case_images: PMC9168403_01
case_images: Axial T1-weighted brain MRI with contrast shows a large left cerebellar
  arteriovenous malformation.
case_images: SNI-13-202-g001.jpg
case_images: PMC9168403_01_SNI-13-202-g001.jpg
case_images: F1
case_images: Eight years prior, he was diagnosed with a left cerebellar arteriovenous malformation during workup for headaches [Figure 1].
case_images: Skull X-rays films (a) anteroposterior view and (b) lateral view showing
  abundant air inside the ventricles (black arrow).
case_images: SNI-13-202-g002.jpg
case_images: PMC9168403_01_SNI-13-202-g002.jpg
case_images: F2
case_images: The skull X-rays showed abundant air inside the ventricles, and we thought that it could be the source of his symptoms [Figure 2].
case_images: Head CT scan reconstruction images (a) coronal brain window view, (b) coronal
  bone window view, and (c) sagittal bone window view showing the left
  ethmoidal meningoencephalocele (yellow arrow) and the fistulous air tract
  extending into the ventricle (red arrow).
case_images: SNI-13-202-g003.jpg
case_images: PMC9168403_01_SNI-13-202-g003.jpg
case_images: F3
case_images: CT scan confirmed the initial radiographic impression of tension pneumoventricle without evidence of an acute traumatic brain injury. Coronal and sagittal head CT reconstructions were performed later by the neuroradiologist and showed a left ethmoidal meningoencephalocele with a fistulous air tract extending into the ventricle, which was not identified on the initial axial images [Figure 3].
case_images: Coronal T2-weighted brain MRI (left and right) shows a small left frontobasal
  meningoencephalocele herniating into the ethmoid (yellow arrow).
case_images: SNI-13-202-g004.jpg
case_images: PMC9168403_01_SNI-13-202-g004.jpg
case_images: F4
case_images: In our investigation toward the etiology of the pneumoventricle, a review of previous head CT scans and MRIs showed that the patient had a small left frontonasal meningoencephalocele extending into the ethmoid which had been previously unnoticed [Figure 4].