Splenogonadal fusion: A rare paratesticular lesion and how to recognise it on ultrasound

Introduction

Splenogonadal fusion (SGF) is a rare developmental disorder that arises due to the close proximity of the developing spleen and gonads in utero. It is classified into continuous and discontinuous subtypes. ¹ In the former, there is continuous splenic or fibrous tissue extending from the normally sited spleen to the left gonad. In the discontinuous subtype, there is no connection between the spleen and ectopic splenic tissue. The majority of reported cases are in males ² with splenic tissue located in the testicle ³ or as a paratesticular mass. ⁴

We describe a case of SGF presenting as a scrotal swelling due to paratesticular ectopic splenic tissue, highlight the pathophysiology and presentation of SGF as well as emphasise the sonographic features that may help to diagnose the lesion.

Case report

A healthy 25-year-old male presented to his primary care physician with an episode of balanitis. A left-sided scrotal swelling separate to the left testicle was also noted on examination and the patient reported that this had been present for “some years” but had gradually increased in size. An epididymal cyst was diagnosed clinically, but an urgent referral to secondary care was made to investigate the scrotal swelling and the possibility of a solid paratesticular lesion.

Testicular tumour markers (alpha-foetoprotein, human chorionic gonadotrophin and lactate dehydrogenase) were all normal. A scrotal ultrasound was performed with an 18 MHz linear probe on a Siemens Acuson S2000 machine. It revealed a 2 cm lesion just superior to, but separate from, the left testicle. It showed a similar reflectivity to the adjacent testicle on B-mode imaging but had marked central vascularity on colour Doppler imaging with a branching pattern (Figure 1). Both testicles were normal in size and sonographic appearance. OPEN IN VIEWER

The case was discussed at a multidisciplinary team (MDT) meeting. Given the long history, a neoplastic paratesticular lesion was considered unlikely, but given the internal vascularity felt still to be feasible on imaging. The decision was therefore made to surgically explore the lesion with a view to biopsy or removal.

The surgery was performed via a scrotal incision, and during the procedure, the lesion was confirmed to be separate from the epididymis and testicle but close to the rete testis. It was therefore not deemed safe to remove it, and the lesion, along with the testicle, was left in situ. Intra-lesional biopsies were obtained and closure performed. Subsequent histolopathologic review revealed splenic parenchyma with well-defined red and white pulp (Figure 2). There was no evidence of malignancy. Given the tissue was obtained from a paratesticular mass, a diagnosis of SGF was made. Figure 3 demonstrates the similarity of the sonographic appearances of the ectopic splenic tissue to the patient's own normal spleen. OPEN IN VIEWER

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Figure 3.

B-mode longitudinal section ultrasound images obtained at the same date demonstrated the similarity in appearances of the (a) ectopic spleen to (b) the patient's normally located spleen (both highlighted by the callipers).

Given the lack of symptoms attributable to the SGF and the fact that the testicles were normal in size and appearance, the patient was discharged and advised to perform regular self-examination.

Discussion

We describe a case of SGF presenting as a paratesticular mass. The vast majority of paratesticular lesions are benign and are usually cystic. Solid lesions are rare but more commonly malignant (16% in one small series ⁵ ). Comprehensive MDT assessment is therefore vital and despite the rarity of SGF, it is important that ultrasound practitioners have an awareness of the imaging features and clinical associations, so that it is not overlooked. Given that SGF can be managed conservatively, diagnosing it prior to invasive surgery (e.g. orchidectomy) is desirable, although somewhat difficult, particularly where SGF presents as an intra-testicular lesion.

SGF occurs due to the close proximity of the dorsal mesogastrium, where the developing spleen is located, to the gonadal mesoderm during the fifth to eighth gestational weeks. The continuous type accounts for 56% of cases, and in around one-third of continuous SGF, there are associated congenital malformations including limb abnormalities, micrognathia, cardiac abnormalities and anal anomalies. ² Discontinuous SGF is very rarely associated with other congenital malformations.

The majority of cases present before the age of 20 and SGF is over 16 times more common in males than in females, ² likely because the male gonad is more clinically apparent than the ovary. Presentation is with left-sided scrotal swelling most commonly. Other common presentations are with a left inguinal hernia or cryptorchidism as the anomaly affects the closure of the processus vaginalis and descent of the gonad. ² Cases are also diagnosed incidentally on autopsy.

