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.

Isolated lissencephaly sequence (ILS) is a condition that affects brain development before birth. Normally, the cells that make up the exterior of the brain (cerebral cortex) are well-organized, multi-layered, and arranged into many folds and grooves (gyri). In people with ILS, the cells of the cerebral cortex are disorganized, and the brain surface is abnormally smooth with an absence (agyria) or reduction (pachygyria) of folds and grooves. In most cases, these abnormalities impair brain growth, causing the brain to be smaller than normal (microcephaly). This underdevelopment of the brain causes severe intellectual disability, delayed development, and recurrent seizures (epilepsy) in individuals with ILS.\n\nMore than 90 percent of individuals with ILS develop epilepsy, often within the first year of life. Up to 80 percent of infants with ILS have a type of seizure called infantile spasms, these seizures can be severe enough to cause brain dysfunction (epileptic encephalopathy). After the first months of life, most children with ILS develop a variety of seizure types, including persisting infantile spasms, short periods of loss of consciousness (absence seizures); sudden episodes of weak muscle tone (drop attacks); rapid, uncontrolled muscle jerks (myoclonic seizures); and episodes of muscle rigidity, convulsions, and loss of consciousness (tonic-clonic seizures).\n\nInfants with ILS may have poor muscle tone (hypotonia) and difficulty feeding, which leads to poor growth overall. Hypotonia also affects the muscles used for breathing, which often causes breathing problems that can lead to a life-threatening bacterial lung infection known as aspiration pneumonia. Children with ILS often develop muscle stiffness (spasticity) in their arms and legs and an abnormal side-to-side curvature of the spine (scoliosis). Rarely, the muscle stiffness will progress to paralysis (spastic paraplegia). Individuals with ILS cannot walk and rarely crawl. Most children with ILS do not develop communication skills.

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.