Orchitis is an inflammatory condition of the testis, most commonly caused by viral (mumps) or bacterial infections. Bacterial orchitis frequently arises from extension of epididymitis (epididymo-orchitis). Ultrasound is the primary imaging modality and typically demonstrates testicular enlargement, heterogeneous echogenicity, and markedly increased vascularity on color Doppler. Reactive hydrocele commonly accompanies acute orchitis. If untreated, orchitis can progress to abscess formation, testicular infarction, and infertility. Mumps orchitis typically presents bilaterally in post-pubertal males. Bacterial orchitis is usually unilateral and associated with urinary tract infections. Chronic orchitis may mimic neoplasms due to focal hypoechoic areas.
Age Range
15-45
Peak Age
30
Gender
Male predominant
Prevalence
Common
The pathophysiology of orchitis varies by infectious agent. Bacterial orchitis typically results from gram-negative organisms (E. coli, Klebsiella) or sexually transmitted pathogens (Chlamydia, Neisseria) reaching the testis retrogradely via the ureter and vas deferens. The inflammatory infiltrate (neutrophils, lymphocytes) causes interstitial edema and congestion, which manifests on ultrasound as testicular enlargement and hypervascularity. Hypervascularity results from inflammatory mediators (prostaglandins, histamine) causing local vasodilation, visualized as increased blood flow on color Doppler. In viral orchitis (mumps), the virus directly infects seminiferous tubules creating interstitial edema, which can lead to bilateral testicular enlargement and infertility from tubular damage. Chronic orchitis develops fibrosis and granulomatous inflammation, appearing as T2 hypointensity on MRI and focal hypoechoic areas on US, making differentiation from neoplasm challenging.
Markedly increased blood flow in testicular parenchyma on color Doppler — reflecting inflammatory hyperemia and serving as the most critical differentiating criterion from torsion (avascular). In orchitis, arterial flow increases and vascular resistance decreases.
In acute orchitis, the testis is diffusely enlarged and loses its normal homogeneous echopattern, becoming heterogeneous. Scattered hypoechoic areas within the parenchyma reflect interstitial edema and inflammatory infiltrate. The affected testis is significantly larger than the contralateral normal testis (at least 20-30% volume increase). The epididymis is also frequently enlarged and hypoechoic (epididymo-orchitis). The degree of testicular enlargement and heterogeneity parallels the severity of inflammation.
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The testis is diffusely enlarged with heterogeneous parenchymal echogenicity; findings are consistent with acute orchitis.
Markedly increased blood flow is observed within the testicular parenchyma on color and power Doppler — reflecting inflammatory hyperemia. Vascular signal density is significantly increased compared to the contralateral normal testis. This finding is the most critical criterion for differentiating torsion (avascular/hypovascular) from orchitis (hypervascular). Hypervascularity may be diffuse or focal. Power Doppler is more sensitive than color Doppler in demonstrating inflammatory hypervascularity even in low-flow states.
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Markedly increased vascularity is observed within the testicular parenchyma on color Doppler; this finding is consistent with inflammatory hyperemia and torsion has been excluded.
Reactive hydrocele (anechoic fluid collection) around the testis and scrotal wall thickening are observed. The hydrocele is typically small to moderate and reflects inflammatory exudation. Scrotal wall thickening (>3 mm) results from edema and inflammatory infiltration. The epididymis appears enlarged and hypoechoic — particularly the epididymal head and body are affected. Peritesticular structures (tunica vaginalis, cremasteric structures) may also be involved.
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Reactive hydrocele and scrotal wall thickening are observed around the testis; findings are consistent with peritesticular inflammation.
On T2-weighted MR images, the affected testis is diffusely enlarged and shows hyperintense signal — reflecting interstitial edema and increased water content. While normal testis shows homogeneous intermediate-to-high T2 signal, in orchitis signal intensity further increases and becomes heterogeneous. The epididymis also appears edematous and T2 hyperintense. Peritesticular fluid (hydrocele) shows bright T2 signal.
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The testis demonstrates diffuse enlargement and increased signal intensity on T2-weighted sequences; findings are consistent with acute orchitis.
Diffusion-weighted imaging (DWI) may show restricted diffusion in the affected testis in acute orchitis — appearing as hyperintensity on high b-value images with decreased signal on ADC maps. Diffusion restriction results from inflammatory cell density and interstitial edema. This finding may help in differentiating orchitis from tumor but is not specific alone. If abscess formation develops, diffusion restriction becomes more pronounced and focal.
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Restricted diffusion is observed in the affected testis on DWI; ADC values are decreased compared to the contralateral normal testis.
