Epididymo-orchitis
- Epididymo-orchitis is infection or inflammation of the epididymis, the testis, or both — basically the scrotum running a fever.
- Ultrasound with color Doppler is the test. The signature is the opposite of torsion: a swollen, painful side with too much blood flow, not too little. The epididymal head grows beyond its normal 10–12 mm and the whole thing lights up.
- Ultrasound is only about 70% sensitive and 88% specific for epididymitis, so a bland scan doesn't exclude it — but the scan's real job is ruling out testicular torsion, and for that it is much stronger.
- Watch for the dangerous companions: an abscess, a missed or self-untwisted torsion, or in older men, an underlying tumor hiding behind the swelling.
Acute scrotal pain is one of those situations where the ultrasound probe is less of a diagnostic tool and more of a smoke detector. Something in there is angry, the patient would very much like it to stop, and your one job is to figure out which angry thing it is — because the answers range from "antibiotics and an apology" to "call the surgeon right now."
Epididymo-orchitis is the common, treatable end of that spectrum. But it lives next door to the emergency, so you have to know it cold.
What it actually is
The epididymis is the little coiled tube draped along the back of the testicle — think of a garden hose neatly looped against the side of a melon. It's where sperm hang out and mature. When bacteria climb up the plumbing (usually from the urinary tract in older men, or a sexually transmitted infection in younger ones), the epididymis gets inflamed, swollen, and furious. That's epididymitis.
If the infection spreads from the hose into the melon itself, the testis inflames too, and now you've got epididymo-orchitis. Isolated orchitis — just the testis, no epididymis — does happen (mumps is the classic example), but the combo is far more common because infection usually arrives via the epididymis first. And because infection arrives from the tail end of the hose, the inflammation typically starts in the tail of the epididymis and marches toward the head, which is a useful pattern to look for.
Findings by modality
Ultrasound
Gray-scale first. The inflamed epididymis looks enlarged and darker than normal — radiologists say hypoechoic, meaning it reflects back fewer of the sound waves, so it shows up gray-dark instead of its usual even speckle. Sometimes it's heterogeneous instead, with bright and dark patches mixed together. The normal epididymal head is the largest part of the organ at about 10–12 mm across; in epididymitis it can swell well past that, and the tail, which is normally a slim structure you have to hunt for, becomes a fat, obvious lump at the lower pole. Look for the extras that come with inflammation: thickened scrotal wall, a reactive hydrocele (more than the normal 2–3 mL between the layers of the tunica vaginalis), and sometimes strands of echogenic fluid.
If the testis is involved, it too enlarges and turns hypoechoic — either diffusely or, more often, in the part adjacent to the inflamed epididymis, since infection spreads by contiguity. A normal adult testis is roughly 5 × 3 × 2 cm with a homogeneous echotexture; orchitis makes it bigger and blotchier.
But the real money shot is color Doppler, the mode that paints moving blood onto the picture. Inflammation means the body floods the area with blood, so the affected side lights up like a Christmas tree — markedly increased flow compared with the calm, normal side beside it. On spectral Doppler that hyperemia shows as fat, high-diastolic-flow waveforms: the inflamed tissue's vessels are dilated and offer little resistance, so blood keeps streaming forward all through diastole rather than slowing to a trickle.
Always scan both testicles in the same setting and compare. The normal side is your built-in control. "Is this a lot of flow?" is impossible to answer in isolation; "is this more flow than the other guy?" is easy.
CT
Nobody orders a CT to diagnose epididymo-orchitis, but you will meet it on a CT ordered for something else — abdominal pain, a "sepsis of unknown source" study, a stone protocol that scanned low. It shows as an enlarged, avidly enhancing epididymis with stranding of the scrotal fat, thickening of the scrotal wall, and sometimes a hydrocele. If you see that, recommend an ultrasound; don't try to exclude torsion or abscess on CT.
MRI
MRI is an occasional problem-solver when ultrasound is equivocal and the question is "abscess or tumor?" Inflamed epididymis and testis enhance avidly and are bright on fluid-sensitive sequences; an abscess is a non-enhancing collection with a thick enhancing rim and restricted diffusion. In practice you will almost never need it for the acute diagnosis.
Why this is really a torsion rule-out
Epididymo-orchitis and testicular torsion present almost identically: sudden-ish painful, swollen scrotum in a guy who is not having a great day. The catch is that they are sonographic opposites.
| Feature | Epididymo-orchitis | Testicular torsion |
|---|---|---|
| Color Doppler flow | Increased on the affected side | Decreased or absent in the testis |
| Gray-scale testis | Enlarged, hypoechoic (if orchitis) | Normal early; enlarged and heterogeneous as it dies |
| Spermatic cord | Thickened, straight vessels | Whirlpool of twisted vessels |
| The clock | Treatable, less time-critical | Surgical emergency — salvage falls with every hour |
That flow column is the whole game. Inflammation adds blood; a twisted spermatic cord chokes it off. So the single most important thing the scan does isn't confirming infection — it's proving the testicle still has its blood supply.
Don't get lulled. A torsion that has spontaneously untwisted ("detorsed") can show increased flow from reactive hyperemia and look exactly like epididymitis. And early or partial torsion can have preserved flow. If the clinical story screams torsion — sudden onset, pain that peaked in minutes, nausea, a high-riding testis — the surgeon doesn't get to relax just because Doppler looks busy. Sweep the cord for a whirlpool before you sign anything off.
The complications you can't miss
Most cases are straightforward, but a few twists turn this from a prescription into a procedure.
- Abscess. If the inflammation organizes into a walled-off pocket of pus, you'll see a complex fluid collection — often with no internal flow at the center (dead pus doesn't take a pulse) but a hyperemic rim. That usually needs drainage, not just antibiotics.
