Acute scrotum may have many causes, such as testicular torsion (complete, incomplete and intermittent), epididymo-orchitis, torsions of testicular or epididymal appendages, acute idiopathic scrotal oedema [1, 2].
In 26% of paediatric patients, the underlying cause of acute scrotum is testicular torsion (TT) [1]. It has an estimated incidence of 3.8/100000 and a bi-modal distribution, during the first year of life and early adolescence [1].
TT can be intravaginal (most common form, secondary to the bell-clapper deformity); extravaginal (typically in neonates, involves twisting of the spermatic cord) and mesorchial (least common form, consists of twisting of an elongated mesorchium) [1].
Testicular pain and swelling may prevent physical examination. Clinical differentiation between causes of acute scrotum may be difficult [3]. Even though there are clinical signs that suggest the aetiology, they are not accurate [2].
US and colour Doppler US are the methods of choice in the differential diagnosis of acute scrotum. They aim to rule in or rule out TT, which requires emergency surgery.
The whirlpool sign is defined as an abrupt change in course of the spermatic cord with a spiral twist of the cord [1]. At US it can have the appearance of a doughnut, a target with concentric rings, a snail shell or a storm on a weather map [2]. It can be located just outside the external inguinal ring, superiorly or posteriorly to the testis [2, 3].
Below the point of spermatic cord torsion, a pseudomass can be identified. It is composed of congested epididymis, proximal vas deferens and vascular bundle. At US it appears as an oval-shaped mass with heterogeneous echotexture [1].
In complete torsion, the whirlpool sign is seen only on B-mode US with absent intra-testicular flow on colour Doppler US. In incomplete torsion, the whirlpool sign is seen in B-mode and colour Doppler US and there is flow in the vessels of the whirlpool sign, distal to it and intra-testicular [2].
The whirlpool sign is the most definitive sign of TT, as it has 100% specificity and sensibility [2]. This is true regardless of colour Doppler US findings [2, 3].
To confirm detorsion, a follow-up US should include disappearance of the whirlpool sign and normalisation of the vascularization [3].
Prompt diagnosis and surgical detorsion are mandatory. Testicular salvage rates are approximately 80-100% within 6 hours, 70% between 6 to 12 hours, 20% after 12 hours and less than 10% after 24 hours [1, 3].
Careful evaluation of the course of the spermatic cord should always be included whenever TT is suspected [3].