Pontocerebellar Hypoplasia: a Pattern Recognition Approach.

Pontocerebellar hypoplasias (PCH) represent a heterogeneous group of very rare disorders with reduced volume of pons and cerebellum. The term is purely descriptive and does not imply a genetic progressive disease. Currently (as of Jan 01, 2020), 13 different types are listed in OMIM (Online Mendelian Inheritance in Man), associated with 19 different genes. However, a large group of similar imaging patterns is known, and it is unclear why some are labeled as PCH, while others are not. The latter include CASK- and VLDLR-associated disorders, some tubulinopathies, certain dystroglycanopathies, a few congenital disorders of glycosylation (CDG) syndromes, several forms associated with rare variants (e.g., DCK1, WDR81, ITPR1), and "cerebellar disruption of prematurity"-an acquired etiology. The objective of this paper is to elaborate a pattern recognition approach, mainly imaging-based, to facilitate a timely and accurate diagnosis, to narrow the differential diagnosis, and to enable targeted additional (genetic) investigations. We describe magnetic resonance imaging (MRI) findings and offer "checklists" for infratentorial findings (e.g., non-lobulated vermis, dragonfly pattern of the cerebellum, cerebellar cysts, brainstem kinking, longitudinal grooves along the brainstem, flat pons) as well as for supratentorial anomalies (e.g., agenesis of corpus callosum, optic atrophy, simplified gyral pattern, and hypomyelination). The clinical context and laboratory investigations need to be considered as well. We also provide a "checklist" for clinical features. A systematic analysis of imaging and clinical features can assist in narrowing the differential diagnosis and permitting more targeted genetic testing. Some imaging patterns are diagnostic.