonstrating the role of CT in the diagnosis of pelvic neoplasms and abscesses (1-8). The information obtained by CT of the pelvis and ischiorectal fossa is, as yet, unmatched by any other modality in diagnostic radiology. Magnetic resonance imaging of the pelvis and ischiorectal fossa demonstrates great potential for diagnosing ischiorectal fossa pathology. Sagittal and coronal reconstruction by CT scanning improve the diagnosis of ischiorectal fossa abnormalities and their relationship to normal anatomic structures surrounding the ischiorectal fossa. Similar anatomic information can also be obtained by direct twodimensional sagittal and coronal images by magnetic resonance imaging. The presence of normal fat in the ischiorectal fossa aids imaging by CT and proton density magnetic resonance imaging. There has been a previous report of a metastatic bronchogenic carcinoma in the ischiorectal fossa
TABLE 1. Ischiorectal
Fossa
Abnormalities
Pathology Neoplastic Cervical carcinoma Rectal carcinoma Vaginal carcinoma Endometrial carcinoma Leiomyosarcoma metastasis Chloroma Chondrosarcoma
FIGURE 9. Transverse CT image of a patient with a history of vaginal bleeding. Vaginal examination revealed a large necrotic mass arising from the cervix. The CT scan shows the abnormal soft tissue density with a necrotic center. The abnormal density extends in the left ischiorectal fossa. At the time of surgery, a large squamous cell carcinoma of the cervix was seen extending through the left levator ani sling into the ischiorectal fossa.
Inflammatory Abscess Granulomatous Osteomyelitis Miscellaneous Hernia Pararectal fistula Total
colitis
No. of cases
3
1 1 1
1 1
1 3 1 1
1 1
16
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10. (A) A rnjiddleaged woman with a pal pable mass in the nerineum and weight loss. i‘he axial CT slice through the ischiorectal fossa shows a soft tissue mass in the right ischiorectal fossa. Biopsy revealed a metastatic leiomyosarcoma. (B) This demonstrates a large, primary leiomyosarcoma in this patient [arrow]. FIGURE
A
(11). Although the exact mechanism of the spread is uncertain, it is probably related to the tendency of malignant cells in metastasize to an area of trauma or subclinical infection (12). A diagnosis of unsuspected metastasis in the ischiorectal fossa may alter the therapeutic management of the primary malignancy. The direct extension of the primary pelvic malignancies into the ischiorectal fossa also has major therapeutic implications. Coronal and sagittal reconstructions help the surgeon to understand extension into, the ischiorectal fossa by
primary pelvic disease and its anatomic relationship to the muscles and bony boundaries of the ischiorectal fossa. It is also important that radiation therapy portals cover the extension of the pelvic malignancies into the ischiorectal fossa. This CT information is extremely valuable and is unobtainable by other conventional radiographic techniques. A case of extension of infection into the ischiorectal fossa from a prosthetic hip joint has been described in the literature (13). Abscesses may also
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FIGURE
11. An axial CT slice shows a large mass with matrix mineralization extending into the obturator internus and externus muscles. The mass extends through the obturator internus muscle into the right ischiorectal fossa (arrow).
reach the ischiorectal fossa following inflammation of anal sinuses, from downward extension of the peripelvirectal abscess, following a tear in the mucous membrane, or from a penetrating wound in the anal region, The natural history of ischiorectal fossa abscesses is that they open spontaneously into the anal canal, the rectum, the skin of the perineum near the anus or anterior abdominal wall, or into all fossae of these areas (15). Because both ischiorectal communicate with each other posterior to the anal canal and around the external sphincter, an abscess in one ischiorectal fossa may spread across the midline and involve a semicircular area around the posterior aspect of the anus. According to the prospective survey done by Read and Abcarian (la), 31% of ischiorectal fossa abscesses rupture spontaneously, 9% are bilateral, and 7% have supralevator extension. Coronal and sagittal reconstructions will detect the supralevator extension preoperatively. When pus is formed in the ischiorectal fossa, it may extend forward, producing edema and spasm of the urethral sphincter. In these cases, urinary difficulties and retention may be the presenting symptoms (16). Rarely, an ischiorectal fossa abscess may actually point above the pubis (15). Because of the marked distensibility of the ischiorectal fossa as well as the paucity of nerves in the fat-filled space, ischiorectal fossa abscesses are not especially painful lesions. The correct CT diagnosis of extension of known granulomatous disease or of herniation of bowel into the ischiorectal fossa may prevent unnecessary surgery in these patients, who can be treated conservatively and their progress monitored on repeat CT scans.
