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1
Kaiser Permanente, 4647 Zion Ave., San Diego, CA 92120.
2
Department of Radiology, University of California, 505 Parnassus Ave., M-396,
San Francisco, CA 94143-0628.
Received May 14, 1999;
accepted after revision July 22, 1999.
Address correspondence to R. B. Goldstein.
Abstract
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MATERIALS AND METHODS. In 1995 we changed our protocol of neonatal cranial sonography to include imaging through the posterolateral fontanelle in all patients. The reports of all sonography performed in the first 15 months of this protocol were reviewed, and two radiologists reviewed the images of all patients in whom a posterior fossa abnormality was diagnosed with posterolateral fontanelle images masked and then with posterolateral fontanelle images available.
RESULTS. In total, 1292 sonograms were obtained in 462 patients. In 200 patients, the sonographic findings were abnormal; of these 200 patients, 24 (12%) had posterior fossa abnormalities (nine posterior fossa hemorrhages, four Arnold-Chiari malformations (type II), two posterior fossa arteriovenous malformations, and nine partial vermian defects). The posterolateral fontanelle images showed the posterior fossa abnormality better than the anterior fontanelle images did in 23 (96%) of the 24 patients, increased confidence in the diagnosis of 18 (75%) of the 24 patients, and was the only technique to reveal the posterior fossa abnormality in 11 (46%) of the 24 patients. Nearly all pathologic correlations with imaging confirmed the posterolateral fontanelle findings except for the diagnosis of inferior vermian agenesis, which was presumed to be a false-positive diagnosis in four patients in whom MR imaging showed no abnormalities.
CONCLUSION. Additional imaging through the posterolateral fontanelle during routine neonatal cranial sonography added considerable benefit. False-positive diagnosis of vermian defects is a troubling problem but may be avoided with careful attention to the midline sagittal sonographic images of the vermis and fourth ventricle.
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In 1995 we added posterolateral fontanelle angled axial images of the cerebellum to the conventional anterior fontanelle images obtained in all cranial sonography performed at our institution. The purpose of this review of our first 15 months' experience is to determine the benefit of adding posterolateral fontanelle imaging to the conventional anterior fontanelle examination.
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We reviewed the reports of all of the cranial sonograms obtained during the first 15 months of using routine posterolateral fontanelle sonography in addition to our standard cranial sonography (February 1995 through April 1996). Each patient whose sonographic report indicated a posterior fossa abnormality was identified.
Scanning was performed portably in the neonatal intensive care unit by experienced sonographers using phased array multiple foci 5.0- or 7.0-MHz sector transducers (XP; Acuson, Mountain View, CA). During each cranial sonogram six to eight images were obtained through the anterior fontanelle in both the coronal and parasagittal planes for a total of 14-16 images, including images of the posterior fossa. After this imaging, additional focused images of the posterior fossa were obtained through the posterolateral fontanelle. The transducer was placed over the posterolateral fontanelle (using the most accessible upside fontanelle) by gently bending the superior portion of the pinna down to find the acoustic window of the posterolateral fontanelle. The transducer was then rotated slightly using real-time imaging to obtain the appropriate angled axial image depicting the cisterna magna, cerebellar hemispheres, vermis cerebelli, fourth ventricle, tectal lamina, and pons. Four to six images were obtained in this plane from superior to inferior, generally imaging from the incisura tentorii cerebelli through the inferior vermis. A normal cerebellum imaged through the posterolateral fontanelle shows the biconvex dorsal margin of the cerebellar hemispheres, normal cerebellar vermis, and the fourth ventricle (Fig. 1A).
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Almost all neonates were imaged through the posterolateral fontanelle during this study period. The only exceptions were some neonates on ECMO (vascular access catheter placement would not allow slight turning of the neck). The precise number of ECMO neonates not examined through the posterolateral fontanelle could not be determined from this retrospective review. No patient in our study with a recognized posterior fossa abnormality was examined only through the anterior fontanelle.
