Pontocerebellar Hypoplasias – An Image-based Diagnostic Algorithm

Background: Pontocerebellar hypoplasias (PCH) represent a heterogeneous group of disorders with reduced volume of pons and cerebellum. The term is purely descriptive. The pathogenesis is variable (pre-/peri-natal acquired, genetic malformative, genetic degenerative). Grouped among the PCH are: PCH types 1–12 (with OMIM entry), forms associated with mutations in CASK, VLDLR, RELN, as well as PCH associated with rare variants in WDR81, ITPR1, DKC1, CNTNAP1, CDG syndromes, dystroglycanopathies, tubulinopathies, and cerebellar disruption in extreme prematurity.

Objective: Elaboration of an MR image-guided diagnostic algorithm to narrow the differential diagnosis and prompt more targeted genetic testing, based on literature reports and personal observations.

Results: A few infratentorial (and possibly associated supratentorial) findings are seminal for some PCH types: Agenesis of corpus callosum and figure eight shape of the mesencephalon (PCH9), non-lobulated cerebellar vermis (VLDLR, RELN, and MAB21L1). A “dragonfly pattern” on coronal view through the posterior fossa (i.e. vermis is less affected than the hemispheres) is typical for PCH2, PCH4, PCH9, but also for cerebellar disruption of prematurity. Other types (as associated with CASK) usually show no “dragonfly pattern”. Cerebellar cysts can evolve in PCH1 and PCH2. Hyperintensity of the cerebellar cortex has been reported in patients with PCH2 and PCH7. In some forms (PCH1B, PCH6) pontine hypoplasia can be missing, those of the cerebellum in PCH10, making recognition as a PCH form very difficult. Reduced supratentorial gyral pattern can be simplified in forms associated with CASK, RELN, and VLDLR. For some PCH forms, a detailed analysis of the general imaging pattern is not yet available due to lack of appropriate images in publications. The clinical context [e.g. peripheral neuropathy and motor neuron degeneration (PCH1), optic nerve atrophy (PCH3, CASK), respiratory failure and congenital contractures (PCH4)] can contribute significantly to a more specific classification.

Conclusion: A systematic MR analysis can assist in the narrowing of the differential diagnosis and a more targeted genetic testing. Some imaging patterns are diagnostic.

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