On T1-weighted images, the orchitis-affected testis appears isointense or mildly hypointense compared to the contralateral normal. Diffuse and avid enhancement is seen on post-contrast sequences — reflecting inflammatory hyperemia. Enhancement may be homogeneous or heterogeneous. In abscess development, a central non-enhancing area (necrotic center) is surrounded by peripheral ring enhancement. The epididymis also enhances. Scrotal wall and tunicae may show enhancement.
Report Sentence
Diffuse and avid enhancement is observed in the testis on post-contrast T1-weighted sequences; findings are consistent with inflammatory hyperemia.
Although CT is not the primary imaging modality for testicular pathology, scrotal findings may be incidentally detected on abdominal-pelvic CT. In orchitis, the affected testis is enlarged and shows increased enhancement on contrast-enhanced CT. Peritesticular fluid (hydrocele), scrotal wall thickening, and reactive inguinal lymphadenopathy may be accompanying findings. CT is particularly useful in evaluating complications (abscess, infarction, Fournier gangrene).
Report Sentence
CT demonstrates testicular enlargement with increased enhancement and peritesticular fluid; findings are consistent with orchitis.
Criteria
Acute inflammation caused by bacterial infection (E. coli, Klebsiella, sexually transmitted agents). Usually arises from extension of epididymitis (epididymo-orchitis). Unilateral, sudden onset, associated with fever and pyuria.
Distinct Features
Epididymis is usually affected first — epididymal enlargement and hypervascularity are prominent. Unilateral involvement is typical. Urine culture may be positive. US shows diffuse hypervascularity of testis and epididymis.
Criteria
Mumps virus-induced orchitis — develops as a complication of mumps in 20-30% of post-pubertal males. Occurs 4-8 days after parotitis. Bilateral involvement is seen in 30% of cases.
Distinct Features
Epididymis is typically not involved (isolated orchitis). Bilateral involvement more common. Higher risk of testicular atrophy and infertility (atrophy in 50% of cases). History of mumps/parotitis is clinical clue.
Criteria
Chronic inflammation or granulomatous reaction — tuberculosis, sarcoidosis, idiopathic granulomatous orchitis, fungal infections. May mimic malignancy due to focal hypoechoic lesions. Testicular atrophy may develop.
Distinct Features
Focal hypoechoic lesion(s) — mimics tumor. Calcifications may be seen (TB). Epididymal involvement common (in TB orchitis). T2 hypointensity on MRI (fibrosis). Clinical correlation and biopsy usually required.
Criteria
Complications developing in untreated or severe orchitis: testicular abscess (focal fluid collection), testicular infarction (ischemia due to pressure within tunica albuginea), Fournier gangrene (necrotizing fasciitis).
Distinct Features
Abscess: thick-walled, avascular central area, DWI restriction. Infarction: wedge-shaped avascular zone (segmental) or diffuse avascular area (global). Fournier: subcutaneous gas, fascial thickening — EMERGENT SURGERY.
Distinguishing Feature
Torsion: DECREASED/ABSENT blood flow on Doppler + whirlpool sign + spermatic cord rotation. Orchitis: INCREASED blood flow (hypervascularity). This differentiation is EMERGENT — torsion requires surgery.
Distinguishing Feature
Seminoma: well-defined, homogeneous hypoechoic solid mass, typically painless. Orchitis: diffuse heterogeneous enlargement with pain and fever. However, chronic orchitis may mimic seminoma with focal lesions — biopsy may be needed.
Distinguishing Feature
Lymphoma: >50 years, bilateral, diffuse hypoechoic infiltration, increased vascularity (similar to orchitis!). Differentiation: no fever/pyuria in lymphoma, tumor markers normal, bilateral involvement more common. Biopsy may be needed.
Distinguishing Feature
Abscess is a complication of orchitis: well-defined, thick-walled, hypoechoic/complex fluid collection, central avascularity with peripheral hypervascularity (ring sign). Marked diffusion restriction on DWI.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
6-monthAcute orchitis is managed with antibiotic therapy. Torsion must be excluded (EMERGENT Doppler). Surgical intervention may be needed if complications develop (abscess, infarction). Chronic/focal orchitis may be confused with malignancy — biopsy may be required. In mumps orchitis, testicular atrophy and infertility develop in up to 50% of cases. Follow-up US is recommended at 4-6 weeks for evaluating treatment response and complications.
Orchitis/epididymo-orchitis usually resolves completely with antibiotic therapy. The critical differential diagnosis is testicular torsion — presence of Doppler vascularity excludes torsion. Complications include abscess formation, testicular infarction, chronic orchitis, and infertility. Mumps orchitis can be bilateral and carries risk of atrophy. If untreated, it can progress to life-threatening conditions such as Fournier gangrene.