The cranial sonograms of the patients with posterior fossa abnormalities were independently reviewed by two radiologists, unaware of clinical diagnosis, in the following fashion: first, with the posterolateral fontanelle images masked, the conventional coronal and parasagittal images obtained through the anterior fontanelle were reviewed. Note was made of the presence or absence of a posterior fossa abnormality; and when a posterior fossa abnormality was present, the examiner was asked to make a specific diagnosis if possible (e.g., cerebellar hemorrhage, intraventricular or extraaxial hemorrhage, ventricular dilatation, vermian agenesis, or other abnormality). Next, the posterolateral fontanelle images were unmasked and the entire sonogram was rereviewed independently by two observers. Again, the presence or absence of posterior fossa abnormality was noted and a specific diagnosis was sought. Finally, with all the images available, each examiner was asked to make an independent subjective assessment of which images better displayed the posterior fossa abnormality and whether adding the posterolateral fontanelle images enhanced the confidence with which he or she made the diagnosis of posterior fossa abnormality.
Clinical follow-up was obtained through discussion with the neonatologists, review of clinical charts, study of any additional imaging, and review of available autopsy results to determine if findings on the posterolateral fontanelle images significantly altered the clinical diagnosis or prompted additional imaging studies on the neonate (including sonography, MR imaging, or CT). Confirmatory studies or follow-up for each patient were noted when available.
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Posterior Fossa Abnormalities
Of the 24 patients with posterior fossa abnormalities, nine had posterior
fossa hemorrhage: six had isolated posterior fossa hemorrhage, one had
hemorrhage as a result of posterior fossa dural arteriovenous malformation,
and two had fourth ventricular and extraaxial blood associated with marked
supratentorial subependymal hemorrhage (SEH) and intraventricular hemorrhage
(grade III SEH). Of the remaining patients, four had findings related to the
Arnold-Chiari malformation II (small posterior fossa, effaced cisterna magna),
two had posterior fossa arteriovenous malformations without hemorrhage, and
nine neonates were thought to have partial (inferior) vermian defects, or
inferior vermian agenesis.
Images of the Lesion
The examiners thought the posterolateral fontanelle images displayed the
posterior fossa abnormality better than the anterior fontanelle images in 23
of the 24 patients. The single abnormality better seen with anterior
fontanelle images was an extremely small and distorted posterior fossa related
to an Arnold-Chiari malformation II. The severely distorted posterior fossa
obscured the usual posterior fossa landmarks, and the conventional anterior
fontanelle images were more easily interpreted in this patient.
The posterolateral fontanelle images increased confidence in the diagnosis of abnormalities in 18 (75%) of the 24 unhealthy patients (Figs. 1A, 1B, 1C, 2A, 2B, 3A, 3B, 3C). These 18 neonates included eight of the nine patients with posterior fossa hemorrhage (Figs. 1A, 1B, 1C and 2A, 2B), including both patients with fourth ventricle hemorrhage associated with Grade III SEH (Fig. 4A, 4B), one patient with Arnold-Chiari malformation II, and all nine neonates with suspected inferior vermian agenesis (Figs. 5A, 5B and 6A, 6B).
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In five (21%) of the 24 patients, although the abnormality was better seen on the posterolateral fontanelle images, the improved visualization was not considered to increase examiner confidence in diagnosis because the lesion was already adequately displayed by the anterior fontanelle images. These five abnormalities included both posterior fossa arteriovenous malformations, two small posterior fossae related to the Arnold-Chiari malformation II, and one 5-cm posterior fossa hemorrhage. In one of the 24 patients, the abnormality of the posterior fossa was better visualized using conventional anterior fontanelle imaging.
