Accepted on 03 Oct 2022
Submitted on 30 Sep 2022
Pearls and pitfalls facilitating the diagnosis of cerebrovascular diseases, tumors, and neurodegenerative and inflammatory disorders in the posterior fossa are reviewed, and incidental ‘leave alone lesions’ with typical magnetic resonance imaging (MRI) findings are discussed.
Cerebrovascular diseases may present with ischemic or hemorrhagic events. The most common lesions in the posterior fossa are small thrombo-embolic infarcts restricted to the cerebellar cortex. Large infarcts affect the PICA – much more frequently than the AICA – or SCA-territories. Symptomatic hemorrhages are most often related to hypertensive bleeding in either the brainstem or deep cerebellar nuclei.
Cerebellopontine angle (CPA) tumors are a classic differential in neuroradiology composed of schwannoma, meningioma, arachnoid cyst, and epidermoid. With cerebrospinal fluid (CSF)-cisternography (such as 3D DRIVE, FIESTA or CISS), routine gadolinium injection is no longer needed for MRI screening of the CPA. Within the fourth ventricle, subependymoma is the most frequent tumor in adults. Malignancy in the posterior fossa in adults usually represents metastatic disease. Apart from parenchymal metastases, the posterior fossa is a predilection site for leptomeningeal carcinomatosis, readily detected with contrast-enhanced black blood weighted images (Figure 1). Primary midline tumors in the thalami, brainstem, or spinal cord typically indicate diffuse midline glioma harboring the H3K27 mutation, in the pons previously named diffuse intrinsic pontine glioma (DIPG). Homogeneous areas of off-midline brain parenchyma infiltration with T2-hypo- to isointense signal abnormalities, restricted diffusion and enhancement surrounded by (limited) edema should raise suspicion for CNS lymphoma. Dysplastic cerebellar gangliocytoma (Lhermitte-Duclos) is a rare benign cerebellar tumor presenting with a typical ‘corduroy’ appearance. A radiologic decision flow chart may serve as diagnostic tool for pediatric posterior fossa tumors.
Leptomeningeal carcinomatosis on contrast-enhanced black blood images. (A) Contrast-enhancing thickening of the right trigeminal nerve, enhancement of the left trigeminal nerve, enhancement in the recesses between pons, superior cerebellar peduncles and cerebellar hemispheres. (B) Sugar coating of the cranial nerves within the inner auditory canals.
Selective neurodegenerative diseases may show specific findings in the posterior fossa. Midbrain atrophy is seen in Progressive Supranuclear Palsy (PSP), while pons atrophy and the hot cross bun sign are seen in multiple system atrophy cerebellar type (MSA-C). The middle cerebellar peduncle (MCP) sign is a distinctive finding in various diseases, including MSA-C. Diffuse cerebellar atrophy is a non-specific finding, but a specific pattern of focal cerebellar atrophy is highly characteristic for CANVAS (Figure 2). Symmetrical white matter changes within the brainstem should raise suspicion for some of the rare adult-onset leukodystrophies.
Sagittal reformat of 3D-FLAIR shows highly characteristic atrophy of the vermian lobules immediately behind the primary fissure in CANVAS.
Inflammatory-infectious diseases affecting the hindbrain (brainstem and cerebellum) are collectively known as rhombencephalitis. Although Listeria infection stands for some of the best-known etiologies, paraneoplastic and auto-immune causes may occur. Isolated cerebellitis usually affects children. Basilar (tuberculous) meningitis presents with thickened and enhancing leptomeninges in the basal cisterns and is often complicated by arteritis-related infarction (Figure 3). CLIPPERS features the characteristic MRI findings of punctate, patchy and linear regions of contrast enhancement predominating in the pons.
Basilar meningitis (left) with marked thickening and contrast-enhancement of the meninges within the basal cisterns and abscess in the interpeduncular cistern. Hydrocephalus (right) with transependymal edema and brain stem infarction on axial T2.
Incidental lesions are often encountered depending on their location. The quadrigeminal plate is a predilection site for lipomas, while enlarged perivascular spaces are often seen in the cerebral peduncles. The pons is a predilection site for capillary telangiectasia. White matter hyperintensities in certain locations are suspicious for demyelinating disease, while indistinct hyperintense changes occurring centrally in the pons are usually of presumed vascular origin. Discrete hyperintense tumefaction may be seen in hypertrophic olivary degeneration, and cerebral peduncle atrophy in Wallerian degeneration. Incidental ‘leave alone’ lesions in the peripheral regions of the posterior fossa include ecchordosis physaliphora and benign enhancing lesion of the foramen magnum.
The author has no competing interests to declare.