Sonography reveals causes of groin pain

Groin pain, whether acute or chronic, is a common clinical presentation that can be caused by a diverse array of disorders involving different anatomic structures. This makes definitive diagnosis difficult for even the most experienced clinician. Imaging can be invaluable in localizing and characterizing otherwise uncertain groin pathology.

Although both CT and MRI may be extremely helpful in particular circumstances, ultrasound remains the initial imaging modality of choice. It provides inexpensive, noninvasive, radiation-free multiplanar imaging with excellent soft-tissue contrast and high spatial resolution. Real-time interaction with the patient allows practitioners to localize pain and to perform dynamic maneuvers.

A sound knowledge of groin anatomy, particularly the inguinal canal and femoral triangle, is a prerequisite for ultrasound assessment of this region. The inguinal ligament, the inferior free edge of the external oblique aponeurosis running from anterior superior iliac spine (ASIS) to pubic tubercle, forms the floor of the inguinal canal and the superior margin of the femoral triangle.

The inguinal canal is approximately 4 cm long in adults, and it runs from the deep ring (a defect in the transversalis fascia, roughly midway between the ASIS and symphysis pubis) inferomedially to the superficial ring (a defect in the external oblique aponeurosis) just superomedial to the pubic tubercle. In men, it allows passage of the spermatic cord and associated structures from the abdomen to the scrotum. In women, it transmits the round ligament to the labium majus.

The inguinal canal also transmits the processus vaginalis in newborns. This is a peritoneal diverticulum that usually becomes obliterated in its entirety in females and in its inguinal portion in males. The scrotal section of the processus vaginalis in males forms the tunica vaginalis.

The femoral triangle is bounded superiorly by the inguinal ligament. Its lateral and medial walls are formed by the sartorius and adductor longus muscles, respectively. Its contents, from lateral to medial, include the femoral nerve, femoral vessels (artery lateral to vein), and femoral canal. The femoral canal contains lymphatics and communicates superiorly with abdominal extraperitoneal space via the femoral ring.

HERNIAS AND RELATED DISORDERS

Inguinal, femoral, and obturator hernias all cause groin pain and can be difficult to diagnose clinically. Ultrasound can identify these types of hernia with a high degree of sensitivity and specificity,1 though some investigators have found a lower sensitivity in the diagnosis of clinically occult hernias.2 Dynamic maneuvers to raise intra-abdominal pressure, such as straining, can be used to improve detection rates. Hernial sac contents can be identified as hyperechoic omental fat, anechoic fluid, or bowel. This latter finding is characterized by a multilaminar signature, either with or without reverberation due to luminal bowel gas.

Differentiation of hernias relies on identification of important anatomic landmarks. Inguinal hernias can be divided into direct and indirect varieties. Indirect inguinal hernias arise at the deep ring and follow the course of the inguinal canal inferomedially. Direct hernias bulge through the posterior wall of the inguinal canal at the superficial ring.

Color and power Doppler ultrasound can distinguish between direct and indirect inguinal hernias by identifying inferior epigastric vessels running superiorly from the external iliac vessels.1,3 Direct inguinal hernias lie medial to the inferior epigastric vessels, while indirect hernias lie laterally (Figures 1 and 2).

Femoral hernias are much less common. These emerge in the femoral canal below the inguinal ligament medial to the femoral vein. Obturator hernias are rarer still, and these emerge between pectineus and obturator externus muscles.

The so-called sportsman's hernia (Gilmore's groin) is not a true inguinal hernia. This related disorder, largely seen in young male athletes, consists of a focally tender bulge of the inguinal canal's posterior wall at the superficial ring upon straining. Eliciting focal tenderness is essential to diagnosis, making ultrasound the ideal imaging modality.4 The validity of this clinical entity is controversial, though many patients obtain relief of chronic groin pain with surgical repair.5

Inguinal hernias in the pediatric population are related to the failure of the processus vaginalis to close normally during the first year or so of life. Bowel may enter the cavity if the processus vaginalis remains widely patent, forming an indirect inguinal hernia that requires surgery. Where the processus vaginalis remains narrowly patent, peritoneal fluid may enter, forming a hydrocele. Hydroceles usually resolve spontaneously, but if they continue to enlarge, surgery may be necessary. Fluid may also become encysted within the processus vaginalis, resulting in a noncommunicating hydrocele of the canal of Nuck. This can continue to cause groin pain through childhood and even into adulthood (Figure 3).6

NEOPLASMS

Primary benign or malignant tumors may arise from any soft tissue in the groin, including skin, fat, nerves, muscles, cartilage, vessels, and lymphoid tissue. Lipoma is the most common benign tumor of the inguinal canal.7 Lipoma lesions are often mistaken clinically for indirect hernias and are easily overlooked at laparoscopic hernia repair.8 Ultrasound shows these masses as discrete encapsulated masses with echogenicity similar to fat (Figure 4). Comparison with the contralateral groin can be helpful in doubtful cases. Dynamic maneuvers are important to exclude hernia.

Secondary involvement of malignancy in the groin can occur as contiguous spread, but it more commonly manifests as metastases to bone or lymph nodes. Inguinal lymph node metastases occur most frequently from tumors of the perineum (vulva, penis, lower vagina, lower rectum, and anus) and lower extremities. Inguinal nodes are often involved in patients with non-Hodgkin's lymphoma.