In 11 (46%) of the 24 patients, one or both examiners saw significant abnormalities exclusively on the posterolateral fontanelle images. These abnormalities included three cases of cerebellar hemorrhage, both cases of fourth ventricular and extraaxial hemorrhage, and six of the nine cases of suspected inferior vermian agenesis. Confirmatory studies were obtained in six of the 11 cases: two of the three posterior fossa hemorrhages were confirmed with autopsy or MR imaging; the other was monitored with serial sonograms showing evolution and gradual resolution of the lesion. In two cases (posterior fossa extraaxial and intraventricular hemorrhage) the abnormalities were not confirmed with additional imaging but were associated with large supraventricular subependymal and intraventricular hemorrhages (grade III SEH) (Fig. 4A, 4B). Of the six cases of possible or probable inferior vermian agenesis detected only on the posterolateral fontanelle images (Figs. 5A, 5B and 6A, 6B), four were evaluated with MR imaging. Three were shown to have a normal vermis cerebelli (false-positive sonographic diagnoses) on MR imaging and one showed inferior vermian agenesis.
In total, follow-up MR images, autopsy, or follow-up sonography was available for 16 of the 24 unhealthy neonates. Six neonates with posterior fossa hemorrhage underwent confirmatory studies (MR imaging, three; autopsy, one; and serial sonograms, two). One neonate with posterior fossa hemorrhage died of complications of necrotizing enterocolitis and disseminated intravascular coagulation and did not undergo either MR imaging or autopsy (Fig. 3A, 3B, 3C). The two patients with posterior fossa dural arteriovenous malformations had confirmed diagnoses with MR imaging. One patient died at 13 days of life from complications related to a large myelomeningocele and did not undergo MR imaging. Nine neonates were thought to have inferior vermian agenesis as a result of an unusually large communication between the fourth ventricle and the cisterna magna. MR imaging was performed in six of these neonates and showed inferior vermian agenesis in two neonates and normal cerebellum in the other four. Neither MR image nor autopsy was performed in the other three neonates thought to have inferior vermian agenesis.
Altered Diagnosis
Adding the posterolateral fontanelle images to our standard cranial
sonograms altered diagnosis or prompted additional evaluation (including
imaging studies) in 12 of the 24 patients identified with posterior fossa
abnormalities. Three posterior fossa hemorrhages and five suspected inferior
vermian ageneses were visible only on the posterolateral fontanelle images. In
three other posterior fossa hemorrhages and one suspected inferior vermian
agenesis the conventional images showed an equivocal area that was interpreted
as definitely abnormal on the posterolateral fontanelle images.
Interobserver Agreement
Good interobserver agreement was seen. The examiners agreed with the
diagnosis and "improvement" or "no improvement" of the
posterolateral fontanelle image for the diagnosis in 23 of the 24 neonates.
The examiners differed in the confidence of the findings on the anterior
fontanelle images in only one patient. One examiner interpreted the images as
having normal findings; the other interpreted the image as showing a
"possible abnormal area." Both examiners interpreted this
patient's posterolateral fontanelle images as showing posterior fossa
hemorrhage.
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It has recently been hypothesized that bringing the high-frequency transducer closer and more perpendicular to many of the posterior fossa structures should improve image clarity. Several recent studies have confirmed improved visualization of the normal posterior fossa using imaging through a variety of posterior acoustic windows, including the foramen magnum [4], the posterior fontanelle [5, 7], and the posterolateral fontanelle [8, 11].
The technique for obtaining images through the posterolateral fontanelle is learned easily and quickly. After only a few "learning cases" the images may be obtained in less than 5 min, and our sonographers consistently obtain the images in less than 2 min, usually in less than 1 min.
On the basis of these early reports and our own experience, we added routine posterolateral fontanelle imaging to our standard cranial sonography. Posterior fossa anatomy is unequivocally better displayed using the posterolateral fontanelle, as are posterior fossa abnormalities. In 23 (96%) of the 24 sonographically diagnosed posterior fossa abnormalities, two examiners independently concluded that the abnormality was better seen using the posterolateral fontanelle images. Even if the posterior fossa abnormality could be identified on the anterior fontanelle images, the examiners in this study concluded that adding the posterolateral fontanelle images resulted in increased confidence in diagnosis in 18 (75%) of the 24 cases.
