TPUS has been widely used as a one of important imaging examinations to assess POP, and has showed advantages in differential diagnosis of the different conditions of anterior and posterior compartments [5, 6].However, it has limitations on demonstration of middle compartment structures because of some technical reasons. On TPUS, the acoustic beam is generally parallel with the cervix during Valsalva maneuver, which inhibit getting optimal images of cervix. And the level of internal cervical orifice is hard to determine with the low frequency transducer used in TPUS, so accurate measurement of cervical length is not feasible in most cases. The introduction of new modality can make up the limitations. The transducer used in TRUS Is different from that of routine one. A 6cm long linear array is lined on the side of the transducer, so the acoustic beam is perpendicular to pelvic organs, which could significantly improve the detection rate and resolution of imaging. Intracavitary high frequency ultrasound had been proved to be useful in visualizing fine structures of anterior department when used in transvaginal way[7, 8] .And until recently, it has been used in transrectal way to evaluate POP, especially middle compartment prolapse. In our study, TRUS showed superiorities to evaluate middle compartment structures in POP patients. First, TRUS can demonstrate entire cervix clearly in most middle compartment prolapse cases both at rest and during Valsalva maneuver, with excellent interobserver repeatability. The detection rates of cervix were higher than 90% on TRUS, whereas the detection rates on TPUS was lower than 50%, and was even less than 10% during Valsalva maneuver. Second, TRUS can reliably investigate the descent of uterus. In TRUS, by being placed in rectum, the transducer does not obstruct the descent of pelvic organs during Valsalva maneuver. So, the degree of descent is closer to that of POP-Q examination. These advantages can provide sufficient information for differentiation of middle compartment prolapse.
Clinically, middle compartment prolapse includes two different entities, uterine prolapse and cervical elongation. And uterine prolapse cases can coexist with cervical elongation 9. In cases of cervical elongation without uterine prolapse, the level I support, the cardinal-uterosacral ligament complex, is not impaired. So, the surgical management for isolated cervical elongation is different from that for uterine prolapse. However, on routine TPUS, the diagnosis of uterine prolapse would be made when the leading edge of cervix is under the level of 15mm above posteroinferior margin of the symphysis pubis at maximal Valsalva. But this concept can not differentiate cervical elongation from uterine prolapse. Recently, Garcia et al[9, 10] proposed that difference of ≥ 15 mm in the distance between uterine fundus and the pubis at rest and during the Valsalva maneuver is useful to identify uterine prolapse from cervical elongation without uterine prolapse. And the same team further proved excellent interobserver reliability of this assessment. In comparison with this indirect evaluation, TRUS can directly measure cervical length, and can show and measure the descent of internal orifice of cervix during valsalva, level of which is close to attachment of sacral ligament, D point of POP-Q system. So TRUS has great potential to differentiate isolated cervical elongation from uterine prolapse. When the cervix is significantly elongated, but the descent of the level of internal orifice is minor, the uterine prolapse can be excluded. To be noted, the original low location of internal orifice at rest, and achieving satisfactory Valsalva maneuver should be taken into consideration during evaluation of the descent. Also, accurate presurgical measurement of the cervical length by TURS can provide further information for trachelectomy in cases hysterectomy is not planned.
Interestingly, our results demonstrated significant difference of cervical length at rest and during maximal Valsalva. In more than 4/5 patients, the cervical parameters were increased during Valsalva maneuver. One of the possible reasons might be that during Valsalva, the cervix will unfold completely. And also, in some cases complicated with severe anterior or posterior compartment prolapse, the cervix maybe be dragged by surrounding prolapsed organs, frequently by bladder. The length of anterior lip is significantly longer than that of posterior one, which may also be caused by the traction of prolapsed bladder. So further study is needed to determine how and when to measure cervical length to keep consistent with the result of POP-Q examinations.
The strength of our study is prospective study design. The limitation is that the study is relatively small number of cases included. However, we believe that 101 cases were enough to verify the advantages of TRUS in the assessment of middle compartment prolapse. And TRUS was new technique for examiners, so the detection rate of cervix had been improved along with the research going on, so the detection rates of our study might be lower than those which could be attained after mastering this technique.
In conclusion, TRUS can significantly raise detection rates of entire cervix, and make the direct evaluation of uterine descent possible in middle compartment prolapse patients. This new technique can be used as a complementary method to TPUS, to attain more comprehensive and accurate presurgical imaging information of middle compartment.