Keywords cranial ultrasound - fontanella mastoidea - brainstem glioma
Introduction
Transcranial ultrasonography is a reliable, inexpensive, and noninvasive tool and
therefore often used for initial bedside diagnostics when neurological or nonspecific
symptoms (e.g., seizure, elevated anterior fontanel [AF], and lethargy) are apparent
in neonates and infants in the first months of life. The standard cranial ultrasound
(CUS) approach is through the AF in sagittal and coronal projections. The standard
transducer frequency for neonatal CUS is 8 to 11 MHz. A better depth penetration can
be reached by decreasing the transducer frequency (5–6 MHz). However, lower frequencies
reduce the object resolution of the ultrasound image.[1 ]
For the depiction of the posterior fossa (PF) in preterm infants and neonates, an
extremely small high-frequency microconvex transducer is used (8–11MHz with the standard
microconvex probes and even higher, up to 15–18 MHz, with linear probes). The mastoid
fontanelles (MFs) are located at the intersection of the parietal, temporal, and occipital
bones.[2 ] To perform the examination using the MF as an acoustic window, the examiner places
the transducer in the mastoid area behind the ear and then moves and changes direction
until able to view the PF. Images should be documented in axial (also transverse)
and coronal planes at differential levels.[1 ]
[3 ]
[Figs. 1A ] and [1B ] show the anatomic structures of a neonatal skull and the axial position of the transducer.
Starting the examination with the probe caudally angled allows visualization of the
cisterna magna, cerebellar vermis, fourth ventricle, and both cerebellar hemispheres.
Sweeping the probe in the rostral direction allows for visualization of the tentorium,
aqueduct, and pedunculi. The posterior horn of the lateral ventricle and the associated
choroid plexus can be detected in the supratentorial area. Near the transducer, the
flow of the transverse and sigmoid sinuses can be shown which can be useful in the
diagnosis of sinus vein thrombosis.[4 ]
[Fig. 2 ] is a coronal view through the MF showing the normal anatomy of the PF. The neonate
may experience an auditory response to pulses of radiofrequency energy which could
cause discomfort.[5 ] For this reason, the views through the MFs should be performed at the end of the
examination.
Fig. 1 (A ) Anatomic structure of a newborn scull (
http://radiopedia.org/images/19305134
); (B ) retroauricular position of the convex probe.
Fig. 2 Coronary cranial ultrasound through the mastoid fontanelle (MF) of a healthy newborn.
Cerebellum (C), fourth ventricle (IV, V), Cisterna magna, temporal lobe (TL), brainstem
(BS).
Case Presentation
Following an uneventful pregnancy, a 5-week-old girl presented with moderate inspiratory
stridor at a local medical facility. On clinical examination, a benign neonatal laryngomalacia
was suspected. However, due to the progression of symptoms, she was admitted to our
hospital. On admission, she showed signs of severe inspiratory stridor but otherwise
uneventful clinical and neurological examination. Echocardiography excluded aortic
anomalies, such as a vascular ring. Elective bronchoscopy demonstrated dysfunctional
inspiratory motility of vocal cords but no signs of tracheomalacia. During the first
3 days of admission, the patient's clinical condition deteriorated, and the infant
developed neurological symptoms including nystagmus, muscular hypotonia, and apnoeic
episodes. On admission to the pediatric intensive care unit, the clinical examination
revealed a bulging fontanelle, muscular hypotonia, and lethargy.
Immediately, point of care ultrasound was performed and showed a heteroechogenic lesion
in the brain stem ([Fig. 3A ]). Subsequently, magnetic resonance imaging (MRI; [Fig. 3B ]) confirmed the infratentorial tumor extension localized in the brainstem. The patient
underwent emergency tumor biopsy and external ventricular drain. The histopathological
results confirmed a diffuse midline glioma, histone H3 K27M mutation, which has a
poor prognosis. Sadly, despite external ventricular drainage, neurological symptoms
of obstructive hydrocephalus progressed, and following review of therapeutic options,
the patient was placed on palliative care at the age of 7 weeks. Standard CUS window
via the AF demonstrated enlarged lateral ventricles but was unable to visualize the
infratentorial structures. Additional CUS views via the MF were obtained which showed
a 2.5 × 1.8 cm heteroechogenic lesion with poorly differentiated borders. The texture
of the mass seemed to be nonhomogeneous, with isolated cystic structures within it.
There was no increased vascularity, and the mass did not appear to invade the surrounding
tissue. All in all, the mass was suspected to be a brain stem glioma with secondary
obstruction of the third ventricle ([Fig. 3A ] and [3B ]).
Fig. 3 (A ) Transcranial ultrasound through the MF showing a heterogenic mass in the PF, localized
in the brainstem. (B ) T1-weighted images shows inhomogeneous mass with displacement of fourth ventricle
(arrows show the border of the tumor). (C and D ) Transcranial ultrasound through the anterior fontanel (AF), axial and coronal view.
Discussion
Brain tumors in neonates and children younger than a year of age are very rare, and
their clinical presentation is often nonspecific and subtle.[6 ]
Satrom et al reported in 2017 about a newborn with respiratory failure caused by a
congenital diffuse intrinsic pontine glioma. The tumor was only detected by MRI scan.[7 ]
Diffuse midline gliomas with H3 K27M mutation have a dismal prognosis regardless of
their location, the histopathological grading, or the extent of tumor resection. The
median survival of these gliomas is less than 1 year.[8 ]
CUS is the primary neonatal neuroimaging technique. It can be performed early and
repeatedly at the bedside and provides instant diagnostic information. The standard
CUS view is obtained through the AF. This enables reliable views of supratentorial
structures, but visualization of the PF is less optimal due to its distance away from
the tranducer.[9 ] The standard views ([Fig. 3C ] and [3D ]) are inferior to the view through the MF since the echogenic tentorium and vermis
may impede visualization. Also, pathological findings, such as the extension of the
lateral ventricle, can complicate the evaluation of the PF. The MF enables a better
view of the PF.[10 ]
[11 ]
A study from 2015 compared the evaluation of the PF using CUS through the AF and in
addition to the PF. The pathological findings were compared with MRI findings. The
study showed that the additional examination via the MF increased the sensitivity,
as well as the specificity, of the cranial ultrasound.[12 ] Recently, the same working group published a detailed description of the examination
of the fossa posterior.[13 ] Specifically, in preterm infants, CUS has reached a new significance due to the
possibility of detecting the smallest bleedings in the cerebellum. CUS examination
for hypoxic–ischemic encephalopathy in term infants should include views through the
MF. Furthermore, the CUS via the MF can be useful when suspecting an intracerebral
infection or malformation, such as Arnold Chiari and Dandy Walker. To our knowledge,
there is no report of a diffuse midline glioma that has been detected via ultrasound
through the MF.
Conclusion
Our case report demonstrated the benefit of using ultrasound view through the MF.
This extended ultrasound window is most valuable for evaluating PF malformations as
it provides highly detailed anatomic views of the cerebellum, fourth ventricle, and
cisterna magna. In the presented case, the lesion and secondary CSF flow obstruction
were detected on bedside examination, guiding further diagnostic modalities (MRI),
as well as indication for release of ventricular dilatation in occlusive HC.