In 11 (46%) of the 24 patients, the posterolateral fontanelle approach allowed detection of significant findings that were not clearly seen on anterior fontanelle imaging, especially in the seven neonates with posterior fossa hemorrhage. One hemorrhage was well seen with anterior fontanelle imaging. The other six were poorly seen or not detected at all using anterior fontanelle imaging, yet confidently identified using posterolateral fontanelle imaging. Six of these hemorrhages were confirmed by autopsy (n = 1), MR imaging (n = 3), or follow-up sonography (n = 2). There were no false-positive diagnoses of posterior fossa hemorrhage. Only three of the nine neonates with posterior fossa hemorrhage had died at the time of this writing. Whereas large and catastrophic hemorrhages have generally been described in the literature, posterolateral fontanelle imaging allowed the detection of some relatively small cerebellar hemorrhages in this study. Two large autopsy studies of low-birth-weight premature neonates have reported an incidence of posterior fossa hemorrhage between 16% and 21% even though this hemorrhage is less common than supraventricular and intraventricular hemorrhage [12, 13]. In one of these studies [13] the findings of cerebellar hemorrhage fell into two groups: large hemorrhages destroying one third or more of the cerebellar parenchyma and small hemorrhages no larger than 5 mm.
The observation of small cerebellar hemorrhages using the posterolateral fontanelle approach in premature neonates has recently been described [14]. The clinical significance of these otherwise undiagnosed hemorrhages is currently under investigation.
Unfortunately, better visualization of structures not well visualized in the past can lead to false-positive diagnoses when one embarks on using this new technique. In this study at least four false-positive diagnoses occurred as a result of adding the posterolateral fontanelle images; all were misdiagnoses of possible inferior vermian agenesis (Fig. 5A, 5B) that deserve mention. These errors were made as a result of misinterpretation of a small apparent communication between the fourth ventricle and the cisterna magna that was mistakenly thought to represent an inferior vermian defect. The erroneous diagnoses were all made early in our study. This imaging pitfall has been described in the fetal sonography literature (angled axial and coronal images of the posterior fossa similar to our posterolateral fontanelle images are easily and commonly obtained) [15]. Angled axial images of the cerebellum can make a prominent vallecula appear similar to a partial (inferior) vermian defect. Barkovich et al. [16], who used MR imaging to investigate Dandy-Walker syndrome, found that the normally formed vermis may tilt forward from its usual position and create the impression of an inferior vermian defect on angled axial and coronal images. We believe that scrutiny of midline sagittal images of the vermis cerebelli obtained through the anterior fontanelle may serve to arbitrate in equivocal cases (Figs. 5B and 6B). A midline sagittal image allows display of the precise cerebellar anatomy because the nodulus of the vermis covering the inferior roof of the fourth ventricle can be better seen, indicating whether the inferior vermis is intact or deficient. This experience emphasizes the caution with which the diagnosis of inferior vermian agenesis should be made.
In conclusion, ours is a retrospective study, and the overall sensitivity of the posterolateral fontanelle images for posterior fossa abnormalities is not evaluated. However, in our review of 15 months of experience with these images, the posterolateral fontanelle images revealed three posterior fossa hemorrhages not detected with standard anterior fontanelle imaging and confirmed three more that were only suspected on the standard images. Nearly every abnormality observed was considered better displayed on the posterolateral fontanelle images, and adding the posterolateral fontanelle images allowed increased confidence in diagnosis in 75% of the posterior fossa abnormalities. Only 1 or 2 min of additional scan time is needed to obtain these images. The potential pitfall of overdiagnosing inferior vermian agenesis should be anticipated and easily avoided.
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