What causes the onion-skin appearance of the spermatic cord on scrotal ultrasound?
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Answer:
The onion-skin appearance of the spermatic cord is the concentric layered pattern seen when the cord has twisted on itself, producing a rounded extratesticular mass of alternating hypoechoic and echogenic layers above the testis. It is one of the ways testicular torsion is recognised, and it is closely related to the whirlpool sign, in which greyscale or colour Doppler shows the spiral twist of the cord and its vessels, a finding with high specificity and, in meta-analysis, better sensitivity than the classical criterion of absent intratesticular flow. Absent or reduced intratesticular Doppler flow remains the primary criterion but has recognised pitfalls: flow may persist in partial or intermittent torsion, is difficult to exclude in the prepubertal testis where normal flow is sparse, and must always be compared with the contralateral side using identical settings with the lowest pulse repetition frequency. Supporting findings are testicular enlargement with heterogeneous echotexture, which indicates infarction and a low likelihood of salvage, an abnormal transverse testicular lie, scrotal wall thickening and a reactive hydrocele. The main differential is epididymo-orchitis, in which flow is increased rather than absent, and torsion of the testicular appendix, which shows a small avascular nodule with normal testicular perfusion. Torsion is a surgical emergency, with salvage rates falling sharply after 6 hours, so imaging must never delay exploration when suspicion is high.
Why is it called so?
The twisted layers of the cord seen in cross-section resemble the concentric layers of an onion.
Pathophysiology
In the bell-clapper deformity the tunica vaginalis invests the testis and the distal cord completely, so the testis hangs free within the tunica and can rotate about the cord. Rotation first occludes the low-pressure venous drainage, causing congestion, oedema and increasing intratesticular pressure, and then compromises arterial inflow, producing ischaemia and eventually haemorrhagic infarction. The cord itself becomes a twisted knot of vessels, nerve and vas surrounded by oedematous connective tissue, and it is this coiled structure, imaged in cross-section, that produces the layered onion appearance and, in the longitudinal plane, the spiral whirlpool. Because venous obstruction precedes arterial loss, early torsion can show preserved arterial signal, which is why the cord must always be examined directly.
Alternative names: Whirlpool sign; spermatic cord twist
Other associated named signs: Whirlpool sign of the cord and the blue dot sign clinical correlate of appendiceal torsion
References
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