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 decreased magnitude of the sensory perception of sound.

A movement disorder which consists of effects including diminished voluntary movements and the presence of involuntary movements.

Involuntary movements characterized by both athetosis (inability to sustain muscles in a fixed position) and chorea (widespread jerky arrhythmic movements).

Inability to initiate changes in activity or movement and to perform ordinary volitional movements rapidly and easily.

Functional neurological abnormalities related to dysfunction of the pyramidal tract.

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.

Truncal ataxia is a sign of ataxia characterized by instability of the trunk. It usually occurs during sitting.

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.

Cerebellar atrophy is defined as a cerebellum with initially normal structures, in a posterior fossa with normal size, which displays enlarged fissures (interfolial spaces) in comparison to the foliae secondary to loss of tissue. Cerebellar atrophy implies irreversible loss of tissue and result from an ongoing progressive disease until a final stage is reached or a single injury, e.g. an intoxication or infectious event.

Spasticity (velocity-dependent increase in tonic stretch reflexes with increased muscle tone and hyperexcitable tendon reflexes) in the muscles of the lower limbs, hips, and pelvis.

A reduced frequency or duration of eye contact.

Difficulty to maintain correct position of the head while standing or sitting. Infant head lag is observed when the head seems to flop around or lags posteriorly behind the trunk. Several articles have maintained that head lag should be absent by age 3 to 4 months.

Muscular hypotonia (abnormally low muscle tone) affecting the musculature of the trunk.

An increased concentration of alanine in the blood.

Abnormally increased level of blood lactate (2-hydroxypropanoic acid). Lactate is produced from pyruvate by lactate dehydrogenase during normal metabolism. The terms lactate and lactic acid are often used interchangeably but lactate (the component measured in blood) is strictly a weak base whereas lactic acid is the corresponding acid. Lactic acidosis is often used clinically to describe elevated lactate but should be reserved for cases where there is a corresponding acidosis (pH below 7.35).

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 pale yellow discoloration of the optic disc (the area of the optic nerve head in the retina). The optic disc normally has a pinkish hue with a central yellowish depression.

An abnormally slow velocity of the saccadic eye movements.

Disorder of the optic nerve.

A reduction in the activity of the mitochondrial respiratory chain complex I, which is part of the electron transport chain in mitochondria.