SIS is a low-cost and low-tech ultrasound procedure where some sterile saline is instilled into the uterine cavity. It separates the anterior and posterior uterine walls and acts as a negative contrast medium to delineate the uterine cavity outline as well as any intracavitary, endometrial or subendometrial pathology. It is an easy procedure to perform with minimal intrusion and discomfort. Nevertheless patients are advised to take a mild analgesic one hour before the procedure. Any clear fluid could be used instead of saline including local anaesthetics especially if a biopsy or polypectomy is contemplated. In such cases the local anaesthetic should be kept in the cavity for 3 minutes before starting the operative procedure.

It is best done immediately after menstruation when only the basal layer should be seen with a double layered measurement of 4 mm. This would avoid any artefacts caused by thick endometrial patches. A higher measurement would indicate incomplete shedding possibly due to endometrial hyperplasia, polyps or intracavitary fibroid. A high percentage of false positive findings would be seen if it is done at other times of the cycle or the catheter is pushed into the uterine cavity causing endometrial fragments detachment. A figure of 27% has been quoted before when the procedure was done after the 10^th day of the cycle [Wolman et al 1999, Gynecol Obstet Invest, 48: 254-258].

 Postmenopausal women on cyclic HRT should have SIS immediately after finishing a withdrawal bleeding. However those on combined HRT could have it at any time as the endometrium thickness is not significantly affected by the combined HRT.

Technique

To gain patients confidence and reduce apprehension and pain thorough explanation should be given before starting the procedure. The bladder should be empty and the patient should be well covered in the lithotomy position. Each step should be explained to the patient beforehand and a simple running commentary should be given. Warm speculum and antiseptic fluid should be used. 

After cleaning the vulva, vagina and cervix with an antiseptic, a soft thin catheter is introduced into the cervical canal and not into the uterine cavity itself. This would reduce discomfort and endometrial detachment which could result in a high false positive diagnosis of polyps. A Goldstein catheter is a good option as it is only 1.8 mm in diameter and its white acorn is applied to the external cervical os to reduce back flow leaking of the fluid. The catheter could be introduced through the side of the bivalve speculum and not down the channel. This would allow easy removal of the speculum without dislodging the catheter. Few millilitres of sterile saline are instilled slowly [to avoid pain] to distend the uterine cavity separating the front and back walls. The fluid will act as an echo-free area to delineate any endometrial pathology. This would allow an exact measurement of the endometrial thickness and would show any polyp, fibroid or synechiae missed during basic scanning. To minimise air artifacts which could interfere with proper visualisation of the cavity the catheter should be primed with saline before insertion into the cervical canal. Using ballooned catheters into the uterine cavity would interfere with complete visualisation and could lead to a high level of false negative findings. Such catheters should be used only in cases with patulous cervical canal, for examination of tubal patency and when high pressure is needed in cases with synechiae and large fibroids. The whole procedure should be videotaped to keep a record and allow further examinations in the future.

The uterus should be scanned initially in the longitudinal plane from one side to the other [from one conua to the other conrua]. The probe is then rotated 90⁰ to scan the uterus in the axial plane from the fundus to the cervical canal. Usually few millilitres of fluid are needed to visualise the uterine cavity and unlike hysteroscopy complete distension of the cavity is not necessary. At the end of the procedure the catheter should be removed gently and the patient allowed to lie down for few minutes if she so wished. She should also be warned that she might have some blood loss for a short while after the procedure. We usually give a 5-day course of 100 mg doxycycline but in case of known allergy metronidazole is prescribed instead. 

Indications

SIS could be helpful in establishing a diagnosis in the following situations:

• Abnormal uterine bleeding which is the most common indication
• Abnormally thick endometrium echoes
• Irregular or indefinite endometrial line
• Indiscriminate intrauterine pathology
• Suspected intrauterine adhesions in cases of amenorrhoea or hypomenorrhoea
• Infertility investigations especially in cases with repeated miscarriages.
• For follow up after hysteroscopic surgery especially after myomectomy and incision of a septum

 It is important to remember that subendometrial pathology including adenomyosis, submucosal fibroids and subendometrial cysts could all show as thick endometrium during basic transvaginal scan examination.

