Cogwheel sign | Radiology Signs

Answer:

The cogwheel sign is the appearance in cross-section of a dilated fallopian tube whose thickened, oedematous longitudinal folds project into the lumen, producing spokes around a fluid-filled centre that resemble a cogwheel. It indicates acute salpingitis with pyosalpinx or hydrosalpinx in the setting of pelvic inflammatory disease. The wall in the acute phase is thicker than 5 mm and shows marked hyperaemia on power Doppler, and the lumen may contain echogenic fluid or debris with fluid-debris levels when pus is present. As inflammation becomes chronic the folds atrophy and flatten, and the tube then shows the beads-on-a-string sign, small mural nodules of 2 to 3 mm around a thin-walled distended tube, which is the chronic hydrosalpinx counterpart. The waist sign, indentations of the opposing walls of the distended tube, helps confirm that a convoluted cystic adnexal structure is tubal rather than ovarian. The important differentials are an ovarian cystic neoplasm, a tubo-ovarian abscess in which the ovary and tube are no longer separable and there is a complex multilocular mass, endometriosis with a haemorrhagic haematosalpinx, and, in the pregnant patient, an ectopic gestation. Recognition matters because untreated pelvic inflammatory disease leads to infertility, chronic pain and ectopic pregnancy, and because a tubo-ovarian abscess may need drainage in addition to antibiotics.

Why is it called so?

The thickened endosalpingeal folds radiating into the lumen look like the teeth of a cogwheel.

Pathophysiology

Ascending infection, usually with Chlamydia trachomatis or Neisseria gonorrhoeae and often polymicrobial, inflames the tubal mucosa. The endosalpinx normally has delicate longitudinal folds; inflammation makes them oedematous and thickened while exudate and pus distend the lumen and occlude the fimbrial end, so the folds are seen in relief within the fluid. Doppler shows the hyperaemia of acute inflammation. With chronicity the mucosal folds are progressively destroyed and fibrose, leaving only small residual nodules, and the tube becomes a thin-walled sac of serous fluid, which is why the acute and chronic appearances differ and why the cogwheel pattern points specifically to active disease.

Alternative names: Cogwheel appearance of the fallopian tube

Other associated named signs: Beads-on-a-string sign of chronic hydrosalpinx and the waist sign of a distended tube

References

1. Molander P, Sjöberg J, Paavonen J, Cacciatore B. Transvaginal power Doppler findings in laparoscopically proven acute pelvic inflammatory disease. Ultrasound Obstet Gynecol. 2001;17(3):233-8. PMID: 11309174.