Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients may show a favorable response to amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

NKX6-2-related disorder is characterized by a spectrum of progressive neurologic manifestations resulting from diffuse central nervous system hypomyelination. At the severe end of the spectrum is neonatal-onset nystagmus, severe spastic tetraplegia with joint contractures and scoliosis, and visual and hearing impairment, all of which rapidly progress resulting in death in early childhood. At the milder end of the spectrum is normal achievement of early motor milestones in the first year of life followed by slowly progressive complex spastic ataxia with pyramidal findings (spasticity with increased muscle tone and difficulty with gait and fine motor coordination) and cerebellar findings (nystagmus, extraocular movement disorder, dysarthria, titubation, and ataxia) with loss of developmental milestones. To date NKX6-2-related disorder has been reported in 25 individuals from 13 families.

Childhood-onset neurodegeneration with cerebellar atrophy (CONDCA) is a severe autosomal recessive neurodevelopmental disorder affecting the central and peripheral nervous system. Patients present in the first year of life with global developmental delay, impaired intellectual development, poor or absent speech, and motor abnormalities. Brain imaging shows cerebellar atrophy. The severity is variable, but death in childhood may occur (Shashi et al., 2018).

Neurodevelopmental disorder with progressive movement abnormalities (NEDPM) is an autosomal recessive complex neurologic disorder characterized by global developmental delay apparent from infancy, moderately to severely impaired intellectual development, poor or absent speech, behavioral abnormalities, and various hyperkinetic movement disorders, including dystonia, spasticity, and cerebellar ataxia, that interfere with gait and cause a stooped posture. The disorder appears to be progressive with age-related deterioration of cognitive and motor function; parkinsonism may develop in older patients. Additional more variable features include seizures, dysmorphic facial features, oculomotor defects, and brain imaging abnormalities (Kaiyrzhanov et al., 2024).