A Fallopian tube filled with water is referred to as a hydrosalpinx. When the end of the the ampulla (end of the Fallopian tube) and the fimbria (delicated fingers) are injured, the end closes. When the glands within the fallopian tube produce a watery fluid and the end is closed, the water gets trapped, filling the tube and causing swelling.
The Fallopian tubes are delicate and therefore susceptible to injury. The fimbria extend from the end of the Fallopian tube to the ovary, and after the egg is released, the fimbria attempt to guide the egg toward the sperm. The fimbria is lined with delicate cells and once the fimbria is damaged, these cells are also damaged, and the fimbria gets fused together and this lining is lost. Once this occurs, the trapped fluid ruins the environment. The injury to the fimbria is usually due to an infection. The most common infections that damage the fimbria are the sexually transmitted diseases gonorrhea and chlamydia. Both these diseases can be present without detection for many years, slowly destroying the fimbria. Other causes are IUDs, endometriosis and abdominal surgeries. Once the body detects an issue, there is an inflammatory response. The combination of inflammation and eventual healing destroys the fimbria and closes off the tubes.
Fluid in the Fallopian tubes can lead to infertility. Fertility treatment can also be thwarted by this condition as well. The trapped fluid, when eventually released, is toxic to early embryo development. Also, enough fluid can collect that the fluid can flow into the uterus, making it difficult for embryos to attach. Fertility drugs can also exacerbate fluid production, filling up the tubes more rapidly.