Acute scrotal ultrasound: a practical guide

Testicular trauma is a common cause of acute scrotal pain. Blunt trauma is the most common mechanism of injury, and is often due to physical assault, motor vehicle accidents, or athletic injuries. Penetrating scrotal injuries are less common, and are potentially due to stab wounds, projectile injuries or bites. Physical examination may be limited due to pain and often reveals varying degrees of edema and ecchymosis that may not correlate with the severity of injury [2]. US has become a pivotal tool in accurately assessing the extent of injury and vascular integrity thereby allowing prompt triage and management.

Blunt and penetrating scrotal trauma can result in testicular rupture and/or fracture, intra- and extra-testicular hematomas, hematoceles, hydroceles, and intratesticular pseudoaneurysms. Scrotal and epididymal hematomas, epididymal fracture and rupture, and traumatic epididymitis can also occur.

Epididymitis and epididymo-orchitis are common causes for acute scrotal pain with approximately 600,000 cases per year in the United States. Occurring most commonly in adolescents and young adults, epididymitis has a prevalence of 14–28% [13]. Orchitis develops in 20–40% of epididymitis due to contiguous spread of infection [14]. Isolated orchitis is rare and most often associated with mumps. In adolescents, most cases of epididymitis are attributed to ascending sexually transmitted diseases, with Chlamydia trachomatis and N. gonorrhoeae the most common pathogens. In those younger than 14 and older than 35 the disease is generally caused by common urinary tract pathogens [15].

Clinically, epididymo-orchitis presents as acute scrotal pain localized to the posterior testis. Symptoms of lower urinary tract infection may be present, a finding that is common in epididymitis, but not torsion [16]. The Prehn sign, defined as relief of pain with elevation of the testes over the symphysis pubis, may be elicited in patients with epididymitis, further distinguishing it from torsion.

Typical gray scale US findings of epididymitis consist of an enlarged hypoechoic epididymis with parenchymal heterogeneity due to edema and hemorrhage [9]. Concomitant orchitis is characterized by an enlarged, heterogeneous testis, although findings may be focal or diffuse. When focal, orchitis is most often located adjacent to an inflamed epididymis. Associated findings of epididymo-orchitis may include skin thickening and reactive hydrocele [17].

Color Doppler is highly sensitive in detecting scrotal inflammation, approaching sensitivity of 100% [18]. Color and power Doppler readily show hyperemia of the epididymis and/or testis (Fig. 3a). While echogenicity may be variable, Doppler flow is invariably increased. Demonstration of a normal spermatic cord and lack of an avascular nodule extending from the upper pole of the testis are important findings to exclude torsion of the testis and appendix testis [13]. Epididymo-orchitis has been associated with decreased resistive indices (<0.5). In severe cases complicated by infarction, there can be reversal of arterial diastolic flow [19].

If incompletely treated, epididymo-orchitis can lead to abscess formation. This may manifest clinically as persistent fevers, elevated white cell count, and development of a palpable scrotal abnormality. Whether intratesticular or paratesticular, US findings typically include a well-defined, ovoid, or rounded mass in the scrotum with variable internal echogenicity due to the ratio of fluid, purulent debris, and gas (Fig. 3b). Application of color Doppler will show an avascular mass with variable flow in the abscess rim and surrounding soft tissues. Follow-up US after incision and drainage or antibiotic therapy will allow assessment of resolution of any fluid collections.

Infection from epididymo-orchitis can spread retrograde via the vas deferens, resulting in deferentitis or vasitis. As the vas deferens extend from the seminal vesicles to the epididymes via the inguinal canal, infection can extend into the peritoneum and be a cause of pelvic pain or referred pain to the scrotum. Therefore, a thorough scrotal US examination should include evaluation of the inguinal region to evaluate the spermatic cord and to assess for indirect inguinal hernia. Physical examination findings in the setting of vasitis can include a firm, tender structure lateral to the testis with extension to an indurated, tender inguinal region due to inflammation, and infiltration along the spermatic cord. US findings mimic those on physical examination and include a thickened hypoechoic, hyperemic spermatic cord with inflammation and phlegmon lateral to the testis (Fig. 3c). Vasitis is commonly unilateral, but can be bilateral. Treatment is similar to epididymitis, and includes antibiotics and analgesia.

In addition to diagnosing vasitis, US may also be used to assess other etiologies involving the spermatic cord and inguinal region which result in pain. Differential considerations for inguinal tenderness include spermatic cord hematoma, hernia, and rarely neoplasms. Lipoma and sarcoma are the most common spermatic cord tumors, but usually present as a painless mass [20]. Spermatic cord hematomas are usually post-traumatic or post-operative. US findings can vary due to age of the hematoma; findings commonly include a heterogeneous mass-like lesion with lack of color Doppler flow. As the hematoma liquefies, a more cystic component will evolve and may contain low-level internal echoes; in this setting, differentiation from abscess may be difficult, but correlation to patient history and laboratory analysis will usually allow correct diagnosis. An indirect inguinal hernia can be a cause of inguinal and scrotal pain, as herniated bowel, fat and vessels can extend into the scrotum. US findings can include an elongated heterogeneous lesion filling the inguinal canal; if peristalsis is shown, the finding is pathognomonic. However, peristalsis may be absent if bowel obstruction has occurred or if only fat and vessels have herniated (Fig. 5a, b). Additional findings can include incomplete or ‘dirty’ shadowing induced from intraluminal air and identification of the stratified layers of the bowel wall. If findings are inconclusive, correlation to CT can be made.

