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What is Pontocerebellar Hypoplasia?
Pontocerebellar hypoplasia is a group of related conditions that affect the development of the brain. People with these conditions have an unusually small and underdeveloped cerebellum, which is the part of the brain that coordinates movement.
A region of the brain called the pons also fails to develop properly. The pons is located at the base of the brain in an area called the brainstem, where it transmits signals from the cerebellum to the rest of the brain.
Researchers have described six forms of pontocerebellar hypoplasia. These forms have somewhat different signs and symptoms and different genetic causes. All forms of this condition are characterized by abnormal brain development, problems with movement, delayed development, and intellectual disability. The signs and symptoms are usually present at birth, and in some cases they can be detected before birth. Many children with pontocerebellar hypoplasia live only into infancy or childhood, although some affected individuals have lived into adulthood.
Six types of Pontocerebellar hypoplasias have been described.
Pontocerebellar Hypoplasia Type 1 (PCH Type 1)
In Pontocerebellar hypoplasia type 1, there is central and peripheral motor dysfunction from birth leading to early death, mostly before 1 year of age. In addition to an abnormally small cerebellum and brainstem including the pons, there is a degeneration of the anterior horn cells. Because of the anterior horn cell involvement, PCH type 1 has some resemblance to infantile spinal muscular atrophy. The hypoplasia of the pons and cerebellum and spinal anterior horn cell degeneration is also associated with pronounced reactive changes (gliosis).
PCH type 1 is associated with reduced fetal movement. The pregnancy sometimes is complicated by polyhydramnios. In most cases, the condition is obvious during the newborn period when the newborn appears floppy and has respiratory insufficiency. At birth, multiple congenital contractures of large joints (arthrogryposis multiplex congenita) may be present. The newborn may show arreflexia, and combined motor signs. PCH type 1 has the hallmark of severe muscle weakness. The associated hypotonia may start prenatally or after birth. Mental retardation and cerebellar signs of visual impairment, nystagmus and ataxia follow the initial presentation.
It has also been found that some patients with PCH type 1 develop the signs of muscle weakness or developmental delay at the age of several months. These late presenting patients have a milder form and may live up to four years. However, the disease is uniformly fatal. Generally, affected babies have a life span not exceeding a few months in most cases.
In all patients, postmortem examinations reveal variable spectrum of cerebellar atrophy, neuronal loss in the anterior horns of the spinal cord, basal ganglia and brainstem suggesting a more widespread neuronal degeneration.
Unlike spinal muscular atrophy, which has been linked to the long arm of chromosome 5, and survival of motor neuron gene (SMN1), no genetic linkage is known yet for PCH type 1. The inheritance of PCH follows an autosomal recessive pattern
Pontocerebellar Hypoplasia Type 2 (PCH Type 2)
In PCH type 2 there is progressive microcephaly from birth combined with extrapyramidal dyskinesia. There is no motor or mental development. Severe chorea occurs, and epilepsy is frequent, while signs of spinal anterior horn involvement are absent in PCH type 2. The main feature distinguishing PCH type 1 from PCH type 2 is that anterior horn cells are spared in PCH type 2.Characteristically, pregnancy is normal. However, at birth, the newborn may show breathing problems or respiratory failure that may require mechanical ventilatory support. Some may have sucking or feeding problems. Most patients with PCH type 2 are born with normal size head. Some already have microcephaly at birth. All affected children have worsening or progression of the microcephaly during infancy. Other features of dysmorphisms are absent. They have impaired mental and motor development. They have abnormal movements termed extrapyramidal movement disorder. All affected children develop marked extrapyramidal dyskinetic movement disorder with predominance of dystonia. Jerky movements and almost continuous dystonic choreoathetotic movements may be seen. These movement abnormalities are usually noticed during the neonatal period of these children.
They have severe to profound mental retardation. No patient with the classical PCH type 2 ever achieved the milestones of sitting, crawling, standing, walking, talking, or developed meaningful social contact skills. Visual fixation is persistently poor and only about one third of these patients are able to fixate and follow. Seizure disorder is frequent. Approximately half of these children may have seizures. A minority may also have hypotonia or hypertonia even as early as the newborn period. Minority may show spasticity.
They are severely handicapped with no voluntary motor function. The children have severe cognitive and language impairment, and with no verbal or non-verbal communication.
There is a near total loss of Purkinje fibers in the cerebellar hemisphere and an undetectable dentate nucleus. Neuronal loss is marked in the basal ganglia and thalamus without any anterior horn cell involvement when autopsy is done. The vermis is also relatively spared. These features are similar to those seen in PCH type 5 and suggest a continuum of pathology between both PCH type 2 and PCH type 5.
The clinical findings, the severity of movement disorder and the developmental delay do not correlate with the degree of pontine or cerebellar hypoplasia on MRI. It is possible that there is a continuum of severe neonatal and infantile types rather than clearly defined groups.
Death during early childhood has been attributed to respiratory and infectious complications.
Pontocerebellar Hypoplasia Type 3 (PCH Type 3)
PCH type 3 has many features in common with PCH type 2. However, in PCH type 3, extrapyramidal symptoms (dyskinesia) are absent. The children may have seizures and microcephaly, which are the result of poor brain development and small size of the cerebellum and pons that affect the overall size of the brain.PCH Type 3 is a unique form described in three sibs of a consanguineous family from the Sultanate of Oman. Clinical features in these affected children include developmental delay, progressive microcephaly with brachycephaly and seizure in the first year, truncal hypotonia with exaggerated deep tendon reflexes, short stature and optic atrophy. One of the three children had thoracic scoliosis contractures of the elbows and knees, and clubfoot. Visual impairment including optic atrophy may be seen in affected patient. Other features include brachycephaly, prominent eyes, and low-set ears. There was no extrapyramidal involvement or dyskinesia.
Imaging studies of the brain showed small brainstem, small cerebellar vermis, and atrophy of the cerebellum and cerebrum. PCH type 3 has been mapped to chromosome 7q11-21 and fine mapping is in progress.
Pontocerebellar Hypoplasia Type 4 (PCH Type 4)PCH type 4 is associated with severe neonatal encephalopathy, microcephaly, myoclonus, and muscular hypertonia. There is a severe loss of neurons in pontine and olivary nuclei in addition to the hypoplasia of the cerebellum and a diffuse gliosis in white matter of both the cerebellum and all areas of the brain. This is a more severe and fatal variant of PCH type 2, which is associated with death within the first few weeks of life, is known as PCH type 4.
Pontocerebellar Hypoplasia Type 5 (PCH Type 5)
PCH type 5 is similar to PCH type 4, but differs in having in-utero fetal seizure-like activity. These patients show evidence of severe olivopontocerebellar hypoplasia and degeneration, dysplastic, c-shaped inferior olivary nuclei, absent or immature dentate nuclei and cell paucity more marked for the cerebellar vermis than the hemispheres.
Pontocerebellar Hypoplasia Type 6 (PCH Type 6)
PCH type 6 manifests as early as the first day of life or within the first month of life as infantile encephalopathy, with generalized hypotonia, lethargic, poor sucking and poor feeding. Recurrent apnea, intractable seizures occur early in the course of this condition.
Neonatal MRI of the brain reveals cerebellar and vermian hypoplasia but normal brain volume while follow-up studies portray evidence of progressive atrophy of the cerebellum, pons, cerebral cortex, and white matter. Activities of mitochondrial complexes I, III, and IV in muscle from this patient were markedly reduced, but activity of complex II was relatively preserved.