Image Interpretation: Evidence of Achilles paratenonitis, characterized by an anechoic halo of fluid circumferentially around the mid-portion of the Achilles tendon, tracking longitudinally along the more superficial aspect of the mid-portion of the Achilles tendon. There is associated hyperemia around the area of paratenon fluid accumulation. The internal architecture of the Achilles tendon is largely unremarkable, without substantial thickening, fusiform change, hypoechogenic inclusions, or partial tearing noted. There is mild edema within Kager's fat pad.

Teaching Pearl: Painful swelling of the Achilles can have multiple potential etiologies: underlying tendon thickening and architecture change secondary to chronic tendinopathy, acute tendinous defect, or inflammation of the paratenon. A detailed history, physical exam, and diagnostic ultrasound can help differentiate these potential pathologies. In this case, diagnostic ultrasound was necessary to make the diagnosis of isolated Achilles paratenonitis without underlying Achilles tendinopathy. Findings were notable for anechoic swelling of the paratenon with associated hyperemia. Tendon changes, such as fusiform thickening, disordered fibrillar architecture, or partial tears, would be suggestive of comitant Achilles tendinopathy but were not present in this instance. In summary, the acute phase of Achilles paratenonitis is characterized by anechoic fluid under the paratenon sheath tracking along the Achilles tendon. Hyperemia may also be present on Doppler imaging. In the chronic phase, the paratenon can become thickened, sometimes creating adhesions between the Achilles and paratenon. 1. van Dijk CN, van Sterkenburg MN, Wiegerinck JI, Karlsson J, Maffulli N. Terminology for Achilles tendon related disorders. Knee Surg Sports Traumatol Arthrosc. 2011 May;19(5):835-41. doi: 10.1007/s00167-010-1374-z. Epub 2011 Jan 11. PMID: 21222102; PMCID: PMC3076576. 2. Ferguson A, Christophersen C, Elattar O, Farber DC. Achilles Tendinopathy and Associated Disorders. Foot Ankle Orthop. 2019 May 10;4(2):2473011419838294. doi: 10.1177/2473011419838294. PMID: 35097320; PMCID: PMC8696945.