Glossary in Congenital Malformations

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#Tectocerebellar dysraphia: Characterized by partial or total vermal agenesis, a severe deformation of the midbrain tectum and a cerebelloencephalocele. The occipital encephalocele contains cerebellar tissue with a severe malformation of the tectum. The tectum may be absent or extend dorsally into the encephalocele. The vermis is either aplastic, or represented by a rudimentary anterior portion. There is laterobasal displacement of the cerebellar hemispheres. The brain stem is displaced caudally. Other malformation may also be present. Microscopically, the cerebellar cortex may be dysplastic. There is also disorganization of the nuclear and fascicular structure of the brain stem and spinal cord. Occipital encephalocele with tectocerebellar dysraphia should be distinguished from rhombic roof ventriculocele and other malformation of the cerebellar vermis.

#Tethered cord syndrome:

Location: Most commonly due to terminal fixation of the conus that prevens the normal upward movement of the spinal cord. It can also occur in the upper cord.
Clinical: Tethering of the spinal cord results in progressive neurologic impairment as a result of recurrent minor traction injuries. Dermatologic signs such as hairy patch skin dimple or sinus, capillary hemangioma or atrophic skin may be found in the lumbar region. May be associated with other abnormalities such as VATER association.
Pathology: The site of attachment is often consisted of a fibrofaty mass with or without neurological tissue or foreign bodies. Heterogous or hamatomatous tissue is frequent seen in the filum terminale; probably related to the pleuripotential cells in the tail bud. Other type of pathology may be seen. Pathogenesis may be related to incomplete separation of neural from mesodermal components.
Tethered cord syndrome can also be seen in adults. For tethered cord in adult, the most common pathologic changes include The most frequent tethering lesions were lipoma in, followed by tight terminal filum, split cord malformation and secondary adhesions in 12 patients, respectively. [Huttmann S et al., J Neurosurg 2001 Oct;95(2 Suppl):173-8].

#Thanatophoric dwarfism:

Clinical: (thana-, Greek, death) Often misdiagnosed as achondroplasia. Infants present with severe hypotonia and skeletal abnormalitis and usually die shortly, in terms of hours, after birth.

GROSS PATHOLOGY: Characteristic abnormally large and hyperconvoluted temporal lobes.

Pathology:

· CNS: Some of the malformation in the brain can be classified as migration disorders. Polymicrogyria could be present. There are characteristic convolutional anomalies and cortical dysplasia in the temporal lobes. Other findings include megalencephaly and sometimes hydrocephalus. The olivary nuclei dentate nuclei are often hyperconvoluted.

· Others: Thanatophoric dwarfism is characterized by chondrodysplasia, micromelia, narrow thorax, and a large head with or without trilobed skull (clover-leaf) anormaly. Malformations of the heart, kidney are also noted.

#Translocations:

· Balanced reciprocal translocation: There is reciprocal recombination between two chromosome arms from two different chromosomes. The overwhelming majority of persons with balanced reciprocal translocations are phenotypically normal.

· Robertsonian translocation: This refers to the recombination of whole chromosome arms.

#Trisomy 13:

· Clinical: Lethal in half of the cases within the first month of life. Incidence is about 1 in 10,000. About 20% are due to unbalanced translocation, with 13/14 translocation being most common.

· Pathology: They have mid-facial malformation. The most distinctive feature is ocular abnormalities including hypotelorism, micropthalamia, iris coloboma, cyclopia, and even absence of the eyes, or retinal dysplasia. Other features include cleft lip and palate. Holoprosencephaly and arhinencephaly occurs in about two-thirds of cases. Patients also have malformations in other systems.

#Trisomy 18:

· Clinical: 80% are seen in female. The features always include mental retardation, poor fetal growth, and failure to thrive. They have characteristic facial features with prominent occiput; facial features are tiny, and the jaw recedes. The ears are low set and malformed. The fists are clenched in a characteristic manner with the index and fifth digits overlapping the third and fourth.

· Pathology: Although several CNS abnormalities have been described, temporal lobe dysplasia is the only consistent abnormalities. The temporal lobe dysplasia consists of abnormal sulci on the external medial and inferior surfaces. The temporal horn was usually large or has diverticular extensions into the white matter. Ammon's horn was malformed, with poor demarcation of the unscrolled granular layer and an elongated undulating pyramidal layer. The dentate nuclei and olivary nuclei are often hyperconvoluted, malformations of other nuclei are also seen. Other abnormalities include holoprosencephaly, arhinencephaly, Chiari II malformation, meningomyelocele, megalencephaly, subarachnoid heterotopia, ventricular dilation and fiber tract abnormality, posterior fossa hypoplasia.

#Trisomy 21- Down’s syndrome:

· Genetics: Trisomy 21 (95% of cases), translocation (less than 5%); mosaicism (less than 1%).

· GROSS PATHOLOGY: Brain weight starts to decrease relative to controls after 1 year of age. The fully-grown brain is small and weighs between 1000 to 1200 grams. It has a characteristic box like appearance. The occipital lobe has steep, almost vertical rising; the frontal lobe is short in anteroposterior diameter. The gyral pattern may be normal but there may also be asymmetry between the two sides and a poverty of secondary sulci. The most common gyral abnormality is narrowness of the superior temporal gyri and perpendicularly oriented temporal gyri. The cerebellum and brain stem can also appear small in face of a small cerebrum.

· Congenital heart diseases are frequently associated with Trisomy 21 and their contribution to the CNS changes are not clear.

· Microscopic features are non-specific: In term babies with Down syndrome, the cellular distribution of neurons in the visual cortex is more diffuse and cell layers are poorly defined. Golgi impregnantion revealed a dramatic cessation of dendritic growth, with actual dentritic shortening or atrophy and represent the most consistent nerual abnormalities in Down syndrome. They also develop characteristic histopathological alterations virtually identical to Alzheimer's disease when they reach their thirties and fourties.

· Alzheimer type changes are frequently seen in the young age.

#Tuberous sclerosis: Tuberous sclerosis is a systemic disease with the brain as the most frequently affected organ. Clinically it is characterized by seizures, mental retardation and adenoma sebaceum. Pathologically, there is cortical tuber, subependymal nodules, and heterotopia in white matter of the brain. Subependymal giant cell astrocytoma (SEGA) is almost always associated with tuberous sclerosis. Review article- [Webb and Osborne, 1995]

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