Ultrasound is the first-line imaging investigation for scrotal swelling. SGF can be of slightly lower or of similar reflectivity to the testicle. Given the rarity of SGF, no ultrasound findings can yet be considered characteristic, but some suggestive features have been described.

In children aged between one and five years, a low reflectivity micronodular pattern is seen in both SGF and in the normally located spleen, due to the development of the lymphoid system and white pulp follicles. ⁶ The central hypervascular appearance with a branching pattern is also seen in SGF ³ and is nicely demonstrated by our case. Malignant lesions would typically demonstrate less organised vascularity. The use of contrast-enhanced ultrasound (CEUS) to diagnose accessory perisplenic tissue is well described, with the key finding being retention of contrast in the delayed phase. ⁷ The CEUS appearances of SGF have also recently been described. ⁸ Similar to normal splenic and perisplenic accessory tissue, SGF showed homogenous arterial phase hyperenhancement and retained contrast in the delayed phase that was greater than the adjacent testicular tissue. Typically, malignant testicular and paratesticular lesions show more disorganised vascularity and would be expected to show washout of ultrasound contrast. CEUS also carries the advantage of avoiding ionising radiation and shows great promise regarding non-invasively diagnosing SGF once the imaging features have been further corroborated.

Following clinical and sonographic assessment, if the possibility of SGF has been raised, a nuclear medicine study can be used to confirm the diagnosis. Performing a ^99mTc sulphur colloid study shows uptake in SGF mirroring the normal hepatic and splenic tracer uptake. ⁹ Cross-sectional imaging can demonstrate the abnormal contiguous splenic tissue in continuous SGF but is not a primary modality to investigate paratesticular lesions.

In the absence of the features suggestive of continuous SGF, the sonographic differential diagnosis for discontinuous SGF in a paratesticular position is that of other solid paratesticular masses. These include the following lesions. Adenomatoid tumours are the commonest epididymal neoplasms, although they can arise from the tunica and rarely from the spermatic cord. They have variable sonographic appearances, but are usually solid, although can appear cystic. A lipoma is the commonest paratesticular non-epididymal lesion, and the ultrasound appearances are similar to lipomas elsewhere in the body. Scrotal haemangiomas are very rare and can appear indistinguishable from varicoceles. The capillary subtype, however, appears as solid hypervascular lesions not dissimilar to the SGF lesion we have demonstrated. Polyorchidism is a very rare congenital anomaly. Ultrasound findings (with and without contrast) are typically that of normal testicular tissue, for example, the presence of traversing vessels mimicking the transmediastinal vessels of normal testes. Finally, the reason that paratesticular solid lesions cannot simply be dismissed is due to the possibility of malignancy, namely, sarcoma. The commonest subtype is a rhabdomyosarcoma, which typically present before adulthood. These are usually larger than benign lesions and more heterogeneous on imaging; however, in early stages, they can appear homogenous and vascular and therefore must be considered in the differential for solid paratesticular lesions.

There are a handful of cases of SGF associated with testicular cancer, ¹⁰ but this may reflect the association between SGF and cryptorchidism, a known risk factor for testicular carcinoma. SGF can therefore be safely managed conservatively, with the need for further intervention based on patient preference and symptoms. Given that SGF can be managed conservatively, a diagnosis before an orchidectomy should be aimed for, but this is challenging with intra-testicular SGF. If the suspicion of SGF is raised on the basis of sonographic features or recognised clinical associations, further characterisation with CEUS is encouraged and the options of a nuclear medicine study ⁹ or biopsy/intra-operative frozen section exist ⁸ to confirm the diagnosis and avoid excisional biopsy or orchidectomy.

Conclusion

SGF is a congenital anomaly that is a rare cause for a testicular or paratesticular mass. Through this case, we highlight the clinical presentation, important clinical associations and ultrasound features of SGF in order to increase awareness amongst clinicians and ultrasound practitioners. This awareness is crucial, as once the possibility of the diagnosis is raised, further characterisation with CEUS can be performed and the diagnosis can be confirmed by a ^99mTc sulphur colloid study and the need for biopsy or invasive surgery obviated. In reality, this scenario is unlikely to be frequently achieved, such as with our case, where proceeding to biopsy of indeterminate paratesticular masses is shown to be safe and effective.