Scrotal US may include evaluation of the perineum in a patient presenting with pain and infectious symptoms; additionally, obtunded patients should also undergo imaging evaluation of the perineum, especially those who have been incapacitated for an indeterminate length of time and those in whom there is clinical suspicion for diabetic coma. A scrotal US examination which includes only the scrotum may miss life-threatening infection or perineal abscess. To properly scan for perineal infection, the US technologist may have to reposition the patient so that the perineum is exposed; it may be necessary for the technologist or patient to retract the scrotum from the field of view.

Imaging findings of perineal abscess include a well-defined collection with variable internal echogenicity due to its contents (Fig. 6a, b); the fluid collection will be avascular with variable peripheral flow and surrounding hyperemia. More diffuse infection raises concern for Fournier’s gangrene, a urologic emergency due to polymicrobial necrotizing fasciitis with mortality rates approaching 75% [21]. Diagnosis may be made on clinical grounds and can include findings of crepitus. Fournier’s gangrene is associated with immunocompromised states, such as chronic diabetes, alcoholism, and advanced age. With US, the key finding is intrascrotal gas manifested by numerous echogenic foci with ring-down or reverberation artifact. This must be differentiated from air in a bowel containing inguinal hernia by showing gas peristalsing in the bowel lumen.

US plays a pivotal role in differentiating acute torsion from epididymo-orchitis. While loss of the cremasteric reflex and absence of Prehn’s are physical examination findings of torsion, there can be overlap on physical examination as both entities present with scrotal tenderness and swelling [22, 23]. Additionally, while most cases of torsion involve younger males, torsion can occur at any age. The degree of venous engorgement, edema, hemorrhage, and arterial compromise depends on the degree of torsion; studies have shown that there must be at least 720 torsions for occlusion of the testicular artery [24]. The degree of testicular ischemia depends on the degree and length of torsion. If diagnosed in the first 6 h, torsion can be successfully treated surgically in nearly 100% of cases; the salvage rate drops to approximately 20% between 12 and 24 h after diagnosis [25]. In adolescents and adults, torsion occurs within the tunica vaginalis due to lack of attachment of the tunica to the posterolateral aspect of the testis; this results in the so-called “Bell-clapper” deformity and allows the testis to be mobile and rotate freely. Torsion can also be iatrogenic in the setting of spermatic cord manipulation or post-herniorrhaphy. Approximately, 8% of cases of testicular torsion are post-traumatic, presumably due to forceful contraction of the cremasteric muscle and testicular rotation [11, 26, 27]. Extravaginal torsion occurs outside of the tunica vaginalis due to non-fixation of the testes and gubernacula; this type of torsion occurs exclusively in newborns and the testis is typically infracted at birth. US findings will include lack of color flow, parenchymal heterogeneity, and hydrocele.

The presence of vascular flow in the testis can be readily depicted at US. When using only color Doppler, the sensitivity, specificity, and accuracy has been reported to be 86, 100, and 97%, respectively [28]. The use of both color and power Doppler further increases sensitivity [29]. However, the presence of color or Doppler flow in the proper clinical setting does not exclude torsion, as cases of incomplete or transient torsion may have flow at the time of US examination. The ability of color, power, and spectral Doppler in diagnosing incomplete torsion remains undetermined as no studies have yet validated their role. Gray scale findings of torsion are also variable and depend on the extent and duration of torsion. In the early phases, the parenchyma will often appear normal; at 4–6 h, the parenchyma may be edematous and slightly hypoechoic [30]. After approximately 24 h, the parenchyma will be heterogeneous due to congestion, infarction, and hemorrhage. Despite the variable gray scale findings in torsion, recognition of a homogeneous testicular parenchyma indicates that a testis has not yet undergone irreversible ischemia or infarction and is a good predictor of viability [31, 32]. Testicular torsion can lead to infarction if left untreated. Similar to torsion, US findings of infarction include lack of Doppler; at gray scale, the parenchyma is usually hypoechoic or heterogeneous (Fig. 7). Infarction may also be segmental if due to an embolic phenomenon, manifesting as a well-demarcated region of hypoechogenicity with lack of flow (Fig. 8a, b).

Torsion can also affect the appendages of the testis and epididymis. Patients typically present with gradual pain and may manifest with a firm bluish nodule at the superior aspect of the testis referred to as the “blue dot” sign. US will show a circular mass adjacent to the testis or epididymis which may show peripheral increased flow on color Doppler examination; reactive hydrocele and skin thickening may also be shown [28].

Testicular tumors rarely present with acute pain; rather, patients may describe a vague discomfort or palpable abnormality. US findings are variable due to tumor type, but typically include a solid parenchymal mass with internal flow. Similarly, most varicoceles do not cause acute pain. Most varicoceles can be diagnosed clinically and are idiopathic. Typical color Doppler US findings include dilated veins which are accentuated with Valsalva maneuver. While most varicoceles are idiopathic, evaluation of the retroperitoneum should be performed if a varicocele is new, right sided or non-decompressible due to the potential of a retroperitoneal mass.