Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease characterized by infantile-onset diarrhea, childhood-onset cataract, adolescent- to young adult-onset tendon xanthomas, and adult-onset progressive neurologic dysfunction (dementia, psychiatric disturbances, pyramidal and/or cerebellar signs, dystonia, atypical parkinsonism, peripheral neuropathy, and seizures). Chronic diarrhea from infancy and/or neonatal cholestasis may be the earliest clinical manifestation. In approximately 75% of affected individuals, cataracts are the first finding, often appearing in the first decade of life. Xanthomas appear in the second or third decade; they occur on the Achilles tendon, the extensor tendons of the elbow and hand, the patellar tendon, and the neck tendons. Xanthomas have been reported in the lung, bones, and central nervous system. Some individuals show cognitive impairment from early infancy, whereas the majority have normal or only slightly impaired intellectual function until puberty; dementia with slow deterioration in intellectual abilities occurs in the third decade in more than 50% of individuals. Neuropsychiatric symptoms such as behavioral changes, hallucinations, agitation, aggression, depression, and suicide attempts may be prominent. Pyramidal signs (i.e., spasticity) and/or cerebellar signs almost invariably become evident between ages 20 and 30 years. The biochemical abnormalities that distinguish CTX from other conditions with xanthomas include high plasma and tissue cholestanol concentration, normal-to-low plasma cholesterol concentration, decreased chenodeoxycholic acid (CDCA), increased concentration of bile alcohols and their glyconjugates, and increased concentrations of cholestanol and apolipoprotein B in cerebrospinal fluid.

Alexander disease, a progressive disorder of cerebral white matter caused by a heterozygous GFAP pathogenic variant, comprises a continuous clinical spectrum most recognizable in infants and children and a range of nonspecific neurologic manifestations in adults. This chapter discusses the spectrum of Alexander disease as four forms: neonatal, infantile, juvenile, and adult. The neonatal form begins in the first 30 days after birth with neurologic findings (e.g., hypotonia, hyperexcitability, myoclonus) and/or gastrointestinal manifestations (e.g., gastroesophageal reflux, vomiting, failure to thrive), followed by severe developmental delay and regression, seizures, megalencephaly, and typically death within two years. The infantile form is characterized by variable developmental issues: initially some have delayed or plateauing of acquisition of new skills, followed in some by a loss of gross and fine motor skills and language during in the first decade or in others a slow disease course that spans decades. Seizures, often triggered by illness, may be less frequent/severe than in the neonatal form. The juvenile form typically presents in childhood or adolescence with clinical and imaging features that overlap with the other forms. Manifestations in early childhood are milder than those in the infantile form (e.g., mild language delay may be the only developmental abnormality or, with language acquisition, hypophonia or nasal speech may alter the voice, often prior to appearance of other neurologic features). Vomiting and failure to thrive as well as scoliosis and autonomic dysfunction are common. The adult form is typically characterized by bulbar or pseudobulbar findings (palatal myoclonus, dysphagia, dysphonia, dysarthria or slurred speech), motor/gait abnormalities with pyramidal tract signs (spasticity, hyperreflexia, positive Babinski sign), or cerebellar abnormalities (ataxia, nystagmus, or dysmetria). Others may have hemiparesis or hemiplegia with a relapsing/remitting course or slowly progressive quadriparesis or quadriplegia. Other neurologic features can include sleep apnea, diplopia or disorders of extraocular motility, and autonomic dysfunction.

Neurodevelopmental disorder with hypotonia and brain abnormalities (NEDHYBA) is characterized by impaired development of motor skills, cognitive function, and speech acquisition beginning in infancy or early childhood. Some affected individuals may have feeding difficulties, seizures, behavioral abnormalities, and nonspecific dysmorphic facial features. Brain imaging shows variable abnormalities, including corpus callosum defects, cerebellar defects, and decreased white matter volume. There is significant phenotypic variability (summary by Duncan et al., 2021).

Neurodevelopmental disorder with seizures and brain abnormalities (NEDSBA) is an autosomal recessive neurologic disorder characterized by global developmental delay and onset of seizures in the first months of life associated with structural brain defects on brain imaging. Additional features may include pigmentary retinopathy with poor visual fixation and spasticity (summary by Duncan et al., 2021).