- Pyocele. Pus filling the space around the testicle, with debris and septations floating in it, sometimes with gas.
- Testicular infarction. Severe orchitis can swell the testis enough inside its tight capsule to choke off its own venous drainage and cause infarction — flow paradoxically drops. A nasty plot twist: a patient who was "just infected" yesterday and now has an avascular testis.
- Chronic thickening. The epididymis can stay lumpy long after treatment, which sets up the next problem.
In an older man — especially if the "infection" stubbornly refuses to resolve on antibiotics — keep an underlying tumor on the list. Inflammation can mask a mass, so a follow-up ultrasound after treatment to confirm everything truly settled is a quietly heroic move.
The numbers
| What | Threshold / value | Why it matters |
|---|---|---|
| Normal epididymal head | 10–12 mm in diameter (the largest part of the epididymis) | Anything clearly bigger, especially with hypoechogenicity and hyperemia, is inflamed |
| Normal adult testis | About 5 × 3 × 2 cm, homogeneously echogenic | Orchitis enlarges it and makes it patchy; compare with the other side |
| Normal fluid in the tunica vaginalis | 2–3 mL | More than this is a reactive hydrocele; a complex collection with debris is a pyocele |
| Ultrasound for epididymitis | Sensitivity 70%, specificity 88% | Moderate: a normal-looking epididymis does not exclude early infection |
| Color Doppler US for torsion (adults, all studies) | Sensitivity 0.86 (95% CI 0.79–0.91); specificity 0.95 (0.92–0.97) | The rule-out that justifies the scan; better still (0.95 and 0.98) in studies after 2010 |
| Whirlpool sign | Sensitivity 92%, specificity 99% | If you see it, the "epididymitis" is a torsion |
How good is the test
Ultrasound for epididymitis, as summarized in the scrotal imaging literature, runs about 70% sensitive and 88% specific. That sensitivity is honestly modest — mild or very early epididymitis can look normal, and the diagnosis is frequently made clinically and treated without any imaging at all. So a scan that shows a normal epididymis with symmetric flow is not proof that there's no infection; it is proof that there is no torsion and no abscess, which is the part that changes management.
For that other job — excluding torsion — the same scan does better: pooled adult sensitivity 0.86 and specificity 0.95 across all studies, and 0.95 and 0.98 in the studies after 2010; a whirlpool sign in the cord is about 92% sensitive and 99% specific. That asymmetry is exactly why the scan is worth doing even when the clinician is already fairly sure it's infection.
Mimics
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Testicular torsion | Painful, swollen hemiscrotum; the epididymis can be enlarged because the cord is engorged | Testicular flow decreased or absent, not increased; whirlpool sign in the cord |
| Detorsed torsion | Reactive hyperemia after the cord untwists looks exactly like inflammation | History of severe pain now easing; thickened cord; the flow may be increased in the testis more than the epididymis |
| Torsion of a testicular or epididymal appendage | Focal pain and reactive hyperemia around the upper pole | A small avascular nodule beside the testis or epididymal head with hyperemia around it; the testis itself is normal |
| Testicular tumor | An older man's "orchitis" that is really a focal hypoechoic, vascular intratesticular mass | Focal rather than diffuse; no epididymal involvement; persists on the follow-up scan after antibiotics |
| Scrotal trauma with hematoma | Painful swelling with heterogeneous parenchyma | The history; hematoma is avascular, not hyperemic; look at the contour of the tunica albuginea |
| Incarcerated inguinal hernia | Painful scrotal swelling with a thickened wall | The "mass" is bowel or fat continuous with the inguinal canal; look for peristalsis and bowel wall |
Reporting
The referrer needs a short list:
- Which structures are involved: epididymis (head, body, tail), testis, or both, and whether the pattern is diffuse or focal.
- Flow status of the testis: increased, normal, or decreased compared with the other side. "Increased" reassures; "decreased" is an emergency and a phone call.
- Complications: abscess (size and location), pyocele, gas, evidence of testicular infarction.
- Anything that shouldn't be there: a discrete intratesticular mass, especially in an older man, with a recommendation for follow-up ultrasound after treatment.
- The torsion sentence: explicitly state that intratesticular arterial and venous flow is present and symmetric, and that the cord shows no whirlpool. Leaving this out is the report equivalent of a smoke detector with no battery.
The one-sentence takeaway
Epididymo-orchitis is the scrotum's inflammatory tantrum, and on ultrasound it announces itself with swelling plus a flood of color Doppler flow — but the reason you reach for that probe in the first place is to make absolutely sure the painful testicle still has blood reaching it, because the look-alike next door is the one that loses organs by the hour.
References
- Lam Shin Cheung J, Bhaduri M. A sound approach to stay on the ball—a review of scrotal pathologies on ultrasound imaging. Br J Radiol 2026;99(1181):847–858. Used for: the ultrasound sensitivity 70% and specificity 88% for epididymitis in Key Points, "The numbers," and "How good is the test"; the normal epididymal head of 10–12 mm, the normal adult testis of about 5 × 3 × 2 cm, and the normal 2–3 mL of tunica vaginalis fluid in Key Points, "Findings by modality," and "The numbers"; the whirlpool sign sensitivity 92% and specificity 99% in "The numbers" and "How good is the test."
- Ota K, Fukui K, Oba K, et al. The role of ultrasound imaging in adult patients with testicular torsion: a systematic review and meta-analysis. J Med Ultrason 2019;46(3):325–334. Used for: the pooled adult color Doppler sensitivity 0.86 (95% CI 0.79–0.91) and specificity 0.95 (0.92–0.97), and the post-2010 sensitivity 0.95 and specificity 0.98, for torsion in "The numbers" and "How good is the test."
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