A generalized-onset seizure is a type of seizure originating at some point within, and rapidly engaging, bilaterally distributed networks. The networks may include cortical and subcortical structures but not necessarily the entire cortex.

A spectrum of malformations of cortical development caused by insufficient neuronal migration that subsumes the terms agyria, pachygyria and subcortical band heterotopia. See also neuropathological definitions for 2-, 3-, and 4-layered lissencephaly.

Cerebellar hypoplasia is a descriptive term implying a cerebellum with a reduced volume, but a normal shape and is stable over time.

Lissencephaly (LIS), literally meaning smooth brain, is characterized by smooth or nearly smooth cerebral surface and a paucity of gyral and sulcal development, encompassing a spectrum of brain surface malformations ranging from complete agyria to subcortical band heterotopia (SBH). Classic lissencephaly is associated with an abnormally thick cortex, reduced or abnormal lamination, and diffuse neuronal heterotopia. SBH consists of circumferential bands of heterotopic neurons located just beneath the cortex and separated from it by a thin band of white matter. SBH represents the less severe end of the lissencephaly spectrum of malformations (Pilz et al., 1999, summary by Kato and Dobyns, 2003). Agyria, i.e., brain without convolutions or gyri, was considered a rare malformation until recent progress in neuroradiology (Bordarier et al., 1986). With this technical advantage, a number of lissencephaly syndromes have been distinguished. Classic lissencephaly (formerly type I) is a brain malformation caused by abnormal neuronal migration at 9 to 13 weeks' gestation, resulting in a spectrum of agyria, mixed agyria/pachygyria, and pachygyria. It is characterized by an abnormally thick and poorly organized cortex with 4 primitive layers, diffuse neuronal heterotopia, enlarged and dysmorphic ventricles, and often hypoplasia of the corpus callosum (Lo Nigro et al., 1997). Kato and Dobyns (2003) presented a classification system for neuronal migration disorders based on brain imaging findings and molecular analysis. The authors also reviewed the contributions and interactions of the 5 genes then known to cause human lissencephaly: LIS1 (PAFAH1B1), 14-3-3-epsilon (YWHAE), DCX, RELN, and ARX. Genetic Heterogeneity of Lissencephaly Lissencephaly is a genetically heterogeneous disorder. See also LIS2 (257320), caused by mutation in the RELN gene (600514) on chromosome 7q22; LIS3 (611603), caused by mutation in the TUBA1A gene (602529) on chromosome 12q13; LIS4 (614019), caused by mutation in the NDE1 gene (609449) on chromosome 16p13; LIS5 (615191), caused by mutation in the LAMB1 gene (150240) on chromosome 7q31; LIS6 (616212), caused by mutation in the KATNB1 gene (602703) on chromosome 16q21; LIS7 (616342), caused by mutation in the CDK5 gene (123831) on chromosome 7q36; LIS8 (617255), caused by mutation in the TMTC3 gene (617218) on chromosome 12q21; LIS9 (618325), caused by mutation in the MACF1 gene (608271) on chromosome 1p34; and LIS10 (618873), caused by mutation in the CEP85L gene (618865) on chromosome 6q22. X-linked forms include LISX1 (300067), caused by mutation in the DCX gene (300121) on chromosome Xq23, and LISX2 (300215), caused by mutation in the ARX gene (300382) on chromosome Xp21. See also Miller-Dieker lissencephaly syndrome (MDLS; 247200), a contiguous gene microdeletion syndrome involving chromosome 17p13 and including the PAFAH1B1 and YWHAE (605066) genes. Lissencephaly caused by mutations in the PAFAH1B1 gene is also called 'isolated' lissencephaly to distinguish it from the accompanying features of MDLS.

A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age.

Underdevelopment of the pons.

Hypotonia is an abnormally low muscle tone (the amount of tension or resistance to movement in a muscle). Even when relaxed, muscles have a continuous and passive partial contraction which provides some resistance to passive stretching. Hypotonia thus manifests as diminished resistance to passive stretching. Hypotonia is not the same as muscle weakness, although the two conditions can co-exist.

Head circumference below 2 standard deviations below the mean for age and gender.

Localized fluid retention and tissue swelling caused by a compromised lymphatic system.

Anterior positioning of the nasal root in comparison to the usual positioning for age.

Inclination of the anterior surface of the forehead from the vertical more than two standard deviations above the mean (objective); or apparently excessive posterior sloping of the forehead in a lateral view.