# Neuroimaging of Pediatric Cerebellum in Inherited Neurodegenerative Diseases
## 1 Introduction
### Pontocerebellar Hypoplasia  
Pontocerebellar hypoplasia (PCH) is a group of neurodegenerative disorders with autosomal recessive inheritance and prenatal onset, mainly affecting the survival and growth of neurons in the cerebellar cortex, dentate nuclei, ventral pontine, and inferior olivary nuclei. Microcephaly, neocortical atrophy, and ventriculomegaly are also usually observed. The common MRI findings include cerebellar hypoplasia and a variable degree of cerebellar and ventral pons atrophy [5].  
To date, there have been identified 19 genes implied in 13 PCH subtypes (OMIM). Like the genetic spinal muscular atrophies, Type 1 is characterized by the loss of motor neurons in the spinal cord [6]. Prenatal ultrasounds are reported to show arthrogryposis and polyhydramnios, while postnatally neonates present with hypotonia, primary hypoventilation, impaired swallowing with consequent feeding difficulties, and progressive microcephaly [7]. In PCH type 2, which represents the most frequent PCH and the best-studied subtype, the distinctive features are dystonia and dyskinesia, severe microcephaly, and, more rarely, pure spasticity [8; 9]. Prenatal ultrasounds are usually unremarkable and pregnancies uneventful. Affected neonates present usually, but not always, with dysphagia, respiratory and feeding difficulties, while extrapyramidal dyskinesia with mixed spasticity develops later. PCH2 is often fatal in early childhood. Affected children develop progressive microcephaly, visual impairment, seizures, and severe cognitive and motor delays. Impaired motor development with a lack of voluntary hand control is not uncommon. PCH type 4, former known as olivopontocerebellar hypoplasia, is a more severe variant of PCH2 with prenatal onset even of clinical manifestations such as polyhydramnios, contractures, and babies usually do not live beyond the neonatal period [10]. PCH1 B, C, and D share a common pathway involving the EXOSOME complex, while PCH2 A, B, C, F, PCH4, and PCH5 are related to mutations in the TSEN complex [11].  
Overall, MRI in PCH patients shows a variable degree of cerebellar hypoplasia with involvement of pons and cerebrum. On coronal imaging, the particular shape of the cerebellum, named \"dragonfly\" or \"butterfly\" appearance, was described by [PERSON] et al. [6]. The cerebellar hemispheres are particularly atrophic, with a substantial sparing of the vermis. Flattening of the caudate nuclear heads can be present [5]. Additional inconstant findings are cerebellar cysts (PCH 1, 2, and 6), genesis of the corpus callosum (PCH 9), simplified gyral pattern (PCH 6 and 10), and hypomyelination (PCH 1, 2, 6, and 9). In several PCH, pons could be relatively preserved, and in PCH10, the cerebellum is mostly normal 11. The presence of C-shaped inferior olive has been reported in PCH2, suggesting a prenatal onset 9 (Figure 1).  
A dragonfly-like pattern has been described in PCH 1, 2 (Figure 1), 4, 6 (Figure 2), and 9.  
Figure 1: PCH 2. Brain MRI scan of a 1-year-old girl with TSEN 54 mutation. MR images demonstrate the characteristic “dragonfly-like” cerebellar pattern: severe hypoplasia of the cerebellar hemispheres with a relative sparing of the vermis. Note the flat ventral surface of the pons and thin corpus callosum.  
Figure 2: PCH6. Brain MRI scan of a 5-year-old boy. The size of the pons is almost normal, the vermis grossly diminished with enlarged interloglial spaces. Vermis and hemispheres are equally hypoplastic.