Introduction:
Cerebellar hypoplasia (CH) is characterized by decreased cerebellum volume with preserved shape. In contrast, cerebellar dysplasia is characterized by an abnormality in the maturation of tissue cells. As a result, the size of the cerebellum is either smaller than normal or is not formed completely.
It is generally associated with several congenital malformations syndromes like Walker-Warburg syndrome, inherited metabolic syndromes like Williams syndrome, and some neurodegenerative disorders such as ataxia telangiectasia. Mutations in the VLDLR (Very Low Density Lipoprotein Receptor) gene usually cause it. The gene makes a protein, such as a very low-density protein, which helps guide the movement of the developing nerve cells to their exact locations in the brain.
Therefore, the mutated gene prevents the developing nerve cells from reaching the parts of the brain where they are needed. Cerebellar anomalies can affect either different parts or all of the cerebellar vermis or one or both of the cerebellar hemispheres, or they may involve the entire cerebellum. Therefore, it is important to differentiate between these various forms because each specifically characterizes a different disorder. Neuroimaging using magnetic resonance imaging (MRI) has been a key diagnostic tool to categorize CH based on the pattern of cerebellar involvement and the presence of associated brainstem and cerebral anomalies.
The cerebellum comprises three lobes that are anterior, posterior, and flocculonodular, and ten lobules and is divided into a midline vermis and two hemispheres. The inferior and middle cerebellar peduncles carry the main inputs to the cerebellum:
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The pontocerebellar and spinocerebellar fibers that form the mossy fibers synapse onto the granule cells.
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The olivocerebellar fibers form the climbing fibers and synapse with the Purkinje cells. All the outputs from the cerebellum are relayed by the three cerebellar nuclei (dentate, interposed, and fastigial) and are carried by the superior fibers.
Cerebellar hypoplasia may be classified based on the extent of cerebellar involvement:
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Unilateral Cerebellar Hypoplasia: It affects only one cerebellar hemisphere with or without the involvement of the vermis, and it usually arises from an antenatal vascular hemorrhagic or ischemic insult. The clinical presentation differs from an asymptomatic incidentally detected pathology to one with severe neurologic deficits. The most common clinical features include developmental and speech acquisition delay, hypotonia, ataxia, and eye movement disorders. However, atypical features such as seizures have also been seen in a few cases. In addition, the involvement of the vermis is often associated with cognitive decline and truncal ataxia.
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Cerebellar hypoplasia with predominant involvement of vermis.
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Global cerebellar hypoplasia includes both the vermis and cerebellar hemispheres.
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Hypoplasia involving the pons and cerebellum is usually associated with an intense intellectual deficit and delayed or absent psychomotor milestones. In most cases, the disease is usually very fatal early in life. There are two basic types of pontocerebellar hypoplasia:
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Type 1: There is congenital central and peripheral motor dysfunction leading to early death, mostly before one year. The abnormally small cerebellum and brainstem, including the pons, are accompanied by degeneration of the anterior horn cells. Due to the involvement of the anterior horn cell, type 1 mimics infantile spinal muscular atrophy. The hypoplasia of the pons and cerebellum and spinal anterior horn cell degeneration is associated with pronounced reactive changes or gliosis. It is also associated with reduced fetal movement. The pregnancy may sometimes be complicated by polyhydramnios.
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Type 2: There is progressive congenital microcephaly along with extrapyramidal dyskinesia. Severe chorea and epilepsy are frequent, while the involvement of the anterior horn is absent in this type. The main feature distinguishing type 1 from type 2 is that anterior horn cells are not involved in type 2. Characteristically, unlike type 1, pregnancy is normal. However, the newborn may show breathing difficulties or respiratory failure at birth, requiring mechanical ventilatory support.
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What Are the Signs and Symptoms of Cerebellar Hypoplasia?
Symptoms of cerebellar hypoplasia include
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Floppy muscle tone.
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Developmental or speech delay.
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Problem with walking and balance.
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Intellectual disability.
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Involuntary side-to-side movement of the eyes.
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Headache.
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Clumsiness.
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Dizzy spells.
What Are the Lab investigations to Be Carried Out?
The investigations to be carried out are:
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Physical Examination: It may show remarkable results if any of the following is observed
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Truncal ataxia.
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Abnormal ocular movements.the
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Hypotonia.
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Intentional tremors.
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An accurate clinical assessment will help arrange the appropriate investigation procedures. Extensive neuroradiological imaging with the magnetic resonance of the brain is the most useful method to identify which investigations are required further. It confirms a cerebellar lesion's clinical suspicion and provides information about its distribution, extent, and morphology.
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Magnetic Resonance Imaging (MRI): Neuroimaging helps differentiate cerebellar hypoplasia from atrophic changes. Hypoplasia, dysplasia, or atrophy can be easily identified if the same subject is examined repeatedly at several intervals. Hypoplasia and dysplasia remain unchanged, whereas the involutional process of atrophy progresses and worsens over time.
What Are the Differential Diagnosis for Cerebellar Hypoplasia?
Differential diagnoses include
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Diffuse cerebellar atrophy.
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Pontocerebellar hypoplasia.
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Unilateral cerebellar hypoplasia.
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Isolated inferior vermian hypoplasia.
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Joubert syndrome or vermian aplasia.
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Dandy-Walker malformation.
What Is the Treatment of Cerebellar Hypoplasia?
The prognosis of the disease and the treatment protocol typically depends upon the underlying cause. For example, cerebellar hypoplasia associated with non-progressive disorders such as abnormal brain formation during fetal development has a relatively better prognosis. In contrast, cerebellar hypoplasia associated with progressive conditions has a poor prognosis.
Conclusion
Cerebellar hypoplasia is characterized by decreased cerebellum volume with preserved shape. The size of the cerebellum is either smaller than normal or is not formed completely. It is generally seen in association with several congenital malformations. The symptoms include floppy muscle tone, developmental or speech delay, problems with walking and balance, intellectual disability, hearing loss, and headache. Neuroradiological imaging with the magnetic resonance of the brain is the most practical method to identify such abnormalities. Cerebellar malformations are now diagnosed with increasing frequency in the fetal and neonatal periods. Therefore, accurate prognostic information to guide parental decision-making has become essential.
