Radiological Diagnosis

LISSENCEPHALY

Lissencephaly is a group of neurodisorders characterised by generalised paucity of gyral and sulcal formation[2] otherwise known as agyria (complete absence of gyri) and pachygyria (presence of relatively few, broad, flat, coarse gyri). This results from abnormal neuronal migration between 8 and 16 weeks of gestation[3]. The brain retains the appearance of fetal brain before 25 weeks of gestation. Patients with lissencephaly are usually microcephalic, although they may be more normocephalic. Profound mental retardation, generalised hypotonia, feeding problems and seizures are usually seen after the newborn period. Patients with band heterotropia exhibit various degrees of developmental delay and seizures in the first decade of life [4].

There are 5 types of lissencephaly [7].

Type 1 (Classic lissencephaly) - associated with MillerDieker syndrome and Norman-Robert's syndrome. Polyhydramnios and failure to thrive are usually associated with this condition

Type 2 (Cobblestone lissencephaly) - is a complex anomaly harbouring different neuronal migrations

Type 3 - comprises patients with true micrencephaly (lissencephalic brain less than 100gms)

Type 4 (Radial microbrain) - size of the brain is reduced despite maintaining normal sulcation and gyration

Type 5 - refers to diffuse polymicrogyria, probably due to cytomegalovirus infection

Type 1 lissencephaly is further divided into 3 subtypes[2]

a) Associated with abnormalities of chromosome 17

b) Associated with X linked mutations

c) Unidentified genetic abnormalities

Type 2 lissencephaly is further divided into 3 subtypes[2]

a) Walker -Warburg syndrome (severe type)

b) Muscle- eye brain disease (moderate type)

c) Fukuyama congenital muscular dystrophy (mild type)

M.R.I is the neuroimaging of choice. Smooth appearance (introduced by Owen in 1868 to describe smooth brains of lower mammalian species) of the brain parenchyma is noted in the parietal and occipital lobes [5],[6]. Bilateral symmetric pachygyria is noted in the frontal lobes and agyria in the parieto-occipital lobes. The Sylvian fissures are vertically oriented and give the appearance of "Figure of 8 " appearance due to narrowing of the cerebrum at the mid region by the Sylvian fissures [1],[7]. There is reduction in the volume of the white matter with presence of lamellated heterotropia (focal collections of ectopic neurons in the cerebral hemispheres) in both the frontal lobes. The lateral ventricles are prominent and show presence of Colpocephaly. Treatment consists of anticonvulsants and genetic counseling.