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About the Author
- Dr. Sushil Mansingani, Third year resident, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
- Dr. Sameer Raniga, Assistant Professor, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
- Dr. Amit Prajapati, Second year resident, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
- Dr. Alpesh Pancholi, Second year resident, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
- Dr. Prakash A. Vohra, Professor and Head, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
- Dr. Vijay Vaidya, Assistant Professor, Department of Radiology, S.S.G. Hospital and Medical College, Baroda.
Lissencephaly is a malformation of the brain in which the brain surface is smooth rather than convoluted. The surface of the brain is formed by a complex series of ridges and valleys. The ridges are called gyri or convolutions, and the valleys are called sulci. In children with lissencephaly, the normal convolutions are absent or only partly formed, so the surface is smooth. Lissencephaly is usually diagnosed based on interpretation of either CT or MRI scan of the brain. It can be suspected based on ultrasound in newborn children. We are presenting a rare case of lissencephaly in a 9 year old mentally retarded female presented with history of recurrent seizures, diagnosed on MRI.
KEYWORDS: Lissencephaly, MRI.
Lissencephaly can be simplified by the word’s Greek roots” lissos” means smooth and “cephalos” means brain. Lissencephaly, which literally means "smooth brain," is a rare brain formation disorder characterized by the lack of normal convolutions (folds) in the brain, and an abnormally small head (microcephaly). It is caused by defective neuronal migration, the process in which nerve cells move from their place of origin to their permanent location. Symptoms of the disorder may include unusual facial appearance, difficulty swallowing, failure to thrive, and severe psychomotor retardation. Anomalies of the hands, fingers, or toes, muscle spasms, and seizures may also occur. Lissencephaly may be associated with other diseases including isolated lissencephaly sequence, Miller-Dieker syndrome, and Walker-Warburg syndrome .
A 9 year old female child presented with delayed milestones, recurrent generalized seizures. On examination child had microcephaly, diplegia and severe mental retardation. The patient was subjected to MRI of the brain. On T1 & T2 weighted axial MRI scans, there is thickened cortex with absence of sulcation & gyration giving smooth surface of the brain with shallow sylvian fissures giving figure of 8 appearance. (Fig 1 & fig 2) The MRI findings are consistent with lissencephaly type I.
Neuronal migration disorder (called cortical developmental anomaly), term referring to a wide spectrum of developmental malformations of the cortex caused by disruption to its normal process of formation, which includes proliferation, migration and organization (lamination, gyration and sulcation).
The term lissencephaly means "smooth brain" and refers to paucity of gyral and sulcal development on the surface of the brain . It is a severe form of agyria pachygyria group of disorders of neuronal migration . This complex process of cell migration can be interfered with by many causes sporadic, unknown, chromosomal or genetic and intrauterine infections . Agyria is defined as absence of gyri on the surface of the brain and is synonymous with "complete lissencephaly". Pachygyria is defined as the presence of a few broad, flat gyri and is used interchangeably with the term incomplete lissencephaly .
Lissencephaly and pachygyria are due to neuronal migration defects, usually genetic or toxic in origin. When the brain forms during the first month of pregnancy, all the nerve cells are located in the center surrounding a fluid-filled cavity called the central canal. During the second month, support cells begin to send branches up to the surface of the brain. During the third and fourth months, the nerve cells climb up these branches to reach the surface. Each wave of nerve cells climbs above the preceding waves so that the last wave of cells is closest to the surface. The developing brain of a 16 or 17 week fetus normally has a smooth, "agyric" appearance with shallow sylvian fissures and almost no surface sulcation. Normally, a large majority of all nerve cells are located at or just under the surface in an area called the cortex. In lissencephaly, many of the nerve cells do not reach the surface. They are stuck in an abnormal position, and so cannot make their usual connections with other nerve cells .
Cortical malformations manifest clinically by producing seizures, mental retardation and focal neurological deficits. Most frequently patients experience medically refractory epilepsy, whose degree of severity and time of onset is variable.
Lissencephaly are classified into types I, II, III, IV and V based on the morphology of the brain and associated brain anomalies. The only common finding among the different types of lissencephaly is that the brain surface looks smooth.
Lissencephaly type I results from a complete arrest of cortical neuronal migration between 12- and 16-weeks gestation. The MRI of the brain has a “figure-8 appearance” on axial images. This appearance results from the smooth brain surface, the large and vertically placed Sylvian fissure, the hypoplastic operculum, and enlarged ventricles with thickened cortex. Cerebral anomalies often associated with lissencephaly type I are hypoplasia of the corpus callosum, colpocephaly, and brainstem hypoplasia . Parenchymal calcifications can be present if lissencephaly is due to intrauterine infections. Neonates with lissencephaly type I do not have ocular or muscle abnormalities [1, 6]. Lissencephaly type I may occur as Isolated lissencephaly syndrome (no specific dysmorphysm); Miller-Dieker syndrome (specific dysmorphysm and deletion of the distal part of the short arm of chromosome 17); and Norman-Robert syndrome (dysmorphic features but no chromosome 17 abnormality). Miller-Dieker syndrome is probably the most common. Lissencephaly type II having an autosomal recessive pattern of transmission, probably results from a defect that involves the external basal lamina and the external layer of the cortex. The MRI of the brain shows findings, characteristic of type I lissencephaly, are less prominent or not present. The cerebral cortex of a neonate with lissencephaly type II shows a very irregular gray-white matter junction .
Lissencephaly type II is often associated with cerebellar abnormalities, hydrocephalus, hypoplasia of the corpus callosum, and hypomyelination. Neonates with lissencephaly type II have ocular anomalies and congenital muscular dystrophy. Lissencephaly type II may occur with: Walker-Warburg syndrome and Fukuyama congenital muscular dystrophy [1, 8].
Lissencephaly types III and IV present with microcephaly. In type III lissencephaly the cortex is thin. A third type of lissencephaly, the cerebro-cerebellar type, occurs without a figure of eight configuration & has microcephaly, enlarged ventricles & hypoplastic cerebellum and brain stem .
Lissencephaly type IV is also called radial microbrain. It has markedly reduced brain size despite normal gyral patterns, absence of destruction or gliosis, normal cortical thickness and normal cortical lamination .
Type V lissencephaly is a diffusely smooth brain caused by abnormal organization of cortical neurons, resulting in diffuse cortical dysplasia, or polymicrogyria (formation of multiple small gyri) .This type of lissencephaly is seen in patients with cortical involvement from CMV infection .
MR imaging is the most appropriate imaging modality to demonstrate cortical malformations and to differentiate various types of lissencephaly as well as associated intracranial anomalies. CT scan can miss some developmental malformations of the cortex and it is superior to MR only in the demonstration of calcified lesions and skull remodeling .
Karyotyping is recommended to detect the chromosomal defect. Differentiation from lissencephaly type II is important for genetic counseling purposes, considering that type II has an autosomal recessive pattern of transmission. No causal treatment is available at this time.
Usually severe mental retardation affects these patients. The prognosis is usually poor, failure to thrive, infantile spasms, and seizures are also expected .
Figure 1 & 2:
T1 & T2W axial MR images shows thickened cortex with absence of sulcation & gyration giving smooth surface of the brain with shallow sylvian fissures giving figure of 8 appearances.
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