In this article, the author reviews and updates information on several anatomical variants of cerebellar hypoplasia in the context of advances in the embryology and genetic programming of cerebellar development. An expanded differential diagnosis based on literature of associated cerebral malformations, neuronal migration disorders, syndromes, genetic diseases, chromosomal abnormalities, and metabolic disorders is discussed, and the clinical, neuroimaging, and pathological findings are reviewed. Features of disorders associated with an enlarged cerebellum and cerebellar dysplasia are briefly mentioned. The difference between primary cerebellar malformations and prenatal disruptions of cerebellar development is discussed. Cognitive and affective disorders associated with cerebellar malformations are highlighted.
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• Cerebellar hypoplasia is a feature of several neurodevelopmental disorders in which the cerebellum is small in size, but normal in shape.
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• The etiology of cerebellar hypoplasia is diverse and includes various syndromes, genetic diseases, chromosomal abnormalities, and metabolic disorders.
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• Prenatal disruptions of cerebellar development may mimic primary cerebellar hypoplasia.
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• Disorganized development of the cerebellum causes cerebellar dysplasia in which the shape and “texture” of the cerebellum appear abnormal due to the presence of abnormal folia pattern or gray matter nodular heterotopia.
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• Macrocerebellum is a rare malformation that may occur as an isolated finding or as part of a few syndromes.
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• Cerebellar hypoplasia, dysplasia, and enlargement are best visualized on MRI.
Historical note and terminology
Small cerebellums were noted by pathologists of the 19th century. Norman was the first to recognize the "granuloprival" form of selective absence of granule cells, though he attributed the change to a degenerative disease process, as did many subsequent authors (109). Numerous genetic defects and associated anomalies, some due to inborn errors of metabolism, are described in association with cerebellar hypoplasia. Cerebellar dysplasia may be associated with cerebellar hypoplasia, hence, their inclusion in this article. Disorders associated with large cerebellar size are rare and are mentioned briefly.
Cerebellar hypoplasia is a feature of several neurodevelopmental disorders and occurs during fetal life. The cerebellar shape (or the affected part) is generally normal or near normal, but its volume is reduced in cerebellar hypoplasia (124). Cerebellar hypoplasia should be distinguished from acquired and progressive cerebellar atrophy in which tissue loss occurs with secondary enlargement of the cerebellar fissures. Differentiating cerebellar atrophy from hypoplasia can be difficult in practice (124; 23; 05); however, newer techniques using diffusion tractography biomarkers are showing an early promise (55). Degeneration of the granular layer may resemble hypoplasia (117). In addition, there is an overlap between cerebellar development and neurodegeneration, which means that they cannot be classified separately at times (101). Atrophic processes beginning in fetal life (eg, in pontocerebellar hypoplasia type 1) are challenging because they are both developmental and degenerative at the same time. Furthermore, many conditions previously regarded as primary malformations are actually acquired disorders in utero (27) and are referred to as cerebellar disruptions (125).
Hypoplasia may affect the vermis selectively with sparing of the lateral hemispheres; the medial borders of the hemispheres may be fused in the midline (as seen in rhombencephalosynapsis), or a space may remain between them. In Chiari I malformation there is cerebellar tonsils herniation; this is not discussed in this article. Chiari II malformation is discussed briefly. Chiari III malformation is associated with cervical encephalocele.
The most common condition is global cerebellar hypoplasia. Selective symmetrical cerebellar hemispheric involvement with sparing of the vermis is more rare than selective vermal involvement and has been described in disruptive cerebellar development secondary to prematurity and in pontocerebellar hypoplasia type 2 (124). Several malformations of the midbrain and hindbrain have been reviewed and are discussed below (01; 05).