Table 1 demonstrates that, on CT, more patients are seen with extension of primary or secondary malignancies into the ischiorectal fossa than infections. This is because patients with neoplasms are usually worked up and followed up by CT scans, whereas patients with ischiorectai abscesses are frequently diagnosed clinically and treated surgically without further radiologic workup. Use of CT scanning of the pelvis and ischiorectal fossa is an invaluable tool in the diagnosis and further management of ischiorectal fossa pathology. It is unmatched by conventional radiologic procedures. Magnetic resonance imaging also demonstrates excellent ischiorectal fossa evaluation and aids in the diagnosis of ischiorectal fossa diseases. The authors express their appreciation to Bonnie Norman for the artwork and to Mary Henry for preparing and editing the manuscript.
REFERENCES 1. Wolverson MK, Jagannadharao B, Sundaram a primary diagnostic method in evaluating abscess. Am J Roentgen01 1979;133:1089-95,
M, et al.: CT as intraabdominal
2. Jaques PF, Staab E, Richey W, et al.: CT assisted abdominal aspiration biopsies in gynecological cies. Radiology 1978;128:651-5.
pelvic and malignan-
3. Hagga JR, Alfidi RJ, Havilla TR, et al.: CT detection and aspiration of abdominal abscesses. Am J Roentgen01 1977;15:441-8. 4. Redman HC: Computed tomography Clin North Am 1977;15:441-8.
of the pelvis.
Radio1
5. Tisnado J, Amendola MA, Walsh JW, et al.: Computed tomography of the perineum. Am J Roentgen01 1981;136:47581.
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6. Brizel HE, Livingston PA, Grayson EV: Radiotherapeutic application of pelvic computed tomography. J Comput Assist Tomogr 1979;3:453-66. 7. Callen PW: Computed tomographic evaluation and pelvic abscess. Radiology 1979;131:171-5.
of abdominal
8. Levitt RG, Sage1 SS, Stanley RJ, Evans RG: Semin Roentgen01 1978;8:193-200. MP, Moss AA, Boyd DD, Royal SA: Coronal and sagittal reconstruction using a 4.8 set CT body scanner. Am J
9. Federle
Roentgen01
1979;135:625-32.
10. Pevsner PH, Kreel L, King DL, Wilson P: Multiple axis image reconstructions from axial transverse data. J Comput Assist Tomogr 1979;3:279-81. 11. Levack B, McCollum CN: Broncheal carcinoma to the ischiorectal fossa. Br Med J 1977;1137-8.
metastasizing
11. Levack B, McCollum CN: Broncheal carcinoma metastasizing to the ischiorectal fossa. Br Med J 1977;1137-8. 12. Fisher B, Fisher ER, Feduska N: Trauma tumor cells. Cancer 1967;20:23-9.
and localization
of
13. Briggs RD, McLauchlan J, Davidson AI: Late infection of a total hip prosthesis presenting as an ischiorectal abscess. Br J Surg 1979;66:291-2. 14. Read DR, Abcarian H: A prospective survey of 474 patients with anorectal abscess. J Dis Colon Rectum 1979;22:566-9. 15. Sherlock D, Ward A: Complications Br J Clin Prac 1981;203-4.
of ischiorectal
abscess.
16. Cassell P: Urinary retention associated with infection in the ischiorectal and supralevator spaces. Br J Surg 1969;56:91820.
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CONTINUING MEDICAL EDUCATION QUESTIONS
Which of the following is not a true statement? a. The ischiorectal fossae communicate with each other posteriorly. b. The ischiorectal fossae are transversed by pudendal vessels and nerves. c. Ischiorectal fossa abscesses are especially painful due to rich nerve supply. d. Ischiorectal fossa abscesses commonly point at the anterior abdominal wall. On CT, the most common chiorectal fossa is: a. Infection. b. Neoplastic. c. Hernia. d. Fistula.
pathology
involving
Which of the following primary malignancies commonly showed extension to the ischiorectal on CT examination? a. Rectum. b. Prostate. c. Leiomyosarcoma. d. Uterine cervix.
the is-
most fossa
Ischiorectal fossa abscesses most commonly: a. Extend to the contralateral ischiorectal fossa. into the rectum and anal b. Drain spontaneously canal. C. Form fistulous communications with the bladder. and present with septid. Spread hematogenously cemia.