Practical implications of the test

• A thin endometrium [< 5mm] with an intact cavity would exclude the possibility of any endometrial pathology and the need for further invasive investigations. Each endometrial wall should be measured separately and the total measurement should not include the fluid in the cavity.
• A universally thick endometrium would be well represented by an office endometrial biopsy.
• A focal abnormal area would indicate a hysteroscopically targeted biopsy. Small focal lesions off the midline may be missed with an office biopsy which samples less than 5% of the cavity [Rodriquez G C et al, 1993, Am J Obstet Gynecol, 168: 55-59]

With such a simple procedure many women would avoid unnecessary invasive operations and the need for general anaesthesia. In our unit saline infusion sonohysterography has replaced diagnostic hysteroscopy as the second line of investigations for abnormal uterine bleeding. This is especially so since previous reports showed that a high percentage of submucosal fibroids could be missed with hysteroscopy which is only resorted to for operative purposes .

 •          SIS revelations

• A normal cavity appears as an anechoic space surrounded by symmetric endometrium on all sides. 
• Focal lesions are easier to see because each single layer of the endometrium is visible separately with saline acting as a background contrast medium.
• Polyps appear as echogenic structures surrounded by the anechoic fluid. The stalk  might be evident at one side.

•      Submucous fibroids push into the cavity with normal overlying endometrium. •They could be echogenic or relatively echo poor lesions with broad base continuous with the myometrium. They might as well cause shadowing.

• Intrauterine adhesions could appear as bright bands across the cavity but as well they could prevent uterine cavity dilatation with saline depending on the severity of the condition and location of the adhesions.

2D sagittal and 3D rendered views of a normal uterus after SIS. The 3D view show a normal triangular cavity which is an important finding especially during investigation of recurrent miscarriages.

Complications

SIS has minimal side effects in a well selected population of patients. Discomfort is the most common one though some patients might feel more cramps similar to period pains. Very rarely vasovagal reactions might occur even after the end of the procedure. These are mere nuisance in young women but could cause hypotension in older patients with atherosclerosis. Genle handling and prompt cessation of the procedure in response to patients' discomfort would reduce these risks. Infection is another risk but is rarely seen if aseptic techniques are used and the patients had no active or recent history of PID. We use routine antibiotics cover as mentioned before in all cases.

3D Ultrasound SIS

Volume acquisition of the uterus during 3D ultrasound examination while injecting saline allows archiving of a complete set of data to be examined in more detail and in different planes. It could also lead to a shorter and less manipulative procedure with smaller amount of saline injected, all contributing to less patients discomfort.

Though different studies showed 2D SIS to be equally diagnostic as 3D SIS, using the multiplanar technique after volume acquisition allows better identification of the exact nature and site of the intracavitary pathology which helps the treating physician planning his or her management. 

 These pictures show the following: 

• 2D sagittal view of the uterus showing a fibroid in the middle of the uterus. It does not show the exact location of the fibroid in relation to the cavity.
• The second photograph shows a 2D AP oblique view of the same uterus after instillation of saline. The fibroid looked like grade 1 submucous fibroid.
• The third photograph shows a coronal view of the same uterus after instillation of saline showing an intracavitary fibroid on the right side upper corner of the cavity. As well, it delineates how much of the fibroid lies within the muscle or the uterine cavity. It was grade 0 in this case. This is an important piece of information prior to hysteroscopic excision.  

It is also important to know that SIS has a very high negative predictive value and could save many patients unnecessary operations.

Contraindications

There are 2 main contraindications to SIS which are:

1. Pregnancy.
2. Pelvic infection