TBC1D24-related disorders comprise a continuum of features that were originally described as distinct, recognized phenotypes: DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures), with profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / developmental delay, and seizures; familial infantile myoclonic epilepsy (FIME), with early-onset myoclonic seizures, focal epilepsy, dysarthria, and mild-to-moderate intellectual disability; progressive myoclonus epilepsy (PME), with action myoclonus, tonic-clonic seizures, ataxia, and progressive neurologic decline; rolandic epilepsy with paroxysmal exercise-induced dystonia and writer's cramp (EPRPDC); developmental and epileptic encephalopathy (DEE), including epilepsy of infancy with migrating focal seizures (EIMFS); autosomal recessive nonsyndromic hearing loss (DFNB); and autosomal dominant nonsyndromic hearing loss (DFNA).

Pyruvate dehydrogenase deficiency is characterized by the buildup of a chemical called lactic acid in the body and a variety of neurological problems. Signs and symptoms of this condition usually first appear shortly after birth, and they can vary widely among affected individuals. The most common feature is a potentially life-threatening buildup of lactic acid (lactic acidosis), which can cause nausea, vomiting, severe breathing problems, and an abnormal heartbeat. People with pyruvate dehydrogenase deficiency usually have neurological problems as well. Most have delayed development of mental abilities and motor skills such as sitting and walking. Other neurological problems can include intellectual disability, seizures, weak muscle tone (hypotonia), poor coordination, and difficulty walking. Some affected individuals have abnormal brain structures, such as underdevelopment of the tissue connecting the left and right halves of the brain (corpus callosum), wasting away (atrophy) of the exterior part of the brain known as the cerebral cortex, or patches of damaged tissue (lesions) on some parts of the brain. Because of the severe health effects, many individuals with pyruvate dehydrogenase deficiency do not survive past childhood, although some may live into adolescence or adulthood.

PRRT2-related disorder, caused by heterozygous pathogenic variants in the gene PRRT2 (associated with aberrant synaptic transmission), is characterized by three core episodic neurologic phenotypes: epilepsy, movement disorder, and migraine. Age at onset and phenotypes range from neonatal/infantile (self-limited [familial] infantile epilepsy), to childhood (childhood absence epilepsy), to adolescence to adulthood (paroxysmal kinesigenic dyskinesia [PKD] or migraine). As individuals with PRRT2-related disorder age, they may exhibit one of more of these core phenotypes in various combinations, either concurrently or sequentially. Additionally, family members with the same pathogenic PRRT2 variant may display different core phenotypes.

People with familial paroxysmal nonkinesigenic dyskinesia experience episodes of abnormal movement that are brought on by alcohol, caffeine, stress, fatigue, menses, or excitement or develop without a known cause. Episodes are not induced by exercise or sudden movement and do not occur during sleep. An episode is characterized by irregular, jerking or shaking movements that range from mild to severe. In this disorder, the dyskinesia can include slow, prolonged contraction of muscles (dystonia); small, fast, "dance-like" motions (chorea); writhing movements of the limbs (athetosis); and, rarely, flailing movements of the limbs (ballismus). The dyskinesia also affects muscles in the torso and face. The type of abnormal movement varies among affected individuals, even among affected members of the same family. Individuals with familial paroxysmal nonkinesigenic dyskinesia do not lose consciousness during an episode. Most people do not experience any neurological symptoms between episodes.\n\nIndividuals with familial paroxysmal nonkinesigenic dyskinesia usually begin to show signs and symptoms of the disorder during childhood or their early teens. Episodes typically last 1 to 4 hours, and the frequency of episodes ranges from several per day to one per year. In some affected individuals, episodes occur less often with age.\n\nFamilial paroxysmal nonkinesigenic dyskinesia is a disorder of the nervous system that causes episodes of involuntary movement. Paroxysmal indicates that the abnormal movements come and go over time. Nonkinesigenic means that episodes are not triggered by sudden movement. Dyskinesia broadly refers to involuntary movement of the body.

Familial paroxysmal nonkinesigenic dyskinesia (PNKD) is characterized by unilateral or bilateral involuntary movements. Attacks are typically precipitated by coffee, tea, or alcohol; they can also be triggered by excitement, stress, or fatigue, or can be spontaneous. Attacks involve dystonic posturing with choreic and ballistic movements, may be accompanied by a preceding aura, occur while the individual is awake, and are not associated with seizures. Attacks last minutes to hours and rarely occur more than once per day. Attack frequency, duration, severity, and combinations of symptoms vary within and among families. Age of onset is typically in childhood or early teens but can be as late as age 50 years.

Glucose transporter type 1 deficiency syndrome (Glut1DS) is a disorder of brain energy metabolism. Glucose, the essential metabolic fuel for the brain, is transported into the brain exclusively by the protein glucose transporter type 1 (Glut1) across the endothelial cells forming the blood-brain barrier (BBB). Glut1DS results from the inability of Glut1 to transfer sufficient glucose across the BBB to meet the glucose demands of the brain. The needs of the brain for glucose increase rapidly after birth, peaking in early childhood, remaining high until about age 10 years, then gradually decreasing throughout adolescence and plateauing in early adulthood. When first diagnosed in infancy to early childhood, the predominant clinical findings of Glut1DS are paroxysmal eye-head movements, pharmacoresistant seizures of varying types, deceleration of head growth, and developmental delay. Subsequently children develop complex movement disorders and intellectual disability ranging from mild to severe. Institution of ketogenic diet therapies (KDTs) helps with early neurologic growth and development and seizure control. Typically, the earlier the treatment the better the long-term clinical outcome. When first diagnosed in later childhood to adulthood (occasionally in a parent following the diagnosis of an affected child), the predominant clinical findings of Glut1DS are usually complex paroxysmal movement disorders, spasticity, ataxia, dystonia, speech difficulty, and intellectual disability.

PRRT2-related disorder, caused by heterozygous pathogenic variants in the gene PRRT2 (associated with aberrant synaptic transmission), is characterized by three core episodic neurologic phenotypes: epilepsy, movement disorder, and migraine. Age at onset and phenotypes range from neonatal/infantile (self-limited [familial] infantile epilepsy), to childhood (childhood absence epilepsy), to adolescence to adulthood (paroxysmal kinesigenic dyskinesia [PKD] or migraine). As individuals with PRRT2-related disorder age, they may exhibit one of more of these core phenotypes in various combinations, either concurrently or sequentially. Additionally, family members with the same pathogenic PRRT2 variant may display different core phenotypes.

Developmental and epileptic encephalopathy-64 (DEE64) is a neurodevelopmental disorder characterized by onset of seizures usually in the first year of life and associated with intellectual disability, poor motor development, and poor or absent speech. Additional features include hypotonia, abnormal movements, and nonspecific dysmorphic features. The severity is variable: some patients are unable to speak, walk, or interact with others as late as the teenage years, whereas others may have some comprehension (summary by Straub et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Pontocerebellar hypoplasia type 17 (PCH17) is a severe autosomal recessive developmental disorder characterized by neonatal hypotonia, severe feeding difficulties, and respiratory insufficiency. Brain imaging shows cerebellar and brainstem hypoplasia. Most affected individuals die in infancy. Those who survive show variable developmental delay. Other features of the disorder include distal hypertonia, poor overall growth, visual defects, autonomic problems, dysmorphic features, and seizures (Coolen et al., 2022). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).