Clonus is an involuntary tendon reflex that causes repeated flexion and extension of the foot. Ankle clonus is tested by rapidly flexing the foot upward.

An abnormal restriction of fetal growth with fetal weight below the tenth percentile for gestational age.

Slow or limited growth after birth.

Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm.

A condition in which the stomach contents leak backwards from the stomach into the esophagus through the lower esophageal sphincter.

A tonic seizure is a type of motor seizure characterized by unilateral or bilateral limb stiffening or elevation, often with neck stiffening.

A bilateral tonic-clonic seizure is a seizure defined by a tonic (bilateral increased tone, lasting seconds to minutes) and then a clonic (bilateral sustained rhythmic jerking) phase.

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.

Loss of developmental skills, as manifested by loss of developmental milestones.

The burst suppression pattern in electroencephalography refers to a characteristic periodic pattern of low voltage (<10 microvolts) suppressed background and a relatively shorter pattern of higher amplitude slow, sharp, and spiking complexes.

An increase in size of the ventricular system of the brain.

A decrease in the level of N-acetyl aspartate in the brain identified by magnetic resonance spectroscopy (MRS).

An increase in the level of lactate in the brain identified by magnetic resonance spectroscopy (MRS).

A myoclonic seizure is a type of motor seizure characterized by sudden, brief (<100 ms) involuntary single or multiple contraction of muscles or muscle groups of variable topography (axial, proximal limb, distal). Myoclonus is less regularly repetitive and less sustained than is clonus.

Interictal refers to a period of time between epileptic seizures. Electroencephalographic (EEG) patterns are important in the differential diagnosis of epilepsy, and the EEG is almost always abnormal during a seizure. Some persons with seizures may show EEG abnormalities between seizures, while others do not. In some cases, multiple interictal EEGs must be recorded before an abnormality is observed. In most cases the electrographic pattern of seizure onset is completely different from the activity recorded during interictal discharge.

An abnormally thin corpus callous, due to atrophy, hypoplasia or agenesis. This term is intended to be used in situations where it is not known if thinning of the corpus callosum (for instance, as visualized by magnetic resonance tomography) is due to abnormal development (e.g. a leukodystrophy) or atrophy following normal development (e.g. neurodegeneration).

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.

A deformity of the chest caused by overgrowth of the ribs and characterized by protrusion of the sternum.

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

Increased height of the central portion of the eyebrow, forming a crescent, semicircular, or inverted U shape.

Diminished amniotic fluid volume in pregnancy.

Atrophy of the optic nerve. Optic atrophy results from the death of the retinal ganglion cell axons that comprise the optic nerve and manifesting as a pale optic nerve on fundoscopy.

A form of loss of vision caused by damage to the visual cortex rather than a defect in the eye.