Normal Testicular Appendix Diameter on Ultrasound
The testicular appendix (appendix testis) is a small Müllerian duct remnant located at the upper pole of the testis, most commonly seen at the groove between the testis and epididymis. Although clinically silent in most patients, it is the most frequently torted intrascrotal appendage and an important cause of acute scrotal pain, particularly in prepubertal boys. Accurate sonographic measurement helps distinguish appendix torsion from the more urgent diagnosis of testicular torsion.
Normal Reference Values
| Measurement |
|---|
| <5 mm |
Clinical Significance
A normal testicular appendix measures less than 5 mm in diameter on ultrasound. When the appendix exceeds this threshold — particularly when associated with increased echogenicity, a surrounding reactive hydrocele, or epididymal enlargement — torsion of the appendix testis should be strongly considered. The classic clinical presentation is gradual onset scrotal pain in a prepubertal boy with a palpable “blue dot” sign, though this sign is present in only a minority of cases.
Distinguishing appendix torsion from testicular torsion is critical, as the former is managed conservatively while the latter requires urgent surgical exploration. Pitfalls include difficulty visualizing a small, non-torted appendix, and the occasional coexistence of an inflamed appendix with epididymo-orchitis, which can confound the diagnosis.
- Torsion of appendix testis — most common cause of appendix enlargement >5 mm
- Epididymo-orchitis — reactive inflammation may involve the appendix
- Testicular torsion — critical differential; requires urgent exclusion with Doppler imaging
- Incidental appendix cyst — small cystic structure at upper testicular pole, typically benign
- Scrotal trauma — appendix may appear enlarged in the setting of blunt injury
Reference: Dogra VS, Gottlieb RH, Oka M et al. Sonography of the scrotum. Radiology. 2003;227(1):18-36.
Imaging Notes
High-frequency scrotal ultrasound (10–15 MHz linear transducer) is the modality of choice for evaluating the testicular appendix. The structure is best identified at the groove between the testis and epididymal head in the longitudinal plane. In the absence of a surrounding hydrocele, visualization can be challenging; a small reactive hydrocele, which commonly accompanies appendix torsion, actually improves conspicuity by providing an acoustic window. Measure the maximum diameter of the appendix in a single plane.
Color and power Doppler are essential adjuncts: absence of flow within an enlarged appendix supports torsion, while hyperemia of the adjacent epididymis may indicate reactive inflammation. Careful real-time examination of testicular vascularity is mandatory to exclude concurrent testicular torsion before attributing pain solely to an appendiceal etiology.
