Evaluation of the Uterine Cavity: HSG, SHG, hysteroscopy: Similar acronyms, different focus
There are three essential components to reproduction: the egg, the sperm and a place where they can meet. The uterus and fallopian tubes are dynamic components and work in concert. The fallopian tubes are quite mobile cradling the ovary during ovulation and directing the egg into the tube as it is ovulated from the ovary itself, much like a vaccum cleaner. In natural conception, the egg is fertilized in the outer part of the fallopian tube. The newly created and now expanding embryo travels down the fallopian tube toward the entrance into the uterine body. This six inch journey takes approximately six days. The embryo is implanted into the welcoming endometrium of the uterus and together, the fetus and uterus will grow at an incredible rate over the next 38 weeks. So crucial are the uterus and fallopian tubes to this process, it is imperative that they are without issue.
There are different ways of evaluating this system. What these diagnostic methods share, other than similar eponyms, is the ability to visualize the structural components of reproduction. Although similar, these tests are not identical and each one has a particular focus.
The hysterosalpingogram (HSG) is a fluoroscopic based test performed in conjunction with a radiologist. A small balloon catheter is placed in the cervix (the lowest part of the uterus) and slowly, a radio-opaque dye is instilled. Very low levels of radiation are used for a short period of time. The dye coats the uterine walls much like milk coats the inside of a glass. With continued instillation, the dye will seep into the upper portion of the uterus and finally into the fallopian tubes. With time (approx 3 minutes), the dye will coat the inside tract of the fallopian tube and then will spill into the abdominal cavity. Essentially, the dye will do the reverse journey that the first egg and then resultant embryo undergo.
The HSG provides information about the contour of the endometrial cavity and the outline of tract of the fallopian tubes. The presence or absence of dye spilling from the very ends of tubes affords information about the ability of the egg’s pontential passage upon ovulation. Structural abnormalities such as hydrosalpinges (swollen tubes) and polyps, fibroids and scar tissue within the uterus may also be defined by this test and later surgically repaired. The HSG provides little information about the contour of the outside of the uterus and the actual mobility of the tubes themselves. Although it is a good test of the uterine cavity, it is not the most sensitive test for all uterine abnormalities.
Its cousin is the sonohysterogram (SHG) and it is an office-based test. A catheter is placed in the cervix identical to that of the HSG. Unlike the HSG, during a sonohysterogram, a small amount of saline is instilled into the uterus separating the apposed layers. A transvaginal ultrasound is used for visualization and no radiation is needed. In actuality, the uterus is a virtual cavity, much like an empty plastic lunch baggie. The pressure of the instilled saline forces the uterine walls apart which allows excellent visualization of the contour of the endometrial cavity. The literature clearly demonstrates that polyps, fibroids or scar tissue within the cavity can decrease embryo implantation rates by as much as 40%. It is for this reason that we must be certain that the endometrial cavity is ideal and receptive to the embryo. The SHG does afford information about the contour of the outside of the uterus (unlike the HSG), but provides less accuracy when assessing fallopian tube patency.
Hysteroscopy is the third way of assessing the uterine cavity. Diagnostic hysteroscopy is the only way to visualize directly the endometrial cavity. It can be performed during an office visit or in the operating suite depending upon the anticipated outcome. Briefly, a small, flexible hysteroscope is placed into the cervical canal using saline to distend the endometrial cavity. Under direct visualization, the hysteroscope is advanced and the full cavity can be assessed as well as can be the ostia (the opening of the fallopian tubes into the uterus). The operating room suite is reserved for patients demonstrated to have pathology on other evaluations, or if the office hysteroscopy is unclear.