Ultrasound performs better than CT and MRI in distinguishing malignant lymphadenopathy in superficial lymph nodes from nodes enlarged by inflammatory disorders. Discriminatory features include a round rather than oval shape (short axis:long axis ratio > 0.5), absence of normal echogenic hilus, and peripheral or mixed vascularity on power Doppler rather than the normal hilar vascularity or apparent avascularity.9 Nodal size is not a reliable discriminator. Lymphomatous nodes were previously reported to have a pseudocystic appearance (hypoechoic with posterior acoustic enhancement), but this finding is much less likely with modern high-frequency transducers.10

Ultrasound is the ideal modality for image-guided biopsy of nodes or other masses.

MUSCULOSKELETAL

Groin pain can be generated by articular or periarticular pathology at the hip joint or symphysis pubis and by traumatic or inflammatory pathology of the various muscles and tendons around the groin. Ultrasound can detect even very small hip joint effusions as well as loose bodies. It can also direct aspiration of joint effusions in suspected septic arthritis.

Several bursae are present around the hip and groin, and these can become inflamed from frictional trauma or other causes. The trochanteric and iliopsoas bursae are most commonly involved in inflammation. Ultrasound of acute bursitis usually shows the bursa to be relatively thin-walled and containing anechoic fluid. The bursal wall usually appears thicker in chronic bursitis, while bursal fluid often contains low-level internal echoes, due to debris and calcifications (Figure 5).11

Avulsion injuries and partial or complete tears of any muscles and tendons around the groin can present as acute or chronic groin pain. Relevant structures include the rectus abdominis, rectus femoris, adductors, hamstrings, gluteals, and tensor fascia lata. Dynamic ultrasound demonstrates such tears well as discontinuities to muscle or tendon fibers and focal hypoechoic areas within tendons.12 Care must be taken to keep the probe at a right angle to the tendon being examined to avoid artifacts. Chronic tear-free tendinopathies are seen as thickening, with reduced echogenicity of the tendon compared with its contralateral counterpart.13

VASCULAR

The combination of gray-scale ultrasound and color flow Doppler readily allows diagnosis of several venous disorders that can present with groin discomfort. These include superficial thrombophlebitis, deep venous thrombosis, saphena varix, and varicoceles of the spermatic cord.

True and false femoral artery aneurysms can be diagnosed with confidence. False aneurysms have become more common, owing to the increasing number of vascular interventional procedures being performed. Ultrasound typically shows turbulent flow within the false aneurysm and the neck through which it communicates with the arterial lumen, and it can guide therapeutic compression or thrombin injection.14

OTHER CAUSATIVE FACTORS

Hematomas are usually secondary to trauma or percutaneous intervention. They may also occur spontaneously in patients with coagulopathies. The appearance of hematomas on ultrasound varies with their age. Acute hematomas appear as mainly hypoechoic fluid collections, becoming mixed echogenicity masses with time. Fluid-fluid levels may be seen as they undergo liquefaction.

Abscesses (Figure 6) also have a variable appearance on ultrasound, and they can be indistinguishable from hematomas. Abscesses are usually predominantly hypoechoic with multiple internal echoes, but if the pus is very viscous, they can appear echogenic and solid. Apparent observation of a reverberation artifact, due to gas, in a suspected abscess must be distinguished from that of gas-filled bowel within a hernia. Ultrasound can also be used to guide diagnostic or therapeutic aspiration or drainage of a suspected abscess.

Ultrasound can show a miscellany of other uncommon disorders that cause groin pain, including granulomata of the epididymis and endometriosis of the inguinal canal.15 The appearance of these conditions on ultrasound is variable, however, and usually nonspecific. Foreign bodies can be detected as highly echogenic structures, a finding that may be likely in intravenous drug users (Figure 7) or metalworkers.

Intra-abdominal pathology such as appendicitis (Figure 8), diverticulitis, and Crohn's disease may occasionally present as groin pain. The lower abdomen and pelvis should be examined if clinical factors are suggestive of such a diagnosis.

Ultrasound can be extremely helpful in patients who develop acute or chronic groin pain following surgery in this region. Imaging can identify collections, such as hematomas, abscesses, seromas, and lymphoceles, in the acute and subacute postoperative period. Ultrasound examination of patients with chronic pain following hernia repair can identify recurrent hernia formation. It may also detect the small mixed echogenicity stitch granuloma related to a surgical suture or the exquisitely tender post-traumatic neuroma that is seen as a spindle-shaped hypoechoic mass, usually in continuity with a hyperechoic nervous structure.11

Groin pain may be caused by a wide variety of factors, including hernias and related conditions and musculoskeletal, neoplastic, vascular, inflammatory, and infectious disorders. Ultrasound can detect most of these pathologies with a high level of accuracy. It is the initial imaging modality of choice, providing radiation-free multiplanar imaging and unrivaled dynamic interaction with the patient. Ultrasound can also be the optimal modality for guiding diagnostic aspiration or biopsy and therapeutic injection or drainage.

Radiologists assessing patients with groin pain require a working knowledge of inguinal anatomy, and an appreciation of the wide spectrum of causative disorders.

DR. HAGAN is a consultant radiologist at Cheltenham General Hospital, U.K. DR. BURNEY is a fellow in interventional radiology at Southampton General Hospital, U.K. DR. WILLIAMS and DR. BRADLEY are consultant radiologists at Southmead Hospital in Bristol, U.K.

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