Spinal Imaging Findings of Open Spinal Dysraphisms on Fetal and Postnatal MRI

Discussion

We describe our experience with spine imaging findings of OSD on fetal and postnatal MR imaging and have made several observations: Concordance within 1 level of the osseous defect was seen in 82% of patients between pre- and postnatal MR imaging. There was a significant correlation between the level of the defect and lateral ventricular size (p = 0.001), with a higher defect level correlating with larger lateral ventricular size. Fetal MR imaging was limited in its ability to identify split cord malformation in this population. Finally, syrinx was noted in only 3% of prenatal studies; however, all were cervical and all were confirmed on postnatal MR imaging.

Existing literature describing the accuracy of prenatal level assignments of the OSD defect, defined as the highest open posterior vertebral arch, is quite limited. One series described equal accuracy of fetal sonography and fetal MR imaging in level assignments for myelomeningocele compared with postnatal imaging, though it was observed that prenatally assigned levels using either technique may vary by ≥2 segments from the postnatal assigned levels in at least 20% of cases.^17,18 Our study has similar findings and adds to the literature our experience in a larger cohort in which we found that the exact level of concordance between fetal and postnatal MR imaging was only 42.9% (51/119) and was within 1 level of concordance in 82% (98/119) of patients. The reasons are multifactorial and may, in part, relate to the very small size of the fetus being imaged, making it challenging to resolve millimetric-sized landmarks, in addition to the lack of ability to resolve the disc spaces in the entire spine for accurate vertebral body counting on fetal MR imaging. In addition, determination of the defect level may be compromised postnatally by the postoperative changes, making a dysraphic defect at the surgical margin versus scar tissue challenging to differentiate. Also, normal anatomic variations in vertebral body counting, such as the presence of 6 lumbarized vertebral bodies, were not taken in to account on fetal MR imaging. This information is important, particularly when selecting candidates for fetal surgery because fetuses with a defect lower than S1 do not meet the selection criteria per the MOMS trial.¹⁹

There is literature describing the correlation of the prenatal defect level with the postnatal neurologic function, making the identification of the defect level prenatally of potential importance in counseling.²⁰ Despite previous sonographic literature describing no significant relationship between defect level and ventricular size, our study illustrates how a higher spinal defect is associated with larger lateral ventricular size on fetal MR imaging in a larger sample size.²¹ This finding is of potential clinical significance because larger ventricles on prenatal imaging have been shown to be associated with an increased need for shunting in those undergoing fetal surgery.²² On the contrary, despite previous literature describing a higher incidence of foot deformity with higher lesion levels, we did not observe a significant correlation between the fetal spinal defect level and the presence of clubfoot deformity in our cohort.²³ This finding suggests that the pathophysiology of this disease process is complex and influenced by multiple factors, and further studies may be helpful in improving prenatal counseling. Our study also adds the observation that there was no significant correlation between defect level and Chiari grade or third ventricle size.

Prenatal diagnosis of split cord malformation, also known as diastematomyelia, has been described in a few isolated cases; however, the diagnostic reliability of fetal MR imaging in a larger population is still unclear.^24,25 Our series adds to the literature by demonstrating a relatively limited ability of fetal MR imaging to identify type II split cord malformation. The lack of our ability to consistently identify split cord malformation prospectively on fetal MR imaging can likely be explained by the very small size of the fetus being imaged. However, it is less clear why cases in which split cord malformation was questioned prenatally were not confirmed on postnatal MR imaging. It is possible that clumped nerve roots from the ventral neural placode or a prominent anterior median fissure of the spinal cord was mistaken for split cord malformation on fetal MR imaging.^26,27 Another possibility is that postoperative changes from OSD closure obscure the underlying split cord malformation on postnatal MR imaging. While the association between type II split cord malformation and open spinal dysraphisms is known and does not preclude fetal surgery, knowing that our ability to diagnose this prenatally is limited may be helpful in counseling. There are notable clinical implications of split cord malformation in this population: Not only do these patients often require more extensive surgical untethering early in life or at the time of primary repair, but they are also at increased risk of tethered cord syndrome and progressive scoliosis postnatally.^{28⇓⇓–31}

Our study also adds to the literature a description of imaging findings on postnatal spine MR imaging in patients with OSD after both prenatal and postnatal repair. Most notably, we observed that syrinx can be seen in association with open spinal dysraphism on fetal MR imaging.³² We describe its incidence in a relatively large sample of fetuses with open spinal dysraphism (3.4%, 4/119) and demonstrate a high specificity of fetal MR imaging, particularly in the third trimester, because all fetuses with a prenatally diagnosed syrinx in our series had the same findings on postnatal MR imaging. The increased incidence of syrinx on postnatal MR imaging (39.5%, 47/119) compared with prenatal MR imaging (3.4%, 4/119) is likely explained by ongoing associated abnormalities of CSF flow dynamics in this population, including hindbrain herniation, hydrocephalus, and tethered cord rather than a missed congenital syrinx.³³ Although postoperative fluid collections were more frequently seen in the postnatal repair group (46.2%, 36/78) compared with the prenatal repair group (5.3%, 2/38), this finding can likely be explained by the relative time of imaging after the operation. We also add our observed incidence of intraspinal arachnoid cysts on postnatal spine MR imaging in this patient population (3.4%, 4/119), which was seen equally in those that underwent prenatal versus postnatal repair.

Our study has some limitations. First, this is a retrospective study, which limits its internal validity. Also, given that this is a single-institution study performed within a certain timeframe, its external validity may be limited as well. Along those same lines, this study is also likely subject to some degree of selection bias because the data were collected from one of the largest referral centers in the country for prenatal repair of open spinal dysraphisms. Also, given that imaging studies were acquired during a 12-year period on multiple different clinical magnets with periodic upgrades, the heterogeneity of scanning parameters may affect our results as well. In clinical practice, sonography is often used in conjunction with fetal MR imaging to determine the defect level of an open spinal dysraphism, and in our practice, we emphasize the sonographic findings for defect level in fetal counseling because some believe that sonography can better delineate the osseous structures of the fetal spine than fetal MR imaging.³⁴ However, given that sonography is highly operator-dependent, which is problematic in clinical practice and in the context of a retrospective analysis, studies in fetal MR imaging of the spine are becoming increasingly important.